Diego Guerra

Aspect Ratio Impact on RF and DC Performance of State-of-the-Art Short-Channel GaN and InGaAs HEMTs

We report a comparison between state-of-the-art GaN and InGaAs HEMTs in terms of the minimum aspect ratio required to limit short-channel effects. DC and RF simulations were carried out through our full-band cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra. Our results indicate that the minimum aspect ratio for GaN devices is 15 for negligible short-channel effects and 10 for reduced short-channel effects. On the other hand, InGaAs devices perform well for lower aspect ratio values such as 7.5 and 4-5 for negligible and reduced effects, respectively. The origin of this difference between GaN and InGaAs HEMTs is believed to be related to the different dielectric constants of the two materials and the corresponding difference in the electric field distributions related to short-channel 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.

Large-signal mm-wave InAlN/GaN HEMT power amplifier characterization through self-consistent Harmonic Balance / Cellular Monte Carlo device simulation

We report the simulation of the large-signal performance of mm-wave FET power amplifiers obtained for the first time through Full Band Monte Carlo particle-based device simulation self-consistently coupled with a Harmonic Balance (HB) frequency domain circuit solver. Due to the iterative nature of the HB algorithm, this FET simulation approach is possible only due to the computational efficiency of our Cellular Monte Carlo (CMC), which uses pre-computed scattering tables. On the other hand, a frequency domain circuit solver such as HB allows the simulation of the steady-state behavior of an external passive reactive network without the need for simulating long transient time (i.e. RC, L/C time constants) typical of time domain solutions. By exploiting this newly developed self-consistent CMC/HB code, we were able to time-efficiently characterize the mm-wave power performance of a state-of-the-art 30-nm gate-length InAlN/GaN HEMT.

Carrier Dynamics Investigation on Passivation Dielectric Constant and RF Performance of Millimeter-Wave Power GaN HEMTs

The effect of the passivation layer dielectric constant and T-gate geometry on the performance of millimeter-wave high-power GaN HEMTs is investigated through a nanoscale carrier dynamics description obtained by full-band cellular Monte Carlo simulation. The effective gate length is found to be increased by fringing capacitances and enhanced by the dielectric constant of the passivation layer in the regions adjacent to the gate for layers thicker than about 5 nm. Detailed simulation results are shown for the carrier energy, velocity, scattering, and electric field profiles along the channel. The output impedance under small- and large-signal operations is also discussed. Our results indicate that the effect of the passivation layer dielectric constant changes the nanoscale carrier dynamics and can strongly affect the radio-frequency performance of deep submicrometer devices.

Large-signal full-band Monte Carlo device simulation of millimeter-wave power GaN HEMTs with the inclusion of parasitic and reliability issues

We report for the first time the simulation of the large-signal dynamic load-line of high-Q matched mm-wave power amplifiers obtained through a Monte Carlo particle-based device simulator. Due to the long transient time of large reactive circuit elements, the time-domain solution of power amplifier high-Q matching networks requires prohibitive simulation time for the already time-consuming Monte Carlo technique. However, by emulating the high-Q matching network and the load impedance through an active load-line, we show that, in combination with our fast Cellular Monte Carlo algorithm, particle-based accurate device simulations of the large signal operations of AlGaN/GaN HEMTS are possible in a time-effective manner. Reliability issues and parasitic elements (such as dislocations and contact resistance) are also taken into account by, respectively, exploiting the accurate carrier dynamics description of the Monte Carlo technique and self-consistently coupling a Finite Difference Time Domain network solver with our device simulator code.

Millimeter-wave power amplifier circuit-device simulations through coupled Harmonic Balance - Monte Carlo particle-based device simulator

We describe the large-signal characterization of mm-wave FET power amplifiers with high-Q matching network performed through full band Monte Carlo particle-based device simulations self-consistently coupled with a Harmonic Balance frequency domain circuit solver. Such circuit-device simulations allow to include the effect of the matching network as well as parasitic elements in the large-signal characterization of mm-wave FETs.

Cellular Monte Carlo study of RF short-channel effects, effective gate length, and aspect ratio in GaN and InGaAs HEMTs

An investigation of RF short-channel effects in state-of-the-art GaN and InGaAs HEMTs, in relation to effective gate length and aspect ratio, is performed through our full band Cellular Monte Carlo simulator. In particular, the short-circuit current gain cut-off frequency, fT, is extracted using two different methods for several gate lengths. The first method relates fT to the electron transit time in the gate region, and from the electron velocity profile allows a direct estimation of fT, the effective gate length Leff, and the investigation of the nananoscale carrier dynamics in the channel. The second extraction methods derives fT through small-signal analysis. Our results indicates that the increasing difference between effective gate length and metallurgical gate length, as the device is scaled, plays a major role in limiting the RF performance. Moreover, maintaining a minimum aspect ratio of 5 for InGaAs HEMTs, and 10 for GaN devices, helps mitigating the short-channel effects.

Linearity analysis of millimeter wave GaN power transistors through X-parameters and TCAD device simulations

A novel approach to perform two-tone and modulated signal linearity analysis with TCAD device simulators is reported. The X-parameters of the simulated device are first extracted through one-tone large-signal coupled circuit-device simulations by exploiting, in our case, a coupled Harmonic Balance - TCAD Monte Carlo particle-based device simulator. Then, large signal linearity figures of merit such as two-tone intermodulation distortion and adjacent channel leakage ratio are obtained by a commercial Circuit Envelope simulator and the one-tone X-parameter compact model. In such a way, the TCAD simulation of a prohibitive number of RF cycles, due to the slowly varying envelope of the modulated carrier, is avoided.

Cellular Monte Carlo study lateral scaling impact of on the DC-RF performance of high-power GaN HEMTs

The effects of access region scaling on the performance of millimeter-wave GaN HEMTs is investigated through nanoscale carrier dynamics description obtained by full band Cellular Monte Carlo simulation. The drain current and transconductance have shown to increase monotonically up to respectively 5500 mA/mm and 1500 mS/mm by symmetrically scaling the source to gate and gate to drain distance from 635 nm to 50 nm. The electric field distribution has been studied for the shorter access regions and it was seen to be still far from the GaN breakdown limit. The access region scaling is found to greatly improve the frequency response of the device as well: from 340 GHz up to 860 GHz. Detailed simulation of the carrier dynamics in the area under the gate showed that these improvements are due to higher transit velocity of electrons at the source end of the gate.