Chapter 1: Semiconductor Diodes

Diodes Simplest Semiconductor Device

It is a 2-terminal device2

BasicoperationIdeallyitconductscurrentinonlyonedirection

andactslikeanopenintheoppositedirection3

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 MaterialsCommon 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) Is(Ge) (Si) (Ge) 0.7(Si)

VD(V)

Is=reverse saturation current

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Operating Conditions

No Bias Forward Bias Reverse Bias

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No Bias ConditionNo 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 0.3V11

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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ZenerRegion

Thediodeisinthereversebiascondition. Atsomepointthereversebiasvoltageissolargethediodebreaksdown. Thereversecurrentincreasesdramatically. Thismaximumvoltageiscalledavalanchebreakdownvoltageandthecurrent iscalledavalanchecurrent.

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Resistance LevelsSemiconductors 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): (1.1)

RD = VD/ID

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Example : DC or Static Resistance refer Figure 1.1 Ideal diode Si diode ID(A) VD(V) RD( ) ID(A) VD(V) RD( ) 20m 2m 0 0 0 0 20m 2m 0.8 0.5 40 250

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

Ideal diode Si diode ID(A) VD(V) RD( ) ID(A) VD(V) RD( ) 0 -10 -2 -10 5M

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

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Figure 1.1Id(mA)ideal-10 20

Si I VD(V)0.5 0.8 -2

2

Si 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

d

Qpt

Tangent line

vD

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AverageACResistance

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 and determined value of forward current.

Figure 1.2

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Ideal diodeVf=0V If =10V/1k =10mA VRlimit =(10m)(1k) =10V

Practical modelVf=0.7V If=(10V 0.7V)/1k =9.3/1k = 9.3mA VRlimit =(9.3m)(1k) =9.3V

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

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Diode TestingA. B. C. Diode Checker Ohmmeter Curve Tracer

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A. Diode CheckerMany DMMs 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. OhmmeterAn 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 TracerA 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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