First slide

Types of semiconductors

Question

A N-type silicon sample of width $2 \times 10^{- 3} m$ , thickness and length $3 \times 10^{- 2} m$ carries a current of 2.4mA, when the voltage is applied across the length of the sample. The free electron density is $10^{22} m^{- 3}$ . Select which one of them is incorrect

Moderate

A

the current density is $20 A / m^{2}$
B

the drift speed is 5cm/s
C

the time taken by electrons to travel the full length of the sample is 5sec
D

All options are incorrect

Solution

$C u r r e n t d e n s i t y = \frac{i}{A} = \frac{2.4 \times 10^{- 3}}{2 \times 10^{- 3} \times 3 \times 10^{- 2}} = 40 A / m^{2}$
$D r i f t s p e e d = V_{d} = \frac{i}{A n e} = \frac{40}{10^{22} \times 1.6 \times 10^{- 19}} = 2.5 \times 10^{- 2} m / s = 2.5 c m / s$
$T i m e = t = \frac{l}{V_{d}} = \frac{3 \times 10^{- 2}}{2.5 \times 10^{- 2}} = 1.2 \sec$

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