The basic function of a BJT is to amplify current. By convention, the direction of transistor relay driver circuit pdf on diagrams is shown as the direction that a positive charge would move.
However, current in many metal conductors is due to the flow of electrons which, because they carry a negative charge, move in the opposite direction to conventional current. On the other hand, inside a bipolar transistor, currents can be composed of both positively charged holes and negatively charged electrons. 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 points in the direction conventional current travels.
PNP transistor comprises two semiconductor junctions that share a thin n-doped region. A discrete transistor has three leads for connection to these regions. BJTs are classified as minority-carrier devices. This allows thermally excited electrons to inject from the emitter into the base region. The physical explanation for collector current is the concentration of minority carriers in the base region. 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. The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency.
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. DC collector current to the DC base current in forward-active region. It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape without being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device.
This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. 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. Die of a KSY34 high-frequency NPN transistor. This effect can be used to amplify the input voltage or current.
The symbol of an NPN BJT. A small current entering the base is amplified to produce a large collector and emitter current. In this “on” state, current flows from the collector to the emitter of the transistor. The symbol of a PNP BJT. A small current leaving the base is amplified in the collector output. Bands in graded heterojunction NPN bipolar transistor.
Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. This barrier arrangement helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias, and thus reduces base current and increases emitter injection efficiency. The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. In the more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively high. Bipolar transistors have four distinct regions of operation, defined by BJT junction biases. By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode.