Note that you can exchange emitter and collector, but for the reasons explain above, beta will be very low (often << 1) and the device breakdown voltage will be low (perhaps 6 V). The choice between all these tradeoffs, as well as the basic current-carrying capability of the transistor is part of the reason why there is a huge number of different models of transistors available. The diode between collector and base terminal is known as collector-base diodes. The diode seen between the emitter-base terminal is referred to as the emitter-base diode. High-level injection and heavy doping induced band narrowing are introduced. This transistor features two diodes that are connected back to back. 8 Bipolar Transistor CHAPTER OBJECTIVES This chapter introduces the bipolar junction transistor (BJT) operation and then presents the theory of the bipolar transistor I-V characteristics, current gain, and output conductance. the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively. The NPN transistor features three terminals: emitter, base and collector. It is possible to make devices with a heavily doped collector -but they would have max. three terminal device forming the basis of a Bipolar Transistor, or BJT for. Therefore in most transistors, the collector is doped quite lightly - but not too lightly, or unwanted series resistance appears in the collector. The collector doping dominates the collector-base breakdown voltage, and only slightly affects transistor gain. Above absolute zero there will be recombination and two depletion regions will form as shown in Figure 4.2. The collector is the largest of the three regions while the base is relatively thin and lightly doped. The base also has to be thin to limit recombination and to increase curent (for a given base voltage.) The three terminals are named the emitter, base and collector. The job of the base is to control this flow of current between emitter and collector. The base has to be doped 'appropriately' - too low, and the unwanted series resistance in the base is too high, degrading performance too high and recombination in the base limits beta. The direction of current depends on whether the transistor is NPN or PNP. This makes 'injection efficiency' (the fraction of current carried by (NPN) electrons from E to B vs. ie with most positive negative (usually black) meter lead on the base the other two leads wil show a conducting diode when the positive lead is placed on them OK - now you know NPN from PNP and which is base. useful) transistor has high gain (beta hFE), and can also withstand a reasonably high voltage on the collector, the E and C are doped differently.įor high gain, the emitter needs to be doped much higher than the base. With a PNP transistor the base will have two diodes facing towards it.
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