The integration of complementary metal-oxide-semiconductor field effect transistors (CMOSFET) with photonics requires photodetectors to operate at hundreds of giga hertz (GHz). This can be achieved by scaling down the size of photodetectors . But a smaller device volume will reduce the light absorption, resulting in a poorer photosensitivity. High gain photodetectors such as avalanche photodetectors are often employed for such applications. Unfortunately, such photodetectors suffer from excessive avalanche noises in particular when the devices operate at high gain. In this work, we develop a novel core-shell nanowire phototransistor that has a gain of 106 and a potential 3dB bandwidth of ~300 GHz. The device is made on a highly doped p-type silicon nanowire that is patterned out of the device layer of a silicon-on-insulator (SOI) wafer. A section of the nanowire is doped to be n-type by self-assembled molecular monolayers [2-3], forming a core-shell pn junction around the nanowire like a structure of “a ring on a finger”. For an appropriate nanowire width, the pn junction will pinch off the nanowire channel without voltage bias. Under light illumination, the channel will open, inducing a high saturation photocurrent. Experimental results show that the nanowire phototransistors show a photoresponsivity of 106 A/W with a potential 3dB bandwidth of 300GHz.
 O. Hayden, R. Agarwal, and C. M. Lieber, “Nanoscale avalanche photodiodes for highly sensitive and spatially resolved photon detection,” Nat. Mater. 5(5), 352–356 (2006).
 Ho, J. C. et al. Controlled nanoscale doping of semiconductors via molecular monolayers. Nat. Mater. 7, 62–67 (2008).
 Guan, B. et al. Nanoscale Nitrogen Doping in Silicon by Self-Assembled Monolayers. Sci. Rep. 5, 12641 (2015).