Heterojunction Phototransistors For Fiber-Optic Communications

AbstractThe theory of operation of the heterojunction phototransistor is reviewed and the limita­ tions on gain and speed-of-response are examined in the context of fiber-optic systems re­ quirements. The response of the base potential is shown to depend on the input optical power yielding a power dependent gain-bandwidth product, f j. Model calculations assuming an optimized device structure with peak response in the l.J Mm - 1.55 Mm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response if a dc bias current is used. A comparison of the S/N of the HPT and an APD is presented to clarify system applicability.IntroductionA continuing need for high performance semiconductor detectors has been generated by the growth of fiber optic communications. The best optical fibers presently available have both low loss and minimum dispersion at ~1.3 Mm, while the lowest loss occurs at ~1.5 Mm. Much work is currently under way to develop reliable sources and detectors capable of operating in the 1.0-1.7 Mm spectral region. The quaternary alloys in InxGa^_xAsyPi v and Al Ga, As Sb,_v are °f interest for this application and encouraging resulrs have been acnievea i^ tne^fabrication of both sources and detectors5"10 . Further developmental work much be completed, however, before practical systems operating at these longer wavelengths can be realized.It is possible, at present, to design systems for operation in the 0.8-0.9 Mm spectral region where the fiber loss is only 1-2 db/km and the dispersion is <10 psec/km-A*1 . Al Ga, As/GaAS DH lasers which emit at wavelengths between 0.8 Mm and 0.87 Mm are the pri­ mary sources for these systems. The detectors generally considered for systems in this wavelength range are either Si PIN or avalanche photodiode (APD) detectors. The PIN detec­ tor, which has no gain, is useful for short distance, low bandwidth transmission, while the avalanche photodiode which does provide gain, is useful in wide bandyyidth applications where the sensitivity of the detection system is limited by the noise of the load resistor- amplifier combination.Several review papers have recently been published which consider the problem of select­ ing a suitable detector for a fiber optic system in some detail11 '18 . The major requirements that must be satisfied are: (a) high responsivity at the operating wavelength of the system, (b) speed of response comparible with the system data rate, (c) minimum introduction of noise by the detection process, and (d) the practical requirement of invariance to changes in ambient conditions. There are also secondary considerations such as power supply require­ ments, physical size, and efficient coupling to the fiber which help to determine the opti­ mum choice.In addition to the PIN and avalanche photodiode, another device that is capable of satis­ fying many of the detector requirements of a fiber communication system is the heterojunc- tion phototransistor. This device has recently received considerable attention for use in optical fiber systems. Heterojunction phototransistors have been fabricated in the AlGaAs/ GaAs19 ~23 and InGaAs P/InP24 "28 materials systems. The performance of these unoptimized de­ vices is very encouraging. In this paper we review the phototransistor device physics, emphasizing the material and structural parameters which are important for the design of devices with high gain and fast response. The applicability of the device to fiber optic systems is assessed based on the gain-bandwidth product and signal-to-noise ratio.