Prof. Shi continuously has high-impact contributions to the ultrafast/high-power photodiodes (PDs) and their applications of wireless communications and millimeter wave (MMW) radar. The ultrafast PD with a miniaturized size usually suffers low responsivity and low saturation power. There are three major ways to overcome the aforementioned problems, which is based on PD epi-layer structure, PD package, and optical source for PD excitations, respectively. Over these years, Prof. Shi has several unprecedented breakthroughs in all these three areas and holds several records in the ultrafast PD. In the previously reported THz PD, a low reverse bias voltage (<-1V) is necessary to avoid inter-valley scattering effect of photo- generated electron and sustain high-speed performance. However, such small bias usually limits its output power. Prof. Shi demonstrated a novel PD structure: near-ballistic uni-traveling carrier photodiode (NBUTC-PD), through his unique design in epi-layer structure of NBUTC-PD, such device can not only increase the optimum bias for maximum speed but also fundamentally break the limitation on bias modulation speed of PD. He demonstrated such PD with record-high saturation current bandwidth product (7500 mA-GHz), photonic wireless linking with extremely-high OOK data rate as 25 Gbit/sec, and radar with record-high resolution (< 1 cm) all at sub-THz regime (100 GHz). The other problem in THz PD is its low responsivity as discussed. Its thin p-type absorber and small active volume usually impedes the light absorption and coupling processes, respectively. Prof. Shi is a pioneer in the flip-chip bonding package of THz PD, which can greatly reduce the optical coupling loss and fold the optical absorption path. Furthermore, Prof. Shi firstly demonstrated a novel absorber design in NBUTC-PD; Type-II hybrid absorber (GaAs0.5Sb0.5/In0.53Ga0.47As), which can significantly enhance the photo-absorption process due to the narrow bandgap (~0.5 eV) in the type-II interface. Combing such novel epi-layer design of PD with the advanced flip-chip bonding package, he demonstrated PD with a record-high O-E bandwidth (~0.33 THz), reasonable responsivity (0.11 A/W), and high saturation current (>10 mA) among all the reported PDs at telecommunication wavelengths. Prof. Shi is also a pioneer in the applications of optical pulse shaper (PS) and ultrafast PD for high-power THz wave generation. By use of PS, which can generate short optical pulse train with a 0.3 THz repetition rate, as light source to excite the THz NBUTC-PD, record-high output power (1 mW) at 0.3 THz from a single PD has been demonstrated. Following is some of our representative papers about such topic: