1-semiconductors

SEMICONDUCTORS AND ELECTRONICS INTRODUCTION

To say that the invention of semiconductor devices was a revolution would not be an exaggeration. Not only was this an impressive technological accomplishment, but it paved the way for developments that would indelibly alter modern society. Semiconductor devices made possible miniaturized electronics, including computers, certain types of medical diagnostic and treatment equipment, and popular telecommunication devices, to name a few applications of this technology. 05 July 2005

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BASIC CONCEPTS An atom consists of a nucleus occupied by protons and neutrons with electrons revolving around the nucleus. Neutrons are electrically neutral, protons have a positive charge and electrons have a negative charge both equal in magnitude. Thus an atom is electrically neutral under normal conditions. Electrons around the nucleus are grouped into energy bands called shells. Each shell consists of one or more energy levels called orbits. The electrons in inner shells have lower energies while those in outer shells have higher energies. The difference of energy between energy levels within a shell is very small as compared to that between two different shells. Thus there are between shells. 05 July 2005discrete energy Engineergaps M S Ayubi 2

The electrons in the inner shells being nearer to the nucleus are tightly bound to it and thus they require greater energy to leave their positions. The electrons in the outer shells contrarily, are loosely bound to the nucleus thus require lesser energy to leave their positions. The outermost shell in an atom is called valence shell and the electrons in this shell are called valence electrons. The atom minus its valence shell is referred to as core of the atom. Thus an atom can be modeled simply as a core surrounded by the valence electrons. If sufficient energy is provided, the valence electrons may leave the atom and move into what is called a conduction band. The energy absorbed by the electron is used to overcome the energy gap between the valence band and the conduction band. The electron in the conduction band is now a free electron and is not associated with any particular atom. 05 July 2005

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Fermi Energy • The highest energy an electron reached if you were to fill the solid with the intrinsic number of electrons at absolute zero. (No added thermal energy) • Meaningful! There is a sea of electrons sitting beneath this energy. – If you bring two solids together with different Fermi energies, the electrons will move around to reach an equilibrium.(Foreshadowing: PN junction) – If you try to put a lower energy electron into a solid (at absolute zero) with a higher Fermi energy, it won’t fit. It cannot be done due to Pauli Exclusion. • If the highest energy electron exactly fills a band, the Fermi Energy is near the center of the bands. 05 July 2005

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eyond 0 K: Fermi-Dirac Statistics • Fermi Energy: The energy state whose probability of being occupied is exactly 1/2 . • Electrons obey Fermi-Dirac statistics, which describe the probability of an electron being present in an allowed energy state. • Note that if there are no states at a given energy (i.e., in the band gap) there will be no electrons, even if there is finite probability. 05 July 2005

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A material that readily allows electric current to flow through it is called a conductor. In conductor atoms the valence and conduction bands overlap each other so that there are a large number of free electrons in the material supporting the current flow. A material that doesn’t readily allow current to flow through it is called an insulator. In insulators the energy gap between the valence band and the conduction band is very wide and the chances of valence electrons crossing it over into the conduction band and hence 05 July 2005 Engineer M S Ayubi becoming free are very limited.

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Semiconductors lie somewhere between conductors and insulators in their ability to conduct electric current. The energy gap between their valence and conduction band is narrower than in the insulators but still it is wide enough to stop electrons from crossing it over normally. However, when supplied with sufficient energy the valence electrons may actually cross the gap over and be free electrons.

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ELECTRONS AND "HOLES“ in Semiconductors At low temperatures, there is little thermal energy available to push valence electrons across the forbidden energy gap, and the semi-conducting material thus acts as an insulator. At higher temperatures, though, the ambient thermal energy becomes sufficient to force electrons across the gap into the conduction band, and the material will conduct electricity. When a voltage is applied to a crystal containing these conduction band electrons, the electrons move through the crystal toward the applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow. There is still another type of current in a pure semiconductor. This current occurs when a covalent bond is broken and a vacancy is left in the atom by the missing valence electron. This vacancy is commonly referred to as a hole. The hole is considered to have a positive charge because its atom is deficient by one electron, which causes the protons to outnumber the electrons. 05 July 2005 Engineer M S Ayubi 9

In the theory just described, two current carriers were created by the breaking of covalent bonds: the negative electron and the positive hole. These carriers are referred to as electron-hole pairs. Since the semiconductor we have been discussing contains no impurities, the number of holes in the electron-hole pairs is always equal to the number of conduction electrons. Another way of describing this condition where no impurities exist is by saying the semiconductor is intrinsic. The term intrinsic is also used to distinguish the pure semiconductor that we have been working with from one containing impurities.

