Combination of a cold anode and a heated cathode, or the semiconductor equivalent, which incorporates a p–n junction; see semiconductor diode. Either device allows the passage of direct current in one direction only, and so is commonly used in a rectifier to convert alternating current (AC) to direct current (DC).
Semiconductor diodes are made from silicon-based materials primarily used in electronic circuits to allow an electrical current to flow in one direction only. Silicon on its own is not useful and it is therefore doped (mixed) with other materials in order to achieve the desired physical and chemical properties. The impurity atoms occupy various positions in the silicon crystal. Two types of junctions are produced by doping: p-type and n-type.
P-type junction Silicon has four valence (free) electrons in its outer shell. It is doped with boron atoms, with only three valence electrons in their outer shells. The silicon atoms will share their valence electrons with the valence electrons of the boron atoms. This results in a matrix of silicon atoms doped with boron atoms, which are short of one electron in order to form a fourth chemical bond with the silicon atoms.
A boron atom can accept an electron moving in from a neighbouring silicon atom. This leaves an electron shortage, or ‘hole’, associated with that silicon atom. The hole then moves on to a new position in the matrix (in reality, electrons move in the opposite direction). As the hole moves from one position to another, it is equivalent to a moving positive charge. The boron-doped silicon is called a p-type material (p stands for ‘positive’, indicating that the electric current is carried by the positively charged holes)
N-type junction Phosphorus has five valence electrons, so if phosphorus atoms are added to wafers of silicon they can donate extra electrons. This is called an n-type material (n stands for ‘negative’, indicating that electric current in the material is carried by the negatively charged electrons).
P–n junction In a diode, wafers of p-type and n-type materials are produced by heating the substrate material and the doping substances in a furnace. An n-type wafer is placed on top of a p-type wafer to form a junction where the two types come into contact. Electrons flow through the junction in one direction only.
Types of diode There are many different types of semiconductor diodes such as light-emitting diodes (LEDs), made from gallium arsenide phosphide and emitting electromagnetic radiation, and light-dependent diodes (photodiodes), allowing a flow of current in the presence of light.
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