Jason S. Ayubi-Moak

The Upper Limit of the Cutoff Frequency in Ultrashort Gate-Length InGaAs/InAlAs HEMTs: A New Definition of Effective Gate Length

Ultrashort gate-length pseudomorphic high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. The RF response and the cutoff frequency fT have been obtained for physical gate lengths ranging from 10 to 50 nm. These results, in turn, have been used in a transit-time analysis to determine the effective gate length in each case. By interpolation, one can make an estimate of the absolute upper limit for fT, which we find to be 2.9 THz in the device studied. Importantly, the effective gate lengths are considerably shorter than the depletion lengths. Thus, in general, any estimate of fT based on the latter quantity is likely too small by a quite significant amount.

Simulation of Ultrasubmicrometer-Gate

Pseudomorphic delta-doped ultrasubmicrometer-gate high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. Reasonable agreement between experimental and numerical results is obtained for a 70-nm gate length. We discuss the scaling of this device to shorter gate lengths and the role played by various dimensions in the structure. Devices with 20-nm gate lengths should produce fTs above 1.5 THz without difficulty. This paper demonstrates the power of particle-based simulation tools in capturing the relevant physics responsible for device operation and key to performance optimization.

\hbox{In}_{0.52} \hbox{Al}_{0.48}\hbox{As/In}_{0.75}\hbox{Ga}_{0.25}\hbox{As/In}_{0.52}\hbox{Al}_{0.48}\hbox{As/InP}

High-electron mobility transistors (HEMTs) have become an important device for high frequency and low noise applications. The performance of these devices has been pushed into the range of several hundred GHz for fT. One question that has been asked is just how high a frequency can be obtained with these devices. To study this question, we have used a full-band, cellular Monte Carlo transport program, coupled to a full Poisson solver to study a variety of InAs-rich, InGaAs pseudomorphic HEMTs and their response at high frequency. We have concentrated on pseudomorphic HEMTs with the structure (from the substrate) InP/InAlAs/InGaAs/InAlAs/InGaAs, with the quantum well composed of In0.75Ga0.25As, and have studied gate lengths over the range 10–70 nm. Various source-drain spacings have also been studied, and the performance of scaled devices evaluated to determine the ultimate frequency limit. Here, the importance of the effective gate length has been evaluated from the properties internal to the device.

Pseudomorphic HEMTs Using a Full-Band Monte Carlo Simulator

In summary, we show that properly scaled HEMT devices can operate will into the THz regime, and provide a viable device option in this spectral region. These results are also important for logic devices desired for operation in the Tbs regime, as the cutoff frequency fT is intimately related to the logic delay time in a switching transistor.

Full-band CMC simulations of terahertz HEMTs

Photonic crystals have shown a great deal of promise for the realization of true integrated optics. Waveguides with small bends may be formed allowing compact integrated photonic circuits to be formed. Full three-dimensional (3D) photonic simulations are required in order to realize very low loss, integrated photonic crystal circuits. Needless to say, the design and fabrication of such fully 3D structures is challenging, and thus efficient simulation tools are necessary to identify the optimum structures for different applications. Researchers at the Department of Defense (DoD) and Arizona State University (ASU) have independently developed parallel Finite Difference Time Domain (FDTD) codes, with the goal of scaling up each simulator for complicated structures such as 3D optical integrated circuits (OIC). As the name implies, FDTD is a popular time-domain method for solving Maxwells equations for the electric and magnetic fields. These two curl equations are solved explicitly in time over half-step intervals, where the values of one set of field values (e.g., electric fields) are used at the successive interval to solve for the other field (e.g., magnetic field) in a time marching fashion. The goal of our current work is to realize a fully parallel FDTD code scalable to 108 FDTD grid points in order to have sufficient resolution to model even a relatively limited number of periods of a given waveguide structure. This requires both a scalable parallel FDTD code, as well as one with the proper boundary conditions and more efficient algorithms to reduce run. The work and results discussed herein address both the scalability and the efficiency of the time-domain algorithm.