Hamid Z. Fardi

Numerical modeling of energy balance equations in quantum well Al/sub x/Ga/sub 1-x/As/GaAs p-i-n photodiodes

The energy balance equations coupled with drift diffusion transport equations in heterojunction semiconductor devices are solved modeling hot electron effects in single quantum well p-i-n photodiodes. The transports across the heterojunction boundary and through quantum wells are modeled by thermionic emission theory. The simulation and experimental current-voltage characteristics of a single p-i-n GaAs/Al/sub x/Ga/sub 1-x/As quantum well agree over a wide range of current and voltage, The GaAs/Al/sub x/Ga/sub 1-x/As p-i-n structures with multi quantum wells are simulated and the dark current voltage characteristics, short circuit current, and open circuit voltage results are compared with the available experimental data, In agreement with the experimental data, simulated results show that by adding GaAs quantum wells to the conventional cell made of wider bandgap Al/sub x/Ga/sub 1-x/As, short circuit current is improved, but there is a loss of the voltage of the host cell, In the limit of radiative recombination, the maximum power point of an Al/sub 0.35/Ga/sub 0.65/As/GaAs p-i-n photodiode with 30-quantum-well periods is higher than the maximum power point of similar conventional bulk p-i-n cells made out of either host Al/sub 0.35/Ga/sub 0.65/As or bulk GaAs material.

Time‐of‐flight studies of minority‐carrier diffusion in AlxGa1−xAs homojunctions

A novel time‐of‐flight technique has been developed for simultaneously measuring minority‐carrier lifetime and diffusivity in homojunctions. A pulsed dye laser produces electron‐hole pairs near the front surface of the device. A delay occurs before the onset of photocurrent due to the diffusion transit time of minority carriers to the junction. An analysis of this effect by both a simplified analytical model and a computer simulation gives similar results for the current as a function of time. A fit of the theory to experimental data on Al0.25Ga0.75As n/p homojunctions produces minority‐carrier lifetime, diffusivity, and diffusion length.

Simulation and modeling of p-n-p-n optical switches

A one-dimensional semiconductor device simulation program (FARDEV) with current boundary conditions is developed to study the steady-state and the transient characteristics of heterostructure four-layer p-n-p-n optical switches. Avalanche effect, radiative recombination, and band-gap discontinuities are included in the model. To demonstrate the use of the simulator, the modeling results are studied for both InP-In/sub 0.53/Ga/sub 0.47/As and Al/sub 0.3/Ga/sub 0.7/As-GaAs devices, and compared with experimental results in the latter case. The effects of optical generation and carrier lifetime on electrical characteristics of p-n-p-n switches are investigated. Simulation and experiment agree on holding current and voltage and on high-current on-state characteristics, while the limitation and simplification of the numerical model leads to discrepancy in the low-current off-state. The simulator is shown to be useful in evaluating the effects of device geometry, material parameters, avalanche mechanism, heterostructure spacing, and light generation on key switching parameters of four-layer p-n-p-n switches. >

Characterization and Modeling of CdS/CdTe Heterojunction Thin-Film Solar Cell for High Efficiency Performance

Device simulation is used to investigate the current-voltage efficiency performance in CdTe/CdS photovoltaic solar cell. The role of several limiting factors such as back contact Schottky barrier and its relationship to the doping density and layer thickness is examined. The role of surface recombination velocity at back contact interface and extended CdTe layer is included. The base CdS/CdTe experimental device used in this study shows an efficiency of 16-17%. Simulation analysis is used to optimize the experimental base device under AM1.5 solar spectrum. Results obtained indicate that higher performance efficiency may be achieved by adding and optimizing an extended CdTe electron reflector layer at the back Schottky contact. In the optimization of the CdS/CdTe cell an extended electron reflector region with a barrier height of 0.1 eV and a doping density of  cm−3 with an optimum thickness of 100 nm results in best cell efficiency performance of 19.83% compared with the experimental data.

Effects of base doping and carrier lifetime on differential current gain and temperature coefficient of 4H-SiC power bipolar junction transistors

4H-SiC NPN bipolar junction transistor (BJT) is studied systematically by performing two-dimensional numerical simulations. Several design issues are discussed. Depending on the doping concentration of the base and the carrier lifetimes, both positive and negative temperature coefficients in the common emitter current gain could exist in 4H-SiC NPN BJTs with aluminium-doped base. The temperature coefficients of the current gain at different base doping concentrations and different carrier lifetimes have been determined. A high base doping concentration can reduce the requirement for the carrier lifetime in order to obtain negative temperature coefficient in current gain. Device simulations are performed to evaluate the carrier lifetimes by fitting the measured output IC –VCE curves. An excellent fitting is obtained and the base electron lifetime and the emitter hole lifetime are extracted to be about 22 and 5.7 ns, respectively.