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eneration and Recombination

Generation = break up of covalent bond to form electron and hole • requires energy from thermal or optical sources (or other external sources) • generation rate: G = Gth + Gopt + ... [cm−3 · s−1] • in general, atomic density >> n, p ⇒ G ≠ f(n, p) (supply of breakable bonds virtually inexhaustible) Recombination = formation of bond by bringing together electron and hole • releases energy in thermal or optical form • recombination rate: R [cm−3 · s−1] • a recombination event requires 1 electron + 1 hole ⇒ R ∝ n · p Generation and recombination most likely at surfaces where periodic structure 05 July crystalline 2005 Engineer is M S broken. Ayubi 11

Thermal equilibrium =steady state + absence of external energy sources

• Generation rate in thermal equilibrium: Go = f(T) • Recombination rate in thermal equilibrium: Ro ∝ no·po In thermal equilibrium: Go = Ro ⇒ nopo = f(T) ni2(T) Important consequence: In thermal equilibrium and for a given semiconductor, np product is a constant that depends only on temperature! Electron-hole formation can be seen as chemical reaction: bond ↔ e− + h+ similar to water decomposition reaction: H2O ↔ H+ + OH− 05 July 2005

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DOPING A semiconductor in its intrinsic form doesn’t support PROCESS much current due to the limited number of electron-

hole pairs. However, its conductivity can be drastically increased by adding small amounts of selected additives to it. These additives are called impurities and the process of adding them to crystals is referred to as doping . When an impurity increases the number of free electrons, the doped semiconductor is negative or Ntype, and the impurity that is added is known as an ntype impurity. However, an impurity that reduces the number of free electrons, causing more holes, creates a positive or P-type semiconductor, and the impurity that was added to it is known as a p-type impurity. Semiconductors doped in this manner are referred to 05 July 2005 Engineer M S Ayubi 13 as extrinsic semiconductors.

N-Type The N-type impurities have 5 valence Semiconductor electrons and are called pentavalent

impurities. This type of impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. Notice the arsenic atom in the center of the Ge lattice in fig. It has 5 valence electrons but uses only 4 of them to form covalent bonds with the Ge atoms, leaving 1 electron relatively free in the crystal structure. Since the N-type semiconductor has a surplus of electrons, the electrons are considered majority carriers, while the holes, being fewer in number, are the minority carriers. 05 July 2005

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Nd ≡ donor concentration [cm−3] • If Nd < < ni, doping irrelevant (intrinsic semiconductor) no = po = ni • If Nd > > ni, doping controls carrier concentrations (extrinsic semiconductor) → no = Nd po = ni2/Nd Note: no > > po: n-type semiconductor Example: Nd = 1017 cm−3 → no = 1017 cm−3, po = 103 cm−3. In general: Nd ∼ 1015 − 1020 cm−3 Chemical reaction analogy: dissolve a bit of KOH into water ⇒ [OH−] ↑, [H+] ↓

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Current Flow in N-type Material Current Flow in the N-type semiconductor, or crystal, is similar to conduction in a copper wire. The +ve potential of the battery will attract the free electrons in the crystal. These electrons will leave the crystal and flow into the +ve terminal of the battery. As an electron leaves the crystal, an electron from the -ve terminal of the battery will enter the crystal, thus completing the current path. Therefore, the majority current carriers in the N-type material (electrons) are repelled by the -ve side of the battery and move through the crystal toward the +ve side of the 05 July 2005 Engineer M S Ayubi battery.

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P-Type The P-type impurities have only 3 Semiconductor

valence electrons and are called trivalent impurities. This type of impurity, when added to a semiconductor material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor. Notice the indium atom in the figure is 1 electron short of the required amount of electrons needed to form covalent bonds with 4 neighboring atoms and, therefore, creates a hole in the structure. Since the P-type semiconductor has a surplus of holes, the holes are considered majority carriers, while the electrons, being fewer in number, are the minority carriers. 05 July 2005

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Na acceptor concentration [cm−3] • If Na > > ni, doping irrelevant (intrinsic semiconductor) → no = po = ni • If Na < < ni, doping controls carrier concentrations (extrinsic semiconductor) → no = Na po = ni2/Na Note: po > > no: p-type semiconductor Example: Na = 1016 cm−3 → po = 1016 cm−3, no = 104 cm−3. In general: Na ∼ 1015 − 1020 cm−3 Chemical reaction analogy: dissolve a bit of H2SO4 into water ⇒ [H+] ↑, [OH−] ↓

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Current Flow in a P-type Material Conduction in the P material is by +ve holes, instead of -ve electrons. A hole moves from the +ve terminal of the P material to the -ve terminal. Electrons from the external circuit enter the -ve terminal of the material and fill holes in the vicinity of this terminal. At the +ve terminal, electrons are removed from the covalent bonds, thus creating new holes. This process continues as the steady stream of holes (hole current) moves toward the -ve terminal. Notice in both N-type and P-type materials, current flow in the external circuit consists of electrons moving out of the -ve terminal of the battery and into the +ve terminal of the battery. Hole flow, on the other hand, only exists within the 05 July 2005 Engineer M S Ayubi 19 material itself.

