The charge-control view easily handles phototransistors, where minority carriers in the base region are created by the absorption of photons, and handles the dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. Due to low-level injection (in which there are much fewer excess carriers than normal majority carriers) the ambipolar transport rates (in which the excess majority and minority carriers flow at the same rate) is in effect determined by the excess minority carriers.ĭetailed 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.

The physical explanation for collector current is the concentration of minority carriers in the base region. 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). 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). 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. 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. In particular, the thickness of the base must be much less than the diffusion length of the electrons.

As well, as the base is lightly doped (in comparison to the emitter and collector regions), recombination rates are low, permitting more carriers to diffuse across the base region. 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. 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. These electrons diffuse through the base from the region of high concentration near the emitter toward the region of low concentration near the collector. This allows thermally excited electrons to inject from the emitter into the base region. 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. In typical operation, 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. 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. 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 ). A discrete transistor has three leads for connection to these regions. The regions of a BJT are called emitter, collector, and base. NPN BJT with forward-biased E–B junction and reverse-biased B–C junctionĬharge flow in a BJT is due to diffusion of charge carriers across a junction between two regions of different charge concentrations. The arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction conventional current travels. 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. On the other hand, inside a bipolar transistor, currents can be composed of both positively charged holes and negatively charged electrons. However, current in many metal conductors is due to the flow of electrons which, because they carry a negative charge, move in the direction opposite to conventional current. 4.1 Active-mode transistors in circuitsīy convention, the direction of current on diagrams is shown as the direction that a positive charge would move.2.3 Transistor parameters: alpha (α) and beta (β).2.2 Turn-on, turn-off, and storage delay.2.1 Voltage, current, and charge control.

Pnp bjt