# Diode current components

I have the following two questions in the derivation of current in a diode.

1) In forward bias, an assumption "abrupt junction" is made and field is equal to 0 is also made. But then we still use built in potential. How is this okay?

2) In most semiconductor classes and also in Millman and Halkias they have left reverse bias case as similar. But can anyone help me out with the actual derivation? Because i don't see how the exponential decrease in hole concentration with distance occurs if the field is non zero in the continuity equation

Any help would be appreciated. Thanks

• Could you clarify what you mean by "field is equal to 0"? I'm not familiar with that assumption for diode analysis. Basic diode analysis usually assumes an abrupt junction (a step function dopant profile) and that there are zero electrons and holes in the depletion region. I also don't understand what you mean about exponential decrease in hole concentration with distance. If you make the previous assumptions then your hole concentration will be a step function as well. 0 inside the depletion region and $N_A$ outside the depletion region. – Matt Aug 8 '18 at 13:29
• The exponential is not from pside to n-side. But rather within the n-side from the junction taken as x=0. By the field I mean the field that causes drift current . – pranav Aug 8 '18 at 13:54
• The field cant be equal to 0. A diode expierences both drift and diffusion currents and cant work if you just assume away the drift current part. You must misunderstand something. I'm still not following this exponential decrease in hole concentration. At the junction the hole concentration is zero, and since hole concentration cant be negative, I dont see what you mean. – Matt Aug 8 '18 at 14:02
• Hmn... you didnt mention where you are assuming the field is equal to zero. Its equal to zero outside the depletion region. Is this what you mean? – Matt Aug 8 '18 at 14:05
• For an abrupt junction we neglect the width of the junction and assume step function right? – pranav Aug 8 '18 at 14:06