In this note I will present details only about the npn transistor. For those who are interested only on the pnp transistor, you can switch the action of the npn transistor and the behavior of current to adjust with the characteristics of the pnp transistor.
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As in the case of a p-n junction,the two n parts of the npn transistor contain an excess of free electrons. In contrast, the p part contains excess holes. As in the case of the p-n junction, in the npn transistor depletion regions develop and junction barriers occur.
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For the correct functioning of the transistor, the first p-n junction is forward biased and the second p-n junction is reverse biased. This results for the first p-n junction to be of low resistance and the second p-n junction to be of high resistance.
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The letters of these elements indicate what polarity voltage to use for correct bias.
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The emitter, which is the first letter in the npn sequence, is connected to the negative side of the battery and the base, which is the second letter (npn), is connected to the positive side.
However, the collector should be connected to the positive end of the battery as the second p-n junction is reverse biased.
The voltage on the collector must also be more positive than the base in order for the transistor to work accurately.
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In the first p-n junction (forward biased junction) the concentration of the electrons in the n region is higher than the hole concentration in the p region. This is because the n region is doped highly as compared to the p region. Thus, the majority carriers in the first p-n junction are electrons.
Now we can understand the principle behind the transistor by considering the two junctions separately.
First junction (Emitter - Base) :
The emitter is connected to the negative terminal of the battery. As discussed above, the majority carriers in this junction are electrons. Thus, the electrons that pass out from the battery's negative terminal enter the n region (emitter) and pass out easily to the p region. In the p region, some electrons combine with holes.
For each electron that fills a hole in the p material, another electron will leave the p material (creating a new hole) and enter the positive terminal of the battery.
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Second junction (Base - Collector):
Since this junction is reverse biased, it works as a barrier to the flow of electrons through the junction. However, a very small current called minority current or reverse current pass through this junction. This current was produced by the electron-hole pairs.
The minority carriers for the reverse-biased p-n junction are the electrons in the p material and the holes in the n- material.
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Current distribution across a npn transistor-
Free electrons are the key to the current flow in the external circuit. Thus, electrons always flow from the negative terminals of the supply batteries to the n-type emitter.
The current generated due to these electrons is known as emitter current (IE).
As explained above, electrons then move from the emitter or the n region to the base or the p region. When the electrons are in the base, they become minority carriers as the majority of the p region are the holes.
Here, some electrons recombine with holes and as explained above for each electron that recombines, another electron moves out through the base. The current thus generated is called the base current IB.
Since the second p-n junction is reverse biased, there is no entrance for the electrons that recombined with the holes in the collector region. To reduce this problem, the base area is made thin and the doping concentration is reduced.
Although there is a reluctance for the movement of holes to the collector region, electrons which are minor carriers in the p region or the base region easily accelerates from the base region to the collector region. This leads to the collector current (IC).
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Note: To hold more heat without damaging and to enable the above electron flow, the emitter is usually made larger than the base.
Finally we can say that the current that the emitter receives from the negative terminal of the battery is distributed to the collector and the base.
Therefore, IE = IB + IC .
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