Semiconductors And Pn Junction

Electronics Properties of crystals, semiconductors, PN Junction

PROPERTIES OF CRYSTALS • What is Crystals? ( 3 physical states ) • Substances in the solid state, will have a molecular structure termed crystalline. • Crystalline structure is one in which the atoms or molecules take up precise positions in a geometric form of lattice.

PROPERTIES OF CRYSTALS • The structure of a solid substance have symmetric & non-symmetric arrangement of its atoms or molecules. Non-symmetric is termed amorphous. • In some crystalline substances, the imperfections in the molecular structure will confer additional properties on the material - luminescence, chemical or electrical effects. • Semiconductors are crystals of substances which exhibit electrical properties due to imperfections in their molecular structure.

PROPERTIES OF CRYSTALS • Examples of semiconductors as : germanium, silicon, selenium, tellurium. • One important electrical property of all semiconductors is that their resistance to electron motion (current flow) decreases as their temperature increases. • From investigations into semiconductors came the development of the transistor, diode, e.t.c.

Properties of semiconductor crystals • A semiconductor normally has the form of a crystal. • Under normal conditions, semiconductors cannot be classified as conductors or insulators. • If a semiconductor crystal is at some temperature above absolute zero there will be: • - some level of internal energy due to the normal molecular motions (vibrations) within the crystal lattice structure - a number of free electrons in the crystal lattice, moving randomly through the lattice and acting as charge carriers Above abs zero

Vibrations Release e

e free moving

Properties of semiconductor crystals • At the absolute zero of temperature (0 K or -273oC), all particles forming the semicon crystal will be at rest and occupy precise positions in a geometric lattice structure, and that: Covalent Bond

• Internal energy (due to molecular motion) will be zero; • There will be no free electrons to act as charge carriers. • It will behave as a perfect insulator.

SEMICONDUCTOR • Types of materials Classification depends on the value of resistivity of materials

Intrinsic conduction • In a semiconductor crystal which is at room temperature the random motion of free electrons, with their associated electric charge, is termed ‘intrinsic conduction’. • In intrinsic conduction the random motion of a free electron will produce a negatively charged particle wherever it is ‘captured’ in the crystal lattice. • The location in the crystal lattice from where the electron has migrated is termed a ’hole’, and this will be positively charged.

Intrinsic conduction • Although intrinsic conduction is taking place within the structure of the semiconductor crystal, there is overall electrical equilibrium, i.e. at all times there will be as many free electrons as there are holes. • Intrinsic conduction, because of its randomness, is not a useful property in a semiconductor crystal. • Two materials commonly used as semiconductor crystal are silicon and germanium. • A crystal of silicon is usually preferred as it has a lower level of intrinsic conduction.

Impurity conduction • The introduction of controlled quantities of impurities into a semiconductors crystal when it is being formed can produce a crystal lattice with a predominance of electrons or holes. • The introduction of impurities into a semiconductors crystal (usually about one part in 1010 parts) is termed ‘doping’. • The materials used for doping are selected so that their atoms will be combined into the crystal lattice without distorting it and that the required predominance of either electrons or holes is obtained.

Impurity conduction • Doping a silicon crystal with arsenic or phosphorus will produce a predominance of electrons. (Majority Carriers => Free Electrons, Minority Carriers => Holes) • Doping a silicon crystal with boron or indium produces a predominance of holes. (Majority Carriers => Holes, Minority Carriers => Free Electrons) • A semiconductor crystal with a predominance of free electrons as an ‘n-type’ crystal. • A semiconductor crystal with a predominance of holes as a ‘p-type’ crystal. • The crystals, n- or p-type are in overall electrical balance.

