Why is the conductivity of n-type semiconductor greater than that of the p-type semiconductor even when both of these have same level of doping?

 

How does the receptivity of (i) a conductor and (ii) a semiconductor vary with temperature?  Give reason for each case.

 

Explain briefly with the hell) of a circuit diagram how V - I characteristics of a p-n junction diode are obtained in (i) forward bias, and (ii) reverse bias.

1.

In spite of having same level of doping the conductivity of an n-type semiconductor is greater than p-type semiconductor because the charge carrier of a n-type semiconductor is electron which is faster to move than the hole which is the charge carrier of a p-type semiconductor. In other words electrons are more mobile than the holes which makes conduction in n-type semiconductor faster.

 

The conductivity of an n-type semiconductor (neglecting minority charge carriers) is given by

σn = neμe

μe is electron mobility

The conductivity of an p-type semiconductor (neglecting minority charge carriers) is given by

σp = peμp

μp is hole mobility

In same level of mobility n = p

We have

μe > μp

=> σn > σp

 

2.

Resistivity of a conductor increases with increases in temperature.

Reason: Increase in temperature lets the kernels of the conductor vibrate and also there occurs thermal vibration in the molecules, these decreases the mean free path of the electrons inside the conductor. Increase in mean free path decreases the relaxation time for the electron. Now as because resistivity of a conductor is inversely proportional to the relaxation time (τ) the resistivity increases with temperature.

ρ = 2me/(ne2τ)

=> ρ α 1/τ

Resistivity of a semiconductor decreases with temperature.

Conductivity of a material is determined by two factors: the concentration of free carriers available to conduct current and their mobility (or freedom to move). In a semiconductor, both mobility and carrier concentration are temperature dependent.

At absolute zero temperature any semiconductor (Si or Ge) behaves as an insulator. Some electron-hole pairs may get generated due to thermal energy at room temperature (300 K). At higher temperature, more electron-hole pairs are generated. The concentration of charge carriers will be higher at higher temperature. Thus, the conductivity of intrinsic semiconductor increases with increasing temperature and the resistivity decreases with increase in the temperature.

3.

i.

Circuit diagram for forward characteristics.

Connections are made as shown in the diagram. The voltmeter is connected in volt 1 V to 12 V. Ammeter is connected in the milliampere range. We vary the voltage across the diode and take the reading for the current in the ammeter and note it down. Then we plot current along the y axis and the potential along the x axis. The points are connected by a free hand curve to obtain the V-I curve.

ii.

Circuit diagram for reversed characteristics

In case or reversed characteristics we follow the similar procedure as of the forward characteristics.

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