Its main function is to remove the majority charges from its junction with the base. The collector-base junction is always in reverse bias. The emitter-base junction injects a large amount of majority charge carrier into the base because it is heavily doped and moderate in size.Ĭollector – The section which collects the major portion of the majority charge carrier supplied by the emitter is called a collector. The emitter is alway connected in forward biased with respect to the base so that it supplies the majority charge carrier to the base. The terminals of the diode are explained below in details.Įmitter – The section that supplies the large section of majority charge carrier is called emitter. The transistor has three terminals namely, emitter, collector and base. The only difference between the NPN and PNP transistor is in the direction of the current. The arrow in the symbol indicates the direction of flow of conventional current in the emitter with forward biasing applied to the emitter-base junction. The symbol of NPN and PNP is shown in the figure below. Similarly, if the material has one layer of N-type material and two layers of P-type material then it is called PNP transistor. The transistor which has two blocks of n-type semiconductor material and one block of P-type semiconductor material is known as NPN transistor. There are two types of transistor, namely NPN transistor and PNP transistor. The emitter based junction of the transistor is connected to forward biased and the collector-base junction is connected in reverse bias which offers a high resistance. These names are given as per the common terminal of the transistor. The right part of the diode is called emitter diode and the left part is called collector-base diode. The base is the middle section which is made up of thin layers. It has three terminals namely emitter, base and collector. 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.The transistor consists two PN diode connected back to back. Beta values normally range between 20 and 200 for most general purpose transistors. 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. The voltage between the Base and Emitter ( V BE ), is positive at the Base and negative at the Emitter. \): 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.
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