Transistor Manufacturing Techniques
Mainly there are two types of transistors –
- Junction transistor
- Point transistor.
These transistors are typically fabricated by means of five basic techniques as follows:
- Diffused Technique
- Point contact Technique
- Fused or Alloy Technique
- Rate-Grown or Grown Technique
- Epitaxial Technique
Diffused TechniqueThis technique is used for the creation of planar transistors. The word planar implies that the fabrication of transistor is done on almost flat wafer plane. Here; in a furnace which include P-type gaseous form impurities, the N-type wafer is heated to high temperature. As a result, the impurities (P-type) get diffused gradually into the surface of the wafer (N-type). Thus, P-type region is created which is the base of the transistor. This system is then enclosed by a mask with holes (apertures) and it is heated once more with the gaseous N-type impurities. As a result, N-type atom diffused all the way through the holes and N-layer which is the emitter of transistor is created over the P-layer (figure2).
At last, a thin layer of silicon dioxide is developed over the entire surface and photo stamped to create aluminium contacts for the leads of base and emitter.
Point Contact TechniqueIt comprise of an N-type semiconductor wafer, its one side is soldered to a metallic base and the other side has a tungsten spring or phosphor bronze which is called Cat’s whisker wire which is hard-pressed against it. The entire arrangement is enclosed in a glass or ceramic envelope. This is done to ensure the mechanical strength. A huge current (200 mA) is passed for 1-100 millisecond extent to create PN junction. Owing to the diffusion of the whisker material and the melting of silicon surface into the surface at the contact point, the PN junction is created there (figure 3). Due to the very small value of capacitance at the point contact junction, these junctions are very helpful for working at high frequencies (10 GHz).
Fused or Alloy TechniqueIn this method, two tiny dots of indium or aluminium (acceptor) are positioned in the opposite side of n-type wafer. Then the whole system is heated to about a temperature which is less than the melting point of wafer material and greater than that of the acceptor.
A tiny portion of indium dissolves and enters into the wafer and thus the p-type material is created in the two sides of the wafer. The PNP transistor is created when it is cooled (figure 4).