Bipolar junction transistors base layer

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. 

Bipolar junction transistors are based on an n-p-n or p-n-p junction sequence. One of the p-n junctions is reverse biased, which suppresses the current between the outer electrodes, commonly called emitter and collector. The middle layer, which is connected to a third electrode (base), is spatially very thin. Initiated by a small control current over the base electrode, charge carriers are injected over the forward biased p-n junction and minority carriers can flood directly over the reverse biased p-n junction. This causes an increased current between emitter and collector. In contrast to FETs, bipolar junction transistors are current controlled. 

FIGURE 8.7 Bipolar junction transistor made with a sequence of -p-n-type semiconductor layers. This system is a three-port irregular multipole with the height of the harrier controlled hy an external voltage applied between the base and the emitter. 

After the discovery Shockley developed a basic transistor theory. Practically he replaced (in 1951) the point contact with a junction system. The bipolar junction transistor may consist of three layers, farthest out emitter and collector of p-type (compare chapter 40 SiHcon) and between them a base layer of n-type (pnp transistor). The... 

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... 

Heterojunction diodes can behave as homojunction diodes or Schottky barriers. They are used, for example, to control direction of carrier injection, induce electron or hole gas layers, and control energy gap on one side of the jrmction. Bipolar junction transistors consist of three layers of semiconductor with alternating doping type where the center layer of the three is relatively thin. The three layers are, respectively, the emitter, base, and collector. A small current emitter to base allows a large current emitter to collector when properly biased. Field-effect transistors have three regions, source, channel, and drain as well as a gate, which controls the conductivity of the channel connecting source to drain. 

Mobile telephones incorporate multilayer III-V epitaxial heterojunction bipolar transistor wafers such as that illustrated in Figure 27.12. The p-n junctions on either side of the base layer are a crucial feature of semiconductor devices, and in the wafer shown in Figure 27.12 (and in other similar wafers), the p-type base layer must be highly doped to provide high-frequency performance. Choice of dopant is critical, e.g. use of a Zn dopant (see below) results in its diffusion into the emitting n-type layers. This problem has been overcome by doping with C which exhibits a low diffusion coefficient C-doped wafers have been used commercially since the early 1990s. 

In Figure 5-la is shown a schematic representation of a silicon MOSFET (metal-oxide-semiconductor field effect transistor). The MOSFET is the basic component of silicon-CMOS (complimentary metal-oxide-semiconductor) circuits which, in turn, form the basis for logic circuits, such as those used in the CPU (central processing unit) of a modern personal computer [5]. It can be seen that the MOSFET is isolated from adjacent devices by a reverse-biased junction (p -channel stop) and a thick oxide layer. The gate, source and drain contact are electrically isolated from each other by a thin insulating oxide. A similar scheme is used for the isolation of the collector from both the base and the emitter in bipolar transistor devices [6],... 

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