Summary

• In a semiconductor, there are two types of carriers: electrons and holes • In thermal equilibrium and for a given semiconductor nopo is a constant that only depends on temperature: nopo = ni2 • For Si at room temperature: ni ≅ 1010 cm−3 • Intrinsic semiconductor: pure semiconductor. no = po = ni • Carrier concentrations can be engineered by addition of dopants (selected foreign atoms): – n-type semiconductor: no = Nd, po = ni2 / Nd – p-type semiconductor: po = Na, no = ni2 / 05Na July 2005 Engineer M S Ayubi 20

PN If we join a section of N-type semiconductor material with JUNCTION

a similar section of P-type semiconductor material, we obtain a device known as a PN JUNCTION. (The area where the N and P regions meet is appropriately called the junction.) The semiconductor should be in one piece to form a proper PN junction, but divided into a P-type impurity region and an N-type impurity region. This can be done in various ways.

GROWN junction

P-type and N-type impurities are mixed into a single crystal. By so doing, a P-region is grown over part of a semiconductor’s length and N- region is grown over the other part (view A in Engineer M S Ayubi fig). 05 July 2005

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FUSED-ALLOY junction

One type of impurity is melted into a semiconductor of the opposite type impurity. For example, a pellet of acceptor impurity is placed on a wafer of N-type germanium and heated. Under controlled temperature conditions, the acceptor impurity fuses into the wafer to form a P-region within it (view B in fig).

POINT-CONTACT construction

It consists of a fine metal wire, called a cat whisker, that makes contact with a small area on the surface of an N-type semiconductor (view A). The PN union is formed in this process by momentarily applying a high-surge current to the wire and the N-type semiconductor. The heat generated by this current converts the material nearest the point of contact to a P-type material (view B). 05 July 2005

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ectrostatics of p-n junction in equilibrium Focus on intrinsic region:

Doping distribution of abrupt p-n junction:

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What is the carrier concentration distribution in thermal equilibrium? First think of two sides separately:

Now bring them together. What happens? Diffusion of electrons and holes from majority carrier side to minority carrier side until drift balances diffusion. 05 July 2005

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Resulting carrier profile in thermal equilibrium:

• Far away from metallurgical junction: nothing happens – two quasi-neutral regions (QNR) • Around metallurgical junction: carrier drift must cancel diffusion – space-charge region (SCR) 05 July 2005

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In a linear scale:

Thermal equilibrium: balance between drift and diffusion Can divide semiconductor in three regions: • two quasi-neutral n- and p-regions (QNRs) • one space charge region (SCR) built-in potential across p-n junction 05 July 2005

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Junction Although the N-type material has an excess of free electrons, Barrier

it is still electrically neutral. This is because the overall number of electrons and protons in the N-type material is equal. By the same reasoning, the P-type material is also electrically neutral. It would seem that if we joined the N and P materials together, all the holes and electrons would pair up. This however, is not the case. Instead some electrons in the N material diffuse into the P material and fill some of the holes near the junction. This process, called junction recombination, reduces the number of free electrons and holes in the vicinity of the junction. Because there is a depletion, or lack of free electrons and holes in this area, it is known as the depletion region. The loss of an electron from the N material creates a positive ion in the N material, while the addition of an electron to the P material creates a -ve ion in that material. These ions are fixed in place in the crystal lattice structure and cannot move. Thus, they make up a layer of fixed opposite charges on the two sides of the junction. 05 July 2005

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An electrostatic field is established across the junction between the oppositely charged ions. The diffusion of electrons across the junction continues until the magnitude of the field increases to the extent that the N-type electrons no longer have enough energy to overcome it. At this point equilibrium is established and, for all practical purposes, the movement of carriers across the junction ceases. For this reason, the field created by the +ve and -ve ions in the depletion region is called a barrier. This action occurs instantly when the junction is formed. Only the carriers in the immediate vicinity of the junction are affected. The carriers throughout the remainder of the N and P material are relatively undisturbed and remain in a balanced condition. 05 July 2005

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PN Junction Diode

The general purpose or rectifier diode is a single PN junction device with conductive contacts and wire leads connected to each region. The schematic symbol of a PN junction diode is shown in figure. The vertical bar represents the cathode (N-type) since it is the source of electrons and the arrow represents the anode (P-type) since it is the destination of the electrons. It makes use of the rectifying properties of a PN junction to convert alternating current into direct current by permitting current flow in only one direction. 05 July 2005 Engineer M S Ayubi 29

