Physics and Simulation of High-Speed Heterostructure Devices
Kizilyalli, Isik Cem F.
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https://hdl.handle.net/2142/69413
Description
Title
Physics and Simulation of High-Speed Heterostructure Devices
Author(s)
Kizilyalli, Isik Cem F.
Issue Date
1988
Doctoral Committee Chair(s)
Hess, Karl
Department of Study
Electrical Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Abstract
In this thesis the transient and steady-state behavior of various GaAs/Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$As heterostructure devices which have shown promise for both high-speed digital and microwave applications is examined. In particular, the following devices are studied in detail: high electron mobility transistor (HEMT), dual-channel high electron mobility transistor, velocity-modulation transistor, and real-space transfer (negative resistance field-effect transistor, charge injection transistor) transistors.
Monte Carlo simulations performed for the high electron mobility transistor demonstrate the importance of nonstationary transport (i.e., velocity overshoot) in these devices. A hydrodynamic model is used in assessing the scaling properties of high electron mobility transistors. The constant voltage scaling study presented here shows a major improvement with respect to all figures of merit when gate and source-to-drain length of high electron mobility transistors are reduced.
Based on the simulations performed on dual-channel high electron mobility transistors and velocity-modulation transistor, it is concluded that recently proposed ideas of multiple channel transfer are valid and enhance the ultimate device speed. The electrons can be switched perpendicular to the layers in tenths of picoseconds due to short distances involved between the channels and high average electron velocities. However, the distribution of electrons along the device channel still plays the dominant role for reaching steady state.
The ensemble Monte Carlo model developed in this thesis reproduces all prominent features of real-space transfer transistors such as negative differential resistance in the drain current, saturation of drain and substrate (injection) current at high source-drain voltages, the negative transconductance in the saturated drain current, and the effect of magnetic fields on the injection current. Transient simulations show that the substrate (injection) establishes on the order of picoseconds.
Hot-electron effects in GaAs devices are so pronounced that their inclusion into device models is imperative. Ensemble Monte Carlo methods and electron temperature models account for these effects but require large computational resources. Standard drift-diffusion device simulators can be augmented to take into account nonstationary effects caused by space- and time-dependent electrical fields by introducing new transport coefficients (i.e., gradient and rate coefficient). In this thesis these coefficients are evaluated and this novel approach to GaAs device modeling is critically reviewed.
A new formulation for the electron ionization coefficient in n-MOSFETs is deduced based on Monte Carlo simulations which account for the nonuniform nature of the electric fields in the n-MOSFET channel.
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