Modeling of short-pulse threshold voltage shifts due to DX centersin AlxGa1-xAs/GaAs and AlxGa1-xAs/InyGa1-yAs MODFET's

The DX-center-related short-pulse threshold voltage shifts (SPTVS) in Al_xGa_1-xAs-based MODFETs is modeled using CBAND, a simulator that solves Poisson equations self-consistently with Schrodinger equations and donor statistics. Using values given in the literature for the DX energy level in Al_xGa_1-xAs this technique gives good agreement between measured and simulated SPTVS for Al_0.3Ga_0.7As/GaAs and Al_0.3Ga_0.7As/In_0.2Ga_0.8As MODFETs. Both simulation and experiment show that the use of Al_0.2 Ga_0.8As in the donor layer reduces the SPTVS relative to the structures using Al_0.3Ga_0.7As. However, the measured shifts at this composition are considerably lower than the simulated values, indicating a DX energy level that may be higher than the value extrapolated from the literature, possibly due to the existence of multiple trap levels. Despite this discrepancy, these results support the use of strained-channel layers and lower Al_x Ga_1-xAs compositions in MODFETs for digital and other large-signal applications requiring good threshold stability     

Characterization of single quantum wells on GaAs/Si grown bymetalorganic chemical vapor deposition

The use of Al_0.5Ga_0.5As/Al_0.55Ga _0.45P intermediate layers and in-situ thermal cycle annealing are shown to be effective in obtaining smooth and sharp heterointerfaces for Al_0.3Ga_0.7As/GaAs single quantum wells (SQWs) grown on Si substrates, as a result of an initial two-dimensional growth and a reduction in threading dislocation density. The best lasing characteristics are obtained in Al_0.3Ga_0.7As/GaAs SQW laser diodes fabricated using the above structure in comparison to those fabricated using the more conventional two-step growth technique     

GaAs/Al0.3Ga0.7As resonant tunneling diodes with atomically flat interfaces grown on (411)A GaAs substrates by MBE

Al_0.3Ga_0.7As (10 nm)/GaAs (5 nm)/Al_0.3Ga_0.7As (10 nm) double barrier resonant tunneling diodes (RTDs) have been fabricated on a (411)A GaAs substrate by molecular beam epitaxy (MBE), which has atomically flat GaAs/Al_0.3Ga_0.7As interfaces over a device area. The (411)A RTDs showed a larger peak to valley current ratio by about 40% at 77K comparing to RTDs grown on a conventional (100) GaAs substrate simultaneously. Reduction of the valley current (0.61 times smaller) is the main cause for this larger peak to valley current ratio, which is probably due to improved interface roughness and defects of the (411)A RTDs.     

Electrical characteristics of an optically controlled N-channelAlGaAs/GaAs/InGaAs pseudomorphic HEMT

Electrical characteristics of an n-channel Al_0.3Ga_0.7As/GaAs/In_0.13Ga_0.87 As pseudomorphic HEMT (PHEMT) with L_g=1 μm on GaAs are characterized under optical input (P_opt). Gate leakage and drain current have been analyzed as a function of V_GS, V _DS, and P_opt. We observed monotonically increasing gate leakage current due to the energy barrier lowering by the optically induced photovoltage, which means that gate input characteristics are significantly limited by the photovoltaic effect. However, we obtained a strong nonlinear photoresponsivity of the drain current, which is limited by the photoconductive effect. We also proposed a device model with an optically induced parasitic Al_0.3Ga_0.7As MESFET parallel to the In_0.13Ga_0.87As channel PHEMT for the physical mechanism in the drain current saturation under high optical input power     

Numerical modeling of energy balance equations in quantum well AlxGa1-xAs/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 _xGa_1-xAs quantum well agree over a wide range of current and voltage, The GaAs/Al_xGa_1-xAs 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_x Ga_1-xAs, 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_0.35Ga_0.65As/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_0.35Ga_0.65As or bulk GaAs material     

Back-gated field effect in a double heterostructuremodulation-doped field-effect transistor

