The slice is now heated whilst exposed to an atmosphere rich in phosphorus. This forms an n-type emitter region by diffusion and the exposed area is again sealed by heating the slice in an oxidising atmosphere to form a layer of silicon dioxide. This results in a deposition of a thin coating of aluminium over the entire face of the slice. Finally the aluminium is removed from the areas in which it is not required by a masking and selective etching operation. The base area of each transistor is sometimes of approximately annular shape surrounding a circular emitter area but in power transistors both base and emitter areas may be in the form of parallel strips.
Planar transistors lend themselves well to mass production.
Planar technology revolutionised silicon transistor manufacture in the 1960s. Leakage currents are very low and the transistors can be designed to work at frequencies well over 1 GHz. In 1963 the process also made possible for the first time mass production of f.e.t.s although the pronciple of this type of transistor had been described by Shockley 11 years earlier. When a conventional pn diode is forward biased some of the majority carriers crossing the junction are neutralised by combination with majority carriers of opposite polarity. This delay is a serious disadvantage in diodes required for operation at microwave frequencies or in high-speed switching.