Consequently, to the CB junction the appear a minority charge carriers, and the reverse bias assists them to cross the junction.īecause the BE junction is forward biased, it has the characteristics of a forward-biased diode. In the case of an npn transistor, the charge carriers arriving at the CB junction are electrons (from the emitter) travelling through the p-type base. Minority carriers are holes from the n-side and electron from the Aside, (see Fig. Majority carriers are, of course, holes from the p-side of a junction and electrons from the n-side. It has already been shown that a reverse-biased junction opposes the flow of majority charge earners and assists the flow of minority carriers. The result is that about 98% of the charge carriers from the emitter are drawn across the CB junction to flow via the collector terminal and the voltage sources back to the emitter.Īnother way of looking at the effect of the reverse-biased CB junction is from the point of view of minority and majority charge carriers. Also, because the base region is very lightly doped, there are few holes in the base to recombine with electrons from the emitter. So, only a very small percentage of the total charge carriers flow out of the base terminal. However, the path from the BE junction to the CB depletion region is much shorter than that to the base terminal. Some of the charge carriers entering the base from the emitter do not reach the collector, but flow out via the base connection, as illustrated in Fig. Because electrons have a negative charge, they are drawn across the CB junction by this bias voltage. The electrons crossing from the emitter to the base arrive quite close to the large negative-positive electric field (or barrier voltage) at the CB depletion region. The reverse bias at the CB junction causes the CB depletion region to penetrate deeper into the base than when the junction is unbiased, (see Fig. Thus, electrons are the majority charge carriers in an npn device. Holes also flow from the p-type base to the n-type emitter, but because the base is much more lightly doped than the collector, almost all of the current flow across the BE junction consists of electrons entering the base from the emitter. The electrons are emitted into the base region hence the name emitter. Note the external bias voltage polarities, The forward bias at the BE junction reduces the barrier voltage and causes electrons to flow from the n-type emitter to the p-type base. For normal operation, the base-emitter (BE) junction is forward biased and the collector-base (CB) junction is reverse biased. 4-5 which shows an npn Transistor Operation with external bias voltages. Note that the junction barrier voltages are positive on the emitter and collector, and negative on the base of the npn device.Ĭonsider Fig. Because of this penetration, the distance between the two depletion regions is very short (within the base). Also, the outer layers are much more heavily doped than the centre layer, causing the depletion regions to penetrate deep into the base, as illustrated. Although it is not shown in the illustration, the centre layer of the transistor is very much narrower than the two outer layers. The high-power Transistor Operation is designed for mechanical connection to a heat sink in order to prevent device overheating.įigure 4-4 illustrates the depletion regions and barrier voltages at the junctions of an unbiased npn transistor. Current levels for high-power transistors range from 100 mA to several amps. Low-power transistors typically pass currents of 1 mA to 20 mA. 4-3 a low-power transistor, and a high power device. Two Transistor Operation packages are illustrated in Fig. Thus, the arrowhead always points from p to n. For a pnp device, the arrowhead points from the p-type emitter to the n-type base. For an npn transistor, the arrowhead points from the p-type base to the n-type emitter. The arrowhead on each symbol identifies the transistor emitter terminal, and indicates the conventional direction of current flow. Circuit symbols for pnp and npn transistors are shown in Fig.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |