LABORATORY REPORTOF

EXPERIMENT E1.01DIODES

GROUP E12

GROUP MEMBER

YAP WAI MINGYAP CHIH HSIUNG

PRESENTED BY

YAP CHIH HSIUNG

INTRODUCTION

Figure 1

P-N junction diode is a two-terminal device consisting of a P-N junction formed either in germanium or silicon crystal. Its circuit symbol is shown in Figure 1. The P-type and N-type regions are referred to as anode and cathode respectively. In Figure 1, arrowhead indicates the conventional direction of current flow when forward-biased. It is the same direction in which hole flow takes place.

Objectives

To understand and explain the response and operation of a- silicon diode- germanium diode, and- Zener diode

and the operation of a limiting circuit.

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THEORY

Figure 2

When the P- and N-type materials exist together in a crystal, a charge redistribution occurs. Some of the free electrons from the N-type material migrate across the junction and recombine with the free holes in the P-type material. Similarly, some of the free holes from the P material migrate across the junction and recombine with free electrons in the N material. As a result, the P material acquires a net negative charge and the N material acquires a net positive charge. These charges create an electric field and a potential difference between the two types of material that will inhibit any further charge movement. The result is a reduction in the number of current carriers near the junction. This happens in an area known as the depletion region. The resulting electric field provides a potential barrier or hill in a direction that inhibits the migration of carriers across the junction. This is shown in Figure 2. In order to produce a current across the junction, the potential barrier or hill must be reduced by applying a voltage of the proper polarity across the diode.

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Working

Figure 3

If a positive potential is applied to the P material relative to the N material as shown in Figure 3, the diode is said to be forward-biased. The depletion region shrinks in size because of the attraction of majority carriers to the opposite side. That is, the negative potential at the right attracts hole in the P region and vice versa. With a lower barrier, current can flow more readily.

Figure 4

Alternatively, if a negative voltage is applied to the P material relative to the N material as shown in Figure 4, the diode is reverse-biased. Free electrons are drawn from the N material toward the right, and holes are drawn to the left. The depletion region gets wider and the diode acts as an insulator.

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V/I Characteristics

Figure 5

Figure 5 shows the static voltage-current characteristics for a silicon and germanium diode.

Forward Characteristics

When the diode is forward-biased and the applied voltage is increased from zero, hardly any current flows through the device in the beginning. It is so because the external voltage is being opposed by the internal barrier voltage vg whose value is 0.7 V for Si and 0.3 V for Ge. As soon as vg is neutralized, current through the diode increases rapidly with increasing applied voltage. A burnout is likely to occur if forward voltage is increased beyond a certain safe limit.

Reverse Characteristics

When the diode is reverse-biased, majority carriers are blocked and only a small current (due to minority carriers) flow through the diode. As the reverse voltage is increased from zero, the reverse current very quickly reaches its maximum or saturation value Io which is also known as leakage current. It is of the order of nanoamperes (nA) for Si and microamperes (mA) for Ge. The value of Io is independent of the applied reverse voltage but depends on

a) temperatureb) degree of doping, and

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c) physical size of the junction.When reverse voltage exceeds a certain value called breakdown voltage VBR, the leakage current suddenly and sharply increases, the curve indicating zero resistance at this point. Any further increase in voltage is likely to produce burnout unless protected by a current-limiting resistor.

Relationship Between Diode Current and Diode Voltage

An exponential relationship exists between the carrier density and the applied potential. It is possible to write a single expression for the density distribution and account for both the forward- and reverse-bias conditions. The expression applies as long as the voltage does not exceed the breakdown voltage. The relationship is described by an equation called Boltzmann diode equation given below:

iD = IqvnkTo

Dexp

1 (1)

The terms are defined as follows:

iD = current in the diode (amperesvD = potential difference across the diode (volts)Io = reverse saturation current (amperes)q = electron charge, 16 10 19. J/Vk = Boltzmann’s constant, 138 10 23. J/°KT = absolute temperature (degrees Kelvin)n = empirical constant between 1 and 2, sometimes referred to as the

exponential ideality factor

Equation (1) can be simplified by defining

VT = kTq

This yields

iD = Iv

nVoD

T

exp

1

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Zener Diode

Figure 6

It is a reverse-biased heavily doped silicon or germanium P-N junction diode which is operated in the breakdown region where current is limited by both external resistance and power dissipation of the diode. Silicon is preferred to Ge because of its higher temperature and current capability. When a diode breaks down, both Zener and avalanche effect are present although usually one or the other predominates depending on the value of reverse voltage. At reverse voltage less than 6V, Zener effect predominates whereas above 6V, avalanche effect predominates.Zener breakdown occurs due to breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current whose value is limited only by the external resistance in the circuit. It is independent of the applied voltage. Avalanche breakdown occurs at higher reverse voltages when thermally generated electrons acquire sufficient energy to produce more carriers by collision. The schematic symbol of a Zener diode is shown in Figure 6.

V/I Characteristics

A typical characteristic is shown by Figure 7. The forward characteristic is simply that of an ordinary forward-biased junction diode. The important points on the reverse characteristic are:

vZ = Zener breakdown voltageIZmin = minimum current to sustain breakdownIZmax = Maximum Zener current limited by maximum power dissipation

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Figure 7

Limiting Circuit

Limiting circuits or clipping circuits are used to eliminate a part of a waveform that lies below or above some reference level. A limiting circuit requires a minimum of two components i.e. a diode and a resistor. Often, dc battery is also used to fix the limiting level. The input waveform can be limited at different levels by simply changing the battery voltage and by interchanging the position of various elements.Such circuits are used in radar and digital computers when it is desired to remove signal voltages above or below a specified voltage level. Another application is in radio receivers for communication circuits where noise pulses that rise well above the signal amplitude are limited down to the desirable level.

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Experiment

Recording of Characteristics

Circuit

Procedure

1. The input voltage values Ue from Table 1 is applied one after the other.

2. The values required to complete Table 1 are determined using the multimeters.

3. The individual characteristics are drawn.

4. An a.c. voltage of 12 V / 50 Hz is applied at the input. The characteristics of the three diodes are displayed on the oscilloscope and recorded in Graticules 1-3.

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Results

1.V1 V2 V3

Ue (V) UF (V) IF (mA) UF (V) IF (mA) UR (V) IR (mA)0.1 0.1 0 0.01 0 0.1 00.2 0.2 0 0.16 0 0.2 00.3 0.3 0 0.21 0.1 0.3 00.4 0.39 0 0.24 0.3 0.4 00.5 0.45 0.1 0.27 0.4 0.5 00.6 0.49 0.2 0.29 0.6 0.6 00.7 0.51 0.3 0.31 0.7 0.7 00.8 0.53 0.5 0.34 0.9 0.8 00.9 0.55 0.6 0.35 1.1 0.9 01.0 0.56 0.8 0.37 1.3 1.0 02.0 0.61 2.8 0.51 2.9 2.0 03.0 0.64 4.9 0.62 4.8 3.0 04.0 0.65 7.0 0.72 6.7 3.8 0.35.0 0.66 9.0 0.81 8.5 4.3 1.46.0 0.67 11.1 0.89 10.4 4.6 3.07.0 0.68 13.1 0.98 12.3 4.7 4.88.0 0.69 15.2 1.05 14.2 4.8 6.79.0 0.69 17.3 1.13 16.1 4.8 8.710.0 0.69 19.3 1.19 17.9 4.9 10.611.0 0.69 21.3 1.19 19.8 4.9 12.612.0 0.69 23.0 1.19 21.8 4.9 14.7

Table 1

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2.

Characteristics of diodes V1 and V2

3.

Characteristic of the Zener diode V3

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4. ax : X via YUF : 1 V/cmIF : 5 mA/cmU(TO) : 0.70 VCoupling : DC

Graticule 1: Characteristic V1; 1N4007

5. ax : X via YUF : 1 V/cmIF : 5 mA/cmU(TO) : 0.30 VCoupling : DC

Graticule 2: Characteristic V2; AA118

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6.ax : X via YUF : 1 V/cmIF : 10 mA/cmU(TO) : 4.7 VCoupling : DC

Graticule 3: Characteristic V3; Zener diode

7. The forward-bias voltage for germanium diode at 0.3 V is lower than that of silicon diode which is 0.7 V. The slope for silicon diode characteristic is steeper than that of germanium. The dynamic resistance is the reciprocal of the slope. Therefore dynamic resistance of silicon diode is less compared with that of germanium diode.

