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E E 339 - CN12-pn jucntion-steady-state current flow

# E E 339 - CN12-pn jucntion-steady-state current flow - 1...

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Unformatted text preview: 1 P-N Junction: Steady-State Current flow P N Junction: Steady State Current flow 1 Forward and Reverse bias Current flow: qualitative analysis Current flow: quantitative analysi Outline Current flow: quantitative analysis Reverse breakdown Zener breakdown Avalanche breakdown (Other) deviations from simple theory Short diodes (quick note) Summary 2 2 Define a “ forward biased ” p-n junction as one for which a positive voltage V = V f = | V f | is “dropped” from the p-side to n-side … and a “ reverse biased ” p-n junction as one for which a “Forward” and “reverse” bias across a p-n junction … and a reverse biased p n junction as one for which a negative voltage V = – V r = – | V r | is “dropped” from the p-side to the n-side . Consistent with the definition of voltage (V = –E F /q ), the (quasi-) Fermi level near the p-contact will be shifted down (up) by an energy –qV relative to those on the n-side reducing (increasing the “band bending” under forward (reverse bias (increasing) the band bending under forward (reverse) bias . As before, essentially all of the band bending and associated electrostatic potential drop will occur across the charged if now smaller (larger) depletion region … except under very high current flows TBD. 3 Figure 5—13, modified Effects of bias on a p-n junction: transition region width and peak electric field, electrostatic potential ν n,p and energy band diagram (a) equilibrium, (b) forward bias, and (c) reverse bias. note different symbol ( ν n,p ) for electrostatic potential vs. that for voltage ( V ) . 4 3 Total electron current I n consists of a current due to the few electrons incident from the p-side I n, → (that fall into the depletion region and are swept across), which is essentially independent of applied voltage V, … Current flow in a p-n junction: quasi- qualitative analysis which is essentially independent of applied voltage V, … … and a current due to electrons incident from the n-side I n, ← , from the high energy tail of the electron distribution with enough energy to overcome the energy barrier , which is proportional to thermal distribution of electrons (with relative T k qV T k V V q B B o e e ) ( E F,p E c,p E v,p q (voltage ,V) E x E v,n ) ( , , o n c p c V V q E E E F,n V o > V under any conditions for which this diagram is reasonable electrons (with relative numbers of low energy electrons minimized for lack of space.) E c,n 5 The total electron charge current from p to n (direction of voltage “drop”) therefore, is of the form o n T k qV o n n n n I e I I I I B , , , , , , equilibrium values due to electrons from right due to electrons from left In equilibrium, V = 0 and the current must vanish, such that | I n, → ,o | = | I n, ← ,o | ≡ I n,o , so that Similarly for hole current from p to n ) 1 ( , T k qV o n n B e I...
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E E 339 - CN12-pn jucntion-steady-state current flow - 1...

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