Impurity conduction

Doping by pentavalent atom - electrons

Doping by trivalent atom - holes

Terms • Trivalent atom - have 3 valence electrons - e.g arsenic & phosphorus - used in doping of p-type semicon • Pentavalent atom - have 5 valence electrons - e.g boron & indium - used in doping of n-type semicon

Terms • Donor atom – pentavalent atom has 1 free electron (doping). • Acceptor atom – trivalent atom has 1 free hole (doping). • N-type semiconductor - doping by pentavalent atom - majority – electrons, minority – holes • P-type semiconductor - doping by trivalent atom - majority – holes, minority – electrons

Diode and Diode Circuit •

All materials can be classified (electrically) into three categories: » » »

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Conductors. Insulators. Semiconductors

Conductors easily allow current to pass through them. Insulators do not allow current to pass through them. Semiconductors are a group of material that posses the property of neither insulator nor good conductor, but somewhere in between Example of semiconductors are silicon (Si) and germanium (Ge). Pure semiconductors are poor conductors, because the low number of free electrons. However, the resistivity can be reduced (so that it conducts more current) by putting in impurities into the pure semiconductors. The process of introducing a small amount of impurities (during manufacturing) into the semiconductors is called doping.

In general, over a limited range of temperatures, the resistance of a conductor increases with temperature increase, the resistance of insulators remains approximately constant with variation of temperature and the resistance of semiconductor materials decreases as the temperature increases.

SILICON AND GERMANIUM ¾

The most important semiconductors used in the electronics industry

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As the temperature of these materials is raised above room temperature, the resistivity is reduced and ultimately a point is reached where they effectively become conductors

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As the temperature of a semiconductor is reduced below normal room temperature, the resistivity increases until, at very low temperatures the semiconductor becomes an insulator

N-TYPE & P-TYPE MATERIALS

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Adding extremely small amounts of impurities to pure semiconductors in a controlled manner is called doping.

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Antimony, arsenic and phosphorus are called n-type impurities and form an n-type material when any of these impurities are added to silicon or germanium

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Indium, aluminium and boron are called p-type impurities and form a p-type material when any of these impurities are added to a semiconductor

2-D view of the Ge crystal lattice

N-type semiconductor is obtained when a pentavalent material is added to a pure Ge crystal

P-type semiconductor is obtained when a trivalent material is added to a pure Ge crystal

MAJORITY & MINORITY CHARGE CARRIERS N-type ------------------------------------+ +

Majority carriers Minority carriers

P-type +++++++++++ +++++++++++ - -

Majority carriers Minority carriers

PN JUNCTION

A PN Junction is piece of semiconductor material in which part of the material is P-type and part is N-type.

Diffusion

During the formation of junction, 2 phenomena take place: 1. A thin layer is set up on both sides of the junction. 2. A junction/barrier potential is developed across the junction Formation of depletion layer: 1. Greater concentration of holes in P than N region 2. Greater concentration of electrons in N than P region The difference in concentration establish a density gradient across the junction resulting in carrier diffusion and terminate the existence by recombination

Depletion layer The area in the region of the junction becomes depleted of holes and electrons due to electron-hole recombination

FORWARD BIASED PN JUNCTION When an external voltage is applied to a PN junction making the P-type material positive with respect to the ntype material, the PN junction is forward biased

As soon as battery connection is made, the holes in P region are repelled by the positive battery terminal & electrons in N region are repelled by the negative battery terminal. Thus both holes & electrons are driven towards the junction where they recombine. This allow easy current flow through the semiconductor as the crystal offers low resistance. FB reduces the thickness of the depletion layer.

Forward characteristics

At room temperature, a potential difference is require before a reasonable amount of forward current starts flowing in a junction. This is known as threshold voltage, Vth. For applied voltage less than Vth, current flow is negligible. As applied voltage increase beyond Vth, forward current increase sharply. If forward voltage increase beyond a certain safe value, it will produce an extremely large current which may destroy the junction due to overheating.

REVISE BIASED PN JUNCTION When an external voltage is applied to a PN junction making the P-type material negative with respect to the ntype material, the PN junction is reverse biased

During the RB, holes are attracted by the negative battery terminal & electrons are attracted by the positive battery terminal. Thus, both holes & electrons move away from the junction. Hence, no electron-hole combination, no current flow and the junction offers high resistance.

Reverse characteristics Practically no current flow, yet there is a small current due to minority carriers which is known as reverse current or reverse current, Is or Io. It also referred as leakage current of PN junction. If reverse bias applied to a junction is increase, a point is reached when the junction breakdown and revere current rises to value limited only by the resistance connected in series with junction. This critical value of voltage is known as breakdown voltage.

