Design and analysis of long absorption-length traveling-wave photodetectors

Long-absorption-length traveling-wave-photodetectors (TWPDs) are requisite for high-power and high-speed applications. In the long absorption length regime, some previously negligible bandwidth limitation factors, such as microwave loss and boundary reflection, become critical. In this paper, we calculate each limiting factor for long-absorption-length TWPDs using a photo-distributed-current model, which can be easily modified to include different effects comparing with the previous model. The simulated device structures are low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) and n-i-n TWPDs for telecommunication wavelength applications. Our simulation results indicate that the carrier trapping time is not the dominant bandwidth limitation factor as in the short device length regime. The device bandwidth is, on the other hand, strongly affected by velocity mismatch, frequency-dependent microwave loss, and boundary reflection. By properly choosing the geometric size of the transmission line in the MSM TWPD structure, the effect of impedance mismatch can be eliminated. Also due to the enhanced microwave velocity and a lower microwave loss, a better bandwidth performance can be found in the MSM TWPD structure. With a longer carrier trapping time and a higher electrical wave velocity, high bandwidth-efficiency product can be expected for LTG-GaAs-based long-absorption length photodetectors.