Fundamentals of BJT operation
The bipolar transistor is basically a simple device, and this section is devoted to a simple and largely qualitative view of BJT operation.
Operation of the BJT:
Let us cosider the PNP transistor:
- A convenient hole injection device is a forward-biased p -n junction. The current in such a junction is due primarily to holes injected from the p region into the n material.
- If we make the n side of the forward-biased junction the same as the n side of the reverse-biased junction, the p -n-p structure of Fig. results.
- With this configuration, injection of holes from the p -n junction into the center n region supplies the minority carrier holes to participate in the reverse current through the n-p junction.
Of course, it is important that the injected holes do not recombine in the n region before they can diffuse to the depletion layer of the reversebiased
junction. Thus we must make the n region narrow compared with a hole diffusion length.
- The structure we have described is a p-n-p bipolar junction transistor. The forward-biased junction which injects holes into the center n region is called the emitter junction, and the reverse-biased junction which collects the injected holes is called the collector junction
- The, center n region is called the base, when we discuss the historical development of transistor fabrication.
- The biasing arrangement in Fig given below is called the common base configuration, since the base electrode B is common to the emitter and collector circuits.
- To have a good p-n-p transistor, we would prefer that almost all the holes injected by the emitter into the base be collected. Thus the n-type base region should be narrow, and the hole lifetime TP should be long.
- This requirement is summed up by specifying Wb < Lp, where Wb is the length of the neutral n material of the base (measured between the depletion regions of the emitter and collector junctions), and Lp is the diffusion length for holes in the base (Dptp)1/2.
- With this requirement satisfied, an average hole injected at the emitter junction will diffuse to the depletion region of the collector junction without recombination in the base.
- A second requirement is that the current IE crossing the emitter junction should be composed almost entirely of holes injected into the base, rather than electrons crossing from base to emitter.
- This requirement is satisfied by doping the base region lightly compared with the emitter, so that the p -n emitter junction of the above fig results.