Bipolar junction transistors collector 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. 

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... 

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. 

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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