Electronics - HBT Home : www.sharetechnote.com
HBT stands for HeteroJunction Bipolar Junction Transistor. As its name implies, it is a kind of BJT (Bipolar Junction Transistor). The only different part is 'HeteroJunction'. What does it mean by this ? Following illustration would answer this question. This would be oversimplified illustration but would give you intuitive understandings of the difference between the conventional BJT and HBT.
Why HBT ?
Why we use HBT ? Why HBT was invented ?
Main motivation for HBT is to overcome various limitations of BJT and especially overcome transition speed limitation of BJT.
To achieve high speed in classical BJT, we need to reduce the base width since the trasit time of the minority carriers through the base is proportional to the square of the base width. (The wider the base width, the longer the transit time).
However, if we reduce the base width, the base resistance will be increased and this increased base resistance will slow down the device response time.
Then, Is there any way to reduce base width without increasing base resistance ?
Yes, there is a way. It is to increase the base doping concentration.
However, there is a problem with increasing the base doping concentration. If we increase the doping concentration too much, the injection into emitter would decreases the current gain.
Therefore, it is impossible to optimize the base thickness(width) and doping concentration for high-speed, high gain and low base resistance.
HBT was invented to overcome this limitation and achieve high speed.