A bipolar junction transistor includes two p-n junctions in a “sandwich” configuration, which may be either p-n-p or n-p-n. Such a p-n-p transistor is shown in Fig. 44-31. The three regions are called the emitter, base, and collector, as shown. When there is no current in the left loop of the circuit, there is only a very small current through the resistor R because the voltage across the base-collector junction is in the reverse direction. But when a forward bias is applied between emitter and base, as shown, most of the holes traveling from emitter to base travel through the base (which is typically both narrow and lightly doped) to the second junction, where they come under the influence of the collector-to-base potential difference and flow on through the collector to give an
increased current to the resistor.
In this way the current in the collector circuit is controlled by the current in the emitter circuit. Furthermore, Vc may be considerably larger than Ve, so the power dissipated in R may be much larger than the power supplied to the emitter circuit by the battery Ve’ Thus the device functions as a power amplifier. If the potential drop across R is greater than Ve, it may also be a voltage amplifier. In this configuration the base is the common element between the “input” and “output” sides of the circuit. Another widely used arrangement is the common-emitter circuit, shown in Fig. 44-32. In this circuit the current in the collector side of the circuit is much larger than that in the base side, and the result is current amplification. The field-effect transistor (Fig. 44-33) is an important type. In one variation a slab of p-type silicon is made with two n-type regions on the top, called the source and the drain; a metallic conductor is fastened to each. A third electrode called the gate is separated from the slab, source, and drain by an insulating layer of Si02• When there is no charge on the gate and a potential difference of either polarity is applied between the source and the drain, there is very little current because one of the p-n junctions is reverse biased.
Now we place a positive charge on the gate. With dimensions of the order of 10-6 m, it takes little charge to provide a substantial electric field. Thus there is very little current into or out of the gate. There aren’t many free electrons in the p-type material, but there are some, and the effect of the field is to attract them toward the positive gate. The resulting greatly enhanced concentration of electrons near the gate (and between the two junctions) permits current to flow between the source and the drain. The current is very sensitive to the gate charge and potential, and the device functions as an amplifier. The device just described is called an enhancement-type MOSFEI’ (metal-oxide semiconductor field-effect transistor).