Design and Simulation of Ge Based Optically Controlled Field Effect Transistor
Optical interconnects can be a viable solution owing to several advantages over their metal counterpart thanks to negligible propagation losses and crosstalk, large bandwidth and noise immunity. While optical communications already dominate the long-distance point-to-point connections, the massive data processing and transmission are stimulating the use of optics towards shorter and shorter distances, down to interconnects between cabinets, boards, chips and even on-chip.
The development of silicon-based optoelectronic integrated circuits comprising electronics, waveguides, passive optical devices and optoelectronic devices based on transistors are under way. One of the most significant innovation lies in the optically controlled field effect transistor (OCFET), a device capable of switching when driven by light (rather than voltage), thanks to a photosensitive gate. This device is expected to radically change the classical approach based on the photodiode followed by the transimpedance amplifier.
The major advantages of the OCFET are low power consumption, device scalability, monolithic integration with electronics, operating frequency limited by the employed technology rather than photodetectors and amplifiers, no need of front-end, with expected benefits in large scale integration optical receivers, distribution of optical clock and on-chip optical interconnects.
The research is just at the beginning and first devices with promising performances in terms of high frequency and integration capabilities are being demonstrated [1-4].
However, the range of physical effects and suitable device structures has not been fully analyzed and there are still many unsolved issues.
In this work we investigate the static and dynamic characteristics of an OCFET based on a MOSFET transistor with a Ge gate. We provide a model of the device, analyze the operation principles and study its performance versus design parameters and operating conditions using TCAD (Technology Computer Aided Design).
Strategies towards best operating conditions and satisfactory trade-off are investigated and discussed along with future perspective and possible fundamental limitations.
Device schematic and its optoelectronic characteristics are shown in fig.1 and 2, respectively.
Fig.1: Device schcematic. In this study we focused on an n-channel OCFET with a gate W=1µm, L=0.18µm, 4nm gate oxide, a p-type Si concentration NSi=2·1017cm-3 and a n-type 200nm thick Ge film with a default concentration NGe=NSi.
Fig.2: IS-VDS characteristics for different light powers with gate bias VGS=0.6 V.
 A. K. Okyay, A.J. Pethe, D. Kuzum et al. "SiGe optoelectronic metal- oxide semiconductor field-effect transistor" Opt. Lett. vol. 32, no. 14, pp. 2022-2024, Jul. 2007.
 S Sahni, X. Luo, J. Liu,Y. Xie, E. Yablonovitch, "Junction field-effect-transistor-based germanium photodetector on silicon-on-insulator " Opt. Lett. vol. 33, no. 10, pp. 1138-1140, May 2008.
 J. Wang, M. Yu, G. Lo, D.L. Kwong, and S. Lee, " Silicon Waveguide Integrated Germanium JFET Photodetector With Improved Speed Performance," J. Lightw. Technol. vol.23, no.12, pp.765-767, Jun. 2011.