An Al_0.3Ga_0.7As/GaAs/Al_0.3Ga_0.7 As double heterojunction field-effect transistor has been fabricated, the novel feature being a pn junction back gate. A device with 2 μm channel length has yielded a change in transconductance by a factor of 2 for a change in back gate voltage of 1 V. The performance of this device shows that this approach could be used in realising novel devices, such as velocity modulation transistors. Also, the change in threshold voltage with back gate bias could be useful in implementing digital circuits     

Short pulse transfer characteristics of AlxGa1-xAs/GaAs andAlxGa1-xAs/InyGa1-yAsmodulation-doped heterojunction FET's

Short-pulse drain current versus gate voltage transfer characteristics measured for modulation-doped HFETs (MODFETs) with four donor-layer-channel-layer combinations-(1) Al_0.3Ga_0.7 As-GaAs, (2) Al_0.2Ga_0.8As-GaAs, (3) Al_0.3Ga_0.7As-In_0.2Ga_0.8As, and (4) Al_0.2Ga_0.8As-In_0.2 a_0.8 As-are compared with the DC transfer characteristics. The measurements are relevant to high-speed switching in HFET circuits. Significant shifts in threshold voltage are observed between the DC and short-pulse characteristics for the structures with n⁺-Al_0.3Ga_0.7As donor layers, while the corresponding shifts for structures with n⁺-Al_0.2Ga_0.8As donor layers are relatively small or virtually nonexistent     

Single- and double-heterojunction pseudomorphic In0.5(Al0.3Ga0.7)0.5P/In0.2Ga0.8As high electron mobility transistors grown by gas sourcemolecular beam epitaxy

In_0.5(Al_0.3Ga_0.7)_0.5 P/In_0.2Ga_0.8As single- and double-heterojunction pseudomorphic high electron mobility transistors (SH-PHEMTs and DH-PHEMTs) on GaAs grown by gas-source molecular beam epitaxy (GSMBE) were demonstrated for the first time. SH-PHEMTs with a 1-μm gate-length showed a peak extrinsic transconductance g_m of 293 mS/mm and a full channel current density I_max of 350 mA/mm. The corresponding values of g_m and I_max were 320 mS/mm and 550 mA/mm, respectively, for the DH-PHEMTs. A short-circuit current gain (H₂₁) cutoff frequency f_T of 21 GHz and a maximum oscillation frequency f_max of 64 GHz were obtained from a 1 μm DH device. The improved device performance is attributed to the large ΔE_c provided by the In_0.5(Al_0.3Ga_0.7)_0.5P/In _0.2Ga_0.8As heterojunctions. These results demonstrated that In_0.5(Al_0.3Ga_0.7)_0.5P/In _0.2Ga_0.8As PHEMT's are promising candidates for microwave power applications     

Electron capture in AlGaAs/AlAs/GaAs double-barrier quantum well structures: Tunneling versus intervalley scattering

We have investigated electron capture times in n-type GaAs/AlAs/Al_0.3Ga_0.7As double-barrier quantum well (DBQW) structures in an electric field along the growth direction. To this end, the time dependence of the photoluminescence (PL) originating from the GaAs wells and the Al_0.3Ga_0.7As barrier layers is analyzed numerically. Theoretical electron capture times are calculated for the investigated DBQW's for the case of several scattering mechanisms. Electron capture mainly occurs via Γ-X intervalley transfer under participation of X-point subbands located at the AlAs tunnel barriers. Good agreement between experiment and theory is obtained when assuming Γ-X backfolding, while impurity scattering, optical phonon-assisted Γ-tunneling, and deformation potential scattering give rise to much larger electron capture times.     

Enhanced spontaneous emission in hydrogen-plasma-passivatedAlGaAs/GaAs vertical-cavity surface-emitting laser structures grown onSi substrate

The effect of hydrogen (H) plasma passivation on a vertical-cavity surface-emitting laser (VCSEL) structure consisting of an active layer of Al_0.3Ga_0.7As/GaAs multi-quantum-well (MQW) grown on an Si substrate is investigated by photoluminescence (PL) measurement. Significant spontaneous emission enhancement is observed at the cavity mode for the H-plasma-passivated sample. This is attributed to the increased MQW PL emission resulting from the incorporation of hydrogen atoms which passivated the electrical activity of the defects-related nonradiative deep centres and increased the minority carrier lifetime     

