Bipolar junction transistors operation

The bipolar junction transistor (BIT) consists of tliree layers doped n-p-n or p-n-p tliat constitute tire emitter, base and collector, respectively. This stmcture can be considered as two back-to-back p-n junctions. Under nonnal operation, tire emitter-base junction is forward biased to inject minority carriers into tire base region. For example, tire n type emitter injects electrons into a p type base. The electrons in tire base, now minority carriers, diffuse tlirough tire base layer. The base-collector junction is reverse biased and its electric field sweeps tire carriers diffusing tlirough tlie base into tlie collector. The BIT operates by transport of minority carriers, but botli electrons and holes contribute to tlie overall current. 

FIGtJRE 7.49 The h vs. Vce plot for a bipolar junction transistor biased for Class A operation is shown with the staticquiescent point halfway between voltage and current limits for symmetrical excursions along the loadline. Strict Class A operation requires that the ends of the loadline not be exceeded. 

This Chapter reviews some of the basic physics and operation of selected circuit elements used in microelectronic devices. For a complete review, see the suggested readings. Basic resistance, capacitance, and inductance were covered in Chapter 2. We focus here on diodes, including clas-sic homojunctions, heterojunctions, and Schottky barriers, because they illustrate most of the important issues in microelectronic materials and because both field-effect and bipolar junction transistors are constructed from them. Once we have discussed diodes, a brief review of these two major classes of transistors is provided. Finally, we finish the review with some of the issues unique to light emitting and laser diodes. 

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... 

Bipolar transistors are realized using either an npn- or pnp-junction sequence. The different segments of the device are named as collector, base, and emitter electrode, respectively. In order to operate the transistor, one of the junctions is forward biased, while the other is biased in reverse. Using a small control current over the base electrode, a significant current between the collector and emitter electrodes is enabled. 

---