P. K. Basu

Estimation of alloy scattering potential in ternaries from the study of two‐dimensional electron transport

A theory of alloy scattering of two‐dimensional electron gas in ternary semiconductors is developed by assuming spherically symmetric square scattering potential, randomly distributed between two kinds of alloy sites. The theory predicts a temperature independent mobility, in agreement with the experimental results for In0.53Ga0.47As. The calculated value at 4.2 K comes close to the experimental value, in which the effect of impurity scattering has been reduced by using an undoped spacer layer. It is concluded that the study of two‐dimensional transport in the devices may lead to a correct estimation of the alloy scattering potential.

Piezoelectric scattering in quantised surface layers in semiconductors

A theory of electron mobility for a two-dimensional electron gas scattered by piezoelectric phonons is developed. The mobility values are then computed for different compound semiconductors at different lattice temperatures. It is found that piezoelectric-scattering-limited mobilities in GaAs, InSb and InAs are smaller than acoustic-phonon-scattering-limited mobility while in strong piezoelectric materials like CdS and ZnO, piezoelectric scattering is more dominant.

Estimated threshold base current and light power output of a transistor laser with InGaAs quantum well in GaAs base

We have solved the continuity equation for electrons in the base of an InGaP?GaAs?GaAs heterojunction bipolar transistor laser (TL) in which the position of an InGaAs quantum well (QW) in GaAs base is variable. The injected minority carrier is related to the two-dimensional carrier in QW via virtual states (VSs). The values for optical gain in the QW are obtained by considering subband energies and envelope functions in presence of strain, polarization dependent momentum matrix element and Lorentzian lineshape. Relating the gain with threshold current and the latter with base current via VS current, the threshold base current and power output from the TL are estimated. Good agreement between the calculated and the experimental threshold base currents is obtained and the match for light output power is satisfactory within experimental uncertainty. Our calculated charge distribution in the base shows similar behaviour as in the charge control analysis of the experimental data.

Modelling of threshold voltage and subthreshold slope of strained-Si MOSFETs including quantum effects

In this paper, the threshold voltage and subthreshold slope of strained-Si channel n-MOSFETs are determined, taking into account quantum-mechanical effects, and the effect of bandgap narrowing due to heavy channel doping and the effect of surface roughness at the Si/SiGe heterointerface for ultra thin channels. Quantum-mechanical effects have been incorporated by considering three components, (1) the modified subband energy of 2D inversion layer charges at the silicon dioxide–silicon interface, (2) the increased effective oxide thickness and (3) the altered value of ground state energy due to surface roughness. The analytical results of threshold voltage and threshold voltage difference are presented with reference to unstrained-Si channel for strained-Si MOSFETs by employing poly-Si gate and titanium nitride gate, the work function of which can be varied over a wide range. In addition, we have predicted the dependence of threshold voltage on different values of oxide thickness, channel doping concentration, and on the molar content, x, of Ge in the Si1−xGex virtual substrate. When compared with the theoretical data of Nayfeh et al our analytical results agree more closely with our experimental results and also with measured and simulated data of threshold voltage for a wide range of devices available in the literature. Furthermore, we have calculated the subthreshold slope of strained-Si channel MOSFETs for different amounts of Ge in the SiGe layer.

Design and Modeling of GeSn-Based Heterojunction Phototransistors for Communication Applications

