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Unformatted text preview: III. Transistors (Introduction & Large Signal Model) 3.1 III. Bipolar-Junction (BJT) Transistors A bipolar junction transistor is formed by joining three sections of semiconductors with alternative different dopings. The middle section (base) is narrow and one of the other two regions (emitter) is heavily doped. The other region is called the collector. Two variants of BJT are possible: NPN (base is made of p-type material) and PNP (base is made of n-type material). Let’s first consider a NPN transistor. B E C B i i E v BE v CE i C v BC- +- +- + p n n + A BJT has three terminals. Six parameters; i C , i B , i E , v CE , v BE , and v BC ; define the state of the transistor. However, because BJT has three terminals, KVL and KCL should hold for these terminals: i E = i C + i B v BC = v BE − v CE Thus, only four of these 6 parameters are independent. The relationships among these four parameters ( i B ,v BE ,i C and v CE ) represent the “ iv ” characteristics of the BJT. At the first glance, a BJT looks like 2 diodes placed back to back. Indeed this is the case if we apply a voltage to only two of the three terminals, letting the third terminal float. This is also the way that we check if a transistor is working: use an am-meter to ensure both diodes are in working conditions. (One should also check the resistance between CE terminals and read a vary high resistance as one may have a burn through the base connecting collector and emitter.) BJT behavior is quite different, however, when voltages are applied to both BE and CE terminals. The BE junction acts like a diode. When this junction is forward biased ( v BE = V D ), electrons flow from the emitter to the base to combine with holes there. But as the emitter region is heavily doped, there is a large number of electrons crossing into the base compared to available holes. As the base region is narrow, most of these electrons diffuse into the collector region. Then, if a positive voltage is applied between collector and base ( v CB > 0 but can be small), these electrons are “collected” by the collector electrode. Note that the BC junction is reverse biased ( v BC < 0) but a large current flows. Because the BE junction acts as a diode: 1) The number of electrons that cross from the emitter into the base and diffuse into collector region depends exponentially on the voltage applied to the BE junction, v BE (similar to the diode equation). 2) For a given v BE , the ECE65 Lecture Notes (F. Najmabadi), Spring 2010 3-1 number of holes traveling from base to emitter (proportional to i B ) is a constant fraction of the number of electrons traveling from emitter to base (proportional to i c ). This ratio depends on doping level, temperature, and other manufacturing parameters. In a BJT, because emitter is heavily doped, there are a lot more electrons (from emitter) than holes (from the base), e.g., on the average, out of every 200 electrons traveling from emitter to base only one combines with a hole and the other 199 travel to the collector. As such, the ratioonly one combines with a hole and the other 199 travel to the collector....
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This note was uploaded on 01/19/2011 for the course ECE 65 taught by Professor Coles during the Spring '08 term at UCSD.
- Spring '08