One is the Emitter-base junction, and the other is Collector- base junction. Thus, there are two junctions of the transistors. Thus, as there are three terminals of the transistor, i.e. In most NPN transistors it will be 1 (Collector), 2 (Base) and 3 (. The transistor as its names suggests transfer resistance from one channel to other channels. The base-collector junction is also forward biased and the transistor acts as a closed switch which is shown in the circuit diagram below. To identify the pins, keep the front flat side facing you and count the pins as one, two etc. Also, the base width is a function of the base-collector voltage. Hence, the base-emitter junction is forward biased i.e. To improve the emitter efficiency and the common-base current gain (), it can be shown that the emitter should be much more heavily doped than the base. As a result, the depletion layer becomes small, and maximum current flows through it. The voltage between the collector and the emitter drops as a result of this. Since all the currents are equal to zero, it is in the ‘OFF’ condition and acts as an open switch which is shown in the circuit diagram below.įor operating in saturation mode, it is biased so that a sufficient amount of base current I B is applied which results in an insufficient amount of collector current I C. Hence, the base-collector junction is also reverse biased. The base-emitter voltage V BE is less than 0.7V which shows that this base-emitter junction is reverse biased. The two regions of operation are shown below in which the shaded part represents cutoff and saturation mode.įor operating in cutoff mode, the base current should be zero. The positive doping of the base, between emitter and collector, is not concentrated. The voltage V EE provides a negative potential at the emitter which repels the electrons in the N-type material and these electrons cross the emitter-base. If it is driven in cutoff mode, it acts as an ‘OFF’ device, and operating in saturation mode acts as an ‘ON’ device. The operation of an NPN transistor can be explained by having a look at the following figure, in which emitter-base junction is forward biased and collector-base junction is reverse biased. Mathematically, it is equal to collector current divided by base current (for BJTs and similar transistors. In the following output characteristics curve, the transistor can work in the active region to operate it as an amplifier.Ī transistor can work in one of the following modes of operations:įor operating as a switch, the area of operation for it is saturation mode and cutoff mode. The current gain of a transistor, how much bigger the collector-emitter current is than the base-emitter current, is a measure of the transistor’s amplification. The figure below shows the input and output characteristics in the CE (Common Emitter) configuration.
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