Nicolas Faralli

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.

Rigid ion model of high field transport in?GaN

Here we report on high field transport in GaN based on the rigid ion model of the electron-phonon interaction within the cellular Monte Carlo (CMC) approach. Using the rigid pseudo-ion method for the cubic zinc-blende and hexagonal wurtzite structures, the anisotropic deformation potentials are derived from the electronic structure, the atomic pseudopotential and the full phonon dispersion and eigenvectors for both acoustic and optical modes. Several different electronic structure and lattice dynamics models are compared, as well as different models for the interpolation of the atomic pseudopotentials required in the rigid pseudo-ion method. Piezoelectric as well as anisotropic polar optical phonon scattering is accounted for as well. In terms of high field transport, the peak velocity is primarily determined by deformation potential scattering described through the rigid pseudo-ion model. The calculated velocity is compared with experimental data from pulsed I-V measurements. Good agreement is found using the rigid ion model to the measured velocity-field characteristics with the inclusion of dislocation and ionized impurity scattering. The crystal orientation of the electric field is investigated, where very little difference is observed in the velocity-field characteristics. We simulate the effects of nonequilibrium hot phonons on the energy relaxation as well, using a detailed balance between emission and absorption during the simulation, and an anharmonic decay of LO phonons to acoustic phonons, as reported previously. Nonequilibrium phonons are shown to result in a significant degradation of the velocity-field characteristics for high carrier densities, such as those encountered at the AlGaN/GaN interface due to polarization effects.

Comparison of N- and Ga-Face GaN HEMTs Through Cellular Monte Carlo Simulations

We compare the performance of GaN HEMT devices based on the established Ga-face technology and the emerging N-face technology. Starting from a state-of-the-art N-face device, we obtain the analogous Ga-face layout imposing the constraint of the same channel charge in both structures, and then, we simulate both the configurations with our full-band cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra. Moreover, we define a modeling approach based on gate-to-2-D electron gas distance and capacitance discussions, which allows a fair comparison between the N- and Ga-face technologies. Full direct current and RF simulations were performed and compared with available experimental data for the N-face device in order to calibrate the few adjustable simulator parameters. Our simulations indicate that N-face GaN HEMTs exhibit improved RF performance with respect to Ga-face devices. Furthermore, the use of an AlN layer in N-face devices results in a reduced alloy scattering and offers a strong back-barrier electron confinement to mitigate short-channel effects, thus improving the cutoff frequency for highly scaled devices.

Emerging N-Face GaN HEMT Technology: A Cellular Monte Carlo Study

This paper aims to investigate the potential of the emerging N-face technology with respect to both the direct current and radio frequency performance of GaN high electron mobility transistor (HEMT) devices. High-frequency high-power state-of-the-art HEMTs were investigated with our full-band cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra. A complete characterization of these devices was performed using experimental data to calibrate the few adjustable parameters of the simulator. The effect of scaling the device dimensions, such as the gate length and the access region lengths, on the device performance was analyzed. In addition, the enhancement-mode configuration of the N-face structure was investigated. Our simulations showed that N-face devices represent an important step in engineering HEMT devices for delivering high power density and efficiency at microwave and millimeter-wave frequencies.

Figures of merit in high-frequency and high-power GaN HEMTs

The most important metrics for the high-frequency and high-power performance of microwave transistors are the cut-off frequency fT, and the Johnson figure of merit FoMJohnson. We have simulated a state-of-the-art, high-frequency and high-power GaN HEMT using our full band Cellular Monte Carlo (CMC) simulator, in order to study the RF performance and compare different methods to obtain such metrics. The current gain as a function of the frequency, was so obtained both by the Fourier decomposition (FD) method and the analytical formula proposed by Akis. A cut-off frequency fT of 150 GHz was found in both the transit time analysis given by the analytical approach, and the transient Fourier analysis, which matches well with the 153 GHz value measured experimentally. Furthermore, through some physical considerations, we derived the relation between the FoMJohnson as a function of the breakdown voltage, VBD, and the cut-off frequency, fT . Using this relation and assuming a breakdown voltage of 80V as measured experimentally, a Johnson figure of merit of around 20 × 1012V/s was found for the HEMT device analyzed in this work.

Full-band CMC simulations of terahertz HEMTs

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.

Ballistic Transport in InP-Based HEMTs

Ballistic transport has been of interest in semiconductor devices for quite some time, and its effect has been used to predict quite-different device performance. Here, we investigate the role of ballistic transport in a short-channel InGaAs/InAlAs HEMT through full-band cellular Monte Carlo simulations. We can examine the contrast in behavior between when scattering mechanisms are present and when they are turned off. When the scattering processes are completely removed, the output characteristics show a distinct change in behavior over all drain voltages. This result is in qualitative agreement with prior arguments, suggesting that triodelike behavior should be expected due to enhanced drain-induced barrier lowering. However, we find that explicit band-structure effects are observable in the output characteristics of the ballistic transistor. We also find that this distinctive behavior gradually disappears as scattering is turned on, particularly in the drain end of the device. We also develop a method of determining the probability that electrons pass through the gate region in a ballistic manner in the presence of realistic scattering. Even when the gate is only 10 nm long, we find that this probability is only on the order of 50% in these devices. We also examine the ballistic ratio in our device as a function of gate length.

Cellular Monte Carlo Modeling of AlxIn1−xSb/InSb Quantum Well Transistors

In this work, an Indium Antimonide (InSb) quantum well transistor is investigated using full-band Monte Carlo simulations. The steady-state characteristic of the device is first analyzed, showing particle transport along the two-dimensional electron gas (2DEG). The small-signal behavior of the device is also investigated. Finally, the noise analysis is performed, allowing for a two-dimensional mapping of the noise within the device.

The upper limits of cut-off frequency in ultra-short gate length InP-based p-HEMTs

Ultrashort gate length pseudomorphic high-electron-mobility transistors (p-HEMTs) based on an InP substrate have been modeled using a full-band cellular Monte Carlo simulator. The RF response has been obtained for lithographic gate lengths ranging from 10 nm to 50 nm and for channel thicknesses of 18 and 10 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 the cut-off frequency, fT, which we find to be 2.9 THz in 18 nm device and 3.1 THz in the 10 nm device.

Simulating Pseudomorphic HEMTs: Optimizing Performance to Achieve Multi-terahertz Operating Frequencies

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.