Chapter1(semiconductor Diodes) Revise

Chapter 1: Semiconductor Diodes

Diodes Simplest Semiconductor Device

It is a 2-terminal device 2

Basic operation Ideally it conducts current in only one direction

and acts like an open in the opposite direction 3

Characteristics of an ideal diode: Conduction Region

Look at the vertical line! In the conduction region, ideally • the voltage across the diode is 0V, • the current is ∞, • the forward resistance (RF) is defined as RF = VF/IF, • the diode acts like a short.

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Characteristics of an ideal diode: Non-Conduction Region

Look at the horizontal line! In the non-conduction region, ideally • all of the voltage is across the diode, • the current is 0A, • the reverse resistance (RR) is defined as RR = VR/IR, • the diode acts like open.

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Semiconductor Materials Common materials used in the development of semiconductor devices: • Silicon (Si) • Germanium (Ge)

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Practical Diode (Silicon vs Germanium) Silicon Germanium Higher *PIV (≈ 1000V) Lower PIV (≈ 400V) Higher current rating Wider temperature range (up to 2000C) Higher forward-bias voltage (0.7V)

Lower current rating Narrow temperature range (lower than 1000C) Lower forward-bias voltage (0.3V)

* PIV = peak inverse voltage

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Comparison of Si and Ge semiconductor diodes ID(mA)

Is(Si)=10nA 0.3(Ge)

0.7(Si)

VD(V)

Is(Ge) (Si)

(Ge)

Is=reverse saturation current

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Operating Conditions

• No Bias • Forward Bias • Reverse Bias

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No Bias Condition No external voltage is applied: VD = 0V and no current is flowing ID = 0A.

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Forward Bias Condition

The Forward bias voltage required for a : • Silicon diode VT ≅ 0.7V • Germanium diode VT ≅ 0.3V

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Reverse Bias Condition

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Actual Diode Characteristics

Note the regions for No Bias, Reverse Bias, and Forward Bias conditions. Look closely at the scale for each of these conditions!

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Zener Diode • A Zener is a diode operated in reverse bias at the Peak Inverse Voltage (PIV) called the Zener Voltage (VZ). • Common Zener Voltages: 1.8V to 200V

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Zener Region

The diode is in the reverse bias condition.  At some point the reverse bias voltage is so large the diode breaks down.  The reverse current increases dramatically.  This maximum voltage is called avalanche breakdown voltage and the current  is called avalanche current.

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Resistance Levels Semiconductors act differently to DC and AC currents. There are 3 types of resistances. • DC or Static Resistance • AC or Dynamic Resistance • Average AC Resistance

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• DC or Static Resistance • The resistance of a diode at a particular operating point is called the dc or static resistance diode. It can be determined using equation (1.1):

RD = VD/ID

(1.1)

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Example : DC or Static Resistance – refer Figure 1.1 Ideal diode Si diode ID(A) VD(V) RD(Ω ) ID(A) VD(V) RD(Ω ) 20m

0

0

20m

0.8

40

2m

0

0

2m

0.5

250

dc resistance of forward-bias region decrease when higher currents and voltage. 18

Ideal diode Si diode ID(A) VD(V) RD(Ω ) ID(A) VD(V) RD(Ω ) 0

-10

∞

-2µ

-10

5M

• dc resistance of reverse-bias region, its open-circuit equivalent.

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Figure 1.1 Id(mA) ideal -10

Si

20

I VD(V)

2 0.5

Si

0.8

-2µ

II

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• AC or Dynamic Resistance • Static resistance is using dc input. If the input is sinusoidal the scenario will be change. • The varying input will move instantaneous operating point UP and DOWN of a region. • Thus the specific changes in current and voltage is obtained. It can be determined using equation (1.2)

rd = ∆VD/ ∆ID

(1.2)

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Diode characteristic

∆Ι

Tangent line

Qpt

d

∆ vD

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•Average AC Resistance

r av

Vd (point to point) Id

AC resistance can be determined by picking 2 points on the characteristic curve developed for a particular circuit.

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• Example: Determine the Vf and If for the diode in Figure 1.2 for each of the diode model. Also find the voltage across the limiting resistor in each case. Asume rd=10 Ω and determined value of forward current.

Figure 1.2

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Ideal diode Vf=0V If =10V/1k =10mA

Practical model Vf=0.7V If=(10V – 0.7V)/1k =9.3/1k = 9.3mA

VRlimit =(10m)(1k) =10V

VRlimit =(9.3m)(1k)

Complete model If=(10V – 0.7V)/1k+10 =9.3/1010 = 9.21mA Vf=0.7V+Ifrd =0.7 +(9.21m)(10) = 792mV

=9.3V VRlimit =(If)(Rlimit) =9.21m(1k) =9.21V

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Diode Testing A. B. C.

Diode Checker Ohmmeter Curve Tracer

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A. Diode Checker Many DMM’s have a diode checking function. A normal diode will exhibit its Forward Bias voltage (VF). The diode should be tested out of circuit.

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B. Ohmmeter An ohmmeter set on a low ohms scale can be used to test a diode. A normal diode will have the following readings. The diode should be tested out of circuit.

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C. Curve Tracer A curve tracer is a specialized type of test equipment. It will display the characteristic curve of the diode in the test circuit. This curve can be compared to the specifications of the diode from a data sheet.

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References: 1) Robert Boylestad, “Electronic Devices and Circuit Theory”, Eighth Edition, 2002 2)Thomas L. Floyd, “ Electronic Devices, Sixth edition”, Prentice Hall, 2002. 3) Puspa Inayat Khalid, Rubita Sudirman, Siti Hawa Ruslan, “ModulPengajaran Elektronik 1”, UTM, 2002. 4) Copyright Supplementary 2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458

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