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Bipolar Junction Transistor

A bipolar junction transistor (BJT) has a silicon or germanium crystal. It would either have a layer of N-type semiconductor sandwiched between two layers of P-type semiconductor or a layer of P-type semiconductor between two layers of N-type semiconductor materials. The first type of transistor is called a PNP transistor, and the second one is called an NPN transistor. The sandwich of semiconductor is very small and sealed inside either a metal or plastic case to protect it from moisture.


Construction of (a) PNP transistors (b) NPN transistors

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