Metal-semiconductor-metal traveling wave photodetectors

Low-temperature-grown GaAs (LTG-GaAs) based photodetectors (PDs) merit a lot of attentions due to their ultrahigh electrical bandwidth performances. We review the advantages and applications of LTG-GaAs based metal-semiconductor-metal traveling wave photodetectors (MSMTWPD) and discuss their ultra-high speed and record high power-bandwidth product performances in both short (∼800 nm) and long (∼1300 nm) wavelength regimes. The MSMTWPD has lower microwave loss and higher microwave velocity than the structure of p-i-n TWPD. The superior microwave guiding structure also implies that the electrical bandwidth is less sensitive to the photo-absorption volume. The measured impulse response and its corresponding frequency response of MSMTWPD are shown under high bias voltage (30 V) and high excitation optical pulse energy (∼71 pJ/pulse) at the wavelength of 800 nm. The obtained peak output voltage (V_p ∼30 V) and electrical bandwidth (190 GHz) product (5.7 THz-V) is the highest among all reported ultrahigh speed PDs. The excellent power-bandwidth product performance is due to short carrier lifetime of LTG-GaAs and superior microwave guiding structure. By utilizing the mid-gap defect states in LTG-GaAs, the photo-absorption in long wavelength regimes (1.3 μm-1.55 μm) can also be achieved. The record high power-bandwidth product performance among all reported high speed PDs in long wavelength regime has also been demonstrated with LTG-GaAs based MSMTWPD. However, compared with the high power performances in short wavelength regime, the demonstrated device exhibits a serious bandwidth degradation effect under ∼1.3 μm wavelength excitation. The different nonlinear behaviors at 1.3 μm wavelength possibly originate from that the photon energy (∼1 eV) is much higher than the subband-gap photon-absorption energy (∼0.7 eV), which will induce significant hot electron, inter-valley scatterings, and serious increasing in carrier lifetime. By contrast, the hot electron effect under short wavelength excitation (∼800 mn) is not evident as the photon energy of excitation wavelength is close to the band-to-band photo absorption energy.