Control bipolar junction transistors

Both bipolar junction transistors (BJTs) and field-effect transistors (FETs) are charge-control devices [15]. The functions of the emitter, base, and collector electrodes of the BJT are replaced by the source, drain, and gate... 

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. 

Transistors as the basic and crucial component for electronic circuits have been vigorously studied since 1947. The essential function of a transistor comes from its ability to control a larger signal by an apphed small signal between one pair of its terminals. This property makes the transistor as an amplifier or switch. According to the mechanism of the generation of carriers in the channel, transistors can be divided into three categories bipolar junction transistor, field-effect transistor, and electrochemical transistor. In electronic textile applications, the EET and ECT are employed to realize fibrous transistors. 

A basic diagram of the bipolar junction transistor (BJT) is shown in Fig. 7.5. Whereas the diode has one PN junction, the BJT has two PN junctions. The three regions of the BJT are the emitter, base, and collector. The middle, or base region, is very thin, generally less than 1 /um wide. This middle electrode, or base, can be considered to be the control electrode that controls the current flow through the... 

It is useful to repeat for emphasis that the current emitter to collector is controlled by current emitter to base as much as by the voltage emitter to base. The collector current increases rapidly with base current but the base current (and therefore collector current) increases exponentially, as in any diode, with base voltage. Therefore the emitter-to-base resistance is moderate and decreases with increasing voltage. In other words, a bipolar junction transistor has a low input impedance (resistance) and is a current amplifier by nature. 

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. 

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... 

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. 

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