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majormajor
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I am trying to understand some aspects of the depletion layer in a semiconductor diode and I have come across some questions that none of the textbooks I have seem to explain. Perhaps some of you could help? (I must add I am not a physisist but an electronics engineer, although I have been taught the usual basic semiconductor theory at uni.)
(1) I understand that when a p and n type materials form a junction, electrons diffuse from the n type material to the p type and holes diffuse from the p type to the n type. This is kind of intuitive enough and I understand how this leaves a positive charge layer in the n-type material that is bound to the crystal lattice and vice versa, a negative charge layer in the p type material due to the holes that have diffused into the n-type material. But what the textbooks don't seem to explain is this: let's just look at the n-type material. OK, so electrons have diffused across the junction, leaving positively charged ions behind. But what happens to the electrons that have left the n-type material? Are they just sitting somewhere in the p-type material (let's assume there is no external electrical field on the junction)? Where are they? Clearly they must form another (mobile) space charge layer somewhere on the p-type side?
(2) What is even more baffling for me is how minority carries can pass through the depletion layer (like in a reverse charged pn junction or the base-collector junction of a BJT). The depletion layer means that near the juction on the n-type side there is a positively charged layer due to the fact that the electrons have left. Surely if somewhere in the n-type layer holes are generated somehow, they will be repelled by the positive space-charge layer and will not be able to cross the junction? I feel this is really a point where I don't understand something, so please could somebody explain (just qualitatively) what is going on here?
Thanks,
MajorMajor
(1) I understand that when a p and n type materials form a junction, electrons diffuse from the n type material to the p type and holes diffuse from the p type to the n type. This is kind of intuitive enough and I understand how this leaves a positive charge layer in the n-type material that is bound to the crystal lattice and vice versa, a negative charge layer in the p type material due to the holes that have diffused into the n-type material. But what the textbooks don't seem to explain is this: let's just look at the n-type material. OK, so electrons have diffused across the junction, leaving positively charged ions behind. But what happens to the electrons that have left the n-type material? Are they just sitting somewhere in the p-type material (let's assume there is no external electrical field on the junction)? Where are they? Clearly they must form another (mobile) space charge layer somewhere on the p-type side?
(2) What is even more baffling for me is how minority carries can pass through the depletion layer (like in a reverse charged pn junction or the base-collector junction of a BJT). The depletion layer means that near the juction on the n-type side there is a positively charged layer due to the fact that the electrons have left. Surely if somewhere in the n-type layer holes are generated somehow, they will be repelled by the positive space-charge layer and will not be able to cross the junction? I feel this is really a point where I don't understand something, so please could somebody explain (just qualitatively) what is going on here?
Thanks,
MajorMajor