FORWARD BIAS

An external voltage applied to a PN junction is called bias. If a voltage source used to supply bias to a PN junction is connected such that its positive terminal is connected to the P-type material and the negative terminal is connected to the N-type material, the bias is known as forward bias. The +ve potential repels holes toward the junction where they neutralize some of the negative ions. At the same time the negative potential repels electrons toward the junction where they neutralize some of the positive ions. Since ions on both sides of the barrier are being neutralized, the width of the barrier decreases. Thus, the effect of the source voltage in the forward-bias is to overcome the barrier potential across the junction and to allow majority carriers to cross the junction. Current flow in the forward-biased PN junction is relatively simple. An electron leaves the negative terminal of the source, enters the N material, where it is the majority carrier and moves to the edge of the junction barrier. 05 July 2005

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For Si and Ge diodes, the typical forward voltage drop is 0.7 and 0.3 volts respectively. Forward voltage drop remains nearly equal for a wide range of diode currents, meaning that diode voltage drop is not like that of a resistor. For most purposes of circuit analysis, it may be assumed that the voltage drop across a conducting diode remains constant. In actuality, however, things are more complex than this. There is an equation describing the exact current through a diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation: qV d /NkT I d = I s (e - 1) 05 July 2005 Engineer M S Ayubi 31

Id = Is (eqV d /NkT - 1) Where, Id = Diode current in amps Is = Saturation current in amps (typically 1 x 10-12 amps) e = Euler’s constant (~ 2.718281828) q = charge of electron (1.6 x 10-19 coulombs) Vd = Voltage applied across diode in volts N = "Nonideality" or "emission" coefficient (typically between 1 and 2) T = Junction temperature in degrees Kelvin 2nd k = Boltzmann’s constant (1.38 x 10-23) The equation kT/q describes the voltage produced within the PN junction due to the action of temperature, and is called the thermal voltage, or Vt of the junction. At room temperature, this is about 26 millivolts. Knowing this, and assuming a "nonideality" coefficient of 1, we may simplify the diode equation and rewrite it as such: Id = Is (eVd /0.026 -1) 05 July 2005

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Because of forward bias, the barrier offers less opposition to the electron and it crosses the junction into the Ptype material. The electron loses energy in overcoming the opposition of the junction barrier, and upon entering the P material, combines with a hole. The hole was produced when an electron was extracted from the P material by the positive potential of the battery. The created hole moves through the P material toward the junction where it combines with an electron.

In forward bias, conduction is by majority current carriers. Increasing the source voltage will increase the number of majority carriers arriving at the junction and will therefore increase the current flow. If the source voltage is increased to the extent that the barrier is greatly reduced, a high current will flow and the junction may be damaged from the resulting heat. 05 July 2005

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REVERSE BIAS

If a voltage source is connected across the PN junction so that its negative terminal is connected to the P-type material, and the positive battery terminal to the N-type material, the bias is known as reverse bias. The negative potential attracts the holes away from the edge of the junction barrier on the P side, while the positive potential attracts the electrons away from the edge of the barrier on the N side. This action increases the barrier width because there are more negative ions on the P side of the junction, and more positive ions on the N side of the junction. This increase in the number of ions prevents current flow across the junction by majority carriers. However, the current flow across the barrier is not quite zero because of the minority carriers crossing 05 July 2005 Engineer M S Ayubi 34 the junction.

With reverse bias, the electrons in the P region are repelled toward the junction by the negative terminal of the source. As the electron moves across the junction, it will neutralize a positive ion in the N region. Similarly, the holes in the N region will be repelled by the positive terminal of the source toward the junction. As the hole crosses the junction, it will neutralize a -ve ion in the P-type material. This movement of minority carriers is called minority current flow, because the holes and electrons involved come from the electron-hole pairs that are generated in the crystal lattice structure, and not from the addition of impurity atoms. 05 July 2005 Engineer M S Ayubi 35

Therefore, when a PN junction is reverse biased, there will be no current flow because of majority carriers but a very small amount of current because of minority carriers crossing the junction. However, at normal operating temperatures, this small current may be neglected. The PN junction is able to offer very little resistance to current flow when forward biased but maximum resistance to current flow when reverse biased. 05 July 2005

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ELECTROSTATICS OF BIASED PN JUNCTION Bias convention for PN junction: V > 0 forward bias V < 0 reverse bias • Potential distribution across PN junction in thermal equilibrium:

• Apply voltage to p-side with respect to n-side: Battery imposes a potential difference across the PN junction 05 July 2005

