/ Takashi Iizuka / Professor
/ Victor I. Ryzhii / Professor
/ Irina I. Khmyrova / Assistant Professor
/ G. Y. Khrenov / Assistant Professor
/ Maxim Yu. Ershov / Research Associate
/ Maxim V. Ryzhii / Research Associate
The research activity of the Computer Solid State Physics Laboratory is aimed at investigating semiconductor electronic and optoelectronic components for prospective computer hardware and future communication systems. The efforts of the members of this laboratory are directed toward the following:
The research results have been published in 21 refereed articles: Semiconductor Science and Technology (5), Solid State Electronics (2), Journal of Applied Physics (3), IEEE Transactions on Electron Devices (1), Japanese Journal of Applied Physics (7), COMPEL (1), and Russian Microelectronics (2).
The results also have been or will be presented at the following international conferences:
The computer programs developed in the laboratory have been used as computer-aided teaching materials in the lecture course ``Theory of Semiconductors'' for sophomore students.
Refereed Journal Papers
The effect of the nonuniformity of the dopant density in the quantum well of an intersubband single quantum-well infrared phototransistor (QWIPT) with triangular emitter and collector barriers is studied theoretically. It is shown that the nonuniformity of the dopant distribution in the plane of the QW, in particular, its random fluctuations, can significantly increase the dark current. However, the nonuniformity does not affect the photocurrent.
The high-frequency operation of a lateral hot-electron transistor is discussed. Analytical expressions for the induced collector current and collector transport factor are found and analyzed. It is shown that lateral geometry of the transistor results in specific features of the frequency dependence of the collector transport factor, namely, slower degradation at high frequencies in comparison with the same parameter of the hot-electron transistor with vertical structure.
An efficient numerical model of heterojunction bipolar transistor high frequency performance is proposed. The developed model is based on the ensemble Monte Carlo particle simulator. The validity and accuracy of the model are verified by comparing of the results of the model prediction with the experimental data. The role of the thickness of the collector junction on the transistor cut-off frequency is investigated and it is found that transistor cut-off frequency as a function of the collector thickness has a maximum.
The gallium arsenide short-channel Gunn diodes have been simulated by an ensemble Monte Carlo particle method. The necessary condition for Gunn oscillations in the short-channel diodes has been determined.
The numerical analysis of GaAs short-channel Gunn diodes is presented. It is revealed that the resonant-tunneling structure (RTS) in a near-cathode region reduces the `dead zone' length and extends the frequency range at which the diode can operate as an oscillator.
A lateral hot-electron phototransistor (LHEPT) utilizing intersubband absorption of radiation is proposed and considered theoretically. The principle of operation of the LHEPT is associated with two-dimensional or quasi-one-dimensional electron gas heating due to absorption of radiation in the base. The photoelectric performance of the LHEPT is evaluated using the proposed model. It is shown that the LHEPT exhibits high responsivity owing to high-frequency near-ballistic hot-electron transport in the LHEPT base and weak electron energy relaxation at low temperatures. LHEPTs can be used for detection of infrared radiation as tunable photodetectors.
Multiple-quantum-well (MQW) phototransistor is considered theoretically. The MQW phototransistor utilizes intersubband optical absorption and exhibits giant photocurrent gain which can lead to very high responsivity and detectivity. This effect is due to the thermionic injection of hot electrons across the emitter barrier and fast electron transit through the MQW base. An analytic theory of the MQW phototransistor utilizing parameters evaluated by Monte Carlo simulation is proposed. Transition from the near ballistic hot electron transport to diffusive transport decreases the responsivity but its value can be significant in this case as well.
We used Monte Carlo simulation to study the temperature dependence of electron impact ionization (II) coefficient in silicon. Based on the results of Monte Carlo calculations, we proposed an empirical models for II coefficient as a function of electric field or average electron energy, which are applicable in a wide range of lattice temperatures.
Multiple-quantum-well Phototransistor (MQWPT) is investigated using Monte Carlo modeling. The MQWPT photocurrent and photocurrent responsivity are evaluated as a functions of the MQWPT structure parameters. It is shown that the MQWPT can exhibit significant photocurrent responsivity, especially under the conditions of near ballistic hot electron transport in the MQWPT base.
Physical mechanisms responsible for the operation and performance of Multiple Quantum Well (MQW) infrared photodetectors are studied using computer modeling. It is shown that the operating mechanism of MQW infrared photodetectors is associated with the redistribution of the potential across an MQW structure under the influence of absorbing radiation, which, in turn, results in the stimulation of electron injection from the emitter contact into the MQW region. The distribution of the electric field under the applied bias is highly nonuniform, with two distinct domains of high field, which controls the electron injection, and low field, supporting the transport of the injected electrons.
Analytical and numerical models of a lateral hot-electron phototransistor (LHEPT) are developed. The principle of operation of the LHEPT is associated with intersubband absorption of infrared radiation in the base,which results in stimulation of the hot-electron injection from the emitter to collector. The responsivity of the LHEPTs based on the structures with two-dimensional channels is evaluated as function of the structure parameters, applied voltages and energy of incident photons.
The transit mode in non-uniformly doped submicrometer GaAs Gunn dioes has been simulated by the ensemble Monte Carlo particle method for a wide range of the diode lengths. It has been found that the oscillation frequency of these diodes can extend to 500 GHz with reasonably high efficiency.
A monolithic wavelength converter of long-wavelength infrared radiation to short wavelength infrared or visible radiation based on a quantum-well structure has been proposed and considered theoretically. The quantum-well converter utilizes intersubband electron transitions in the emitter quantum well. The threshold intensity of long-wavelength infrared radiation necessary for the laser generation is evaluated as a function of the device structural and physical parameters.
Electrical and optical properties of quantum-well infrared phototransistors (QWIPTs) utilizing intersubband electron transitions from a single quantum well (QW) are studied theoretically. Dependencies of the electron concentration in the QW, dark current, photocurrent and responsivity on applied bias voltage are evaluated. The obtained results can be used for the optimization of the QWIPT performance.
Effect of the modulation of the electron density in the quantum well (QW) of intersubband single quantum-well infrared phototransistor (QWIPT) on its performance is considered theoretically. We show that the sheet electron concentration can significantly differ from the sheet concentration of the donors in the QW. The sheet electron concentration can increase with applied bias, which leads to an increase of the dark current, photocurrent, responsivity and detectivity of the QWIPT. The effect of the electron tunneling from the QW is also discussed.
A quantum well infrared photodetector which converts long-wavelength infrared radiation into short-wavelength infrared (or visible) radiation is considered. The quantum well infrared photodetector utilizes an intraband absorbtion of radiation in a quantum well in the emitter and interband emission in either classical or quantum well in the collector. It is shown that the efficiency of energy conversion of long-wavelength radiation into short-wavelength radiation can be about unity.
We have analyzed the transport of electrons in the base of a heterojunction bipolar transistor operating in the coherent regime. The analysis is based on the Boltzmann transport equation for the one-electron distribution function. The influence of electron scattering on the transistor current gain in the frequency range beyond the usual cut-off frequency is included in the proposed model. It is demonstrated that the scattering of the electrons in the base region strongly affects the current gain of the heterojunction bipolar transistor in the extended frequency range. Conditions on the relationship between the electron transit times in the base and collector regions and electron scattering rate in the base, which are necessary for the implementation of the current gain beyond the usual cut-off frequency, are found.
A method to evaluate high-frequency performance of heterojunction bipolar transistors based on Fourier analysis of the non-stationary collector current response is proposed. The nonequilibrium electron transport and velocity overshoot effects are taken into account by ensemble Monte Carlo particle simulation. The influence of the emitter and collector capacitances on the high-frequency performance are also included in our consideration. It is found that the conventional method for cut off frequency calculation underestimates its value. Experimental results reported in the literature are compared with the results of numerical simulation. The comparison shows high accuracy and validity of the developed method.
The high-frequency operation of a lateral hot-electron transistor (LHET) with near ballistic transport in the base and collector is considered. The high-frequency efficiency of the hot-electron transport in the base and collector is evaluated. It is shown that due to two-dimensional electron gas high-frequency properties of the LHET collector are quite different from the properties of the hot-electron transistors with vertical structure.
Electron transport characteristics of strained $Si_{1-y}C_{y}$ random alloy grown on Si (100) substrate are studied theoretically using a Monte Carlo technique. The value of alloy scattering potential has a strong influence on the low-field electron mobility. Valley repopulation effect combined with decreased scattering rate of electrons in strained $Si_{1-y}C_{y}$ material can give rise to the increase of in-plane drift electron velocity with carbon concentration, in spite of the enhancement of alloy scattering. Electron transport characteristics have been calculated over a wide range of electric fields and temperatures.
A procedure for fitting of Monte Carlo calculated impact ionization coefficient to experimental data has been proposed. This procedure has been applied for optimization and sensitivity analysis of fitting parameters of impact ionization model for electrons in Si. Strong correlation between threshold energy and preexponential factor of the impact ionization model and redundancy of power exponent have been found. A wide range of data on impact ionization coefficient can be fitted by adjusting the parameters of the microscopic impact ionization model.
In this paper we present the picture of the physical effects in the Quantum Well Infrared Photodetectors (QWIPs) utilizing intersubband electron transitions. Our study is based on the numerical model, which allows one to find the distributions of the physical quantities in the QWIP structure and calculate the external device characteristics. The operation of QWIP is associated with the nonuniform distribution of the potential and other related physical quantities. We show that the contact and distributed effects play an important role in determining the operation and characteristics of the QWIPs.
The effect of random fluctuations of the donor density in the quantum well of an intersubband single quantum-well infrared phototransistor (QWIPT) with triangular emitter and collector barriers is studied theoretically. It is shown that the fluctuations of the donor distribution in the plane of the QW can significantly increase the dark current. It reduces the photocurrent-to-dark current ratio and detectivity.
