;High-speed and high-power photodiodes (PDs) serve as the key component in the millimeter-wave-over-fiber (MoF) or THz wireless communication system. In order to boost the speed of PD up to THz regime, downscaling the depletion layer thickness and device active area is an essential way to minimize both the internal carrier transient time and RC-limited bandwidth. However, the miniaturized size of device usually results in serious device-heating and thermal failure under high-power operation. The primary source of self-heating is the input electrical power, which equals to the product of dc reverse bias of PD and its output photocurrent. To have the PD sustain high-speed and high-power performance even under zero-bias operation should thus be one of effective solutions to minimize this thermal issue. Uni-traveling carrier photodiodes (UTC-PDs), which have only fast electron as active carriers under small external applied electric field (~ 10 kV/cm), is one of attractive choices to meet the above-mentioned application under zero-bias operation. Such device structure has demonstrated an excellent 3-dB O-E bandwidth (>110 GHz) under 1.2 mA output photocurrent with a moderate saturation output power (-18.6 dBm at 2mA) at 100 GHz. In this work, we demonstrate a novel design of UTC-PD, which can further enhance its zero-bias performance. By using type-II (GaAs0.5Sb0.5/InP) absorption/collector interface and AlxInyGa1-x-yAs graded bandgap structure in the collector layer, the current blocking (Kirk) effect can be minimized. State-of-the-art high-speed performance (~140 GHz 3-dB O-E bandwidth at 2mA output photocurrent) and output power (-13.9 dBm at 8 mA) at sub-THz regime (160 GHz) under zero-bias operation has been successfully demonstrated.