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How does potential distribution inside junction change as a result of bias? V can drop across five regions : • metal/p-QNR contact • p-QNR • SCR • n-QNR • metal/nQNR contact In which region does V drop most? Essentially, all applied voltage July 2005 SCR: Engineer M S Ayubi drops05across

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V-I characteristics  ofFora Forward PN Bias The forward current If is JUNCTION

zero when the voltage across the Si PN junction is zero. Increasing the bias voltage Vf has very little effect on If until Vf reaches 0.7 volts (barrier potential) at the knee of the curve. Beyond this point, Vf remains nearly steady at 0.7 volts while If increases rapidly. The normal operation for a forward biased PN junction is above the knee of the curve. Increasing Vf far above the barrier potential causes a very high If that can effectively damage the PN junction. 05 July 2005

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For Reverse Bias

When the reverse voltage Vr across the PN junction is zero, there is no reverse current Ir. Increasing Vr has very less effect on Ir until Vr reaches what is called the breakdown value Vbr. Beyond this point Ir increases very rapidly while the voltage across the PN junction increases very little above Vbr. Break down, with some exceptions, is not a normal mode of operation for most PN junction devices as the high Ir generates considerable heat that can damage the PN junction.

Temperature Effects on the V-I Characteristics

For a forward biased PN junction, as temperature increases, the forward current If increases for a given value of forward bias voltage Vf. For a reverse biased PN junction, as temperature increases, the reverse current Ir increases for a given value of reverse voltage Vr. Ir approximately doubles for o every05 July 102005 C rise in temperature. Engineer M S Ayubi 40

-I characteristics (cont.) PN Junction current equation: I = Io(eqV/kT − 1)

Physics of forward bias:

• potential difference across SCR reduced by V ⇒ minority carrier injection in • minority carrier diffusion through QNR’s QNR’s • minority carrier recombination at surface of QNR’s • large supply of carriers available for injection ⇒ I ∝ eqV/kT

Physics of reverse bias:

• potential difference across SCR increased by V ⇒ minority carrier extraction from QNR’s • minority carrier diffusion through QNR’s • minority carrier generation at surface of QNR’s • very small supply of carriers available for extraction ⇒ I saturates to small value 05 July 2005

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I-V characteristics I = Io(eqV/kT − 1)

Key dependences of PN junction current: I = qAn2i( 1/Na Dn/Wp − xp + 1/Nd Dp/Wn − xn) (eqV/kT − 1) • I ∝ n2iN (eqV/kT −1) ≡ excess minority carrier concentration at edges of SCR – in forward bias: I ∝ n2i/N eqV/kT : the more carrier are injected, the more current flows – in reverse bias: I ∝ −n2i/N : the minority carrier concentration drops to negligible values and the current saturates • I ∝ D: faster diffusion ⇒ more current • I ∝ 1/WQNR: 05 July 2005 Engineer M S Ayubi 42 shorter region to diffuse through ⇒ more current

Diode For every device there are defining characteristics that industry has Characteristics found to be useful when describing them. Even for such a simple device as a diode there are many hundreds of types that have been specifically designed for: Switching Rectifying Power High frequency Low leakage

Properties:

Si or Ge, determines the voltage drop across the diode and the drift current. Maximum reverse voltage, PRV (peak reverse voltage) or PIV (peak inverse voltage). If, maximum forward current. Junction capacitance, reverse recovery time. Capacitance depends on the size and geometry of the junction, the capacitance can be thought of as in parallel with the junction. A large If typically means a large junction capacitance: what happens if you try to use this diode at high frequencies? 05 July 2005

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emperature Sensitivity of Diodes The current through a forward biased diode is a function of temperature and thus a diode can be used as a temperature sensor. We have two choices, constant current or constant voltage. I = Io(eeV/kT −1) In constant current mode, dV/dT ~ k/e ~0.002 V/°C.

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Meter Check of a Diode

Since we know that a diode is essentially nothing more than a one-way valve for electricity, it makes sense we should be able to verify its one-way nature using a DC (battery-powered) ohmmeter. Connected one way across the diode, the meter should show a very low resistance. Connected the other way across the diode, it should show a very high resistance ("OL" on some digital meter models):

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Diode Ratings

 In addition to forward voltage drop (VF ) and peak inverse voltage (PIV), there are many other ratings of diodes important to circuit design and component selection. Semiconductor manufacturers provide detailed specifications on their products -diodes included- in publications known as datasheets. Datasheets for a wide variety of semiconductor components may be found in reference books and on the internet. A typical diode datasheet will contain figures for the following parameters: 05 July 2005

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Maximum repetitive reverse voltage = VRRM, the maximum amount of voltage the diode can withstand in reverse-bias mode, in repeated pulses. Ideally, this figure would be infinite. Maximum DC reverse voltage = VR or VDC, the maximum amount of voltage the diode can withstand in reverse-bias mode on a continual basis. Ideally, this figure would be infinite. Maximum forward voltage = VF , usually specified at the diode's rated forward current. Ideally, this figure would be zero: the diode providing no opposition whatsoever to forward current. In reality, the forward voltage is described by the "diode equation.“ 05 July 2005

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Maximum (average) forward current = IF the maximum average amount of current the diode is able to conduct in forward bias mode. This is fundamentally a thermal limitation: how much heat can the PN junction handle, given that dissipation power is equal to current (I) multiplied by voltage (V or E) and forward voltage is dependent upon both current and junction temperature. Ideally, this figure would be infinite. Maximum (peak or surge) forward current = IFSM or IF(surge), the maximum peak amount of current the diode is able to conduct in forward bias mode. Again, this rating is limited by the diode junction's thermal capacity, and is usually much higher than the average current rating due to thermal inertia (the fact that it takes a finite amount of time for the diode to reach maximum temperature for a given current). Ideally, this figure would be infinite. (AV ),

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Maximum total dissipation = PD, the amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P=IE) of diode current multiplied by diode voltage drop, and also the dissipation (P=I2R) of diode current squared multiplied by bulk resistance. Fundamentally limited by the diode's thermal capacity (ability to tolerate high temperatures). Operating junction temperature = TJ , the maximum allowable temperature for the diode's PN junction, usually given in degrees Celsius (0C). Heat is the "Achilles' heel" of semiconductor devices: they must be kept cool to function properly and give long service life. Storage temperature range = TSTG, the range of allowable temperatures for storing a diode (un-powered). Sometimes given in conjunction with operating junction temperature (TJ ), because the maximum storage temperature and the maximum operating temperature ratings are often identical. If anything, though, maximum storage temperature rating will be greater than the maximum operating temperature rating. 05 July 2005

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Thermal resistance = R(θ ), the temperature difference between junction and outside air (R(θ )JA) or between junction and leads (R(θ )JL) for a given power dissipation. Expressed in units of degrees Celsius per watt (0C/W). Ideally, this figure would be zero, meaning that the diode package was a perfect thermal conductor and radiator, able to transfer all heat energy from the junction to the outside air (or to the leads) with no difference in temperature across the thickness of the diode package. A high thermal resistance means that the diode will build up excessive temperature at the junction (where it is critical) despite best efforts at cooling the outside of the diode, and thus will limit its maximum power dissipation. Maximum reverse current = IR, the amount of current through the diode in reverse-bias operation, with the maximum rated inverse voltage applied (VDC). Sometimes referred to as leakage current. Ideally, this figure would be zero, as a perfect diode would block all current when reverse biased. In reality, it is very small compared to the maximum forward current. 05 July 2005

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Typical junction capacitance = CJ , the typical amount of capacitance intrinsic to the junction, due to the depletion region acting as a dielectric separating the anode and cathode connections. This is usually a very small figure, measured in the range of picofarads (pF).  Reverse recovery time = trr, the amount of time it takes for a diode to "turn off" when the voltage across it alternates from forward-bias to reverse-bias polarity. Ideally, this figure would be zero: the diode halting conduction immediately upon polarity reversal. For a typical rectifier diode, reverse recovery time is in the range of tens of microseconds; for a "fast switching" diode, it may only be a few nanoseconds. Most of these parameters vary with temperature or other operating conditions, and so a single figure fails to fully describe any given rating. Therefore, manufacturers provide graphs of component ratings plotted against other variables (such as temperature), so that the circuit designer has a better idea of what the device is capable of. 05 July 2005

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Load  What is Lines diode?

the current through the

A traditional means of finding the operating point of a non-linear circuit is through load lines. The object is to partition the circuit into a set of sources and a load and then to simultaneously find solutions for both. Of course the same end can be achieved by knowing the equation for operation of the non-linear element. While load lines are not really that useful in designing circuits, you see them often and they are useful in developing a physical intuition of how circuits operate. 05 July 2005

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The load line approach. Consider the 1k resistor to be a load of the diode. Plot the current through the resistor as a function of the diode voltage. Note this is simple since the resistor is now sandwiched between 2 voltage sources.

