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High frequency response is improved in a heterojunction phototransistor by using a small area emitter and a high conductivity base.
English (United States)
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High frequency response is improved in a heterojunction phototransistor by
using a small area emitter and a high conductivity base.
Heterojunction phototransistors have found limited use in fiberoptic
applications due to the need for simultaneous high speed response and low
noise gain. With previous designs the emitter-base capacitance has been
proportional to the active area, making it difficult to minimize emitter-base
capacitance. With a large emitter-base capacitance it is necessary to operate
the transistor at a relatively large bias current to achieve optimum frequency
response. This bias current in turn increases the shot noise at the emitter and
The figure illustrates a heterojunction phototransistor constructed to minimize
the emitter-base capacitance while at the same time retaining high speed
performance. The device is constructed using steps similar to those ordinarily
used to construct heterojunction phototransistors. Three epitaxial layers are
grown on a suitable semiconductor substrate 10: a lightly doped collector layer
16, a heavily doped thin base layer 14, which may have a larger band gap than
the collector, and a moderately or lightly doped emitter layer 12 with a larger
band gap than the base. Additional layers could be grown to facilitate making
contact to the three regions. The area of the emitter is reduced by using an etch
that preferentially attacks the wide gap emitter material to create a small mesa
onto which the emitt...