Transistors, which were invented in 1947, are one of the most important devices ever created. They should be ranked in terms of importance with the telephone, the automobile and the printing press. A transistor is a solid-state, semiconductor device that allows an exact amount of current to flow through a circuit enabling it to open or close the circuit. Transistors can also be used for amplification, a signal ion, switching, voltage stabilization, signal modulation and various other functions. A transistor acts as a variable valve which, based on the input voltage, controls the current it draws from a connected voltage source. These instruments form a very important part of an IC chip, which may have millions of transistors in it. A typical chip is quite small and could sit comfortably on a pinhead.

Transistors are now the most important component of all digital circuits, including computers. It is the solid-state analog to the triode electron tube; transistors have replaced electron tubes for just about all common applications. A transistor is an arrangement of semiconductor materials that have the same physical boundaries.

Transistors are made out of various materials such as silicon, gallium arsenide, and germanium, into which a process called doping gives it impurities. In n-type semiconductors the impurities or dopants will cause an excess of electrons, or negative charges. In p-type semiconductors the dopants will cause a deficiency of electrons which results in an excess of positive charge carriers or holes.

A transistor is has three terminals in it. The current flowing across two of the terminals can be used to control the greater current in the third terminal. While selecting a transistor there are various things you need to consider such as the polarity as the PN junctions can be arranged in different ways like NPN, PNP, P Channel and N Channel.

There are various types of transistors including: Bipolar junction transistors (BJT), bipolar TR transistors, junction field effect transistors (JFET), metal-oxide semiconductor field-effect transistors (MOSFET), insulated gate bipolar transistors (IGBT), complementary metal-oxide semiconductors (CMOS), metal semiconductor field effect transistors (MESFET), Temperature Protected Field-Effect Transistors (TempFET), radio frequency transistors (RFT), high electron mobility transistors (HEMT), heterojunction field-effect transistors (HFET), pseudomorphic high electron mobility transistor (PHEMT) and Darlington transistors.

Transistors are packaged and designated as TO-3, TO-8, TO-202, etc.

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