technology novel devices
“fast” recombining carriers favored (see Figure 2). It follows that it is possible that the intrinsic spontaneous recombination time in the base of the transistor can be “clamped” at the same order of magnitude as the quantum well base region transit time, typically 10 ps.
We have studied the behavior of our transistor laser in more detail by considering its small-signal linear optical response. A damping factor is included in our calculations. One insight gained from this effort is there are at least three approaches to reducing the resonance peak: speeding the base spontaneous recombination lifetime; increasing the natural resonance of the system; and reducing the ratio between the base current and the threshold current. Analysis also reveals that it is possible to realize a “critically damped” condition that eliminates carrier-photon resonance.
The small-signal linear optical response of our device has been calculated for spontaneous recombination lifetimes of 2, 10, 50 and 250 ps. For these calculations we have assumed a photon lifetime of 2.5 ps and a value of five for the ratio of base current to threshold current.
Figure 2. (a, top) Bandwidth (20 GHz) (b, above) Eye diagram (13.5 Gbit/s)
Further reading G. Walter et al. Appl. Phys. Lett. 85 4768 (2004)
M. Feng et al. Appl. Phys. Lett. 87 131103 (2005)
G. Walter et al. Appl. Phys. Lett. 94 231125 (2009)
M. Feng et al. Appl. Phys. Lett. 95 033509 (2009)
H. W. Then et al. J. Appl. Phys. 107 094509 (2010)
describing the dynamical interaction between population inversion and cavity electromagnetic energy.
The unwanted self-resonance seen is these masers also plagues today’s laser diodes used for data communication. Here it causes a hike in the bit error rate, which is countered with expensive, complex peripheral circuits. Typically passive low-pass filters, such as Bessel filters with a fixed cut-off frequency, are employed to “filter out” the resonance frequencies. But this addition comes with a big performance penalty: it restricts the laser’s transmission bandwidth to below its resonant frequency.
One of the great strengths of our novel, three-port device is that it can produce resonance-free semiconductor laser behavior. This stems from the incredibly fast base spontaneous recombination lifetime, which is typically just 29 ps. To realize this we use an approach that would fail in today’s workhorse for data communication, the p-i-n double heterojunction laser. This involves building a structure that tilts the injected carriers and diffuses them across a thin, oppositely doped quantum-well base active region. Slowly recombining carriers are removed, and
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www.compoundsemiconductor.net November / December 2010
In addition, we have measured and fitted an optical frequency response to our transistor laser. This highlights the absence of carrier-photon resonance, which results from the “fast” base spontaneous recombination lifetime. In agreement with our model, there is a slight resonance in the output of our laser, which is less than 3 dB and only seen at higher bias. What is pleasing is that a resonance- free response of the tilted-charge transistor laser is observed at a range of biases: (a) IB (c) 60 mA; and (d) 100 mA.
= 30 mA; (b) 40 mA;
It is worth noting that the Statz and deMars coupled carrier-photon equations do not include parasitic charging delays. To cater for this, we have developed a physically based transistor model, which includes parasitic charging delays to predict microwave frequency response and digital eye-diagrams.
A consequence of the multi-port capability of the transistor laser is the need to re-formulate Kirchoff’s law, which is widely used to analyze and design conventional circuits. In order to cater for our transistor laser, this law must include energy conservation, and not simply current and charge. We have recently done just this, and published a paper detailing these efforts in the Journal of Applied Physics. Our novel transistor laser clearly holds great promise. It is still early days, but what is clear is that this multi-port structure offers a vast improvement in topological and device-to-device system design freedom. Thanks in part to these attributes, it promises to offer a leap in the performance of electrical-optical integrated circuits that is impossible to conceive with either the transistor, or even more limited two-terminal diode.
This work is sponsored by DARPA and ARO
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