Quantum-well Hall devices in Si-delta-dopedAl0.25Ga0.75As/GaAs and pseudomorphic Al0.25Ga0.75As/In0.25Ga0.75As/GaAsheterostructures grown by LP-MOCVD: performance comparisons

Characterized herein are quantum-well Hall devices in Si-delta-doped Al_0.25Ga_0.75As/GaAs and pseudomorphic Al_0.25Ga_0.75As/In_0.25Ga _0.75As/GaAs heterostructures, grown by low-pressure metal organic chemical vapor deposition method. The Si-delta-doping technique has been applied to quantum-well Hall devices for the first time. As a result high electron mobilities of 8100 cm^-2/V·s with a sheet electron density of 1.5×10¹² cm^-2 in Al_0.25Ga_0.75As/In_0.25Ga_0.75 As/GaAs structure and of 6000 cm^-2/V·s with the sheet electron density of 1.2×10¹² cm^-2 in Al_0.25Ga_0.75As/GaAs structure have been achieved at room temperature, respectively. From Hall devices in Al_0.25Ga_0.75As/In_0.25Ga_0.75 As structure, the product sensitivity of 420 V/AT with temperature coefficient of -0.015 %/K has been obtained. This temperature characteristic is one of the best result reported. Additionally, a high signal-to-noise ratio corresponding to the minimum detectable magnetic field of 45 nT at 1 kHz and 75 nT at 100 Hz has been attained. These resolutions are among the best reported results     

Fabrication and numerical analysis of AlGaAs/GaAs tandem solarcells with tunnel interconnections

High-efficiency (20.2% at 1 sun, AM 1.5) Al_0.4Ga_0.6As/GaAs tandem solar cells are successfully fabricated by molecular-beam epitaxy. The interconnection between the AlGaAs top cell and the GaAs bottom cell consists of a GaAs tunnel junction sandwiched between AlGaAs layers and provides a high-quality tunnel junction. Numerical analysis suggests that an efficiency of 30% can be realized by increasing the carrier lifetimes of AlGaAs layers to 10^-8 s. An efficiency of 35% is expected to be obtainable by replacing the GaAs tunnel junction with an Al_0.33 Ga_0.67As tunnel junction     

Insulator passivation of In0.2Ga0.8As-GaAssurface quantum wells

The electronic passivation of (100) In_0.2Ga_0.8 As-GaAs surface quantum wells (QWs) using in situ deposition of an amorphous, insulating Ga₂O₃ film has been investigated and compared to standard Al_0.45Ga_0.55As passivation. Nonradiative lifetimes τ_r=1.1±0.2 and 1.2±0.2 ns have been inferred from the dependence of the internal quantum efficiency η on optical excitation density P₀' for the Ga₂O₃ and Al_0.45Ga_0.55As passivated In_0.02 Ga_0.8As-GaAs surface QW, respectively. Beyond identical internal quantum efficiency, the amorphous Ga₂O₃ insulator passivation simplifies device processing, eludes problems arising from lattice-mismatched interfaces, and virtually eliminates band bending in electronic and optoelectronic devices based on a low dimensional system such as quantum wells, wires, and dots     

Dislocation scattering in n-type modulation dopedAl0.3Ga0.7As/InxGa1-xAs/Al0.3Ga0.7As quantum wells

The mobility due to misfit dislocation scattering in n-type modulation doped Al_0.3Ga_0.7As/In_xGa_1-xAs/Al _0.3Ga_0.7As quantum wells is discussed. Initially, the dislocations are modeled as an array of orthogonal charged lines. The scattering potential is introduced, including both the coulombic and piezoelectric components. The expression for the mobility limited by dislocation scattering is established, and the anisotropic characteristics of mobility and its variation with various material and device parameters are presented and discussed     

Al0.3Ga0.7As/GaAs HEMT's under opticalillumination

Theoretical and experimental work for the DC and RF performance of depletion mode Al_0.3Ga_0.7As/GaAs HEMTs under optical illumination is presented. The photoconductive effect increasing the 2-DEG channel electron concentration and photovoltaic effect in the gate junction are considered. Optical tuning of a 2 GHz HEMT oscillator and optical control of the gain of a 2 to 6 GHz HEMT amplifier are presented and potential applications are described     

