Ashwin Ashok

Electrothermal Monte Carlo Simulation of GaN HEMTs Including Electron–Electron Interactions

A Monte Carlo device simulator was developed to investigate the electronic transport properties in AlGaN/GaN high-electron mobility transistors (HEMTs). Electron-electron interactions were included using a particle-particle-particle-mesh coupling scheme. Quantum corrections were applied to the heterointerface using the effective potential approach due to Ferry. Thermal effects were also included by coupling the particle-based device simulator self-consistently with an energy balance solver for the acoustic and optical phonons. The electrothermal device simulator was used to observe the temperature profiles across the device. Hot spots or regions of higher temperatures were found along the channel in the gate-drain spacing. Results from electrothermal simulations show self-heating degradation of performance at high source-drain bias. More importantly, the observed nonequilibrium phonon effects may play an important role in determining the thermal distribution in these HEMTs, resulting in reliability issues such as current collapse.

Importance of the Gate-Dependent Polarization Charge on the Operation of GaN HEMTs

We investigate the influence of the gate-voltage dependence of the polarization charge on the electron sheet charge density in the channel and how it reflects on the device transfer and output characteristics in GaN HEMTs. We find that a 10% increase in the polarization charge is needed to match the experimental data when the gate-voltage dependence of the polarization charge is included in the theoretical model. This information is important for calibration in commercial device simulators and for better understanding of the quality of the GaN/AlGaN interface.

Modeling GaN HEMTs using thermal particle-based device simulator

A Monte Carlo device simulator was developed to investigate the electronic transport properties in AlGaN/AlN/GaN High Electron Mobility Transistors (HEMTs). Two different polarization models were considered to introduce electromechanical coupling and changes in the piezoelectric polarization charge at the interface and their effect on the device characteristics was compared. The influence of the gate-voltage dependence of the polarization charge on the electron sheet charge density in the channel was determined. Our investigations suggest that 10% increase in the polarization charge is needed to match the experimental data when the gate voltage dependence of the polarization charge is included in the theoretical model. This information is important for calibration in commercial device simulators and for better understanding of the quality of the GaN/AlGaN interface [1].

Monte Carlo simulation of GaN n+nn+ diode including intercarrier interactions

Gallium nitride (GaN) is becoming increasingly more attractive for a wide range of applications, such as optoelectronics, wireless communication, automotive and power electronics. Switching GaN diodes are becoming indispensable for power electronics due to their low on-resistance and capacity to withstand high voltages. A great deal of research has been done on GaN diodes over the decades but a major issue with previous studies is the lack of explicit inclusion of electron-electron interaction, which can be quite important for high carrier densities encountered. Here we consider this electron-electron interaction, within a non-parabolic band scheme, as the first attempt at including such effects when modeling nitride devices. Electron-electron scattering is treated using a real space molecular dynamics approach, which exactly models this interaction within a semi-classical framework. Results in particular focus on the strong effect of carrier-carrier scattering on the drain side of the gate, where rapid carrier relaxation occurs.

Spin polarization in GaAs/Al0.24Ga0.76As heterostructures

The spontaneous spin polarization of a quantum point contact (QPC) formed by the lateral confinement of a high-mobility two-dimensional electron gas in a GaAs/AlGaAs split gate heterostructure is investigated. Self consistent calculations of the electronic structure of the QPC are performed using the spin-polarized density functional formalism of Kohn and Sham. Spin polarization occurs at low electron densities and exchange interaction is found to be the dominant mechanism driving the local spin polarization within the QPC. The cascading scattering matrix approach is utilized to compute the conductance and a conductance anomaly at ∼0.5 (2e 2 /h) has been observed. In addition to this, the sheet density dependence of the 0.7 conduction anomaly is investigated.

Importance of the Gate‐Dependent Polarization Charge and the Electron‐Electron Interactions on the Operation of GaN HEMTs

We investigate the influence of the gate‐voltage dependence of the polarization charge on the electron‐sheet density in the channel and how it reflects on the device transfer and output characteristics. We also investigate the role of the electron‐electron interaction on the magnitude of the drain current. We find that 10% increase in the polarization charge is needed to match the experimental data when these two effects are included in the theoretical model.

Drift Diffusion Modeling of Solar Cells

A three dimensional n+−p−p+ silicon Solar cell has been simulated using a Drift‐Diffusion model which involves the self consistent solution of the Poisson and Continuity equations. Light does not enter through all the regions of the structure as metal contacts exist to tap the current from the solar cell and these contacts prevent the light from entering through them. Shadowing effect of the metal contact has been included in the code which plays an important role in the calculation of the efficiency of solar cell as it is one of the factors that determines the number of effective photons contributing to the generation of electron hole pairs in the device. The regions not having any contact on them will act as a window for the light to enter the device and are prone to surface recombination whose effect has also been included in the code. This effect tends to reduce the efficiency as some of the carriers are lost even before entering the structure. Recombination current in solar cells is the major limiter...

Bias Induced Strain Effects, Short-Range Electron - Electron Interactions and Quantum Effects in AlGaN/GaN HEMTs

In this paper we present state of the art modeling of GaN HEMTs, which includes for the first time simultaneous consideration of the electromechanical coupling, short-range Coulomb and quantum mechanical size quantization effects.

Monte Carlo Simulation of GaN Diode Including Intercarrier Interactions

Gallium Nitride (GaN) is becoming increasingly more attractive for a wide range of applications, such as optoelectronics, wireless communication, automotive and power electronics. Switching GaN diodes are becoming indispensable for power electronics due to their low on-resistance and capacity to withstand high voltages. A great deal of research has been done on GaN diodes over the decades but a major issue with previous studies is the lack of explicit inclusion of electron-electron interaction, which can be quite important for high carrier densities encountered. Here we consider this electron-electron interaction, within a non-parabolic band scheme, as the first attempt at including such effects when modeling nitride devices. Electron-electron scattering is treated using a real space molecular dynamics approach, which exactly models this interaction within a semi-classical framework. It results in strong carrier-carrier scattering on the biased contact of the resistor, where rapid carrier relaxation occurs.

Thermal modeling of gan HEMTs

Thermal effects were investigated to get a better understanding on the role of self-heating effects on the electrical characteristics of AlGaN/GaN HEMTs This is implemented by solving simultaneously the acoustic and optical phonon energy balance equations and also takes into account the coupling of the two subsystems The electro-thermal device simulator was used to observe the temperature profiles across the device Hot spots or regions of higher temperatures were found along the channel in the gate-drain spacing These preliminary results from the electro-thermal simulations suggest that the thermal effects do not have a drastic impact on the electrical characteristics, the current reduction falls between 5–10% over the simulated range of voltages However, the non-equilibrium phonon effects might play an important role in determining the thermal distribution in these HEMTs and thus, resulting in reliability issues such as current collapse.