The earliest electronic circuits were made of distinct components – resistors, capacitors, diodes and transisitors. It was soon realised however, that all these components could be miniaturised and combined into a single wafer of silicon material – an integrated circuit. There are two main types of integrated circuit:
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Linear/analogue circuits in which currents are amplified.
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Digital or switching circuits. Digital circuits are themselves divided into two types. The first is called transistor transistor logic or TTL, and the second is called complementary metal oxide semiconductor logic or CMOS. The first allows faster on off switching of the circuit but requires are larger power supply than the second. On the other hand, CMOS circuits allow a higher density of components to be achieved – more components on chips of the same size.
Four basic steps are needed to create regions on a silicon wafer where components can be placed.
a) Oxidation – a layer of silicon oxide (glass) is grown on top of the silicon wafer by heating in a furnace with oxygen and steam.
b) Photomasking – a light sensitive plastic film is formed on the surface of the silicon oxide, which hardens when ultraviolet light hits it. A mask containing the circuit is placed over the strip and ultraviolet light shone through it. The soft part of the plastic, not hardened by exposure to ultraviolet light, are now washed away, along with the unprotected silicon oxide film. The remaining plastic coating is chemically removed.
c) Diffusion – unprotected layers of silicon are changed to n or p type by diffusion of a phosphor gas mixture in a furnace.
d) Metallization – oxides are grown on the surface as before, and by a similar masking process a thin aluminium layer is placed over the top layer to connect transistors and diodes.