Charge control model of the double delta-doped quantum-wellfield-effect transistor

The charge control properties of Al_0.3Ga_0.7As-GaAs-Al_0.3Ga_0.7 As quantum-well field-effect transistor (FET) with double δ-doped planes are studied theoretically. A simple capacitor-like charge control model for the doubleδ-doped quantum-well FET's has been proposed and verified through self-consistent calculation. The threshold voltage and the capacitance can be related to the structure through simple analytical equations. The effective separation between capacitor plates is found to be the thickness of AlGaAs layer d+d_i +δ plus a correction term to account for the distribution of N_2DEG inside the GaAs quantum well. For small well widths, only the ground-state subband is occupied and there is a simple linear relation between N_2DEG and the gate bias V_G. For larger well widths, electrons occupied the lowest subband, then the next higher energy subband, and the relation between the N_2DEG and the V_G can be divided into piece-wise linear regions     

Quantum-confined Stark effect modulators at 1.06 μm on GaAs

Electroabsorption modulation is achieved at or near a wavelength of 1.06 μm with In_xAl_yGa_1-x-yAs/In _xGa_1-xAs multiple-quantum-well (MQW) structures grown on GaAs substrates. The lattice mismatch (close to 2%) between the MQW and the substrate is accommodated by a compositional-step-graded buffer array. A dislocation density of less than 10⁷/cm² is estimated for the MQW region. For 80-to-100 Å well widths, a maximum electroabsorption coefficient of 8000 cm^-1 with an applied voltage of 15 V is obtained     

Improved performance of submicrometer-gate GaAs MESFETs with an Al0.3Ga0.7As buffer layer grown by metal organicvapor phase epitaxy

Submicrometer-gate MESFETs were fabricated with a GaAs active layer and an Al_xGa_1-xAs buffer layer grown by metalorganic vapor-phase epitaxy. To investigate the effect of buffer layer composition on device performance, microwave FETs with GaAs and Al _0.3Ga_0.7As buffer layers were compared. Electron Hall mobility in the n-GaAs active layer was found to be unaffected by the Al content or carrier concentration in the buffer layer. However, a considerable improvement in the maximum available gain to as much as 5.2 dB was obtained at 26.5 GHz for FETs with a p-Al_0.3Ga_0.7 As buffer layer; this was 1.5 dB higher than the gain obtained with a p-GaAs buffer layer. The improvement is due to a 20-30% reduction in both drain conductance and drain-gate capacitance     

Hot electron and DX center insensitivity ofAl0.25Ga0.75As/GaAs HFET's designed for microwavepower applications

We show in this work that, although designing Al_xGa_1-xAs/GaAs HFET's for microwave power applications requires a large barrier layer bandgap (hence x>0.2), the presence of a large concentration of electrically active DX centers in the barrier layer does not hinder the device reliability. The existence of a remarkable quantity of DX centers in the Al_0.25Ga_0.75As barrier layer is for the first time revealed by means of room temperature electroluminescence, and their concentration is evaluated by measuring the threshold voltage shift induced by hot electron stress at cryogenic temperatures     

Electrical and optical properties of deep-red top-surface-emittinglasers

By studying thermal behavior of all-MBE surface-emitting lasers, barrier heights and optimum cavity design parameters are obtained. The barrier heights for holes between hetero-interfaces of Al_0.3Ga_0.7As-Al_0.65Ga_0.35As and AlAs-Al_0.65Ga_0.35As (Δx=-0.35) are measured to be 77 meV at zero bias for the deep-red top-surface-emitting laser. The barrier height decreases linearly with forward bias voltage, explaining the nonlinearity in current-voltage characteristics of the top-surface-emitting laser. The contribution of electrons to electrical resistance is estimated to be negligibly small compared to that of holes for the structure consisting of Δ_x =0.35. Minimum threshold current and maximum differential quantum efficiency observed around 200 K indicate slight mismatch between gain maximum and Fabry-Perot resonance for the deep-red top-surface-emitting laser