Ch 2b 2006

Ch 2b 2006 - The current-voltage(IV characteristics of the...

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The current-voltage (IV) characteristics of the ideal diode are modeled by the ideal diode equation. First we will start with a qualitative explanation of the IV characteristics. We will begin with the equilibrium band diagram of a pn junction The filled and empty dots in this figure represent a crude approximation of the carrier distribution (e- filled, h+ open) on the two sides of the junction. All of these carriers have thermal energy above 0 K. We start by considering the n side e-, most of these have insufficient energy required to “climb” over the potential hill that results from the electric field that is pushing the e- back into the n region (opposing e- diffusion). Most of the time, when an e- makes it from the n-side into the scr it will be reflected back into the n-side because of this energy barrier. A few of the higher energy e- can make it over the (reduced) energy barrier and they will diffuse into the p region (and quickly recombine with the majority carrier h+). EE 329 Introduction to Electronics 61
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region, sees no energy barrier at all and is quickly swept into the n region by the electric field in the scr. The few number of e- that drift from the p side to the n side (remember they are minority carriers in the p side) will be exactly matched by the few e- in the n side that have sufficient energy in which to diffuse over the energy barrier into the p side (at equilibrium DRIFT e- = DIFFUSION e-). The situation with the h+ is exactly the same, except the energy barrier is reversed. The h+ see an energy barrier when they try to diffuse from the p side to the n side, but any h+ that make it to the edge of the scr on the n side will be quickly swept into the p side by the electric field (at equilibrium DRIFT h+ = DIFFUSION h+). Now what happens when we apply a forward bias to the pn junction (NOTE – a forward bias is when we make the p side of the junction MORE positive and n side MORE negative ). See figure below. What forward bias does is to move the bands closer together (i.e. using two hands on either side of the junction – we move them closer together under forward bias). What is the biggest change in the new band diagram? Now have a reduced energy barrier for e- wanting to diffuse from the n to p side and h+ in the opposite direction. The reduction in the energy barrier is linear with respect to the applied bias, but remember that carrier concentration increases exponentially as one moves away from the band edges, therefore we expect that any reduction in energy barriers means a corresponding exponential increase in e- or h+ flow. EE 329
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Ch 2b 2006 - The current-voltage(IV characteristics of the...

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