What is a semiconductor? Distinguish between intrinsic and extrinsic conduction in a semiconductor - Leaving Cert Physics - Question 8 - 2016

Intrinsic conduction occurs in a pure semiconductor, which contains equal numbers of electrons and holes. Extrinsic conduction, on the other hand, occurs in a doped semiconductor where the balance of electrons and holes is altered; doping with group III elements creates p-type semiconductors (fewer electrons), while doping with group V elements creates n-type semiconductors (more electrons).

(i) A pure semiconductor can be converted to a p-type semiconductor by doping it with a group III element, such as boron. This process introduces holes, or positive charge carriers, by creating more vacancies in the electronic structure.

(ii) Conversely, to convert a pure semiconductor into an n-type, it can be doped with a group V element, like phosphorus, which adds extra electrons to the system, increasing the number of negative charge carriers.

At the p-n junction, when p-type and n-type materials are brought together, electrons from the n-type material will diffuse into the p-type material and recombine with holes. This recombination results in the formation of a depletion region, which is devoid of charge carriers, and creates an electric field that opposes further diffusion.

A depletion layer is a region at the p-n junction where charge carriers (electrons and holes) are depleted due to recombination. This layer possesses high resistance and helps establish the electric field necessary for diode operation.

On the p-n junction diagram, the p-type side will be positively charged due to the presence of holes, the n-type side will be negatively charged because of the excess electrons, and the depletion region will be neutral as it lacks charge carriers.

To protect a diode from excessive current in forward bias, a resistor can be placed in series with the diode. This resistor will limit the amount of current that can flow through the diode, thus preventing it from being damaged.

If the terminals of the battery were reversed, the diode would enter reverse bias. In this condition, it allows minimal current to flow, often in the range of microamperes, thus protecting the diode from damage under normal circumstances.

A rectifier is a device that converts alternating current (AC) to direct current (DC). It allows current to flow in one direction, effectively blocking the reverse flow, which is essential for powering DC devices.

The key property that makes a diode useful in a rectifier circuit is its ability to conduct current in one direction while blocking it in the opposite direction. This unidirectional conductivity allows it to effectively convert AC to DC.