What is a Transistor?

A transistor is a semiconductor device used to amplify and switch electronic signals.  The development of the transistor grew directly out of large strides in diode technology during World War II.  It is the fourth most used electronic component next to resistors, capacitors, and diodes.  The transistor has at least three terminals, labeled; base, collector and emitter.  It works by applying a voltage or current to one pair of the transistor's terminals which changes the current flowing through another pair of terminals.  In other words a transistor is an electronic component which can control a large amount of current or voltage with a small amount of current or voltage.  If the controlled output current or voltage is much more than the controlling input current or voltage, the transistor will provide amplification of a signal.  It does so by sandwiching one semiconductor between two other semiconductors.  Because the current is transferred across a material that normally has high resistance (i.e. a resistor), it was a "transfer-resistor" or transistor.  The primary type of transistor in use today is known as a bipolar junction transistor(BJT), which three layers consists of  of semi-conductor material produced by the addition of impurities; such as arsenic or boron, to silicon.  Two layers of which have extra electrons, and one layer which has gaps in it.  There are two varieties of this (BJT); the npn transistor and the pnp transistor, where the n and p stand for negative and positive, respectively.  These impurities affect the way electric current moves through the silicon.  The only difference between the two is how they are biased in a circuit.
The schematic symbol for both bipolar types is shown below.


                                       PNP Transistor                                    NPN Transistor                                            

The transistor revolutionized the field of electronics, the first important use of the transistor was in hearing aids and paved the way for smaller and cheaper radios and calculators, also in the 1960s transistors were integrated into silicon chips, laying the groundwork for the technology that would eventually allow personal computers to become a reality.  There are several different types of transistors, classified by how the internal mobile charges (electrons and holes) function.  The main categories are bipolar junction transistors (BJTs) and field-effect transistors (FETs).  Field-effect transistors can be called unipolar because their main conduction is by one carrier type, the majority carrier.  Therefore, field-effect transistors are either n-channel (majority electrons) or p-channel (majority holes).  FETs are used in high frequency oscillations circuits.  Bipolar transistors have many uses including switching, voltage/current regulation, amplification, high frequencies circuits and in high-power applications. There is of course much much more to learn about transistors.

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