Why there is exponential growth in current with respect to voltage in p-n junction diode under forward bias configuration?
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2 Answers
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To briefly summarize the whole physics:
P-n diode has excessive holes in one side of the junction and excess free electrones on the other but both sides are electrically neutral. Initially they diffuse to create an equilibrium of numbers of electron and hole , but in this way They create electric potential on the opposite side Off the flow. This Presents prevent further flow of electrons. But when forward bias is used the potential is lowered and a huge flow is again initiated. The exact mathematical structure(the exponential behaviour) can be obtained by writing down the equations of motion and solving them.
Consult the book on Solid State Physics by Ashcroft and Mermin or by Kittel for the mathematical description.
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$\begingroup$ As per me in normal state there is a formation of depletion layer due to combination of electrons with holes and they repel other electrons to combine with holes on the other side.but when we apply voltage across diode in forward bias configuration it gives energy to electron and thus electron crosses the depletion layer and combine with holes to the other side.now as they are combined with holes now these electron should futher repel the other electrons to combine with holes but we see exponential growth rather than increase in resistance. $\endgroup$ Commented May 21, 2016 at 15:00
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$\begingroup$ That is why we connect the diode to a source all the time. When electrons and holes combine they do create a repelling force but this is overcome by the supply of charges from(and to) the battery. $\endgroup$– AriCommented May 21, 2016 at 15:08
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$\begingroup$ The exponential growth is not with time. Rather it is with voltage. So for a steady voltage you get a definite current at all time. $\endgroup$– AriCommented May 21, 2016 at 15:09
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But by combination of electrons with holes there will be increase in repulsion forces of electrons this will create more resistance. so currrent should not increase exponentially with voltage
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$\begingroup$ No. Whenever a electron-hole pair vanishes the battery compensates for it. So that no extra repulsion is created. That's what the battery is there for. To make sure that the voltage across the junction is fixed. $\endgroup$– AriCommented May 21, 2016 at 15:15
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$\begingroup$ I didnt get it.how will battery compensate it.will it provide extra potential to overcome repulsion $\endgroup$ Commented May 21, 2016 at 15:18
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$\begingroup$ That is sure there will be repulsion if electron and hole will combine as we can se in depletion layer.depletion layer also work on this cocept as it also create replusive force/backward potential $\endgroup$ Commented May 21, 2016 at 15:20
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$\begingroup$ No. Why does it need to provide extra potential? It will supply negative charge to one side and positive to the other. $\endgroup$– AriCommented May 21, 2016 at 15:21
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$\begingroup$ I mean to say as depletion layer is formed due to combination of holes and electron.when we forward bias we are increasing the combination of electrons and holes they will further create more repulsive force.repulsive force of depletion layer+replusive force of new combined electrons.thus there should be increase in resiatance $\endgroup$ Commented May 21, 2016 at 15:26