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A transistor is a three terminal device. One terminal is called emitter, one collector and in between them is base. Now, during biasing the junction between emitter and base is made forward biased and the junction between collector and base is made reverse biased.

My question is that if one of the junction of a transistor is reverse biased, how does the transistor allow current to flow through it because the reverse biased junction (diode) doesn't allow the current to flow through it?

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electronics.stackexchange.com may possibly be a better home for this question(v1). –  Qmechanic Jan 8 '13 at 13:55
    
You can try William Beaty's discussion: amasci.com/amateur/transis.html –  Steve B Jan 8 '13 at 15:15
    
Cross-posted to electronics.stackexchange.com/q/53345 –  Qmechanic Mar 8 '13 at 0:25
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3 Answers

In reverse bias , minority carriers can contribute current not majority carriers.So during reverse bias of the collector - base diode the electrons acts as minority carrier in npn transistors and because of that current conduction takes place.

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Simply put, and speaking about a NPN. The base emitter junction is forward biased to allow current to be injected into the emitter. The transistor uses this small injected current and amplifies it in the current that flows from Collector to Emitter. The mechanisms for that are complicated and I won't cover it. But in essence if you also want the base collector junction to be forward biased then the amplified current would also flow into the base. You wouldn't have a transistor then as these currents need to be kept seperate.

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It's surprisingly difficult to find a nice simple description of how a transistor works. This description is from my old physics book - I suspect this may be oversimplified and I'm sure a complete description would run to lots of equations!

Anyhow, this is what an NPN transistor looks like:

Transistor

so as you say, the collector-base junction is reverse biased and no current flows.

Although it isn't clear from the picture, the base is very thin and lightly doped so the hole density is quite low. As soon as you apply a voltage to the base, electrons flow from the emitter into the base and start combining with holes. These electrons can then cross the base-collector junction and a current flows between the emitter and the collector. As you increase the base voltage further more electrons flow into the base from the emitter, so more flow into the collector and more current flows. This is how the small current between the emitter and the base can control the much larger current between the emitter and the collector.

A PNP transistor works the same way but in reverse.

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You say that the base is lightly doped... isn't it also true that the collector and emitter are similarly lightly doped, albeit with a differential material (that makes it n-type)? I mean, my expectation is that the hole density in the base is the same as the extra electron density in the others. I'm wondering if there's some more sophisticated picture that I don't have the background for, or if I'm just reading too far into that. –  AlanSE Jan 8 '13 at 17:55
    
As I recall, the collector layer is moderately doped and the emitter layer is heavily doped. –  John Rennie Jan 8 '13 at 18:06
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