Basic principle and operation of semiconductor device - bipolar junction transistor

TRANSISTOR CONSTRUCTION

The transistor is a three-layer semiconductor device consisting of either two n - and one p -type layers of material or two p - and one n -type layers of material. The former is called an npn transistor , and the latter is called a pnp transistor . Both are shown in Fig. 3.3 with the proper dc biasing. We will find in Chapter 4 that the dc biasing is necessary to establish the proper region of operation for ac amplification. The emitter layer is heavily doped, with the base and collector only lightly doped. The outer layers have widths much greater than the sandwiched p - or n -type material. For the transistors shown in Fig. 3.2 the ratio of the total width to that of the center layer is 0.150/0.001 = 150:1. The doping of the sand- wiched layer is also considerably less than that of the outer layers (typically, 1:10 or less). This lower doping level decreases the conductivity (increases the resistance) of this mate- rial by limiting the number of “free” carriers.

For the biasing shown in Fig. 3.3 the terminals have been indicated by the capital letters E for emitter , C for collector , and B for base . An appreciation for this choice of notation will develop when we discuss the basic operation of the transistor. The abbreviation BJT, from bipolar junction transistor , is often applied to this three-terminal device. The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppo- sitely polarized material. If only one carrier is employed (electron or hole), it is considered a unipolar device. 

TRANSISTOR OPERATION

The basic operation of the transistor will now be described using the pnp transistor of Fig. 3.3a . The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. In Fig. 3.4a the pnp transistor has been redrawn without the base-to- collector bias. Note the similarities between this situation and that of the forward-biased diode . The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p - to the n -type material. Let us now remove the base-to-emitter bias of the pnp transistor of Fig. 3.3a as shown in Fig. 3.4b . Consider the similarities between this situation and that of the reverse-biased diode. Recall that the flow of majority carriers is zero, resulting in only a minority-carrier flow, as indicated in Fig. 3.4b . In summary, therefore: One p–n junction of a transistor is reverse-biased, whereas the other is forward-biased.

In Fig. 3.5 both biasing potentials have been applied to a pnp transistor, with the resulting majority- and minority-carrier flows indicated. Note in Fig. 3.5 the widths of the depletion regions, indicating clearly which junction is forward-biased and which is reverse-biased. As indicated in Fig. 3.5 , a large number of majority carriers will diffuse across the forward- biased p–n junction into the n -type material.

The question then is whether these carriers will contribute directly to the base current I_B or pass directly into the p -type material. Since the sandwiched n -type material is very thin and has a low conductivity, a very small number of these carriers will take this path of high resistance to the base terminal. The magnitude of the base current is typically on the order of microamperes, as compared to milliamperes for the emitter and collector currents. The larger number of these majority carriers will diffuse across the reverse-biased junction into the p -type material connected to the collector terminal as indicated in Fig. 3.5 .

The reason for the relative ease with which the majority carriers can cross the reverse-biased junction is easily understood if we consider that for the reverse-biased diode the injected majority carriers will appear as minority carriers in the n -type material. In other words, there has been an injection of minority carriers into the n -type base region material. Combining this with the fact that all the minority carriers in the depletion region will cross the reverse-biased junction of a diode accounts for the flow indicated in Fig. 3.5 . Applying Kirchhoff’s current law to the transistor of Fig. 3.5 as if it were a single node, we obtain

and find that the emitter current is the sum of the collector and base currents. The collector current, however, comprises two components—the majority and the minority carriers as indicated in Fig. 3.5 . The minority-current component is called the leakage current and is given the symbol I_CO ( I_C current with emitter terminal O pen). The collector current, there- fore, is determined in total by

For general-purpose transistors, I_C is measured in milliamperes and I_CO is measured in microamperes or nanoamperes. I_CO , like I_s for a reverse-biased diode, is temperature sen- sitive and must be examined carefully when applications of wide temperature ranges are considered. It can severely affect the stability of a system at high temperature if not con- sidered properly. Improvements in construction techniques have resulted in significantly lower levels of I_CO , to the point where its effect can often be ignored.

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