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Transistor A three-terminal active semiconductor device that provides current amplification. There are two general types of transistors in use today: (1) the bipolar transistor (often called the bipolar junction transistor, or BJT), and (2) the field-effect transistor (FET). The bipolar transistor, composed of two closely coupled P-N junctions, is bipolar in that both electrons and holes are involved in the conduction process. A bipolar transistor has base, emitter and collector electrodes, and is a current-controlled device (able to deliver a change in output voltage in response to a change in input current) with a low input impedance. This type of transistor is widely used as an amplifier and is also a key component in oscillators, high-speed integrated circuits, and switching circuits. Like diodes, all transistors are intrinsically light-sensitive (accordingly, most are packaged in opaque containers of some sort). Bipolar transistors designed specifically to take advantage of this light sensitivity are called phototransistors. Meanwhile, the field-effect transistor is a unipolar device, as its conducting process primarily involves only one kind of charge carrier. It can be built either as a metal-oxide-semiconductor field-effect transistor (MOSFET) or as a junction field-effect transistor (JFET). A field-effect transistor has gate, source, and drain electrodes and is a voltage-controlled device with a high input impedance.
A bit of history Vacuum-tube triodes work for many purposes, but were slow, bulky, fragile, and consumed copious amounts of power. For years, researchers around the globe tried to make a solid-state version of the device in an attempt to enable the creation of smaller, faster, less power-hungry electronics. The field-effect transistor was patented by the German scientist Julius Lilienfeld in 1926, although he likely never got it to work. Meanwhile, the German physicist Robert Pohl made a solid-state amplifier in 1938 using salt as the semiconductor -- it worked, but reacted to signals too slowly to be of any use. Finally, three scientists (John Bardeen, Walter Brattain, and William Shockley) working at Bell Laboratories discovered how to make the first workable solid-state transistor on December 23, 1947. Their first working device was a "point-contact transistor" -- Shockley's team had modified a "cat's whisker"-style diode by placing two fine metal wires close together on the surface of a piece of N-type germanium. The result could also be called a "triode", because it has three terminals -- the two free ends of the diodes and their common junction. A voltage applied to the junction controls a current flowing through the other two terminals. Point-contact transistors, though, were essentially laboratory curiousities -- they were hard to make (performance depended on the exact placement of the wires on the germanium), and none too reliable (since they responded nearly as much to their surroundings as to their input signals). Research continued. A month after the birth of the point-contact transistor,
Shockley realised that Russell Ohl's P-type
and N-type semiconductors
in effect made it possible to build a solid-state analog of
the vacuum tube triode. The solution was to sandwich a thin
P-type semiconductor
between two N-type pieces,
resulting in two P-N
junctions (i.e., two diodes)
face to face. A current applied
to the P-type layer could then
control the current between the
two N-type regions. The resulting
bipolar transistor proved much more reliable than the
point-contact transistor. In the bipolar transistor (as in
all modern transistors), the vital junctions between the
N-type and P-type
layers are buried deep within the semiconductor crystal
where they cannot be affected by their surroundings. |
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