Physics · Ch 14 — Electronic Devices
Junction Transistor: Structure of npn and pnp Transistors
Junction Transistor: Structure of npn and pnp Transistors
Structure. A junction transistor is a three-layer semiconductor sandwich, formed from a single crystal, in which a very THIN, lightly doped layer of ONE type is sandwiched between two much WIDER, more heavily doped layers of the OPPOSITE type. Two distinct arrangements are possible, giving the two families of junction transistor: an n-p-n transistor, with a p-type base sandwiched between an n-type emitter and an n-type collector; and a p-n-p transistor, with an n-type base sandwiched between a p-type emitter and a p-type collector. Each of the three regions is brought out to its own external lead: the EMITTER (), the BASE (), and the COLLECTOR ().
Why the three regions differ. The three regions are NOT built identically, and each asymmetry matters for the transistor action of Section 9.19. The EMITTER is heavily doped, so it can supply ('emit') a large number of majority carriers into the base. The BASE is made very THIN and only lightly doped, so that the great majority of carriers injected from the emitter can cross it without recombining, reaching the collector rather than being lost as base current. The COLLECTOR is made physically LARGER than the emitter (it must both collect the carriers that cross the base and dissipate, as heat, most of the electrical power the transistor handles), and is moderately doped.
Two junctions, two names. The junction between the emitter and the base is called the EMITTER-BASE JUNCTION; the junction between the base and the collector is called the BASE-COLLECTOR JUNCTION. In normal (active-mode) operation, described fully in Section 9.19, the emitter-base junction is forward biased while the base-collector junction is reverse biased -- an asymmetric biasing arrangement that is only possible BECAUSE the transistor's own internal structure (emitter/base/collector doping and size) is itself asymmetric; a symmetric device could never be biased this selectively. …
What this figure shows. Two transistor cross-sections are drawn side by side. The LEFT one, an n-p-n transistor, shows three doped regions in a row: a moderately doped, WIDE n-type EMITTER region on the left, a very THIN, lightly doped p-type BASE region in the middle, and a moderately doped, WIDE n-type COLLECTOR region on the right (drawn wider than the emitter, since it must dissipate more heat), each connected to its own external lead labelled , , respectively. The RIGHT cross-section, a p-n-p transistor, is drawn the same way but with the doping types reversed: p-type emitter, thin n-type base, p-type collector. Beneath each cross-section, the corresponding circuit SYMBOL is drawn: a vertical base line with two angled leads coming off it for emitter and collector, one of the two leads carrying a small arrowhead -- for the n-p-n symbol the arrowhead is on the EMITTER lead and points OUTWARD, away from the base; for the p-n-p symbol the arrowhead is on the emitter lead and points INWARD, toward the ba …