Semiconductor Diode

Abstract

The simplest electronic device is referred to as a diode. It consists essentially of two different materials in contact such that electric charge flows easily in one direction but is impeded in the other. Despite its simplicity, it performs an important role in electronic systems from the simple to the complex. In this chapter, we will discuss the nature and characteristics of the solid-state diode (i.e. one based on semiconductor material) as well as employ it in the design of modern electronic systems.

Problems

• 1. Briefly explain the following:

  • (i) The formation of n-type and p-type material

  • (ii) Increased conductivity in n-type and p-type material

  • (iii) Why there is conduction across a forward-biased p-n junction

  • (iv) Why there is little or no conduction across a reverse-biased p-n junction

  • (v) What happens when n-type and p-type material are brought into contact

  • (vi) The effect of temperature on the conductivity of intrinsic semiconductor material

  • (vii) Why a conductor conducts better than a semiconductor

  • (viii) Why a semiconductor conducts better than an insulator

  • (ix) The cause of the barrier potential in an np junction

• 2. Determine the dynamic resistance of a semiconductor diode for 0.5 mA current through the diode.

• 3. A Zener diode has V_Z = 6.8 V at 25^oC. If the diode has T_C = 0.03%/^oC, determine the Zener voltage at 63^oC.

• 4. An 10 V Zener has a power rating of 400 mW. Determine the maximum reverse current I_Zmx.

• 5. An LED is driven by a 9 volt battery in series with a resistor R₁. Determine the value of R₁ if the LED is green.

• 6. In the circuit of problem 5, if the LED is replaced by a silicon diode, what is the value of the current for the same value of R₁?

• 7. Sketch the output waveform for each of the circuits shown in Fig. 1.72. In each case V_i is a symmetrical square wave input signal of amplitude ±10 volts.

• 8. For the circuits in problem 7, sketch the output waveform for a sine wave input signal of amplitude ±15 volts.

• 9. Sketch the following circuits:

  • (i) Full-wave rectifier using a centre-tapped transformer

  • (ii) Full-wave bridge rectifier operating into a load

  • (iii) Clipping circuit using one Zener diode

  • (iv) Clamping circuit with a peak positive output of +0.7 volts

• 10. Sketch the output waveform for each of the circuits shown in Fig. 1.73. In each case the input is a symmetrical square wave input signal of amplitude ±12 volts.

Fig. 1.72

Circuits for Question 7

Fig. 1.73

Circuits for Question 10

• 11. With the aid of a circuit diagram, explain the operation of a half-wave rectifier operating into a resistive load. Draw a voltage/time diagram of the output waveform. Explain the effect of a filter capacitor being placed across the load.

• 12. With the aid of a circuit diagram, explain the operation of a full-wave rectifier operating into a resistive load. Draw a voltage/time diagram of the output waveform. Explain the effect of a filter capacitor being placed across the load.

• 13. Compare the advantages and disadvantages of using a bridge rectifier with a normal transformer or two diodes and a centre-tapped transformer to realize a full-wave rectifier.

• 14. Design a half-wave unregulated power supply delivering 15 volts DC at 0.5 A with less than 2 V peak-to-peak ripple.

• 15. Using a centre-tapped transformer, design a full-wave unregulated power supply delivering 12 volts DC at 2 A with less than 1 V peak-to-peak ripple.

• 16. Re-design the circuit in problem 16 using a 15 volt transformer and a bridge rectifier.

• 17. A 6 volt Zener diode with a minimum current requirement of 5 mA is to be used in a voltage regulator. The supply voltage is 22 volts and the variable load current has a maximum value of 15 mA. Calculate the required series resistance and the minimum power rating of the Zener diode.

• 18. A 16 volt Zener diode with a minimum current requirement of 5 mA and a power rating of 800 mW is to be used in a voltage regulator. The supply voltage is 24 volts and the load current is variable. Calculate the required series resistance and the minimum load resistor that can be connected to the supply.

• 19. A 5 volt Zener diode has a minimum current requirement of 10 mA and is to be used in a voltage regulator. The supply voltage is 20 volts ±10% and the fixed load current is 20 mA. Calculate the series resistance required, the minimum power rating of the Zener diode and the minimum instantaneous power dissipated in the Zener.

• 20. A 12 volt Zener diode has a minimum current requirement of 6 mA and is to be used in a voltage regulator. The supply voltage is 30 volts ±5% and the fixed load current is 15 mA. Calculate the series resistance required and the maximum instantaneous power dissipated in the Zener diode.

• 21. A 12 volt Zener diode with a minimum current requirement of 4 mA and a power rating of 600 mW is to be used in a voltage regulator. The supply voltage is 20 volts and the load current is variable. Calculate the required series resistance and explain what occurs if the supply is short-circuited.

• 22. A 9 volt Zener diode has a minimum current requirement of 3 mA and is to be used in a voltage regulator. The supply voltage is 24 volts ±5% and the variable load current has a maximum value of 18 mA. Calculate the series resistance required and the minimum power rating of the Zener diode.

• 23. A 6 volt Zener diode with a 1 W power rating has a minimum current requirement of 5 mA and is to be used in a voltage regulator. The supply voltage is 18 volts ±5% and the load current is variable. Calculate the series resistance required and the maximum current that can be delivered by the supply.

• 24. Design a Zener regulated power supply to deliver 12 volts at a current of 25 mA using an 18 volt transformer and a bridge rectifier.

• 25. Using the basic configuration of Fig. 1.60, design a circuit to deliver (i) 3 V from a 5 V DC supply and (ii) 6 V from a 12 V DC supply.

• 26. Determine how to arrange the components in the polarity indicator circuit of Fig. 1.62 if only one LED is available.

• 27. Using the basic Zener regulator configuration and a BZX55C2V0 2 volt Zener, design a circuit that supplies 1.3 volts from a 5 V supply that can replace 1.3 V mercury cells. Use an 1N4148 silicon diode to effect the voltage reduction from 2 V to 1.3 V.

• 28. The circuit shown in Fig. 1.74 is that of a mains wiring fault detector. Determine the condition of the leds (on/off) for the following fault conditions: (i) live disconnected; (ii) live and neutral interchanged; (iii) ground disconnected; (iv) live and ground interchanged; (v) neutral disconnected; and (vi) all connections good

  Fig. 1.74

  

  Circuits for Question 28

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• 29. Resistor R₁ and LED D₁ in Fig. 1.75 constitute a blown fuse indicator for a power supply. Explain the operation of the arrangement and determine a suitable value for the resistor

  Fig. 1.75

  

  Circuits for Question 29

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• 30. Discuss how the circuit shown in Fig. 1.76 can be used for testing fuses in an automobile without their removal and determine a suitable value of R₁

  Fig. 1.76

  

  Circuit for Question 30

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