Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 33 : Mobility of Electrons and Holes
Example 5
  The average distance travelled by a carrier between collisions is called the mean free path . The electron mobility in GaAs at 77 K (the temperature at which nitrogen becomes a liquid) is 30 m $ ^2$/V-s. If the effective mass of electron is 0.067 $ m_0$, determine the mean free path.
  Solution
  The relaxation time can be calculated from mobility
 
$\displaystyle \tau = \frac{m\mu}{q}= \frac{0.067\times 9.1\times 10^{-31}\times 30}{1.6\times 10^{-19}} = 11.375\times 10^{-12}\ \ {\rm s}$
  The thermal velocity is given from kinetic theory to be
 
$\displaystyle v_{th} = \sqrt{\frac{3kT}{m}} = \sqrt{\frac{3\times 1.38\times 10^{-23}\times 77}{0.067\times 9.1\times 10^{-31}}}= 22.92\times 10^4\ \ {\rm m/s}$
  Thus the mean free path is $ v_{th}\tau = 2.6\times 10^{-6}$ m.
 

 Back