<P> Extrinsic semiconductors are components of many common electrical devices . A semiconductor diode (devices that allow current in only one direction) consists of p - type and n - type semiconductors placed in junction with one another . Currently, most semiconductor diodes use doped silicon or germanium . </P> <P> Transistors (devices that enable current switching) also make use of extrinsic semiconductors . Bipolar junction transistors (BJT), which amplify current, are one type of transistor . The most common BJTs are NPN and PNP type . NPN transistors have two layers of n - type semiconductors sandwiching a p - type semiconductor . PNP transistors have two layers of p - type semiconductors sandwiching an n - type semiconductor . </P> <P> Field - effect transistors (FET) are another type of transistor which amplify current implementing extrinsic semiconductors . As opposed to BJTs, they are called unipolar because they involve single carrier type operation--either N - channel or P - channel . FETs are broken into two families, junction gate FET (JFET), which are three terminal semiconductors, and insulated gate FET (IGFET), which are four terminal semiconductors . </P> <P> Other devices implementing the extrinsic semiconductor: </P>

What are n-type and p-type semiconductors explain by examples