We propose the use of Ge1-xSnx heterojunction phototransistors (HPTs) as efficient optical receivers on Si substrates and analyze their performance. Our designs use n-Ge/p-Ge1-xSnx/n-Ge1-xSnx layers pseudomorphically grown on Si wafers via a Ge virtual substrate, which offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology. By incorporating Sn into the Ge photon-absorbing layer to shrink the bandgap, the photodetection range can be significantly extended to the mid-infrared (MIR) region with a considerably enhanced optical response. The use of HPT structures provides optical conversion gain to further enhance the optical responsivity, thereby enabling efficient photodetection in the short-wave infrared region. We develop theoretical models to calculate the composition-dependent band alignments, the band structures (by taking into account the nonparabolic effect), the absorption coefficient, and the optical responsivity for the proposed GeSn HPTs. As the Sn content increases, the conduction band nonparabolicity becomes increasingly significant and considerably impacts the optical absorption coefficient. Moreover, analysis of the spectral response for the Ge1-xSnx HPTs shows that efficient photodetection covering the entirety of the fiber-optic telecommunication bands, as well as the emerging 2-μm MIR communication band, can be achieved. These results indicate that the proposed Ge1-xSnx HPTs are attractive for use as high-responsivity CMOS-compatible photodetectors in communication applications.

Polar optic phonon scattering of two dimensional electron gas in a rectangular potential well

Abstract The mobility of a two-dimensional electron gas (2DEG) in a rectangular potential well formed in a AlGaAs/GaAs/AlGaAs structure scattered by polar-optic phonon is calculated by an iterative solution of Boltzmann equation. The values are significantly different from those calculated by using a relaxation time. The polar-optic mobility is found to dominate over acoustic mobility over a temperature range of 100–300 K when the well thickness is about 10 nm. The mobility values for 2DEG are, however, found to be lower than the bulk values.

Modeling Resonance-Free Modulation Response in Transistor Lasers With Single and Multiple Quantum Wells in the Base

We have developed the expressions for terminal currents in transistor lasers (TLs) having a single quantum well (SQW) as well as multiple quantum wells (MQWs) of different well and barrier widths in the base by solving a continuity equation relating the bulk carrier density with the 2-D carrier density via virtual states (VS). The gain in the quantum well (QW) is obtained by considering strain, 2-D density-of-states, polarization-dependent momentum matrix element, Fermi statistics, and Lorentzian broadening. A calculated value of 7.06 mA of threshold base current for three 16-nm-wide QWs in the base indicates a substantial reduction from the calculated and experimental value of 21.5 mA for a 16-nm-wide InGaAs QW in GaAs base. A similar reduction is also obtained for three QWs of different widths having variable barrier widths. The estimated modulation bandwidths (BWs) are higher in the case of MQW structures than in the SQW TL. Above threshold, the effective base recombination time, including spontaneous and stimulated processes, gives rise to a fast recombination process in the base, which leads to resonance-free modulation response. The estimated recombination time compares favorably with the value reported from the analysis of experimental data.

Modeling of current gain compression in common emitter mode of a transistor laser above threshold base current

We have obtained the expressions for the terminal currents in a heterojunction bipolar transistor laser the base of which contains a quantum well (QW). The emitter-base junction is assumed to be abrupt, leading to abrupt discontinuity in quasi-Fermi level at the interface. The expressions for the terminal currents as a function of collector-emitter and base-emitter voltages are obtained from the solution of the continuity equation. The current density in the QW located at an arbitrary position in the base is related to the virtual state current density. The threshold current density in the QW is calculated by using the expression for gain obtained from Fermi golden rule. The plot of collector current (IC) versus collector-emitter voltage (VCE) for different values of base current shows the usual transistor characteristics, i.e., a rising portion after a cut-in VCE, and then a saturation behavior. The dc current gain remains constant. However, as the base current exceeds the threshold, a stimulated recombi...