Once the diode curve has been drawn (perhaps from a data sheet), then we can explore the circuit to find an operating point. Since the resistor is linear we know that the VI curve will be a line and thus we only need to locate two points. It is easiest to find the points where the non-linear device is strongly on and off. Note that these points do not need to be reachable by a circuit. With the diode off, the full voltage drop is across the diode and the current is zero. With the diode perfectly conducting, there is no voltage drop across it and the current is only limited by the resistor. The operating point is simply the overlap of these two curves. It can now easily be seen what happens to the operating point as: Increase the voltage ⇒ load line moves up. Increase the resistance ⇒ slope of the load line changes. 05 July 2005

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Zener Diode

Zener diode is a special type of diode that can handle breakdown without failing completely. When forward-biased, zener diodes behave much the same as standard rectifying diodes: they have a forward voltage drop which follows the "diode equation" and is about 0.7 volts. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that zener voltage point. So long as the power dissipated by this reverse current does not exceed the diode's thermal limits, the diode will not be harmed. Zener diodes are manufactured with zener voltages ranging anywhere from a few volts to hundreds of Engineer M S Ayubi 54 volts.05 July 2005

This zener voltage changes slightly with temperature, and like common carbon-composition resistor values, may be anywhere from 5 percent to 10 percent in error from the manufacturer's specifications. However, this stability and accuracy is generally good enough for the zener diode to be used as a voltage regulator device in common power supply circuit: Please take note of the zener diode's orientation in the above circuit: the diode is reverse-biased, and intentionally so. If we had oriented the diode in the "normal" way, so as to be forward-biased, it would only drop 0.7 volts, just like a regular rectifying diode. 05 July 2005

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If we want to exploit this diode's reverse breakdown properties, we must operate it in its reverse-bias mode. So long as the power supply voltage remains above the zener voltage (12.6 volts, in this example), the voltage dropped across the zener diode will remain at approximately 12.6 volts. Like any semiconductor device, the zener diode is sensitive to temperature. Excessive temperature will destroy a zener diode, and because it both drops voltage and conducts current, it produces its own heat in accordance with Joule's Law (P=IE). Therefore, one must be careful to design the regulator circuit in such a way that the diode's power dissipation rating is not exceeded. Interestingly enough, when zener diodes fail due to excessive power dissipation, they usually fail shorted rather than open. A diode failed in this manner is easy to detect: it drops almost zero voltage when biased either way, like a piece of wire.05 July 2005 Engineer M S Ayubi 56

Summ ary •According to the classical Bohr model, the atom is

viewed as having a planetary-type structure with electrons orbiting at various distances around the central nucleus. •The nucleus of an atom consists of protons and neutrons. The protons have a positive charge and the neutrons are uncharged. The number of protons is the atomic number of the atom. •Electrons have a negative charge and orbit around the nucleus at distances that depend on their energy level. An atom has discrete bands of energy called shells in which the electrons orbit. Atomic structure allows a certain maximum number of electrons in each shell. These shells are designated 1, 2, 3, and so on. In their natural state, all atoms are neutral because they have an equal number of protons and electrons. 05 July 2005

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•The outermost shell or band of an atom is called the valence band, and electrons that orbit in this band are called valence electrons. These electrons have the highest energy of all those in the atom. If a valence electron acquires enough energy from an outside source such as heat, it can jump out of the valence band and break away from its atom. •Semiconductor atoms have four valence electrons. Silicon is the most widely used semiconductive material. •Materials that are conductors have a large number of free electrons and conduct current very well. Insulating materials have very few free electrons and do not conduct current under normal circumstances. Semiconductive materials fall in between conductors and insulators in their ability to conduct current. 05 July 2005

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•Semiconductor atoms bond together in a symmetrical pattern to form a solid material called a crystal. The bonds that hold a crystal together are called covalent bonds. Within the crystal structure, the valence electrons that manage to escape from their parent atom are called conduction electrons or free electrons. They have more energy than the electrons in the valence band and are free to drift throughout the material. When an electron breaks away to become free, it leaves a hole in the valence band creating what is called an electron-hole pair. These electron-hole pairs are thermally produced because the electron has acquired enough energy from external heat to break away from its atom. •A free electron will eventually lose energy and fall back into a hole. This is called recombination. But, electron-hole pairs are continuously being thermally generated so there are always free electrons in the material. 05 July 2005

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•When a voltage is applied across the semiconductor, the thermally produced free electrons move in a net direction and form the current. This is one type of current in an intrinsic (pure) semiconductor. •Another type of current is the hole current. This occurs as valence electrons move from hole to hole creating, in effect, a movement of holes in the opposite direction. •An n-type semiconductive material is created by adding impurity atoms that have five valence electrons. These impurities are pentavalent atoms. A p-type semiconductor is created by adding impurity atoms with only three valence electrons. These impurities are trivalent atoms. 05 July 2005

