Multi-Hundred Gigahertz Photodetector Development Using LT GaAs
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MULTI-HUNDRED GIGAHERTZ PHOTODETECTOR DEVELOPMENT USING LT GaAs Y. CHEN*, S. WILLIAMSON**, T. BROCK**, F.W. SMITH***, AND A.R. CALAWA*** *Ultrafast Science Laboratory, University of Michigan,Ann Arbor,MI 48109-2099 (Current address: AT&T Bell Laboratories, Holmdel, NJ 07733) **Ultrafast Science Laboratory, University of Michigan, Ann Arbor, MI ***Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173-9108 ABSTRACT We report on the development of a new, integrable photoconductive-type detector based on low-temperature-grown GaAs. The detector has a response time of 1.2 ps and a 3-dB bandwidth of 375 GHz. The responsivity is 0.1 A/W. This is the fastest photodetector reported to date. We discuss the unique properties of this device, including its performance as functions of both light intensity and bias voltage. INTRODUCTION Much progress has been made over the past several years in the development of high-speed photodiode detectors. A detection bandwidth of 105 GHz together with a responsivity of 0.1 A/W have been reported for a metalsemiconductor-metal (MSM) photodiode.[1] The most common approach to increasing the bandwidth in MSM photodiodes (at least up to 100 GHz) is to shorten the carrier transit time by reducing the electrode spacing. However, achievement of bandwidths > 100 GHz requires more than simply reducing further the electrode spacing. Monte Carlo simulation of the intrinsic response for a photodiode with 0.1-prm electrode spacing, for example, shows a response tail persisting for picoseconds and having an integrated energy comparable to the main signal.[2,3] This tail is caused by the long transit time of the photogenerated holes, which is almost 10 times that of electrons. The response times of photoconductive detectors, on the other hand, can be quite fast because they are determined solely by the carrier lifetime of the material that is used. Recently, low-temperature-grown GaAs (LT GaAs) has been applied to ultrafast [4] and high-power [5] optical switching. The subpicosecond carrier lifetime,[6] high mobility (> 200 cm 2 N.s), and high breakdown field strength (>100 kV/cm) of LT GaAs make this material ideal for electrical pulse generation and gating. Such applications have so far required use of moderate-to-high peak optical powers since the switching efficiency, defined as the ratio of electrical output power to optical input power, is still < 1%. However, as we have found, the switching efficiency tends to be influenced more by the electrode dimensions than by the intrinsic characteristics of the material. We now report a LT GaAs-based photoconductive detector that takes advantage of the high breakdown field capability of LT GaAs to greatly improve sensitivity. In a photodiode, a reduction in electrode spacing improves speed with little change in sensitivity. In a photoconductive detector, by contrast, such a reduction increases the sensitivity with little change in speed. Decreasing the carrier transit time across the semiconductor gap to a value comparable to the Mat
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