Therefore if a transistor has a Beta value of 50, then for every 50 electrons flowing between the emitter-collector terminals one electron will flow from the base terminal.īy combining the expressions for both Alpha, α and Beta, β the current gain of the transistor can be given as:Īs seen from the equations above, electron mobility between the Collector and Emitter circuits is the only link between these two circuits. Figure 10.2 shows a cross-sectional view of one possible geometry. The N-type epitaxial layer is used as the base region. This device is made using the NPN base diffusion for both the emitter and collector regions. Beta values normally range between 20 and 200 for most general purpose transistors. All the principles that applied to NPN's also apply to PNP’s, with the exception that emitter is at a higher potential than base and base at a higher potential than collector. One type PNP transistor that can be made by the six-mask process is called a lateral PNP.
NPN transistors are good amplifying devices when the Beta value is large. The current gain of the transistor from the Collector terminal to the Base terminal is signified by Beta, ( β ). The current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is a function of the electrons diffusing across the junction. The ratio of the collector current to the emitter current is called Alpha (α). Since the physical construction of the transistor determines the electrical relationship between these three currents, (Ib), (Ic) and (Ie), any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ). Note: “Ic” is the current flowing into the collector terminal, “Ib” is the current flowing into the base terminal and “Ie” is the current flowing out of the emitter terminal. The current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as A nother way to display a NPN Transistor is shown in Figure 2 below. The Base terminal is always positive with respect to the Emitter. A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. The voltage between the Base and Emitter ( V BE ), is positive at the Base and negative at the Emitter.
Electrons have high mobility than holes and. \): NPN Transistor schematic.įor a bipolar NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter. In npn transistors electrons are majority charge carriers and in pnp transistors holes are majority charge carriers.
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