Modeling dc gain performance of 4H‐SiC BJTs

Purpose – To model the differential dc gain, base resistance, and current voltage performance of 4H‐Silicon Carbide (SiC) bipolar junction transistors (BJT) operating at and above room temperature. Accurate modeling will result in improved process efficiency, interpretation of experimental data, and insight into device behavior.Design/methodology/approach – The PISCES two dimensional device simulation program is used to allow for modeling the behavior of 4H‐SiC BJT. The physical material parameters in PISCES such as carriers mobility and lifetime, temperature dependent bandgap, and the density of states are modified to accurately represent 4H‐SiC. The simulation results are compared with the measured experimental data obtained by others. The comparisons made with the experimental data are for two different devices that are of interest in power electronics and RF applications.Findings – The simulation results predict a dc current gain of about 25 for power device and a gain of about 20 for RF device in ag...

Numerical modelling and characterization of high-frequency high-power high-temperature GaN/SiC heterostructure bipolar transistors

Device modelling is used in the characterization of GaN/SiC heterostructure bipolar transistors (HBTs), operating at high power, high frequency and high temperature. The differential DC current gain was simulated to be constant for an emitter current over several orders of magnitudes and decreased significantly with increasing temperature. The current gain as a function of temperature was obtained from a maximum of 300 000 at room temperature to a value of about 200 at 300°C. These simulated results are in agreement with experimental data obtained by others. Simulated results show a maximum cut-off frequency of 6 GHz for the actual device at a current density of 3000A cm−2. It is shown that high-temperature device modelling is essential in the design and optimization of GaN/SiC HBTs for high-power high-frequency high-temperature operation.

Detailed formulation of energy balance equations in single quantum well devices

An energy balance equation model coupled with drift‐diffusion transport equations are solved in heterojunction p‐i‐n diodes with embedded single quantum well to model hot electron effects. A detailed formulation of hot electron transport is presented. In the well, the carrier energy levels are estimated from the analytical expressions applied to a quantum well with finite height. Both bound and free carriers are modeled by Fermi‐Dirac statistics. Both size quantization and the two dimensional density of states in the well are considered. Thermionic emission is applied to the heterojunctions and quantum wells boundary. Energy transfer among the charge carriers and crystal lattice is modeled by an energy relaxation lifetime. Two sets of devices are simulated. First, the simulated kinetic energy and carrier density profiles were compared with published Monte Carlo results on an GaAs n+/n/n+ diode. Second, the current‐voltage characteristics of an embedded single quantum well AlGaAs/GaAs p‐i‐n structure was compared with measured data. Both comparisons are satisfactory and demonstrate the usefulness of the model for studying quantum well structures.

Modeling submicrometer GaAs MESFETs using PISCES with an apparent gate-length-dependent velocity-field relation

An empirical velocity-field relationship, based on Monte Carlo simulation, is integrated into the PISCES drift-diffusion simulation program in order to analyze short-gate GaAs MESFETs. The current-voltage characteristics are compared with 2D Monte Carlo simulation results on a 0.2 mu m gate length and with measured I-V characteristics of a 0.32 mu m gate-length GaAs MESFET. The comparison and the analysis made support the accuracy of the modified drift-diffusion model and show that it is computationally efficient for simulation of short-gate devices. >

Numerical modeling of hot electron GaAs/Al x Ga 1−x As quantum well photovoltaic

The effects of carrier escape from quantum well (QW) and the interaction of hot electrons with crystal lattice are investigated in GaAs/AlxGa1−xAs QW hot carrier solar cells where the cooling dynamics in photo-excited structures affect the cell efficiency. For modeling and characterization of quantum well hot electron solar cells the thermionic emission theory of heterostructures coupled with energy balance and non-isothermal driftdiffusion equations are solved numerically. Simulation data show that hot carriers escape from multi quantum well results in a higher maximum conversion point in structures with long energy relaxation lifetime which may lead to a higher efficiency in hot electron solar cells.