High School Physics

# The semiconductor Diode – Revision notes

A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential I-V relationship and therefore we can not describe its operation by simply using an equation such as Ohm’s law.

## How does a diode work?

When the diode’s anode terminal is more positive than its cathode terminal by at least 0.7 volts for a silicon device, the diode is classed as being “forward biased” allowing current to flow through the device.

However, when the anode is made more negative than the cathode, the diode is classed as being “reversed biased” and blocks the flow of current up to its reverse breakdown voltage at which point the diode loses control.

The diode’s two connections are known as the Anode, (A) and Cathode, (K). The cathode (negative end) is often marked with a band for identification.

Semiconductor diodes are formed using either Silicon or Germanium semiconductor materials.

Diodes are classed as either small signal diodes for use in a variety of small voltage applications or classed as power diodes for use in rectifying and mains-powered circuits.

Germanium diodes, unlike silicon diodes, only require a forward-biasing voltage of 0.3 volts or greater for conduction to occur.

### Connection of diodes in series

Diodes being one-way devices cannot be connected together in series randomly. Only circuit, “A” below will conduct current.