PHYSIC FORM 5 CHAPTER 4: ELECTRONICS 4.2 SEMICONDUCTOR DIODES

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PROPERTIES OF SEMICONDUCTORS

Conductors are materials which allow current to flow through

them easily.

Reason : conductors have free electrons which can drift

between their atoms. Hence, conductors have low resistance.

Semiconductors are a group of materials that can conduct

better than insulators but not as good as metal conductors

Semiconductors can be pure element such as silicon,

germanium, boron, tellurium.

At 0 Kelvin it behaves as an insulator. When the temperature

increases, the conductivity of the electricity will increase

because its resistance will be lowered.

IN TERMS OF RESISTANCE

METALS INSULATORS

Good conductors of electricity because they have free electrons that can move easily between atoms

The resistance of metals is generally very low.

Poor conductors of electricity because they have too few free electrons to move about.

The resistance of insulators is very high

TWO TYPE OF CHARGE CARRIERS

TYPE OF CHARGE CARRIERS

Hole

Electron

which is negatively charge

which is positively charge

CHARACTERISTICS OF A SILICON ATOM

Figure on the top shows the outer electrons in a silicon crystal which all are involved in perfect covalent bonds, leaving no free electrons to conduct electricity.

There are four electrons in the outermost shell of a silicon atom and they are shared between four other neighbouring atoms to form four covalent bonds.

Each of the covalent bonds has a pair of electrons. Every atoms shares one electron with each of its neighbours.

These holes are said to be carriers of positive

charge

At very low temperature, pure silicon crystal is an insulator and has a high

resistance to current flow.

As the temperature of pure silicon crystal increases, the energy of the vibrating atoms

in the silicon crystal causes some electrons to break free.

For every electron that is broken free, there is a hole in the bonding structure

between the atoms of the crystal. (atom X)

One outer electron from the neighbouring atom (Y) will fill the hole and at the same time will produce a hole at Y.

When the valence/outer electron moves to the left, the hole ‘move’ to the right

This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature

Doping is a process of adding a small amount of impurities into the crystalline lattice of semiconductors to increase their conductivity.

Atoms of the impurities added should have almost the same size as the atoms of the semiconductors.

2 types of semiconductors can be obtained : p-type semiconductor n-type semiconductor

DOPING

Similarities

Undergoes the ‘doping’ process

Made from pure silicon or

germanium

Has electrons and holes as

charge carriers

Boron, indium, gallium, aluminium

Doping substance Antimony, arsenic, phosphorus

Acceptor atom/trivalent Type/ valency of atom Donor atom/ pentavalent

Hole Majority charge carriers Electron

Electron Minority charge carriers Hole

Current flow

p-type semiconductor n-type semiconductor

A semiconductor diode is also called a p-n junction diode.

It consists of a p-type semiconductor in contact with an n-type semiconductor.

The regions of p-type and n-type materials are called anode and cathode respectively.

SEMICONDUCTOR DIODES

WHAT IS THE P-N JUNCTION?

• A p-n junction (depletion layer) is formed when a n-

type and p-type semiconductors are joined

together.

At the p-n junction, electrons from the n-side move to the p-side and recombine with

the holes.

• As a result of this flow, the n-side has a net positive

charge, and the p-side has a net negative charge.

WHAT IS THE DEPLETION LAYER?

The region around the

junction is left with neither

holes nor free electrons.

• This neutral region which has no charge

carriers is called the depletion

layer.

• This layer which has no

charge carrier is a poor

conductor of electricity.

Figure shows the depletion layer and junction voltage of a diode.• In order for electric current to flow through the

diode, the voltage applied across the diode must exceed the junction voltage.

• The junction voltages for germanium and silicon are approximately 0.1V and 0.6V respectively.

WHAT IS FORWARD-BIASED ? The p-type of the diode is

connected to the positive terminal and the n-type is connected to the negative terminal of a battery.

The diode conducts current because the holes from the p-type material and electrons from the n-type material are able to cross over the junction.

A light bulb will light up. The depletion layer is narrow,

and the resistance of the diode decreases.

WHAT IS REVERSE-BIASED ?

The n-type is connected to the positive terminal and the p-type is connected to the negative terminal of the battery.

The reversed polarity causes a very small current to flow as both electrons and holes are pulled away from the junction.

When the potential difference due to the widen depletion region equals the voltage of the battery, the current will cease. Therefore the bulb does not light up.

When a p-n junction diode is in a forward-biased

arrangement, it only allows the current to flow from the anode to the cathode. It is

said to act as a valve.

A diode can convert alternating current into direct

current. This is known as rectification. So, a diode can

act as a rectifier.

An alternating current is a current which changes its

direction with a certain frequency. This is due to the

alternate change of the polarity of the power supply

DIODE AS A RECTIFIER

A complete cycle of alternating current consists of 2 half cycles; a positive half-cycle negative half-cycle

There are 2 ways to convert an alternating current into a direct current: Half-wave rectification Full-wave rectification

The current can only flow in the forward direction through the diode.

In the first half-cycle, the diode is forward-biased. The current can flow through the diode

In the second half-cycle, the diode is reverse-biased. The diode blocks the current.

The process of rectification using a diode which allows current to flow in the half-cycle is known as half-wave rectification

Half-wave rectification

FULL-WAVE RECTIFICATION The arrangement of

diodes in Figure 4.38 is called a bridge rectifier, because it reverses the negative half of each alternating current cycle instead of blocking the flow of the current.

During the forward half of

each cycle, diodes X are

forward-biased but diodes Y are reverse-biased.

Therefore, diodes X conduct current (solid arrows) but

diodes Y block current.

During the reverse half, diodes Y are

forward-biased but diodes X are reverse-biased.

Therefore, diodes Y conduct current

(broken arrow) but diodes X block current.

As a result, the current always

flows in the same direction through

the load regardless of which way it

leaves the power supply.

The process of rectification using 4 diodes to allow current to flow in a complete cycle and in the same

direction is called full-wave

rectification.

The output from a rectifier circuit can be smoothed by connecting a capacitor across the load, as shown in Figure 4.39

During the forward peaks (positive half-cycles), the capacitor is charged up. Energy is stored in the capacitor.

In between the forward peaks(negative half-cycles), the capacitor releases its charge (discharges). It discharges partly through the load. The energy stored in the capacitor acts as a reservoir and maintains the potential difference across the load.

SMOOTHING

Click icon to add pictureFigure 4.40 shows

the potential difference for half-wave rectification with smoothing.A capacitor with

great capacitance produces a smoother

current. This is because the

capacitor can store more charge.