Mainly there are two types of transistors –
- Junction transistor
- Point transistor.
Due to the small size and ruggedness, junction transistor is mainly used. They are of two types – PNP and NPN transistors (figure 1).
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
This 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 Technique
It 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 Technique
In 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).
Rate-Grown or Grown Technique
In this technique, a single crystal is drawn from a melt of Ge or Si which include p-type impurities. The Czochralski technique or floating technique is used for this purpose. Here in Czochralski technique, a single semiconductor seed is submerged in the molten semiconductor which is in graphite crucible. After that, it is slowly withdrawn and at that time, the rod which is holding the seed is gradually rotating. At first, P-type impurities are added, and then it changes to N-type. Thus, PN junction is grown. The apparatus used is shown below.
The name came from the Greek words ‘epi’ and ‘taxis’-‘Epi’ means on and ‘taxis’ means arrangement. Here, a very lean n-type Semiconductor or p-type semiconductor layer can be grown on a highly doped material (substrate) which is of the same semiconductor (figure 6). The layer formed can be any of the three (base, emitter or collector). The junction thus created has low resistance.