Direction: A semiconductor has following parameters. µa = 7500 cm2/ V - s,
µp = 300 cm2/ V - s,
ni = 3.6 × 1012 cm–3
-
When conductivity is minimum, the hole concentration is—
-
- 36 × 1012 cm–3
- 1.8 × 1013 cm–3
- 1.44 × 1011 cm–3
- 9 × 1013 cm–3
Correct Option: A
Given data
µn = 7500 cm2/ V-s
µP = 300 cm2/ V-s
ni = 3.6 × 1012 cm–3
we know that overall conductivity is given by relation
| σ = q (µn n + µP P) where, n = | P |
| or σ = 9 µn. | + 9. µP. P …(A) | P |
for max. or min. value of conductivity, differentiate equation (A) w.r.t. hole or electron concentration and put equals to zero.
| = - | + 9µP | dP | P2 |
| 0 = – | + 9µP | P2 |
| or P = ni |  |  | 1/2 | |
| µP |
| or P = 3.6 × 1012 | | | 1/2 | |
| 300 |
or P = 7.2 × 1012 (25)1/2
or P = 7.2 × 5 × 1012
or P = 36.0 × 1012 cm–3
Hence alternative (A) is the correct choice
