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Question:
Published on: 28 November, 2022

What is the advantage of heterojunction bipolar transistor (HBT) over BJT? Describe the advantage of operational mechanism of HBT.

Answer:

The advantage of heterojunction bipolar transistor over BJT is High speed switch device and enhancement of output power.

Emitter injunction efficiency of BJT is limited by the fact that carriers can flow from the base into the emitter region, over the emitter junction barrier, which is reduced by the forward bias. Therefore, a more suitable BJT for high frequency would have a heavily doped base and lightly doped emitter. This is just the opposite of the traditional BJT. To accomplish such a radical different transistor deign, we need some other mechanism instead of doping to control the relative amount of injections of electrons and hole across the emitter junction. If transistors are made in materials that allow heterojunction to be used, the emitter injection efficiency can be increased without strict requirements on doping. An n-p-n transistor made in a single material (homojunction) is contrasted with a heterojunction bipolar transistor (HBT), in which the emitter is a wider band gap semiconductor. It is possible in such a structure for the barrier for electron injection (qVn) to be smaller than the hole barrier (qVp). Since carrier injection varies exponentially with the barrier height, even a small difference in these two barriers can make a very large difference in the transport of electrons and holes across the emitter junction.

Fig. 16 constant of carrier injection at the emitter of (a) a homo junction BJT and (b) a heterojunction BJT

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