The purpose of this work is to propose and evaluate two-terminal and three-terminal QW laser-phototransistor structures, which can generate a laser radiation in short-wavelength infrared range of spectrum modulated by long-wavelength infrared radiation. The mechanism of the modulation under consideration is compared with that associated with the heating of the electrons by infrared radiation.
The high-speed characteristics of InP/InGaAs HBTs with two different collector structures have been investigated using an ensemble Monte Carlo particle simulator. A dramatic decrease of the collector delay time has been observed for HBT with non-uniformly doped ($i-p^+-i-n^+$) collector structure. The collector delay time is reduced due to the extension of the velocity overshoot region in the collector. The parameters of HBT with proposed collector structure ($i-p^+-i-n^+$) has been optimized in order to obtain the ultimate high-frequency performance under the high collector voltage.
High-frequency operation of InP/InGaAs HBTs with various collector structures has been investigated using a Monte Carlo particle simulation. It is shown that vertical scaling of collector does not provide a substantial improvement in HBT overall speed performance due to the drastic increase of the collector capacitance charging time with reducing the collector layer thickness. A considerable improvement of HBT high-frequency performance has been obtained in HBT with buried subcollector and non-uniform collector doping profile.
The effect of structure parameters and operation conditions on the base-collector capacitance and intrinsic transit time of HBT with buried subcollector has been investigated by using a numerical model. It is shown that non-planar geometry of HBT with buried subcollector leads to specific features of voltage dependence of the base-collector capacitance. The non-uniform collector doping profile has been proposed to reduce collector transit time and to optimize HBT structure for low-voltage operation.
The InP/InGaAs HBTs with various structures have been investigated using an ensemble Monte Carlo simulation to understand how HBT high-frequency performance can be improved by using a vertical scaling of the collector layer. Results of numerical simulation have shown that the vertical scaling of collector layer alone does not result in a considerable improvement in overall speed performance of HBT.
A novel quantum intersubband photodetector - lateral hot-electron phototransistor (LHEPT) - is proposed and its physical and mathematical models are considered. Using the proposed model the upper limits of the responsivity and detectivity are estimated. It is shown that the LHEPTs can be utilized as high efficiency photodetectors for long-wavelength infrared radiation.
High-efficient numerical model of HBT is developed and implemented to calculate the ultimate speed performance of submicrometer HBTs.
The model of a heterostructure bipolar transistor incorporating RT collector structure is developed and considered. The aim of the paper is the numerical analysis of the optical control effects. The RTBT under consideration combines bistable electrical operation and optically controllable bistability effect that seems to be attractive in optoelectronic applications.
This paper presents a theoretical analysis of physical mechanisms responsible for operation and performance, in particular, optical gain, of infrared multiple-quantum-well (MQW) photodetectors. The influence of the device structure on the distribution of potential, which, in turn, determines the carrier injection and transport properties, is discussed. We used Monte Carlo method to study the hot electron effects in MQW phototransistor proposed earlier by the authors. It is shown that the nature of the hot electron transport in the base of MQW phototransistor plays a crucial role in determining device characteristics.
Physical mechanisms responsible for the operation and performance of Multiple Quantum Well (MQW) infrared photodetectors are studied using computer modeling. It is shown that the operating mechanism of MQW infrared photodetectors is associated with the redistribution of the potential across an MQW structure under the influence of absorbing radiation, which, in turn, results in the stimulation of electron injection from the emitter contact into the MQW region. The distribution of the electric field under the applied bias is highly nonuniform, with two distinct domains of high field, which controls the electron injection, and low field, supporting the transport of the injected electrons.
This paper deals with the modeling of lateral hot electron phototransistors (LHEPTs) based on lateral field-effect-controlled semiconductor structures with the two-dimensional electron gas. The optical gain, responsivity and detectivity of the LHEPTs based on the structures with GaAs and InAs two-dimensional channels are evaluated as functions of the structure parameters, applied voltages and energy of incident photons. It is shown, that the LHEPTs can be used as high efficiency voltage-tunable photodetectors for infrared radiation.
InGaAs/GaAs Quantum Well Infrared Photodetectors (QWIP) are studied theoretically using a numerical modeling. It is shown that triangular emitter and collector barriers provide better transport conditions and higher QWIP performance as compared to rectangular barriers. The photoconductive gain differs considerably from the dark current gain, due to a redistribution of the potential in the QWIP under the influence of infrared radiation.
A quantum well infrared photodetectors which transform long-wavelength infrared radiation into short-wavelength radiation are proposed and considered. It is shown that the intensity of output short-wavelength radiation can significantly exceed the intensity of input long-wavelength radiation, so that the optical gain can be much more than unity. The mechanism of the optical gain is connected with the injection of extra electrons from the emitter caused by photoexcitation of the electrons from the quantum well.
An integrated wavelength converter of long-wavelength infrared radiation to short wave-length infrared or visible radiation based on a quantum-well structure has been proposed and considered theoretically.
A novel quantum well device, which can convert long-wavelength infrared radiation into laser short-wavelength infrared or visible radiation is proposed and its performance is evaluated.
Thesis Advisor: Victor I. Ryzhii.
Others