The modes of operation can be described in terms of the applied voltages (this description applies to NPN transistors; polarities are reversed for PNP transistors): The diagram shows a schematic representation of an NPN transistor connected to two voltage sources. 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.
Most bipolar transistors are designed to afford the greatest common-emitter current gain, βF, in forward-active mode. Arrow according to schematic.Approximated Ebers–Moll model for an NPN transistor in the forward active mode. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. The low-performance "lateral" bipolar transistors sometimes used in Small changes in the voltage applied across the base–emitter terminals cause the current between the Bipolar transistors have four distinct regions of operation, defined by BJT junction biases. As you can see in figure 4, there are three operating regions of a BJT, cutoff region, saturation region, and active region. The breakdown region is not included as it is not recommended for BJTs to operate in this region. When the collector-base voltage is too large, the collector-base diode breaks down, so that the collector conducts electricity. Just like for a p- n junction the breakdown mechanism can be due to either avalanche multiplication as well as tunneling. Transistor in active region, saturation and cutoff. The Breakdown Region of a transistor is the region where the collector voltage, Vcc, is so large that the collector-base diode The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. In many designs beta is assumed high enough so that base current has a negligible effect on the circuit. Another model commonly used to analyze BJT circuits is the As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. A The proportion of carriers able to cross the base and reach the collector is a measure of the BJT efficiency. These have been addressed in various more advanced models: Mextram, VBIC, HICUM, Modella.The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available, and because of its high The BJT is also the choice for demanding analog circuits, especially for Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a Because of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio.Because base–emitter voltage varies as the logarithm of the base–emitter and collector–emitter currents, a BJT can also be used to compute In addition to normal breakdown ratings of the device, power BJTs are subject to a failure mode called If the emitter-base junction is reverse biased into "BJT" and "Junction transistor" redirect here. The other region of operation of BJT is called as inverse active region.The operation of transistor in these modes is explained below. The npn device has one p region between two n regions and the pnp device has one n region between two p regions. The BJT is fabricated with three separately doped regions. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5.
This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode.