A bipolar junction transistor (BJT) is a type of transistor, an amplifying or switching device constructed of doped semiconductors. Bipolar transistors are so named because the main conduction channel employs both electrons and holes to carry the main electric current.
Overview
The BJT is a three layer sandwich of differently doped sections, either N-type|P-type|N-type (NPN transistors) or P-type|N-type|P-type (PNP transistors). The center layer bipolar junction transistor circuits is called the base of the transistor and is made from lightly doped, high resistivity material. By varying the voltage across the base-emitter terminals very slightly, the current allowed to flow between the emitter and a third terminal known as the collector(which are both heavily doped and hence low resistivity regions) can be varied. This effect can be used to amplify the input current. BJTs can be thought of as voltage-controlled current sources but are usually characterized as current amplifiers due to the low impedance at the base. Early transistors were made from germanium but most modern BJTs are made from silicon.
Basics of transistor operation
An NPN bipolar transistor can be considered as two diodes connected anode to anode. In normal operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. In an npn-type transistor for example, electrons from the emitter wander (or "diffuse") into the base. These electrons in the base are in the minority and there are plenty of holes with which to recombine. The base bipolar junction transistor amplification is always made very thin so that most of the electrons diffuse over to the collector before they recombine with holes. The collector-base junction is reverse biased to prevent the flow of holes, but electrons are swept into the collector by the electric field around the junction. The proportion of electrons able to penetrate the base and reach the collector is approximately constant in most conditions. However, the heavy doping (low resistivity) of the emitter region and light doping (high resistivity) of the base region mean that many more electrons are injected into the base, and therefore reach the collector, than there are holes injected into the emitter. The base current is the sum of the holes injected into the emitter and the electrons that recombine in the base - both small proportions of the total current. Hence, a small change of the base current can translate to a large change in electron flow between emitter and collector. The ratio of these currents (Ic/Ib, usually called β or h21e) is typically 100 or more.
It is important to keep the base region as thin and as free from defects as possible, in order to minimize recombination losses of the minority carriers.
Transistors in circuits
The diagram opposite is a schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 what is bipolar junction transistor mA) from C to E, VBE must be equal to or slightly greater than the cut-in voltage. The cut-in voltage is usually between 600 mV and 700 mV for silicon based BJTs. This applied voltage causes the lower p-n junction to 'turn-on' allowing a flow of electrons from the emitter into the base. Because of the electric field existing between base and collector (caused by VCE), the majority of these electrons cross the upper p-n junction into the collector to form the collector current, IC. The remainder of the electrons exit the base connection to form the base current, IB . As shown in the diagram, the emitter current, IE, is the total transistor current which is the sum of the other terminal currents. That is:
- IE = IB + IC
(Note: in this diagram, the arrows representing current point in the direction of the electric or conventional current - the flow of electrons is in the opposite direction of the arrows since electrons carry negative electric charge). The ratio of this collector current to this base current is called the DC current gain. This gain is usually quite large and is often 100 or more. It should also be noted that the base current is related to VBE exponentially. For a typical transistor, increasing VBE by just 60 mV increases the base current by a factor of 10!
Transistors have different regions of operation. In the "linear" region, collector-emitter current is approximately proportional to the base current but many bipolar junction transistor times larger, making this the ideal mode of operation for current amplification. The BJT enters "saturation" when the base current is increased to a point where the basic bipolar junction transistor operation external circuitry prevents the collector current from growing bipolar junction transistor notes any larger. At this point, the C-B junction also becomes forward biased. A residual voltage drop of approximately 100 mV to 300 mV (depending on the amount of base current) then remains between collector and emitter.
Less commonly, bipolar transistors are operated with emitter and collector reversed, thus a base-collector current can control the emitter-collector current. The current gain in this mode is much smaller (i.e., 2 instead of 100), and it is not a value that is controlled by manufacturers so it can vary dramatically among transistors.
A transistor is said to operate in the "cut off" region when the base-emitter voltage is too small for any significant current to flow. In typical BJTs manufactured from silicon, this is the case below 0.7 V or so. BJTs that operate only in 'cut off' and 'saturation' regions can by viewed as electronic switches.
Because of its temperature sensitivity, the BJT can be used to measure temperature. Its nonlinear characteristics can also be used to compute logarithms. The germanium transistor was more common in the 1950s and 1960s, and while it exhibits a lower "cut off" voltage, making it more suitable for some applications, it also has a greater tendency to exhibit thermal runaway. The Heterojunction Bipolar Transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems.
Vulnerabilities of transistors
Exposure of the transistor to ionizing radiation causes radiation damage. Radiation causes a buildup of 'defects' in the base region that act as recombination centers. This causes gradual loss of gain of the transistor.
See also
- Electronics
- Transistor
External links
- A good introduction to BJTs (Note: this site shows current as a flow of electrons, rather than the convention of showing it as a flow of holes, so the arrows may appear the wrong way around)
- Characteristic curves
- A water analogy (See also hydraulic analogy)
- http://www.play-hookey.com/semiconductors/transistor.html
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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