Transistors are semiconductor devices that have at least three terminals. A small current or voltage through the middle terminal is used to control the current flow through the others. They function as electronic valves. Their primary uses are as switches and amplifiers. Transistors are based on PN junctions, and are modelled as back-to-back diodes.
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PN junctions are created from a combination of p-type and n-type semiconductors. P stands for positive, and n for negative. Semiconductors have holes that are positively charged, and free electrons or negative charges. The holes are formed by electrons being forced from their positions due to an electric field.
A semiconductor that has been doped to form a p-type has holes as the majority carriers, and electrons are minority carriers. Doping means that impurities are deliberately applied to the semiconductor in order to increase its conductivity. An applied voltage causes the electrons and holes to move, and the holes recombine with free electrons from the external circuit. The electrons are so few in number that their effect is negligible, and so only the movement of the holes is considered. P-types therefore have excess positive charge.
A semiconductor that has been doped to form an n-type has free electrons as the majority carriers, and holes as the minority carriers. When an applied voltage causes both of them to move, the holes are so few in number that only the flow of the free electrons is considered. N-types therefore have excess negative charge.
Transistors come in several types, with bipolar ones being the most popular. Bipolar transistors are formed from a combination of a PN junction with another junction. The transistor layers are the emitter, base, and collector, and they each have terminals or leads. The middle layer is the base, and it controls the flow of current through the other two layers, so that it functions like a gate. The base is doped to be thinner than the collector and emitter, and the effect is that a small current through an emitter-base layer will cause a much larger current through the emitter-collector layer. This is why transistors make very good amplifiers.
Diodes are made from PN junctions. Transistors are therefore considered to be two diodes, where one of them is deliberately placed backwards. The base-emitter junction or diode part of the transistor will not conduct unless the voltage exceeds 0.6 volts.
An Npn transistor has an n-type followed by an p-type followed by another n-type. Basically, the emitter injects free electrons into the base, and most of these will flow to the collector.
The normal or conventional current flow diode model is that of the base-emitter part being conducting or forward-biased, which means that the current flows in the same direction as the applied voltage. The base-collector part is reverse-biased, so the current flow is from the collector to the base. The voltage at the collector must be more positive than that of the emitter, which just means that the collector voltage must be larger than the emitter voltage.
A Pnp transistor has a p-type followed by an n-type followed by another p-type. Here, the emitter injects holes into the base, and most of them continue on to the collector. The diode model of a Pnp transistor is different from that of an Npn. The base-collector part is conducting or forward biased. The base-emitter portion is reverse biased -- the current must flow from the emitter to the base. Finally, in a Pnp transistor circuit, the emitter must be more positive in voltage than the collector. This means that the emitter voltage must be larger than the collector voltage.
Transistors are a basic part of circuits where switching, amplification, or both is desired. Television, speakers, radios, cell phones, computers, and microwaves are just a few of the devices where they may be found.
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