# In addition to a drop in the speed of bipolar

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In addition to a drop in , the speed of bipolar transistors also degrades in saturation (Chapter 11). Thus, electronic circuits rarely allow operation of bipolar devices in this mode. As a rule of thumb, we permit soft saturation with mV because the current in the B-C junction is negligible, provided that various tolerances in the component values do not drive the device into deep saturation. It is important to recognize that the transistor simply draws a current from any component tied to its collector, e.g., a resistor. Thus, it is the external component that defines the collector voltage and hence the region of operation. Example 4.16 For the circuit of Fig. 4.36, determine the relationship between and that guarantees operation in soft saturation or active region. Solution In soft saturation, the collector current is still equal to . The collector voltage must not fall below the base voltage by more than 400 mV: (4.98) Thus, (4.99)

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 154 (1) 154 Chap. 4 Physics of Bipolar Transistors Q 1 R I C C V BE V CC V CC R C V BE 400 mV Acceptable Region (a) (b) Figure 4.36 (a) Simple stage, (b) acceptable range of and . For a given value of , must be sufficiently large so that still maintains a reasonable collector voltage. Exercise Determine the maximum tolerable value of . In the deep saturation region, the collector-emitter voltage approaches a constant value called (about 200 mV). Under this condition, the transistor bears no resemblance to a controlled current source and can be modeled as shown in Fig. 4.37. (The battery tied between C and E indicates that is relatively constant in deep saturation.) 800 mV 200 mV B E C Figure 4.37 Transistor model in deep saturation. 4.6 The PNP Transistor We have thus far studied the structure and properties of the transistor, i.e., with the emitter and collector made of -type materials and the base made of a -type material. We may naturally wonder if the dopant polarities can be inverted in the three regions, forming a “ ” device. More importantly, we may wonder why such a device would be useful. 4.6.1 Structure and Operation Figure 4.38(a) shows the structure of a transistor, emphasizing that the emitter is heavily doped. As with the counterpart, operation in the active region requires forward-biasing the base-emitter junction and reverse-biasing the collector junction. Thus, and . Under this condition, majority carriers in the emitter (holes) are injected into the base and swept away into the collector. Also, a linear profile of holes is formed in the base region to allow diffusion. A small number of base majority carriers (electrons) are injected into the emitter or recombined with the holes in the base region, thus creating the base current. Figure 4.38(b) illustrates the flow of the carriers. All of the operation principles and equations described for transistors apply to devices as well.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 155 (1) Sec. 4.6 The PNP Transistor 155 V BE V CE x x 2 x 1 Density h p n + p Hole V BE V CE + e h

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