8. When a diode is forward-biased and the applied voltage is increased from zero, hardly any current flows through the diode in the beginning. It is so because the applied voltage is being opposed by the turn-on voltage vg. As soon as the voltage exceeds vg, current through the diode increases rapidly with increasing applied voltage.

9. When a Zener diode is reverse-biased, majority carriers are blocked and only a small current flow through the diode. As the reverse voltage is increased from zero, the reverse current increases slowly until it reaches its minimum current to sustain breakdown IZmin. When the reverse voltage exceeds the voltage corresponding with IZmin, the reverse current suddenly and sharply increases but its reverse characteristic is not exactly vertical as in normal diodes.

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Limiting Circuit 1

Circuit

Procedure

1. Ue = 20 VPP, f = 1 kHz is applied to the circuit input.

2. The input and output voltages are displayed on the oscilloscope. The curves are drawn in Graticule 4.

3. The experiment record and exercises are completed.

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Results

1. ax : 0.5 ms/DivaYA : 5 V/DivaYB : 5 V/DivUe : 20 VPP

fe : 1 kHzUa : 9 VP

Coupling : AC

Graticule 4: Ue; Ua

2. During the positive half-cycle of the input, the diode is forward-biased. Hence current flows across resistor R3 and output is obtained across it. Difference between the input and output is due to the forward voltage drop across diode V4.During the negative half-cycle of the input, the diode is reverse-biased. There exists a small leakage current Io. Reverse current across diode V4 and resistor R3 remains at Io even though reverse input voltage increases. Therefore, voltage across R3 is constant when reverse input voltage increases beyond a particular amount.

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Limiting Circuit 2

Circuit

Procedure

1. Ue = 20 VPP, f = 1 kHz is applied to the circuit input.

2. The input voltage, Ua and U1 are displayed on the oscilloscope. The curves are drawn in Graticule 5 and 6.

3. The experiment record and exercises are completed.

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Results

1.ax : 0.5 ms/DivaYA : 5 V/DivaYB : 5 V/DivUe : 20 VPP

fe : 1 kHzUa : 5.5 V, -6 VCoupling : AC

Graticule 5: Ue; Ua

2.ax : 0.5 ms/DivaYA : 5 V/DivaYB : 5 V/DivUe : 20 VPP

fe : 1 kHzUa : 0.35 V, -0.3 VCoupling : AC

Graticule 6: Ue; U1

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3. During positive input half-cycle, V5 is shorted (being forward-biased) but V6 acts like an open upto 5.5 V. Therefore, it goes into breakdown and holds the output voltage constant till input voltage falls below 5.5 V in the later part of the half-cycle. At that point, V6 comes out of the breakdown and again acts like an open across which the entire input voltage is dropped.

During negative input half-cycle, V6 is shorted (being forward-biased) but V5 acts like an open upto -6 V. Therefore, it goes into breakdown and holds the output voltage constant till input voltage falls below (in magnitude) -6 V in the later part of the half-cycle. At that point, V5 comes out of the breakdown and again acts like an open across which the entire input voltage is dropped.

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References

1. C. J. Savant, Jr., Martin S. Roden, Gordon L. Carpenter,“Electronic Design: Circuits and Systems”, Second Edition, The Benjamin/Cummings Publishing Company, Inc.

2. B. L. Theraja, A. K. Theraja,“A Text Book of Electrical Technology”, Nirja Construction and Development Co. (P) Ltd.

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Contents

Introduction 1Objectives 1

Theory 2Working 3V/I Characteristics 4Forward Characteristics 4Reverse Characteristics 4Relationship Between Diode Current and Diode Voltage 5Zener Diode 6V/I Characteristics 6Limiting Circuit 7

Experiment 8Recording of Characteristics 8Circuit 8Procedure 8Results 9Limiting Circuit 1 13Circuit 13Procedure 13Results 14Limiting Circuit 2 15Circuit 15Procedure 15Results 16

References 18

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