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•The process of adding pentavalent or trivalent impurities to a semiconductor is called doping. •The majority carriers in an n-type semiconductor are free electrons acquired by the doping process, and the minority carriers are holes produced by thermally generated electron-hole pairs. The majority carriers in a p-type semiconductor are holes acquired by the doping process, and the minority carriers are free electrons produced by thermally generated electronhole pairs. •A PN junction is formed when part of a material is doped n-type and part of it is doped p-type. A depletion region forms starting at the junction that is devoid of any majority carriers. The depletion region is formed by ionization. 05 July 2005

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•There is current through a diode only when it is forwardbiased. Ideally, there is no current when there is no bias nor when there is reverse bias. Actually, there is a very small current in reverse bias due to the thermally generated minority carriers, but this can usually be neglected. •Avalanche occurs in a reverse-biased diode if the bias voltage equals or exceeds the breakdown voltage. •A diode conducts current when forward-biased and blocks current when reversed-biased. •The forward-biased barrier potential is typically 0.7 V for a silicon diode and 0.3 V for a germanium diode. These values increase slightly with forward current. •Reverse breakdown voltage for a diode is typically greater than 50 V. •An ideal diode represents an open when reversed-biased and a short when forward-biased. 05 July 2005

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PN JUNCTION Half-Wave APPLICATIONS

The PN junction diode can be used to convert AC into DC Rectifier as it conducts when forward biased and doesn’t conduct when reverse biased. If we place this diode in series with a source of AC power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished. The simplest rectifier circuit is a half-wave rectifier which consists of a diode, an ac power source, and a load resister. 05 July 2005

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During the positive half-cycle of the input signal, the top of the transformer is positive w.r.t ground. The diode is forward biased and current flows through the circuit. When this current flows through the load resistor, it develops a negative to positive voltage drop across it, which appears as a positive voltage at the output terminal. When the AC input goes negative, the top of the transformer becomes negative w.r.t ground. The diode becomes reverse biased and minimum current flows through the diode. For all practical purposes, there is no output developed across the load resistor during the negative alternation of the input signal. Since only the positive half-cycles appear at the output this circuit converted the AC input into a positive pulsating DC voltage. The frequency of the output voltage is equal to the frequency of the applied AC signal since there is one pulse out for each cycle of the ac input. For example, if the input frequency is 60 hertz, the output frequency is 60 pulses per second (pps). If the diode connections are reversed, a negative output voltage is obtained. 05 July 2005

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ll Wave Rectifier

If we need to rectify AC power so as to obtain the full use of both half-cycles of the sine wave, a full-wave rectifier circuit configuration must be used.

Center-tap Design

This design uses a transformer with a center-tapped secondary winding and two diodes. This circuit's operation is easily understood one half-cycle at a time.

Consider the first half-cycle, when the source voltage polarity is (+) on top and (-) on bottom. At this time, only the top diode is conducting; the bottom diode is blocking current, and the load "sees" the first half of the sine wave, positive on top and negative on bottom. 05 July 2005

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Only the top half of the transformer's secondary winding carries current during this halfcycle: During the next half-cycle, the AC polarity reverses. Now, the other diode and the other half of the transformer's secondary winding carry current while the portions of the circuit formerly carrying current during the last half-cycle sit idle. The load still "sees" half of a sine wave, of the same polarity as before: positive on top and negative on bottom: 05 July 2005 Engineer M S Ayubi 66

One disadvantage of this full-wave rectifier design is the necessity of a transformer with a center-tapped secondary winding. If the circuit in question is one of high power, the size and expense of a suitable transformer is significant. Consequently, the center-tap rectifier design is seen only in low-power applications.

Full Wave Bridge Design

The full-wave bridge rectifier design is built around a four-diode bridge configuration.

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Current directions in the full-wave bridge rectifier circuit are as follows for each half-cycle of the AC waveform: Positive half cycle

Negative half cycle

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Diode The basic diode switch is shown Switch in figure. When the input to

this circuit is at zero potential, the diode is forward biased because of the zero potential on the cathode and the positive voltage on the anode. The diode conducts and acts as a straight piece of wire because of its very low forward resistance. In effect, the input is directly coupled to the output resulting in zero volts across the output terminals. Therefore, the diode, acts as a closed switch when its anode is positive with respect to its cathode. If we apply a positive input voltage (equal to or greater than the positive voltage supplied to the anode) to the diode’s cathode, the diode will be reverse biased. In this situation, the diode is cut off and acts as an open switch between the input and output terminals. Consequently, with no current flow in the circuit, the positive voltage on the diode’s anode will be felt at the output terminal. 05 July 2005

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ltage Regulator Zeners are diodes that have variable resistance. Specifically, zeners have a constant current output over a range of input voltages. Thus, by providing a constant current to a circuit, zeners can be used as voltage regulators. A simple voltage regulator. Poor ripple suppression, requires a zener with high power rating, and variations with load impedance. 05 July 2005

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