Transistor collector electrode

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... 

A particular type of biosensor can be developed by putting a membrane in contact with the semi-conducting layer of a field effect transistor. If the membrane incorporates an enzyme adapted to transform a particular analyte (Fig. 19.8), reaction of that enzyme will modify the polarity at the surface of the insulating layer. This will in turn modify the conduction between the source and the collector of the field effect transistor. The current flowing through these two electrodes (source and collector) serves as the signal. 

Because of their fragility and occasional unreliability, the point-contact electrodes were eventually replaced with three layers of adjacent semiconducting surfaces, each of which corresponded to an element in the triode vacuum tube the emitter layer (for the heated filament which is the source of electrons), the base (for the grid that controls the electron flow), and the collector, for the triode plate that receives the electrons. The areas where the layers join one another are called junctions, and transistors made in this way are known as junction transistors. 

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. 

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... 

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. 

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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