First slide

Diodes

Question

A potential barrier of 0.5V exists across a p-n junction. An electron with speed $5 \times 10^{6} m / s$ approaches the p-n junction from the n-side, with what speed will it enter the p-side?

Moderate

A

$2.7 \times 10^{5} m / s$
B

$5.4 \times 10^{5} m / s$
C

$5 \times 10^{6} m / s$
D

$5 \times 10^{5} m / s$

Solution

$A s t h e e l e c t r o n c r o s s e s t h e p o t e n t i a l b a r r i e r, i t s s p e e d d e c r e a s e s .$
$U \sin g C o n s e r v a t i o n o f M e c h a n i c a l E n e r g y, \Rightarrow \frac{1}{2} m V_{i}^{2} = e V + \frac{1}{2} m V_{f}^{2}$
$\Rightarrow \frac{1}{2} (9.1 \times 10^{- 31}) {(5 \times 10^{6})}^{2} = (0.5 \times 1.6 \times 10^{- 19}) + \frac{1}{2} (9.1 \times 10^{- 31}) V_{f}^{2}$
$\Rightarrow V_{f} = 2.7 \times 10^{5} m / s$

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