17-01-2012, 12:17 PM
Small-signal modelling of the transistor laser including the quantum capture
and escape lifetimes
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The transistor laser was rst introduced in [1], and it was
predicted that this device could have a high-speed modula-
tion bandwidth of up to 100 GHz [2]. In the transistor laser
(TL), a quantum well (QW) is embedded in the base of the
bipolar junction transistor (BJT) and acts as an optical col-
lector. The stimulated recombination causes compression in
the I-V characteristics of the transistor, and the current gain
of the transistor ( IC=IB) decreases ( stim < spon). The
interesting feature of the TL is the potential for an enhanced
small-signal modulation bandwidth due to the reduced car-
rier lifetime in the base region. In [2], the authors present
a model based on the charge control method and laser rate
equations, which predicts a large intrinsic modulation band-
width. However the model does not di erentiate between the
bulk carriers and the QW carriers in the carrier rate equa-
tion, and signi cantly over estimates the bandwidth. A more
complete model would include the e ects of the capture and
escape lifetimes in the QW by using two-level rate equations
[3]. Integrating the rate equations with the di usion pro-
cess in the base requires the introduction of the concept of
virtual states localized in the QW, together with the quan-
tum capture and escape e ects [4]. In this work, a transistor
laser model including di usion, quantum capture and escape,
and the laser rate equations is developed to calculate the
minority carrier density distribution and the photon dynam-
ics. We demonstrate that transistor lasers modulated by a
small-signal base current are limited by the same relaxation
oscillations that limit conventional semiconductor lasers, but
with an improvement in carrier dynamic e ects.
