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A large number of majority carriers will diffuse across the forward-biased p-n junction into the n-type material. The widths of the depletion regions, indicating clearly which junction is forward-biased and which is reversed-biased. In figure both biasing potentials have been applied to a pnp transistor, with the resulting majority and minority-carrier flow indicated. “One p-n junction of a transistor is reversed biased, while the other is forward biased.”įigure : Reverse based junction of a pnp transistor. Recall that the flow of majority carriers is zero, resulting in only a minority-carrier flow. Let us now remove the base-to-emitter bias of the pnp transistor of figure (a) as shown in figure (similar to reverse-biased diode). The depletion region has been reduced in width due to applied bias, resulting in a heavy flow of majority carriers from p- to the n-type material.įigure : Forward-biased junction of a pnp transistor. In figure the pnp transistor has been redrawn without the base-to-collector bias (similar to forward-biased diode). The operation of the npn transistor is exactly the same if the roles played by the electron and holes are interchanged. The basic operation of the transistor is described using pnp transistor as shown in figure (a). The term bipolar junction transistor (BJT) reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material. The terminals have been indicated by the capital letters E for emitter, C for collector, and B for base. The doping of the sandwiched layer is also considerably less than that of the outer layer (typically, 10:1 or less).This lower doping level decreases the conductivity (increases the resistance) of this material by limiting the number of “free” carriers. The ratio of the total width to that of the center layer is 150:1. The outer layers have widths much greater than the sandwiched p- or n-type material. The emitter layer is heavily doped, the base lightly doped, and the collector only lightly doped. Both are shown in figure with proper biasing.įigure: Types of transistors: (a) pnp (b) npn. The former is called npn transistor, while latter is called an pnp transistor. Transistor is a three-layer semiconductor device consisting of either two n- and one p-type layer of material or two p- and one n-type layers of material.

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