Construction of a Transistor
Following are some manufacturing techniques used in the construction of a transistor −
Diffusion Type
In this method, the wafer of semiconductor is subjected to some gaseous diffusion of both N type and P type impurities to form emitter and collector junctions. First, base-collector junction is determined and photo-etched just prior to base diffusion. Later, the emitter is diffused on the base. Transistors manufactured by this technique have better noise figure and improvement in current gain is also seen.
Grown Type
It is formed by drawing a single crystal from melted silicon or germanium. The required concentration of impurity is added during crystal drawing operation.
Epitaxial Type
A very high purity and thin single-crystal layer of silicon or germanium is grown on a heavily doped substrate of the same type. This improved version of the crystal forms the collector on which the emitter and base junctions are formed.
Alloy Type
In this method, the base section is made of a thin slice of N type material. At the opposite sides of the slice, two small dots of Indium are attached and the complete formation is kept to a high temperature for a shorter time. The temperature would be above melting temperature of Indium and below Germanium. This technique is also known as fused construction.
Electrochemically Etched Type
In this method, on the opposite sides of a semiconductor wafer, depression is etched in order to reduce the width of the base region. Then a suitable metal is electroplated into the depressions area to form emitter and collector junctions.
