Alpha, [math] \alpha [/math], is the ratio of collector current to emitter current and is Because emitter current is the sum of collector current and base current one . The diode connecting base to emitter is the important one here; it matches the . relationship between voltage and current, transistors are non-linear devices. . If you supply a current over the maximum rating, the transistor might blow up. It looks to be if VC is bigger than VB, then the current will flow thorough the between base and collector that is reverse based when Vc>Vb.
Typically, the emitter region is heavily doped compared to the other two layers, whereas the majority charge carrier concentrations in base and collector layers are about the same collector doping is typically ten times lighter than base doping .
By design, most of the BJT collector current is due to the flow of charge carriers electrons or holes injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.
In typical operation, the base—emitter junction is forward-biasedwhich means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base—collector junction is reverse-biased.
In an NPN transistor, when positive bias is applied to the base—emitter junction, the equilibrium is disturbed between the thermally generated carriers and the repelling electric field of the n-doped emitter depletion region.
This allows thermally excited electrons to inject from the emitter into the base region. These electrons diffuse through the base from the region of high concentration near the emitter toward the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type, which makes holes the majority carrier in the base.
To minimize the fraction of carriers that recombine before reaching the collector—base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority-carrier lifetime. As well, as the base is lightly doped in comparison to the emitter and collector regionsrecombination rates are low, permitting more carriers to diffuse across the base region. In particular, the thickness of the base must be much less than the diffusion length of the electrons.
Bipolar junction transistor - Wikipedia
The collector—base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector—base junction.
The thin shared base and asymmetric collector—emitter doping are what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series.
Voltage, current, and charge control[ edit ] The collector—emitter current can be viewed as being controlled by the base—emitter current current controlor by the base—emitter voltage voltage control.
These views are related by the current—voltage relation of the base—emitter junction, which is the usual exponential current—voltage curve of a p—n junction diode.
Detailed transistor models of transistor action, such as the Gummel—Poon modelaccount for the distribution of this charge explicitly to explain transistor behaviour more exactly. However, because base charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in circuit design and analysis. In analog circuit design, the current-control view is sometimes used because it is approximately linear.
However, to accurately and reliably design production BJT circuits, the voltage-control for example, Ebers—Moll model is required. As with diodesbipolar transistors are rated for maximum allowable reverse-bias voltage across their PN junctions.
VEBthe maximum reverse voltage from emitter to base is approximately 7 V for some small signal transistors. Some circuit designers use discrete BJTs as 7 V zener diodes with a series current limiting resistor.
Bipolar junction transistor
Transistor inputs to analog integrated circuits also have a VEB rating, which if exceeded will cause damage, no zenering of the inputs is allowed. The rating for maximum collector-emitter voltage VCE can be thought of as the maximum voltage it can withstand while in full-cutoff mode no base current.
This rating is of particular importance when using a bipolar transistor as a switch. A typical value for a small signal transistor is 60 to 80 V.
In power transistors, this could range to V, for example, a horizontal deflection transistor in a cathode ray tube display. A maximum value for collector current IC will be given by the manufacturer in amps. Typical values for small signal transistors are 10s to s of mA, 10s of A for power transistors.
Transistor Ratings and Packages (BJT)
Understand that this maximum figure assumes a saturated state minimum collector-emitter voltage drop. If the transistor is not saturated, and, in fact, is dropping substantial voltage between collector and emitter, the maximum power dissipation rating will probably be exceeded before the maximum collector current rating. Just something to keep in mind when designing a transistor circuit! Ideally, a saturated transistor acts as a closed switch contact between collector and emitter, dropping zero voltage at full collector current.
In reality, this is never true.
Transistor Ratings and Packages (BJT) | Bipolar Junction Transistors | Electronics Textbook
Manufacturers will specify the maximum voltage drop of a transistor at saturation, both between the collector and emitter, and also between base and emitter forward voltage drop of that PN junction. Collector-emitter voltage drop at saturation is generally expected to be 0. Low voltage transistors, low VCEshow lower saturation voltages.
The saturation voltage is also lower for higher base drive current. Bipolar transistors come in a wide variety of physical packages. Package type is primarily dependent upon the required power dissipation of the transistor, much like resistors: Figure below shows several standardized package types for three-terminal semiconductor devices, any of which may be used to house a bipolar transistor.
There are many other semiconductor devices other than bipolar transistors which have three connection points. Note that the pin-outs of plastic transistors can vary within a single package type, e. TO in Figure below. It is impossible to positively identify a three-terminal semiconductor device without referencing the part number printed on it, or subjecting it to a set of electrical tests.