Question

#2 At one instantaneous moment, the electron concentration in n-type silicon at 300 K varies linearly from 1e17 cm-3 at x = 0 to 6e16 cm-3 at x = 2 µm. Calculate the electron current density in the silicon sample at that moment. Assume µn= 1000 cm2/V/s. #3 Explain why the assumed mobility in question #2, above, is valid and reasonable.  NOTE: I just need to know the 2nd part of the question that why 'mu' = 1000 is valid and reasonable.

Answer 1

Since electron mobility is a function of doping density and according to your problem doping density varies depending on the position, therefore, an average value of electron mobility is used.

You can find the values from the graph (courtesy: ecee.colorado.edu)

see at x=0 where the electron concentration is 10¹⁷, electron mobility is 1200 cm2/V/s

and for electron concentration 10¹⁶, electron mobility is 800 cm2/V/s

therefore an average of 1000 cm2/V/s is an good approximation.