43 operation of bipolar transistor in active mode in

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4.3 Operation of Bipolar Transistor in Active Mode In this section, we analyze the operation of the transistor, aiming to prove that, under certain conditions, it indeed acts as a voltage-controlled current source. More specifically, we intend to show that (a) the current flow from the emitter to the collector can be viewed as a current source tied between these two terminals, and (b) this current is controlled by the voltage difference between the base and the emitter, . We begin our study with the assumption that the base-emitter junction is forward-biased ( ) and the base-collector junction is reverse-biased ( ). Under these condi- tions, we say the device is biased in the “forward active region” or simply in the “active mode.” For example, with the emitter connected to ground, the base voltage is set to about 0.8 V and the collector voltage to a higher value, e.g., 1 V [Fig. 4.6(a)]. The base-collector junction therefore experiences a reverse bias of 0.2 V. V BE = +0.8 V V CE = +1 V 1 D V BE = +0.8 V V CE = +1 V D 2 I 2 I 1 (a) (b) Figure 4.6 (a) Bipolar transistor with base and collector bias voltages, (b) simplistic view of bipolar transistor. Let us now consider the operation of the transistor in the active mode. We may be tempted to simplify the example of Fig. 4.6(a) to the equivalent circuit shown in Fig. 4.6(b). After all, it appears that the bipolar transistor simply consists of two diodes sharing their anodes at the base terminal. This view implies that carries a current and does not; i.e., we should anticipate current flow from the base to the emitter but no current through the collector terminal. Were this true, the transistor would not operate as a voltage-controlled current source and would prove of little value. To understand why the transistor cannot be modeled as merely two back-to-back diodes, we must examine the flow of charge inside the device, bearing in mind that the base region is very
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 128 (1) 128 Chap. 4 Physics of Bipolar Transistors thin. Since the base-emitter junction is forward-biased, electrons flow from the emitter to the base and holes from the base to the emitter. For proper transistor operation, the former current component must be much greater than the latter, requiring that the emitter doping level be much greater than that of the base (Chapter 2). Thus, we denote the emitter region with , where the superscript emphasizes the high doping level. Figure 4.7(a) summarizes our observations thus far, indicating that the emitter injects a large number of electrons into the base while receiving a small number of holes from it. n p n V BE = +0.8 V V CE = +1 V + e h + n p n V BE = +0.8 V V CE = +1 V + h + e Depletion Region n p n V BE = +0.8 V V CE = +1 V + h + Depletion Region e (c) (a) (b) Figure 4.7 (a) Flow of electrons and holes through base-emitter junction, (b) electrons approaching col- lector junction, (c) electrons passing through collector junction.
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