Electronic – Basic operation of a bipolar junction transistor


I've tried really hard to understand the basic operational principle of a transistor. I've referred to many books and been to forums but have never got a convincing answer.

Here are the things I want to understand:

A transistor is similar to a reverse biased diode unless a voltage is applied to the Base. Since the Emitter-Base junction is forward-biased, there will be conduction of – say – electrons (npn). What happens then? Is it true that these electrons from the Base break the barrier of the Collector-Base junction and then the combined current passes to the Emitter? (IB + IC = IE)

And why is that we are getting more current? Where is amplification? It can't be like creating something out of nothing. I know I'm missing some crucial point here. Can somebody explain me clearly in simple terms?

It has been a week I'm tryin to understand this. 🙁

Best Answer

When electrons flow through a forward-biased diode junction, such as the base-emitter junction of a transistor, it actually takes a non-zero amount of time for them to recombine with holes on the P side and be neutralized.

In an NPN transistor, the P-type base region is constructed so as to be so narrow that most of the electrons actually pass all of the way through it before this recombination occurs. Once they reach the depletion region of the reverse-biased base-collector junction, which has a strong electrical field across it, they are quickly swept away from the base region altogether, creating the collector current.

The total current through the base-emitter junction is controlled by the base-emitter voltage, which is independent of the collector voltage. This is described by the famous Ebers-Moll equation. If the collector is open-circuit, all of this current flows out the base connection. But as long as there's at least a small positive bias on the collector-base junction, most of the current is diverted to the collector and only a small fraction remains to flow out of the base.

In a high-gain transistor, fewer than 1% of the electrons actually recombine in the base region, where they remain as the base-emitter current, which means that the collector current can be 100× or more the base current. This process is optimized through careful control of both the geometry of the three regions and the specific doping levels used in each of them.

As long as the transistor is biased in this mode of operation, a tiny change in base-emitter voltage (and a correspondingly small change in base-emitter current) causes a much larger change in collector-emitter current. Depending on the external impedance connected to the collector, this can also cause a large change in collector voltage. The overall circuit exhibits power gain because the output power (ΔVC × ΔIC) is much greater than the input power (ΔVB × ΔIB). Depending on the specific circuit configuration, this power gain can be realized as either voltage gain, current gain, or a combination of both.

Essentially the same thing happens in a PNP transistor, but now you have to think of the holes (the absence of an electron) as being the carrier of a positive charge that drifts all of the way through the N-type base to the collector.