ELECTRONICS (5.2 Semiconducter Diode)

What is a semiconductor?

A material with electrical conductivity between a conductor and an insulator.

Examples of semiconductor materials.

• Silicon

• Germanium

What is doping?

The process of adding foreign atoms into the crystal structure of a pure semiconductor to change its electrical conductivity.

What is a p-type semiconductor?

A semiconductor formed by doping a pure semiconductor with suitable impurity atoms, in which holes are the majority charge carriers.

What is the majority charge carrier in a p-type semiconductor?

Holes (positive charge carriers).

What is an n-type semiconductor?

A semiconductor formed by doping a pure semiconductor with suitable impurity atoms, in which electrons are the majority charge carriers.

What is the majority charge carrier in an n-type semiconductor?

Electrons (negative charge carriers).

What is a hole in a semiconductor?

The absence of an electron in a covalent bond, which behaves as a positive charge carrier.

What is a p-n junction?

The boundary formed when p-type and n-type semiconductor materials are joined.

What is a semiconductor diode?

An electronic component that allows electric current to flow in one direction only.

What are the two terminals of a semiconductor diode?

• Anode (+)

• Cathode (−)

What is the function of a semiconductor diode?

To control the direction of current flow in an electrical circuit.

What is forward bias?

A condition where the positive terminal of the battery is connected to the anode and the negative terminal is connected to the cathode.

What happens when a diode is forward biased?

Holes move towards the n-type semiconductor.

Electrons move towards the p-type semiconductor.

The depletion layer becomes thinner.

The junction voltage decreases.

The resistance of the diode becomes very low.

Current flows through the diode.

The bulb lights up.

What is reverse bias?

A condition where the positive terminal of the battery is connected to the cathode and the negative terminal is connected to the anode.

What happens when a diode is reverse biased?

Holes and electrons move away from the depletion layer.

The depletion layer becomes wider.

The junction voltage increases.

The resistance of the diode becomes very high.

Almost no current flows through the diode.

The bulb does not light up.

Depletion layer

A region around the p-n junction that has very few mobile charge carriers.

What is the difference between forward bias and reverse bias?

Forward bias Reverse bias

(Positive to anode) (Positive to cathode)

(Negative to cathode) (Negative to anode)

(Thin depletion layer) (Wide depletion layer)

(Low resistance) (Very high resistance)

(Current flows) (Almost no current flows)

(Bulb lights up) (Bulb does not light up)

What is rectification?

The process of converting alternating current (AC) into direct current (DC).

Why is rectification necessary?

Many electronic devices require DC to operate, whereas electricity supplied to homes is AC.

2 types of rectification

1 Half-Wave

2 Full-Wave

What is half-wave rectification?

The process of allowing only one half-cycle of an alternating current to pass through a load.

How does half-wave rectification work?

During the positive half-cycle:

• The diode is forward biased.

• Current flows through the diode and load.

During the negative half-cycle:

• The diode is reverse biased.

• Current does not flow through the load.

What happens to the negative half-cycle?

It is blocked by the reverse-biased diode.

What is full-wave rectification?

The process of converting both positive and negative half-cycles of AC into current flowing in the same direction through a load.

What is a bridge rectifier?

An arrangement of four semiconductor diodes used to produce full-wave rectification.

How many diodes are used in a bridge rectifier?

Four diodes

How does a bridge rectifier work during the positive half-cycle?

• Two diodes are forward biased.

• The other two diodes are reverse biased.

• Current flows through the load in one direction.

How does a bridge rectifier work during the negative half-cycle?

• The other pair of diodes becomes forward biased.

• The first pair becomes reverse biased.

• Current still flows through the load in the same direction.

What happens if one diode in a bridge rectifier is burnt or disconnected?

The normal full-wave rectification process is disrupted. Depending on which diode fails, one half-cycle may be lost, producing a half-wave-like output.

Why is a capacitor used in a rectification circuit?

To smooth the pulsating DC output.

How is a capacitor connected in a rectification circuit?

In parallel with the load resistor.

How does a capacitor smooth the DC output?

When the output voltage increases:

• The capacitor charges.

• Electrical energy is stored in the capacitor.

When the output voltage decreases:

• The capacitor discharges.

• Stored energy maintains the potential difference across the load resistor.

• The output voltage does not fall to zero immediately.

What is the difference between pulsating DC and smooth DC?

Pulsating DC: Current flows in one direction, but its magnitude changes with time.

Smooth DC: Current flows in one direction with an almost constant magnitude.

What happens when capacitance increases?

A larger capacitance generally produces better smoothing because the capacitor can store more charge and discharge more slowly.

What happens if no capacitor is connected?

The output remains pulsating DC with greater fluctuations.

What is the difference between half-wave and full-wave rectification?

Half-wave Full-wave

(Uses one diode in a basic circuit) (Uses four diodes in a bridge rectifier)

(Only one half-cycle is used) (Both half-cycles are used)

(Lower average DC output) (Higher average DC output)

(More fluctuations) (Less fluctuation for the same basic conditions)

(Less efficient) (More efficient)