Understanding Transistors

Bipolar transistors, also called BJTs (bipolar junction transistors), are three-terminal devices that can function as electronic switches or as signal amplifiers. Figure below shows the schematic symbols for the two types of bipolar transistors. The schematic symbols show the three terminals of the transistor.
The leg with the arrow is called the emitter, the collector is the other slanted leg, and the base is the leg connected perpendicular to the line connecting the emitter and the collector. If the arrow of the schematic symbol points outward, the transistor is an NPN transistor. If the arrow points inward, the transistor is a PNP transistor.

NPN PNP Transistors

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.

History on Transistors

Originally, devices that needed to amplify a signal (such as radios and the first computers) used the vacuum-tube amplifier. This was the original “triode”: a glass tube containing a heater filament, a heated emitter plate (a.k.a., the cathode) that emits electrons, a collector plate (a.k.a., the anode) that collects the electrons once they have accelerated through the tube, and a metal grid in between. The name “triode” came from the fact that the device had three active elements — the anode, the cathode, and the grid. Small changes to the voltage of the grid cause large changes in the electron current flowing to the collector plate. Remove the grid, and the device becomes a vacuum-tube diode.

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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