Dissertation Defense, Weike Wang

Title: Characterization of InGaAs Metal-Oxide-Semiconductor Field-Effect Transistors

After about fifty years of development in silicon metal-oxide-semiconductor field-effect transistor (MOSFET), it is more and more difficult to continue transistor scaling nowadays due to limitations in lithography, power consumption, and reliability. Recently great effort has been put into searching alternative channel structures or materials for future high-performance and low-power logic applications. Huge progress has been made in research of several novel devices, including carbon-nanotube (CNT) field-effect transistors (FETs), silicon nanowire FETs, graphene FETs, and planar FETs with alternative channel materials such as Ge, InAs, InSb,  and InGaAs. This dissertation discusses about the electrical characterization of the interface traps, analysis of the inversion charges, electron mobility and junction leakage currents of Al2O3/InxGa1−xAs (x = 0.53, 0.65 or 0.75) MOSFETs.

Charge pumping has been used to characterize the interface traps between Al2O3 and InGaAs in an n-channel inversion-mode MOSFET. By analyzing the charge pumped under gate voltage pulses of different rise and fall times, the interface trap density is extracted across the bandgap, with an average value between mid 1012 cm−2eV−1 and low 1013 cm−2eV−1. The majority of interface traps in indium-rich InGaAs metal-insulator-semiconductor structures have been identified as donors, which limit the off-state performance of InGaAs MOSFETs such as subthreshold slope, drain-induced barrier lowering, and on/off current ratio. The result helps explain the promising on-state performance of the Al2O3/InGaAs MOSFETs and the need to further improve the interface so that its off-state performance can be on par with that of the Si MOSFET.

The electron mobility in Al2O3/InGaAs MOSFETs has been analyzed for scattering by oxide charge as well as interface charge and roughness, and compared with measured transfer characteristics from depletion to inversion. The analysis shows that in heavy inversion the electron mobility is mainly limited by interface roughness. The extracted interface roughness from the measured data is two to seven times that of the interface between high-κ dielectric and Si, assuming the correlation lengths are comparable. Therefore, to fully benefit from the high bulk mobility of InGaAs, its interface roughness with the gate oxide needs to be further improved.

Finally, the reverse junction leakage current has been analyzed by calculating diffusion, generation, and tunneling currents, and compared with measurement at room temperature. It is found that the leakage current increases with In mole fraction. Generation and tunneling currents dominates in medium- and high-bias regions, respectively.

Doctoral Committee Members:
Prof. James C. M. Hwang (Chair)
Prof. Miltiadis K. Hatalis
Prof. Boon S. Ooi
Prof. Marvin H. White
Prof. Peide D. Ye