Physics · Ch 16 — Semiconductor Devices

Bipolar Junction Transistor (BJT)

16.4

Bipolar Junction Transistor (BJT)

A junction transistor is a semiconductor device having TWO junctions and THREE terminals; because current in it is carried by both electrons and holes together, it is called a BIPOLAR JUNCTION TRANSISTOR (BJT). It consists of three doped regions forming two back-to-back p-n junctions, of which there are two structural types: the n-p-n transistor, formed by growing a thin p-type layer between two relatively thick n-type layers, and the p-n-p transistor, formed by growing a thin n-type layer between two relatively thick p-type layers. The three regions are always named, in order, the EMITTER (E), the BASE (B), and the COLLECTOR (C) -- Fig. 16.18(b). Their circuit symbols (Fig. 16.18a) both draw the emitter lead with an arrowhead: pointing OUTWARD, away from the base, for an n-p-n transistor, and pointing INWARD, towards the base, for a p-n-p transistor -- this arrow direction always indicates the direction of conventional current flow in the emitter. A transistor can be thought of, geometrically, as two p-n junction diodes connected back to back sharing the base as their common middle region (the two-diode analogy, Fig. 16.18c) -- though as the next section makes clear, simply wiring two separate diodes together this way does NOT reproduce genuine transistor action, because the shared base must be a single, extremely thin, lightly-doped region, not two independent junctions.\n\nThe three regions differ deliberately in doping and geometry, and this asymmetry is essential to transistor action: the EMITTER is a thick, HEAVILY doped layer, whose job is to supply a large number of majority carriers into the device; the BASE is thin and LIGHTLY doped (compared to the emitter); and the COLLECTOR is on the far side of the base, also lightly doped -- about ten times more lightly than the base -- and has a LARGER area than either the emitter or the base, so it can collect the majority of the carriers the emitter supplies and also help dissipate any heat generated. Depletion regions form at both junctions (emitter-base and base-collector); because of the differing doping levels, the emitter-base junction has LOW resistance while the base-collector junction has HIGH resistance. In the standard biasing arrangement used for normal ('active region') operation, the emitter-base junction is FORWARD biased and the base-collector junction is REVERSE biased -- this combination is what permits the easy flow of majority carriers from the emitter, all the way through the transistor to the collector.\n\nFor an n-p-n transistor, electrons (the emitter's majority carriers) form the emitter current $I_E$ . The current through the forward-biased E-B junction is large, and -- despite the B-C junction being reverse biased -- the current through it is ALSO large; this seemingly surprising fact is explained by the base's extreme thinness: once the forward bias $V_{BE}$ exceeds the barrier potential (about 0.6-0.7 V for silicon), a large number of electrons are injected from the emitter into the base (Figs. 16.19b-c). Once inside the thin base, these electrons split into two paths (Fig. 16.19d): the great majority diffuse straight across the thin base region and, on reaching the base-collector depletion region, are swept by its field into the collector, forming the COLLECTOR CURRENT $I_C$ (about 95% of $I_E$ ); only a small minority recombine with the base's own (sparse) majority carriers and leave via the base terminal, forming the much smaller BASE CURRENT $I_B$ (about 5% of $I_E$ ). Overall, $I_E=I_B+I_C$ , and since $I_B$ is so small, $I_C\approx I_E$ to a good approximation. A p-n-p transistor works identically, except that holes (not electrons) are the majority carriers doing the work.\n\nA transistor can be connected into a circuit in three possible …

Figure 16.18aFig. 16.18(a): Circuit symbols of a BJT (n-p-n and p-n-p)