CH1012
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11
TRANSISTORS
The combination of n-type and p-type semiconductors in contact forms a p-n junction.
When a battery is connected +ve to p and -ve to n current flows in the n ® p direction.  Current does not flow in the p ® n direction.  The p-n junction was the starting point for the formation of non-valve electronic devices.
A transistor is a n-p-n (or p-n-p) device which is the basis of the semiconductor industry and resulted in the replacement of valves.
To see the difference in power requirements one of the earliest valve computers was called ENIAC (1947), Eniac required 150 000W to operate and yet it has only a fraction of the compute power of a transistor based Pentium 4 Notebook.