Tor A. Fjeldly

Conducting laboratory experiments over the Internet

We report on an interactive on-line laboratory for remote education called Automated Internet Measurement Laboratory (AIM-Lab), which utilizes the Internet and the World Wide Web. AIM-Lab allows efficient use of laboratory equipment in both regular and laboratory courses, especially in a distance-learning environment. Our approach is based on newly developed software packages and commercial measurement equipment. As an example, we describe an application of remote experiments on semiconductor device characterization, which can be freely accessed on the Web.

A Physics-Based Analytical Model for 2DEG Charge Density in AlGaN/GaN HEMT Devices

In this brief, we present a physics-based analytical model for 2-D electron gas density <i>ns</i> in AlGaN/GaN high-electron mobility transistors. The proposed model accounts for the interdependence between Fermi level <i>Ef</i> and <i>ns</i>. The model is developed by considering the variation of <i>Ef</i>, the first subband <i>E</i>₀, the second subband <i>E</i>₁, and <i>ns</i> with applied gate voltage <i>Vg</i>. The proposed model is in very good agreement with numerical calculations.

Precise Modeling Framework for Short-Channel Double-Gate and Gate-All-Around MOSFETs

A precise modeling framework for short-channel nanoscale double-gate (DG) and gate-all-around (GAA) MOSFETs is presented. For the DG MOSFET, the modeling is based on a conformal mapping analysis of the potential distribution in the device body arising from the interelectrode capacitive coupling, combined with a self-consistent procedure to include the effects of the inversion charge. The DG interelectrode coupling, which dominates the subthreshold behavior of the device, can also be applied with a high degree of precision to the cylindrical GAA MOSFET by performing a simple geometric scaling transformation to account for the difference in gate control in the two devices. Near threshold, self-consistent procedures invoking Poissons equation in combination with boundary conditions and suitable modeling expressions for the potential are applied to the two devices. In strong inversion, these solutions converge to those of the respective long-channel devices. The drain current is calculated as part of the self-consistent treatment. The results for both the electrostatics and the current are in excellent agreement with numerical simulations.

Analytical Modeling of Surface-Potential and Intrinsic Charges in AlGaN/GaN HEMT Devices

A surface potential (SP)-based analytical model for intrinsic charges in AlGaN/GaN high electron mobility transistor devices is presented. We have developed a precise analytical method to calculate the Fermi-level position Ef from a consistent solution of Schrodingers and Poissons equations in the quantum well, considering the two important energy levels. The accuracy of our Ef calculation is on the order of femto-volts for the full range of bias voltage. The SP calculated from Ef is used to derive an analytical model for intrinsic charges in these devices. The model is in excellent agreement with experimental data.

Robust Surface-Potential-Based Compact Model for GaN HEMT IC Design

We present an accurate and robust surface-potential-based compact model for simulation of circuits designed with GaN-based high-electron mobility transistors (GaN HEMTs). An accurate analytical surface-potential calculation, which we developed, is used to develop the drain and gate current model. The model is in excellent agreement with experimental data for both drain and gate current in all regions of device operation. We show the correct physical behavior and mathematical robustness of the model by performing various benchmark tests, such as DC and AC symmetry tests, reciprocity test, and harmonic balance simulations test. To the best of our knowledge, this is the first time a GaN HEMT compact model passing a range of benchmark tests has been presented.

CMOS RF Modeling, Characterization and Applications

RF MOS measurements, F. Sischka and T. Gneiting MOSFET modelling and parameter extraction for RF ICs, M. Je et al MOSFET modelling for RF IC design, Y. Cheng RF CMOS noise characterization and modelling, C.-H. Chen and M.J. Deen SOI CMOS transistors for RF and microwave applications, D. Flandre et al RF CMOS reliability, S. Naseh and M.J. Deen.

Enhanced GaAs MESFET CAD model for a wide range of temperatures

We describe a new and enhanced GaAs MESFET model suitable for implementation in computer aided design (CAD) software packages such as, for example, SPICE. The model accurately reproduces both above-threshold and subthreshold characteristics of GaAs MESFETs in a wide temperature range, from 77 K to 350/spl deg/C. The current-voltage characteristics are described by a single continuous, analytical expression for all regimes of operation. The physics-based model includes effects such as velocity saturation in the channel, drain induced barrier lowering, finite output conductance in saturation, bias dependent series source and drain resistances, effects of bulk charge, bias dependent average low-field mobility, frequency dependent output conductance, backgating and sidegating, and temperature dependent model parameters. The output resistance and the transconductance are also accurately reproduced, making the model suitable for analog CAD. >

Monte Carlo simulation of short channel heterostructure field-effect transistors

Self-consistent Monte Carlo simulation was used to study self-aligned, planar-doped AlGaAs-GaAs heterostructure field-effect transistors (HFETs) with gate lengths varying from 0.1 to 1.0 mu m and two different depths of the implanted contacts. The drain output conductance in saturation as well as the threshold voltage shift are found to be approximately inversely proportional to the square of the gate length for gate lengths smaller than 0.5 mu m. The predominant physical mechanism behind these short-channel effects at such gate lengths is the injection of electrons from the contacts into the GaAs buffer region beneath the two-dimensional channel. The critical parameter for the onset of large short-channel effects is the ratio between the source-region-drain-region separation and the contact depth. Hence, an optimum depth of the contacts should be found as a tradeoff between short-channel effects and parasitic series resistances. Simulated current-voltage characteristics exhibit pronounced negative differential resistance at large gate voltages because of real space transfer of channel elections into the AlGaAs layer and subsequent collection by the gate electrode. Simulated on and off transients have similar durations, but a trend toward shorter switch-off times exists. >

Compact Subthreshold Current Modeling of Short-Channel Nanoscale Double-Gate MOSFET

A physics-based compact subthreshold current model for short-channel nanoscale double-gate MOSFETs is presented. The potential is modeled using conformal mapping techniques in combination with parabolic approximations. For subthreshold conditions, we have assumed that the electrostatics is dominated by capacitive coupling between the body electrodes. Hence, the potential is obtained as an analytical solution of the 2-D Laplace equation. The current modeling is based on drift-diffusion theory. The modeling results are in good agreement with those of numerical simulations without the use of adjustable parameters.

Compact Charge-Based Physical Models for Current and Capacitances in AlGaN/GaN HEMTs

This paper presents physics-based compact models for the C-V and I-V characteristics of AlGaN/GaN HEMT devices. The contribution of only the first energy level in the triangular quantum well at the AlGaN/GaN interface (where most of the charge carriers of the 2-DEG channel reside) is considered, which resulted in an accurate and simple unified charge control model. Based on this, analytical models of the drain current, the gate charge, and the gate capacitances have been developed. The models cover all the different operating regimes of a device. The excellent agreements between the model and measured C-V and I-V characteristics of devices with different gate lengths have demonstrated the validity of the model.