Higher Physics

Semiconductor Diodes

Light Emitting Diode 1

An LED is a forward biased diode

When a current flows, electron-hole pairs combine at the p-n junction.

The doping of the p-type and n-type materials is carefully tuned so that there is excess energy when electron-hole pairs combine.

Light Emitting Diode 2

The recombination energy of the electron-hole pairs is released by de-excitation of the electrons

This leads to photon emission

Photon Energy, E = hf

where h – Planck’s Constantf – light frequency

Light Emitting Diode 3

Example

Find the recombination energy for the following LEDs –

a) red – λ = 650 nm

b) blue – f = 4.5 x 1014 Hz

Photodiode

A photodiode has a p-n junction where electron-hole pairs are generated by absorbed photons from incident light

Photodiodes can be operated in two different modes

Photovoltaic Mode 1

Photodiode has no bias voltage applied, but is illuminated by a light source.

Electron-hole pairs are produced, giving a potential difference

The output voltage increases as the irradiance of the source increases

V

Light source

Irradiance, I

Voltage, V

Photovoltaic Mode 2

Output voltage can be used to power devices e.g. photo cell for calculator

Can be connected in series to give larger voltage outputs

In this mode the photodiode operates in exactly the opposite way to an LED

Photoconductive Mode 1

In this mode the photodiode is connected in reverse bias.

If it is kept dark, it acts a reverse-biased p-n junction and will not conduct.

If it is illuminated, the junction will release electrons and create electron-hole pairs.

This provides a number of free charge carriers in the depletion layer, decreasing the resistance and enabling a current to flow.

Photoconductive Mode 2

A greater irradiance gives more free charge carriers and therefore less resistance.

The photodiode acts as a light dependent resistor (LDR)

Because the electron-hole pairs recombine quickly LDRs have a very fast response time, allowing them to be used in situations where light levels change rapidly.

Irradiance, I

Resistance, R

MOSFET 1

Stands for –

MetalOxideSemiconductorFieldEffectTransistor

MOSFET 2

Operate like npn transistors (have threshold voltage to ‘switch on’)

Work in a different way, due to charge distributions inside the transistor material.