Investigation of static and dynamic characteristics of optically controlled field effect transistors

Abstract

In recent years, scaling down the dimensions of electronic devices has driven dramatic improvements in the performance of electronic devices. With the increase in device speed and the introduction of exascale computing, the communication bottleneck has become one of the greatest challenge in both short and long distance communications. Traditional metal interconnects are efficient at short distances, but their excessive power dissipation and delay in global lines, makes them unsuitable for the ever-growing bandwidth demand. Currently, Optical interconnects (OIs) provides a solution to the communication bottleneck in long distance communications with their superiority in noise free, low loss, power efficiency and faster data transfer. However, in shorter distances, energy per bit is still higher compared to metal interconnects, due to the large power consumption by the receiver circuits. Since the receiver circuit after the photodetector, consumes most of the power, it is important to minimize the power consumption of the circuits, or if possible, to introduce receiver-less detection. Phototransistors, monolithically integrated to silicon electronics provides a possibility to replace power hungry receiver circuits in short distance (inter chip and intra-chip) communications. Although many phototransistors were reported with III-V compound semiconductors, it is still not easy and cost efficient to integrate them with the silicon photonics. In this context, Ge is becoming increasingly popular in silicon based photonic devices. Due to its strong absorption in the NIR region and its relative ease of integration with Si electronics, it is a promising candidate in fabricating CMOS compatible integrated photoreceivers. In this work, an optically controlled field effect transistor (OCFET) with Ge as an NIR absorbing gate is designed simulated using ISE-TCAD. The static and dynamic charateristics of the OCFET are studied in terms of Ion/Ioff ratio, responsivity and bandwidth as functions of doping concentrations, channel length, optical power, Ge gate thickness, gate bias and Ge carrier lifetime. A maximum simulated responsivity of 100A/W and the fall time (tfall) as low as 100ps are obtained. The OCFET in different inverter configurations with parameters like load resistor, W/L ratio of the load MOSFET and CMOS configuration are investigated. A proof of concept of OCFET is investigated by connecting a Ge-on-Si photodiode with the MOSFET gate terminal. The Ge-on-Si photodiode and the trench MOSFET designed and fabricated and OCFET concept is investigated under dark and illuminated conditions. The final part of this thesis is dedicated to the design, fabrication and characterization of an optical JFET with 4μm and 8μm channel length and Ge thin film of 500nm as the gate. The current-voltage characteristics of the Optical JFET is investigated with open gate and applied gate bias. In the open gate configuration, the device exhibits a signal to noise ratio of 14dB at 0dBm (1mW) optical power compared to 3.7dB at -10dBm (100μW) optical power. The responsivity with floating gate was 5.3A/W, which decreases to 0.13A/W with applied gate bias of -1V.