Anyway, so, because the ratio, J sub n to J sub p, depends on exponential Delta E_G over KT, the current gain, the emitter injection efficiency, and therefore, the current gain, increases exponentially with your bandgap difference. You can refer to the lecture on p-n heterojunction to refresh your memory on this. It does not depend on the conduction band offset or the valence band offset specifically but combined difference in the bandgap energy only. So, in that case, the entire current is diffusion current and the hole end electron current density ratio depends only on the total difference between the bandgap. This is the case when the conduction band of the base region, the p-type region, lies higher in energy than the maximum point of the conduction band in the n-type emitter region. Now, if you recall, the ratio of the electron current and the hole current across this heterojunction is dependent on the exponential of the Delta E sub G, the bandgap difference. There is an offset in the conduction band Delta E sub c, and there is also an offset in the valence band Delta E sub v. But in the emitter-base junction, you have this band discontinuity. So, the base-collector junction is the same homojunction with the same bandgap for both base and collector. So, single heterojunction transistor, it has a heterojunction for the base-emitter junction, and the typical band diagram is shown here. So, depending on that, you could have a single heterojunction transistor or a double heterojunction transistor. You could have only one of these two p-n junction replaced with a heterojunction or you can replace both junctions with heterojunction. ![]() So, you can incorporate heterojunction in two different ways in bipolar transistor because the bipolar junction transistor have two p-n junctions. So, in a heterojunction, if you remember, is a junction between a two different semiconductor material that has two different bandgap, among other things. So, is there a way to improve the gain without really increasing the resistance in the base region? One possible way is provided by using the heterojunction. However, too low doping density in the base region could increase the resistance of the base region, and eventually that would hurt the transistor performance. The current gain in a BJT is usually limited by the emitter injection efficiency, and you can try to improve the gain and also the frequency response by reducing the base doping and also reducing the base width. ![]() In this video, we will discuss heterojunction bipolar transistor or HBT.
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