The PN junction • When n-type doping material (e.g. arsenic) is diffused into the exposed parts of the surface of a p-type silicon crystal, a thin layer of n-type material will form (- ions) at the surface where diffusion has taken place. H dif

e dif

layer +

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from H from e

The PN junction • ‘Junction’ is the thin layer of n-type material in the p-type or p-type material in the n-type silicon crystal. • Although the term ‘junction’ is used, the pand n-type materials form part of a continuous crystal structure.

The PN junction • The silicon semiconductors containing the ‘junction’ is termed a ‘diode’ because two electrode regions, of different polarity, have been created within the one crystal structure. • If an e.m.f. is applied to the diode with the positive terminal connected to the n-type region of the crystal and the negative terminal to the p-type region, the diode will become an almost perfect insulator and permit no current to flow. • If an. e.m.f. is applied to the diode with the positive terminal connected to the p-type region and the negative terminal to the n-type region, the diode will act as a conductor and permit current to flow.

The PN junction • If the applied e.m.f. is alternating then current will only flow when the applied e.m.f. is positive at the p-type region, and the diode will act as a ‘rectifier’, having an a.c. input but a d.c. output. • When it acts as a conductor the diode is said to be ‘forward biased’. • When it acts as an insulator the diode is said to be ‘reverse biased’. • When a diode is reverse biased and the voltage is increased beyond a certain value, a sudden and substantial flow of reverse current takes place.

The PN junction • The voltage at which the reverse flow of current commences is termed the ‘avalanche’, ‘breakdown’ or ‘zener’ voltage. • Within certain limits, this reverse action is not harmful and can be usefully used in electrical circuits. • Important uses for diodes are in electrical power supplies for converting a.c. to d.c., voltage regulation ( stabilization), switching device.

The PN junction • Sketches curves of voltage and current for a diode when forward and reverse biased.

Passive components • Components in an electrical circuit which do not of themselves act to change or modify circuit conditions are termed ‘passive’. • Examples of ‘passive’ components, resistors, thermistors, inductors, capacitors, quartz crystal, silicon temperature sensors. • Quartz crystals are used in the form of a thin plate in resonant circuits and such circuits are used to produce oscillation.

Passive components • The use of a quartz crystal produces a more stable and steadier frequency on a voltage than the equivalent circuit containing inductor, resistor and capacitors. • From the development and use of silicon in strain gauges, it was found that, after appropriate preparation, a silicon crystal could be used as a highly sensitive solid-state temperature sensor. • A silicon temperature sensor is a pure silicon crystal, and the relationship between its resistance and its temperature is almost linear, which gives it an advantage over the thermistor with its exponential relationship.

Active components • Components which can of themselves act to change or modify circuit conditions are termed ’active’. • Examples of ‘active’ components as, diodes, rectifiers, zener diodes, LED, transistors.

DOPING • • •

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The type of material that is added to the pure semiconductor will determine whether it will become n-type or p-type semiconductor. N-type semiconductor is produced if the impurity is either phosphorus (P), arsenic (As), or antimony (Sb) - all from group 5 of periodic table. The introduction of either one of these impurities into a pure semiconductor produces more free electron in the semiconductor. P-type semiconductor is produced if the impurity is either aluminium (Al), boron (B), or gallium (Ga) - all from group 3 of periodic table. The introduction of either one of these impurities into a pure semiconductor produces more "hole" in the semiconductor. A hole is a condition where there is absence of one electron, which gives the effect of more positive charge.

PN JUNCTION •

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A p-n junction is piece of semiconductor material in which part of the material is p-type and part is n-type. In order to examine the charge situation, assume that separate blocks of p-type and n-type materials are pushed together. Also assume that a hole is a positive charge carrier and that an electron is a negative charge carrier. At the junction, the donated electrons in the n-type material, called majority carriers, diffuse into the p-type material and the acceptor holes in the p-type material diffuse into the n-type material as shown by the arrows in Figure 2.2. Because the n-type material has lost electrons, it acquires a positive potential with respect to the p-type material and thus tends to prevent further movement of electrons. The p-type material has gained electrons and becomes negatively charged with respect to the n-type material and hence tends to retain holes. Thus after a short while, the movement of electrons and holes stops due to the potential difference across the junction, called the contact potential. The area in the region of the junction becomes depleted of holes and electrons due to electron-hole recombination's, and is called a depletion layer, as shown in Figure 2.3.

PN JUNCTION

PN JUNCTION