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Unformatted text preview: 1 PN Junction: SteadyState 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 ” pn junction as one for which a positive voltage V = V f =  V f  is “dropped” from the pside to nside … and a “ reverse biased ” pn junction as one for which a “Forward” and “reverse” bias across a pn junction … and a reverse biased p n junction as one for which a negative voltage V = – V r = –  V r  is “dropped” from the pside to the nside . Consistent with the definition of voltage (V = –E F /q ), the (quasi) Fermi level near the pcontact will be shifted down (up) by an energy –qV relative to those on the nside 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 pn 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 pside I n, → (that fall into the depletion region and are swept across), which is essentially independent of applied voltage V, … Current flow in a pn junction: quasi qualitative analysis which is essentially independent of applied voltage V, … … and a current due to electrons incident from the nside 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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 Spring '11
 LeonardRegister
 Pn junction, depletion region

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