Bipolar junction transistor
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A bipolar junction transistor (BJT) is a semiconductor made for amplifying, switching, voltage stabilization and many other functions. A transistor consists of layers of diffrent kind of semiconductors, called a junction.
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Types
Common transistor types are NPN and PNP transistors. The lettering tells the structure of the junction. N denotes N-type semiconductor and P denotes P-type semiconductor. The easy way to tell the schematic diagram apart is PNP is Pointing In (the arrow) and NPN is Not Pointing In.
Transistors amplify current, for example they can be used to amplify the small output current from a logic chip so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.
A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). A pure silicon crystal is an insulator, but it becomes conductive when an impurity is added. In an N-type semiconductor, the impurity is arsenic. Arsenic makes the material conductive by donating free electrons.
P-type semiconductor has boron as the impurity, which makes the crystal conductive.
A transistor is formed by laying layers of these two diffrent kind of semiconductors on top of eachother, forming a junction.
Characteristics
Transistors have different regions of operation. In the "linear" region, collector-emitter current is approximately proportional to the base current but many 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 external circuitry prevents the collector current from growing 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.
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.
Math
The gain has the measure β or hFE.
Darlington configuration
A darlington pair behaves like a single transistor with a very high current gain. The total gain of the darlington is the product of the gains of the individual transistors, hFEtot = hFE1 x hFE2. Some power transistors have a low gain and can require several amperes of base current to drive high currents, using two transistors in a darlington configuration is a simple way to achieve useful gain.
The base-emitter voltage drop is twice that of a normal transistor, VBEtot = VBE1 + VBE2.
Darlington pairs can be built using discrete components or can be bought in packages that look like normal transistors. Darlington arrays also exists in chip form like the ULN 2803 that also contains protection diodes.
See also
- MOSFET transistors.
Further reading
