# Solid-state devices and valves

### Semiconductor materials

The manufacturing process of solid-state devices is complex: it requires the refinement of silicon and germanium to an extremely high purity and then the introduction of a very small but closely controlled amount of an impurity. This process is known as 'doping'. It is this impurity that gives the base material (silicon or germanium) its semiconductor properties which provide transistor and diode action.

N-type material
In the manufacturing process, impurity atoms having an outer ring containing five electrons are introduced into the crystal lattice. The resulting material is known as 'N-type'. Typical impurities are phosphorus and arsenic.

P-type material
If the impurity added has only three electrons in its outer ring then a gap is left in the lattice which could be filled by a free electron. Such a gap is called a 'hole' and the resulting material is known as 'P-type'. Typical impurities are boron and aluminium.

The doped material of both N and P types is electrically neutral because each one of the individual atoms present is itself electrically neutral.

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