Exercises: Semiconductors, p-n Junction and Transistors

Silicon is a semiconductor which can be doped.

1. a) What is meant by doping?
2. b) Name one element used to make n-type silicon and one used to make p-type silicon.
3. c) State the majority charge carriers in n-type and in p-type material.

1. a) What is the depletion layer in a p-n junction?
2. b) Explain in terms of charge carriers how it forms.
3. c) State whether the depletion layer gets wider or narrower when the junction is reverse biased.

A transistor can act as an electronic switch.

1. a) Name the three terminals of an n-p-n transistor.
2. b) State which terminal is the input that controls the switch.
3. c) Describe what happens to the current between collector and emitter when the base voltage is below its threshold of about $0.6\ \text{V}$.

Pure silicon has a small number of free charge carriers at room temperature. A sample of silicon is doped with phosphorus.

1. a) Explain how a phosphorus atom, which has five valence electrons, contributes to conduction in silicon (four valence electrons).
2. b) Explain why the doped sample is still electrically neutral overall.
3. c) Explain why the resistance of pure silicon decreases as temperature rises.

A transistor is used in a circuit to switch on an LED in a night-light when it becomes dark. A light-dependent resistor (LDR) and a fixed resistor form a potential divider connected to the base.

1. a) Explain how the voltage at the base changes as light level falls, if the LDR is in the lower half of the divider and its resistance rises in the dark.
2. b) Describe how this turns the LED on.
3. c) Why is a transistor used rather than connecting the LED directly to the divider?

A silicon p-n junction has a potential barrier of about $0.7\ \text{V}$ across its depletion layer. It is connected in series with a $330\ \Omega$ resistor and a $5.0\ \text{V}$ supply in forward bias.

1. a) Explain how the depletion layer changes in forward bias.
2. b) Calculate the current in the circuit, assuming the diode drops $0.7\ \text{V}$.
3. c) Calculate the power dissipated in the resistor.

In a p-n junction, electrons diffuse from the n-side to the p-side and holes in the opposite direction.

1. a) Describe the formation of the depletion layer and the resulting electric field direction.
2. b) Explain why diffusion does not continue indefinitely.
3. c) Explain why the junction conducts in forward bias but practically not in reverse bias.
4. d) Explain why a diode can be used to convert alternating current into direct current, sketching in words the output of a half-wave rectifier.

An n-p-n transistor switches a relay coil of resistance $120\ \Omega$ from a $12\ \text{V}$ supply. The base is connected to a $5.0\ \text{V}$ source through a resistor $R_B$. The base-emitter voltage is $0.7\ \text{V}$ when conducting.

1. a) Calculate the current through the relay coil when the transistor is fully on (ignore the collector-emitter voltage).
2. b) The transistor's current gain is $h_{FE} = 100$. Calculate the minimum base current needed.
3. c) Calculate the largest value of $R_B$ that will ensure full switching on.
4. d) Explain why a diode is often connected across a relay coil.

A student claims: "Doped semiconductors are charged because they have extra electrons or holes."

1. a) Evaluate this claim.
2. b) Explain why doping increases conductivity by several orders of magnitude.
3. c) Compare, with reasons, the effect of raising temperature on the resistance of a metal and of a pure semiconductor.
4. d) Explain why transistors in a processor are made from semiconductors rather than metals.