Semiconductor laser theory

Introduction to Semiconductor Lasers Brief History Principle of Lasers Semiconductor Laser Materials for Semiconductor Lasers Special Features Applications Basic Theory Introduction Band Structure E-k Diagram and Effective Mass Density of States Carrier Concentration Intrinsic and Extrinsic Semiconductor Transport of Charge Carriers Excess Carriers Diffusion and Recombination: The Continuity Equation Basic p-n Junction Theory I-V and Capacitance-Voltage Characteristics of p-n Junction Heterojunctions and Quantum Structures Introduction Alloys Heterojunctions Quantum Structures Quantum Wells Quantum Wires and Quantum Dots Strained Layers Band Structures Introduction Band Theory: Bloch Functions The k.p Perturbation Theory Neglecting Spin Spin-Orbit Interaction Strain-Induced Band Structure Quantum Wells Waveguides and Resonators Introduction Ray Optic Theory Reflection Coefficients Modes of a Planar Waveguide Wave Theory of Light Guides 3-D Optical Waveguides Resonators Optical Processes Introduction Optical Constants Absorption Processes in Semiconductors Fundamental Absorption in Direct Gap Intervalence Band Absorption (IVBA) Free-Carrier Absorption Recombination and Luminescence Nonradiative Recombination Carrier Effect on Absorption and Refractive Index Excitons Models for DH Lasers Introduction Gain in DH Lasers Threshold Current Effect of Electric Field in Cladding on Leakage Current Gain Saturation Rate Equation Model Rate Equations: Solution of Time-Dependent Problems Modulation Response Temperature Dependence of Threshold Current Quantum Well Lasers Introduction Structures Interband Transitions Model Gain Calculation: Analytical Model Recombination in QWs Loss Processes in QW Lasers MQW Laser Modulation Response of QW Lasers Strained QW Lasers Type II Quantum Well Lasers Tunnel-Injection QW Laser Quantum Dots Introduction QD Growth Mechanisms and Structures Introductory Model for QD Lasers Deviation from Simple Theory: Effect of Broadening Subband Structures for Pyramidal QDs Refined Theory for Gain and Threshold Modulation Bandwidth: Rate Equation Analysis Tunnel-Injection QD Lasers Quantum Cascade Lasers Introduction A Brief History Basic Principle Improved Design of Structures Nonradiative Inter- and Intrasubband Transitions Some Design Issues Frequency Response Terahertz QCL QD QCL Vertical-Cavity Surface-Emitting Laser Introduction Structures and Basic Properties Elementary Theory of VCSEL Requirements for Components Characteristics of VCSELs Modulation Bandwidth Temperature Dependence Tunnel Junction QD-VCSEL Microcavity Effects and Nanolasers Single-Mode and Tunable Lasers Introduction Need for Single-Mode Laser Limitation of FP Laser Distributed Feedback DBR Laser DFB Laser Tunable Lasers Characteristics of Tunable Lasers Methods and Structures for Continuous and Discontinuous Tuning Tunable Vertical-Cavity Surface-Emitting Laser Nitride Lasers Introduction Polar Materials and Polarization Charge Quantum-Confined Stark Effect Early Work and Challenges Some Useful Properties of Nitrides First Laser Diode Violet c-Plane Laser Blue and Green Lasers Nonpolar and Semipolar Growth Planes Group IV Lasers Introduction Need for Si (Group IV) Lasers Problems Related to Group IV Semiconductors: Indirect Gap Recent Challenges Use of Heterostructure for Direct Bandgap Type I Structure Ge Laser at 1550 nm Mid-Infrared Laser Based on GeSn Incorporation of C Transistor Lasers Introduction Structure and Basic Working Principle Principle of Operation: Model Description Gain Compression Frequency Response Appendix I Appendix II Problems, a Reading List, and References appear at the end of each chapter.

Analytical theory of a small signal modulation response of a transistor laser with dots-in-well in the base

We have developed the theory for the threshold base current, light power output and small signal modulation response of a transistor laser, the base of which contains a quantum well (QW) with a layer of quantum dots (QDs) inserted within it. Our theory involves the solution of the continuity equation for an arbitrary location of the QW in the base, and the solution of coupled rate equations for carriers and photons in the QD. The excitonic recombination model is assumed to be valid in the QD. The estimated threshold base current for InAs QDs embedded in the InGaAs QW is found to be smaller than the value for the InGaAs QW in the GaAs base. The values of the modulation bandwidths for different values of the base current however do not reach the high values obtained for the QW system.