Saturation The ideal transistor model is based on the ideal p-n diode model and provides a first-order calculation of the dc parameters of a bipolar junction transistor. To further simplify this model, we will assume that all quasi-neutral regions in the device are much smaller than the minority-carrier diffusion lengths in these regions, so that the "short" diode expressions apply. The use of the ideal p-n diode model implies that no recombination within the depletion regions is taken into account. Such recombination current will be discussed in section 5.
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This is called conventional current. However, current in many metal conductors is due to the flow of electrons. Because electrons carry a negative charge, they move in the direction opposite to conventional current. In this article, current arrows are shown in the conventional direction, but labels for the movement of holes and electrons show their actual direction inside the transistor.
The arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction in which conventional current travels. Function[ edit ] This section may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts , without removing the technical details.
An NPN transistor comprises two semiconductor junctions that share a thin p-doped region, and a PNP transistor comprises two semiconductor junctions that share a thin n-doped region. N-type means doped with impurities that provide mobile electrons, while P-type means doped with impurities that provide holes that readily accept electrons. The regions of a BJT are called emitter, base, and collector.
Typically, the emitter region is heavily doped compared to the other two layers, and the collector is doped much lighter than the base 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 heavily doped 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 of an NPN device, the base—emitter junction is forward-biased , which 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.
When forward bias is applied to the base—emitter junction, the equilibrium between the thermally generated carriers and the repelling electric field of the n-doped emitter depletion region is disturbed.
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. In a PNP device, analogous behaviour occurs, but with holes as the dominant current carriers. Having a lightly doped base ensures recombination rates are low. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector—base junction is reverse-biased, and so negligible electron injection occurs from the collector to the base, but carriers that are injected into the base and diffuse to reach the collector-base depletion region are swept into the collector by the electric field in the depletion region.
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 control , or 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 model , account 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. For translinear circuits , in which the exponential I—V curve is key to the operation, the transistors are usually modeled as voltage-controlled current sources whose transconductance is proportional to their collector current.
In general, transistor-level circuit analysis is performed using SPICE or a comparable analog-circuit simulator, so mathematical model complexity is usually not of much concern to the designer, but a simplified view of the characteristics allows designs to be created following a logical process. Turn-on, turn-off, and storage delay[ edit ] Main article: Baker clamp Bipolar transistors, and particularly power transistors, have long base-storage times when they are driven into saturation; the base storage limits turn-off time in switching applications.
A Baker clamp can prevent the transistor from heavily saturating, which reduces the amount of charge stored in the base and thus improves switching time. The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. A thin and lightly-doped base region means that most of the minority carriers that are injected into the base will diffuse to the collector and not recombine.
It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications. Both injection efficiency and recombination in the base reduce the BJT gain. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0. It is less than unity due to recombination of charge carriers as they cross the base region.
Measuring Ebers-Moll model parameters in transistors
Transistors characteristically have multiple modes of conduction. We can view these phenomena in the two-diode model of a bipolar junction transistor BJT. Two diodes whose anodes join to form a center tap are analogous to an NPN transistor insofar as ohmmeter readings accurately represent the real device. Two diodes with cathodes connected to a common node are analogous to a PNP transistor. NPN transistors are preferred due to increased mobility of electrons compared to holes and also because they are compatible with a negative ground system. Because two diodes are separate components and cannot share in common a semiconducting layer, they do not function as an amplifier, go into oscillation or perform switching action in the manner of actual transistors. When in forward-active mode, the collector diode is reverse-biased so ICD is virtually zero.
Bipolar junction transistor
Dojin Holt, Reinhart, and Winston. It is typically the emitter efficiency, which limits the current gain ebres transistors made of silicon or germanium. Sometimes it is also called Giacoletto model because it was introduced by L. In the reverse active mode, we reverse the function of the emitter and the collector. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way. General bias modes of a bipolar transistor While the forward active mode of operation is the most useful bias mode when using a bipolar junction transistor as an amplifier, one cannot ignore the other bias modes especially when using the device as a digital switch. We now turn our attention to the recombination current in the quasi-neutral base and obtain it from the continuity equation 2.