Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 35 : Diffusion
Example 12
  Electron concentration in a semiconductor varies linearly from a value $ 1\times 10^{24}$ per m $ ^3$ to $ 6\times 10^{23}$ per m $ ^3$ over a distance of 0.1 cm. If the diffusion constant is 0.025 m $ ^2$/s, find the diffusion current density in the sample.
  Solution
 
$\displaystyle \frac{dn}{dx} = \frac{10^{24}-6\times 10^{23}}{0.001}= 4\times 10^{26}\ \ {\rm m}^{-4}$
  Thus the diffusion current density
 
$\displaystyle j$ $\displaystyle =$ $\displaystyle eD_n\frac{dn}{dx}$
$\displaystyle =$ $\displaystyle 1.6\times 10^{-19}\times 0.025\times 4\times 10^{26}$
$\displaystyle =$ $\displaystyle 1.6\times 10^6\ \ {\rm A/m}^2$
 

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