The Bipolar Transistor

The frequency response or switching speed of the bipolar transistor is governed by the same processes which control the speed of thep—n junction, the capacitance associated with the movement of charge into and out of the depletion regions. To achieve high frequencies the dimensions of the active areas and parasitic circuit elements must be reduced. The two critical dimensions are the width of the emitter contact and the base thickness, W. The cutoff frequency,, is the frequency at which = 57 / - b /t > where is the emitter-to-coUector delay time and is the sum of the emitter... 

The power MOSFET is the most common choice as a power switch. Its cost and saturation loss are comparable to the bipolar transistor in most applications and it switches five to ten times faster. It is also easier to use in a design. 

The symbol is shown to the right of the cross section. One metal electrode is called the source, one is the gate, and one is the drain, similar to the emitter, base, and collector in the bipolar transistors of the previous chapter. 

We have compared radiation hard CMOS (1.2 fim feature size fabricated by UTMC) and bipolar integrated circuitr " (Tektronix, but AT T or Westinghouse yield similar results) to determine the best technology for this application. Fig. 2.14 shows the calculated noise performance vs. shaping time for a detector capacitance of 22 pF. At short shaping times the shot noise contribution of the bipolar transistor s base current becomes small... 

When packaged as a discrete component the FET looks much the same as the bipolar transistor. Its circuit symbol and connections are given in the Appendix. However, it is the bipolar transistor which is much more widely used in electronic circuits as a discrete component. 

The bipolar transistor consists of three pieces of semiconductor material sandwiched together, as shown in Fig. 3.97. The structure of this transistor makes it a three-terminal device having a base, collector and emitter terminal. 

The bipolar transistor, which began the microelectronics revolution, consists of two diodes joined back to back by a thin common semiconductor layer (the base). Thus, bipolar transistors are referred to as p-n-p or n-p-n depending upon whether the common base layer is n or p type. Consider a typical n-p-n bipolar junction transistor, shown schematically in Figure 3.24 along with the electrical coimections for its operation. When turned on the emitter junction is forward biased. This injects electrons into the base and holes into the emitter. The holes injected into the emitter recombine there and are of no value to operation of the device. The electrons emitted into the base may either recombine, contributing to base current, or transit the base. If the base is thin, most of the electrons pass through it without recombining, reach... 

---