Ultrahigh-power-bandwidth product and nonlinear photoconductance performances of low-temperature-grown GaAs-based metal-semiconductor-metal traveling-wave photodetectors

Maximum-output-power and bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors (PDs). In this paper, we report record high-peak output voltage (/spl sim/ 30 V) together with ultrahigh-speed performance (1.8 ps, 190 GHz) observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak output power and ultrahigh-electrical bandwidth performances were achieved due to superior MSM microwave guiding structure, short carrier trapping time, and the capability to take high bias voltage (/spl sim/ 30 V) with a LTG-GaAs layer. Under such a high bias voltage, a significant nonlinear photocurrent increase with the bias voltage was observed. The nonlinear photoconductance and ultrahigh-output power-bandwidth performances opens a new way in the application of high-performance optoelectronic mixers and photomixer devices.

The Two major trends in the development of ultrahighspeed photodetectors (PDs) are improving bandwidthefficiency product and obtaining high-output power-bandwidth product [1]. For the PDs with high-output power-bandwidth product, they can find applications in low-cost photoreceiver circuits without electrical amplifiers [1], microwave photonics systems [2], and optoelectronic generation of high-power microwaves and millimeter waves [3]. Maximum-output-power and electrical bandwidth performances are two tradeoff parameters in the design of high-speed PDs [1]. By reducing optical modal absorption constants and increasing photoabsorption lengths (volumes) in edge-couple PD structures, the device's output power can increase significantly [4], [5]. However, in these high-power PDs, the electrical bandwidth would decrease seriously due to R-C or velocity mismatch bandwidth limitations and serious high-frequency microwave loss [6]. There are two major ways to increase the product performance of output current (power) and electrical bandwidth. One is to distribute the photocurrents along edge-coupled PDs, such as the velocity matched distributed photodetector (VMDP) [7]; the other is to increase the carrier velocity using a unitraveling carrier PD [8]. In this letter, we report record-high peak output voltage-bandwidth product performance (, 190 GHz, 5.7 THz-V) of low-temperature-grown GaAs (LTG-GaAs)-based MSM traveling wave photodetector (MSM-TWPD) [9] under high optical excitation energy () and a high bias voltage level (30 V), which was measured using an electrical–optical (EO) sampling station based on a femtosecond Ti:sapphire laser at 800 nm. By utilizing the MSM microwave guiding structure, a large photoabsorption volume can be achieved without obvious electrical bandwidth degradation [6]. The serious space charge screening effect under high-power illuminations in most high-power PDs has also been reduced with LTG-GaAs photoabsorption layer due to its short carrier trapping time and the fact that most of the collected photogenerated carriers are from the regions near metal contacts where least space charge screening effects occur [1], [10]. The capability to take high dc bias voltages (30V) in a MSM-TWPD also reduces the external circuit saturation effect [10] and improves its maximum output peak voltages. Compared with the maximum allowable bias voltage of molecular beam epitaxy (MBE) annealed MSM-TWPDs [11], the demonstrated rapid thermal annealing (RTA) annealed devices show superior capability to take even higher bias voltages and to deliver even higher output powers. Under such a high dc bias voltage, a significant nonlinear photocurrent increase has also been observed, which are possibly originated from carrier capture-time increasing [12] or avalanche breakdown effects. Fig. 1. FWHM of the measured transient responses of a 12--long MSM TWPD under different bias voltages with a fixed optical excitation energy (71pJ/pulse)