Steady-state Carrier Distributions Research Articles

Modeling of photocurrent and lag signals in amorphous selenium x-ray detectors

A mathematical model for transient photocurrent and lag signal in x-ray imaging detectors has been developed by considering charge carrier trapping and detrapping in the energy distributed defect states under exponentially distributed carrier generation across the photoconductor. The model for the transient and steady-state carrier distributions and hence the photocurrent has been developed by solving the carrier continuity equation for both holes and electrons. The residual (commonly known as lag signal) current is modeled by solving the trapping rate equations considering the thermal release and trap filling effects. The model is applied to amorphous selenium (a-Se) detectors for both chest radiography and mammography. The authors analyze the dependence of the residual current on various factors, such as x-ray exposure, applied electric field, and temperature. The electron trapping and detrapping mostly determines the residual current in a-Se detectors. The lag signal is more prominent in chest radiographic detector than in mammographic detectors. The model calculations are compared with the published experimental data and show a very good agreement.

Specific features of the steady-state carrier distribution and holding current in an optically triggered SiC thyristor

A numerical model making it possible to analyze the specific features of the steady-state distribution of carriers and the nature of the holding current for the axially symmetric case typical of optically triggered high-power SiC thyristors has been developed. The planar and axially symmetric models are compared. The relative contributions of the field and diffusion mechanisms of turn-on spread are analyzed. It is demonstrated that the diffusion mechanism of turn-on spread does not noticeably contribute to the steady-state carrier distribution and the field mechanism plays a major role. The data obtained are in satisfactory agreement with the first experimental results for the holding current in optically triggered SiC thyristors.

Cathodoluminescence measurement of grain boundary recombination velocity in vapour grown p-CdTe

A cathodoluminescence (CL) based method for measuring the minority carrier diffusion length (L) and reduced recombination velocity(Sred) of an individual grain boundary is presented. The technique is based on the van Roosbroeck model for the steady-state carrier distribution near a free surface in a semi-infinite solid. Values of L = 0.55±0.03 μm and Sred = 0.23±0.02 are obtained for vapour grown p-type CdTe. The effect of the electron beam generation volume on the accuracy of measurement is also discussed. Intermediate beam voltages (e.g. 15 kV for CdTe) are likely to produce the most reliable results. The method can be used to characterise the electrical activity of grain boundaries in thin-film solar cells.

Carrier Dynamics in Tunneling Injection Quantum Dot Lasers

We describe a detailed model for carrier dynamics in tunneling injection quantum dot lasers. The model includes the spatial dependence of the mobile carriers and the coupling between mobile carriers and confined carriers. The Poisson and Schrodinger equations are solved together, which enables consideration of band structure modification due to an electron-hole interaction. The model is solved both in steady state and for temporal perturbations. Steady-state carrier distributions and the band structure are presented, as well as large and small signal modulation responses.

Modeling of MOS-Side Carrier Injection in Trench-Gate IGBTs

A compact trench-gate insulated-gate bipolar transistor model that captures MOS-side carrier injection is developed. The model retains the simplicity of a 1-D solution to the ambipolar diffusion equation, but at the same time, captures the MOS-side carrier injection and its effects on steady-state carrier distribution in the drift region and on switching waveforms.

Monte Carlo simulation of terahertz quantum cascade laser structures based on wide-bandgap semiconductors

Wide-bandgap semiconductors such as GaN∕AlGaN and ZnO∕MgZnO quantum wells are promising for improving the spectral reach and high-temperature performance of terahertz quantum cascade lasers, due to their characteristically large optical phonon energies. Here, a particle-based Monte Carlo model is developed and used to quantify the potential of terahertz sources based on these materials relative to existing devices based on GaAs∕AlGaAs quantum wells. Specifically, three otherwise identical quantum cascade structures based on GaN∕AlGaN, ZnO∕MgZnO, and GaAs∕AlGaAs quantum wells are designed, and their steady-state carrier distributions are then computed as a function of temperature. The simulation results show that the larger the optical phonon energies (as in going from the AlGaAs to the MgZnO to the AlGaN materials system), the weaker the temperature dependence of the population inversion. In particular, as the temperature is increased from 10to300K, the population inversions are found to decrease by factors of 4.48, 1.50, and 1.25 for the AlGaAs, MgZnO, and AlGaN structure, respectively. Based on these results, the AlGaN and MgZnO devices are then predicted to be in principle capable of laser action without cryogenic cooling.

Comparison of SOI Power Device Structures in Power Converters for High-Voltage, Low-Charge Electrostatic Microgenerators

An inertial generator based on a moving-plate capacitor can provide energy for medical sensors from low-frequency (human body) motion. The energy exists as a very small charge (4 nC) at high voltage (300 V). An initial proposal for power processing used a carefully scaled lateral power MOSFET-diode pair, which results in a low, but sufficient, energy yield. It was found that increasing the area of the MOSFET to reduce conduction loss is highly detrimental to the energy yield because of capacitive loading of the generator. This paper examines alternative device topologies which may greatly increase the energy yield for a given system size by increasing both the generation efficiency and the conversion efficiency. An insulated gate bipolar transistors (IGBT) and a MOS-triggered thyristor, both based on previous silicon-on-insulator MOSFET designs, are examined for their switching speed and losses using physics-based finite-element simulation. The scaling criteria to achieve optimum system effectiveness are discussed. The small charge available from the generator results in a brief conduction period which does not allow the devices to reach their steady-state carrier distributions. Nevertheless the IGBT, and especially the MOS-triggered thyristor, switch on faster than the MOSFET, run at higher current densities, and provide improved efficiency. This allows the devices to be reduced in area leading to less capacitive loading on the generator. It also allows a reduction in the value and volume of the circuit inductor without a conduction loss penalty. We describe the device behavior in detail for the various phases of the conversion cycle and illustrate device/circuit tradeoffs graphically. Requirements are outlined for the development of power devices for microgenerators in implanted medical sensors.

Gavels, a velocity-space GaAs transport model—II. Velocity overshoot simulation

Velocity overshoot in GaAs is simulated using a time-dependent implementation of the GAVELS (Gallium Arsenide VELocity-Space) transport model. A solution technique is described which combines the steady-state carrier distributions with analytically derived relaxation parameters. The technique produces fast results and lends intuitive insight into the effect of the steady-state fine structure and relaxation parameters. Simulated responses to field steps in the 0–20 kV cm −1 range compare more favorably with Monte-Carlo results than simulations using fitted relaxation rates and Maxwellian steady-state distributions. The new results demonstrate the significance of the steady-state fine structure as well as a more sophisticated application of the GAVELS methodology.

Nonlinear modulation of hot carrier mobility and optical absorption in photoexcited polar semiconductors

The non linear interaction between the steady state carrier distribution in a photoexcited polar semiconductor and the population generated by a second light source is studied theoretically. We analyze the changes in mobility and optical absorption as a function of the intensities and energies of the light sources and as a function of the intensity of an applied electric field. We find that the interaction between the two carrier populations generated by the light sources can serve as a mechanism of modulation for the carrier mobility and optical absorption.

On steady-state carrier distributions in semiconductor devices

On steady-state carrier distributions in semiconductor devices