This field is so strong due to the large potential gradient in the collector material mentioned earlier, that the electrons are swept across the depletion layer and into the collector material, and so towards the collector terminal. Once here, they come under the influence of the strong electric field across the base/collector junction. This means there is a large concentration of electrons in the base region and most of these electrons are swept straight through the very thin base, and into the base/collector depletion layer. However, because the emitter region is very heavily doped, many more electrons cross into the P type base region than are able to combine with the available holes. These holes attract electrons across the forward biased base/emitter junction to combine with the holes. Therefore holes are injected into the P type material. When the base emitter junction is forward biased, a small current will flow into the base. This means that most of the voltage between collector and base is developed across this thin high resistivity layer, creating a high voltage gradient near the collector base junction. This makes the depletion layer between base and collector quite wide once power is applied.Īs mentioned above, the collector is made up of mainly heavily doped, low resistivity material with a thin layer of lightly doped, high resistivity material next to the base/collector junction. Fig 3.3.2 How a Transistor Amplifies Current.ĭuring normal operation, a potential is applied across the base/emitter junction so that the base is approximately 0.6v more positive than the emitter, this makes the base/emitter junction forward biased.Ī much higher potential is applied across the base/collector junction with a relatively high positive voltage applied to the collector, so that the base/collector junction is heavily reverse biased.
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