Transistor Manufacturing Techniques
The following four techniques are employed to manufacture transistors:
1. Grown technique.
A PNP transistor is manufactured by drawing a single crystal from the melt of Germanium or silicon having p-type impurities. When the crystal is drawn, n-type impurities are added to form the base region of the transistor. Then p-type impurities are added. Similarly, the NPN transistor is constructed.
2. Alloy or fused technique.
In this technique, a thin wafer of N-type material is taken. Then two small dots of Indium (trivalent impurity) are placed on the opposite side of the wafer. Now, the whole system is heated for a short time to a temperature higher than the melting point of Indium but below the melting point of wafer material.
The Indium melt and penetrates little in the wafer. Thus, both the side of the wafer becomes p-type whereas the middle part of the wafer is n-type which acts as the base of the PNP transistor. Similarly, the NPN transistor can also be manufactured.
A flat wafer surface of n-type Silicon (known as n-type substrates) is heated in a furnace having p-type impurities in the gaseous form. p-type impurities diffuse into the wafer surface and form the base of the transistor. The flat surface of the wafer acts as a collector.
Now the system is covered with a mask having a small aperture over the base region. This system is now heated in the gaseous environment of n-type impurities. n-type impurities after passing through the Aperture deposit over the base and from the n-type layer which acts as the emitter of the transistor.
A silicon dioxide layer is Grown over the surface and then photoetched in order to make aluminum contacts for both the emitter and base leads.
In this technique, a very thin and high Purity single crystal layer of n-type silicon is grown on a heavily doped substrate of the same material. This path acts as a collector of the transistor. Now p-type impurities are diffused which form the base of the transistor. Then n-type impurities are defused to form the emitter of the transistor as shown in the figure.