Fabio Alessio Marino

Kinetics of Buffer-Related RON-Increase in GaN-on-Silicon MIS-HEMTs

This letter reports an extensive analysis of the charge capture transients induced by OFF-state bias in double heterostructure AlGaN/GaN MIS- high electron mobility transistor grown on silicon substrate. The exposure to OFF-state bias induces a significant increase in the ON-resistance (Ron) of the devices. Thanks to time-resolved on-the-fly analysis of the trapping kinetics, we demonstrate the following relevant results: 1) Ron-increase is temperature- and field-dependent, hence can significantly limit the dynamic performance of the devices at relatively high-voltage and high temperature (100 °C-140 °C) operative conditions; 2) the comparison between OFF-state and back-gating stress indicates that the major contribution to the Ron-increase is due to the trapping of electrons in the buffer, and not at the surface; 3) the observed exponential kinetics suggests the involvement of point-defects, featuring thermally activated capture cross section; and 4) trapping-rate is correlated with buffer vertical leakage-current and is almost independent to gate-drain length.

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.

Development of a new high holding voltage SCR-based ESD protection structure

A new silicon controlled rectifier low voltage triggered (SCR-LVT), to be adopted as protection structure against electrostatic discharge (ESD) events, has been developed and characterized. A high holding voltage has been obtained thanks to the insertion of two parasitic bipolar transistors, achieved adding a n-buried region to a conventional SCR structure. These two parasitic transistors partially destroy the loop feedback gain of the two main npn and pnp BJTs, resulting in an increase of the sustaining (holding) voltage during the ESD event. A strong dependence of the holding voltage with the ESD pulse width has also been observed, caused by self-heating effects. 2D device simulations (DESSIS Synopsys) have been performed obtaining results that perfectly fit the measurements over a wide temperature range (25 degC-125 degC). Using device simulation results , the factors that influence the holding voltage, in terms of temperature dependence, but also in the behavior of the parasitic BJTs, are explained. A guideline to change the SCR holding voltage, related to the SCR design layout without any change to process parameters, is also proposed.

Simulation of polarization charge on AlGaN/GaN high electron mobility transistors: Comparison to electron holography

The effects of polarization charge on the electrostatic potential distribution across the heterostructure of a AlGaN/GaN high electron mobility transistor device have been investigated. Simulations were performed using a full-band cellular Monte Carlo simulator, which included electronic dispersion and the phonon spectra. Quantum effects were taken into account using the effective potential method. Experimental extraction of potential profiles across the device was carried out using off-axis electron holography. Based on comparison to simulations, the differences between the theoretical predictions and experimental results could be explained, thereby providing better understanding of device operation.

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.

Breakdown investigation in GaN-based MIS-HEMT devices

Breakdown mechanisms in AlGaN/GaN HEMT devices are here analyzed, placing particular emphasis in the analysis of GaN based device grown on silicon substrate. Based on combined experimental data and bi-dimensional numerical simulation we demonstrate that many physical mechanisms can contribute to increase the leakage current leading to the final breakdown of the device. In particular we show how band-to-band phenomena, rather than impact ionization, can be responsible of the premature breakdown even in double-heterostructure HEMTs.

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.