Question

Two wafers of extrinsic Si have the same dopant concentration, but one wafer is n-type and the other is p-type. The resistivities of the two wafers are measured under the same test conditions and found to be different. Which one has the larger resistivity? Explain.

Answer 1

The conductivity of an N type semiconductor is given by the equation:

(As in N-type n >> p)

Here n- Concentration of Dopant Ions

e – charge on an electron

– mobility of an electron

Since , hence resistivity _N =;

Similarly for the P type semiconductor resistivity _P is given by:

P ₌ ,

Here p is the concentration of acceptor ions

And _h is the mobility of holes.

Now the ratio of _N and _P is given by :

= =

Since it is given that concentration is same i.e. n=p hence

= .

Now in a silicon semiconductor mobility of electrons is nearly 2.2 times of that of mobility of holes at room temperature i.e.

_e 2.2 _h

(This is because of the fact that the effective mass of hole is greater as compared to the effective or rest mass of an electron).

Hence 2.2 or _p>_N.

So we can hence conclude that for the same concentrations and test conditions resistivity of P-type wafer is greater as compared to the N-type wafer.