A doped semiconductor or a semiconductor with suitable impurity atoms added to it is called extrinsic semiconductor. Extrinsic semiconductors are of two types:
a) N- type semiconductor
b) P-type semiconductor
N- type semiconductor
When a pure semiconductor of Ge or Si is doped with a controlled amount of pentavalent atoms, say arsenic or phosphorous or antimony or bismuth. We get N-type semiconductor or donor type semiconductor. It is called n-type semiconductor because the conduction of electricity in such semiconductor is due to motion of electrons i.e. negative charges, or n-type carries. It is called donor type, because the doped impurity atom donates one free electron to semiconductor for conduction. In n-type semiconductor electrons are majority carriers and holes are minority carriers.
P- type semiconductor
When a pure semiconductor of Ge or Si is doped with a controlled amount of trivalent atoms, say indium or boron or aluminum. We get p-type semiconductor or donor type semiconductor or acceptor type semiconductor. It is called p-type semiconductor because the conduction of electricity in such semiconductor is due to motion of holes i.e. positive charges. It is called acceptor type semiconductor because the doped impurity atom creates a hole in semiconductor which accepts the electron resulting conduction in p-type semiconductor. In p-type semiconductor, holes are majority carriers and electrons are minority carriers.
Effect of temperature on conductivity of semi conductor
With the increases in temperature, more number of covalent bonds is broken, resulting large increase in current carrier concentration (i.e. electrons and holes). Due to which the conductivity of semiconductor increases.
1) When a p-type crystal is brought into close contact with n-type crystal, the resulting arrangement is called p-n junction or junction diode.
2) In equilibrium state, the current does not flow in the junction diode.
3) The p-n junction diode is equivalent to a capacitor in which the depletion layer acts as a dielectric.
4) The current flows in the semiconductor when it is forward biased which is due to movement of electrons and holes both in opposite direction.
5) The effective current in a junction diode is unidirectional, which is in the direction of flow of positive charge.
It is a layer created around the junction which is devoid of free charge careers and has immobile ions.
Characteristics of p-n junction diode
The graphical relations between forward bias voltage and forward current are called forward characteristics. The graphical relations between reverse bias voltage and reverse is called reverse characteristics of p-n junction.
It is the forward voltage beyond which the current through the junction starts to increase rapidly with the linear variation. But below the knee voltage, the characteristics curve is non linear. The knee voltage for Si is 0.7 V and for Ge is 0.3 V .