Space charge width pn junction

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Homework Statement



Calculate the width of the space charge region in a pn junction when a reverse biased voltage is applied. Consider a silicon pn junction at T=300K with doping concentrations of Na=10^16 cm^-3 and Nd=10^15 cm-3. Assume that Ni=1.5*10^10 cm-3 and VR=5V

Homework Equations


W=((2Ef(Vbi+Vr)/e)((Na+Nd)/((Na*Nd))))^1/2

The Attempt at a Solution



I correctly calculated the in built potential to be 0.635V however, I do not know how to calculate the fermi energy
 
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Check that formula again. It does not involve the Fermi energy.
 
phyzguy said:
Check that formula again. It does not involve the Fermi energy.

What is the value of epsilon in the formula attached?
 

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It's the permittivity of the material.
 

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