Monte Carlo simulation of semiconductor devices

Abstract

This paper gives a review of applications of the Monte Carlo technique and recent literature on Monte Carlo modeling of semiconductor devices. The emphasis of the original research results reported in this paper is on self-consistent ensemble Monte Carlo simulation of GaAs/AlGaAs Heterostructure Field-Effect Transistors (HFETs) and novel HFET structures. We consider both electron and hole transport keeping in mind possible applications for complementary devices and circuits. Hole transport properties in GaAs are treated using corrected expressions for the scattering rates and a very accurate analytical description of the valence bands valid to about 1 eV. We review the corrected rates. Our simulations demonstrate superlinear behavior in the velocity-field relationship at low temperatures, and a temperature maximum is found in the low-field mobility between 40 K and 60 K at a doping density of 10 16 cm −3. We describe in detail our two-dimensional self-consistent Monte Carlo simulator and demostrrate the usefulness of Monte Carlo simulations for developing and validating simple device models utilized in computer-aided design tools for VLSI circuits. From this perspective, the unified charge-control model is also briefly outlined. Short-channel effects in self-aligned HFETs are shown to be caused by the injection of charge from the contact regions into the buffer beneath the device channel for gate lengths shorter than about 0.5 μm, given an adequately large aspect ratio. For possible incorporation in charge-control models, the threshold voltage shift as well as the output conductance in saturation are found to be approximately inversely proportional to the gate length under the same conditions. Our simulations show that a p-i-p + buffer structure improves the carrier confinement to the channel, reducing the output conductance by about 80% in a 0.3 μm device. Two device concepts recently proposed, aimed at increasing carrier velocities, are studied. The Variable Threshold HFET (VTHFET) as well as the Split-Gate HFET (SGHFET) utilize a gate voltage swing that is made to depend on lateral position. This position dependence raises the resistivity and thus the electric field and the velocity near the source contact. For a certain doping configuration, a p-type-like VTHFET is shown to exhibit a 78% increase in the current-gain cutoff frequency f T, a 59% increase in the maximum transconductance, and substantially higher K-factor compared with a conventional 0.5 μm GaAs/AlGaAs HFET. In n-type VTHFETs, the only pronounced improvement is broader and flatter high g m region. The much larger improvement using p-type VTHFETs is shown to be associated withthe lower hole mobility, the absence of satellite valleys, and the smaller velocity overshoot. The n-VTHFET also suffer from a big shift in overall threshold voltage. The remarkable modulation of velocity and energy profiles achieved could in its own right warrant applications in other n-type devices, such as real-space transfer devices. Guidelines for succesful utilization of the VTHFET concept in other materials and for other device geometries are given.