Highly sensitive ultraviolet (UV) photodetectors are highly desired for industrial and scientific applications. However, the responsivity of silicon photodiodes in the UV wavelength band is relatively low due to high-density Si/SiO2 interface states. In this paper, a coplanar avalanche photodiode (APD) was developed with a virtual guard ring design. When working in Geiger mode, it exhibited a strong UV response. The responsivity of 4 × 103 A/W (corresponding to a gain of 8 × 106) at 261 nm is measured under the incident power of 0.6 μW with an excess bias of 1.5 V. To the best of our knowledge, the maximum 3-dB bandwidth of 1.4 GHz is the first report ever for a Si APD when working in the Geiger mode in spite of the absence of an integrated CMOS read-out circuit.

Graphene/semiconductor hybrid photodetectors have attracted much attention recently for their excellent optoelectronic properties. However, high background current and 1/f noise limit their sensitivity. In this paper, the phenomenon of photo-induced quantum capacitance was found based on the transfer-free graphene/Cu2O/Cu structure. Under illumination, the Fermi level in graphene can be modulated by the photogenerated holes trapped in Cu2O nanoparticles, which causes the capacitance response phenomenon. The maximum capacitance response of 104 F/W with the minimum detectable light power of 5.3 fW was achieved at room temperature.