Charge control, DC, and RF performance of a 0.35-μmpseudomorphic AlGaAs/InGaAs modulation-doped field-effecttransistor

The authors describe a study of charge control in conjunction with DC and RF performance of 0.35-μm-gate-length pseudomorphic AlGaAs/InGaAs MODFETs. Using C-V measurements, they estimate that a two-dimensional electron gas (2DEG) with density as high as 1.0×10¹² cm^-2 can be accumulated in the InGaAs channel at 77 K before the gate begins to modulate parasitic charges in the AlGaAs. This improvement in charge control of about 10-30% over a typical AlGaAs/GaAs MODFET may partially be responsible for the superior DC and RF performance of the AlGaAs/InGaAs MODFET. At room temperature, the devices give a maximum DC voltage gain g _m/g_d of 32 and a current gain cutoff frequency f_T of 46 GHz. These results are state of the art for MODFETs of similar gate length     

Pseudo two-dimensional simulation of a three heterojunction GaAs/AlGaAs MODFET

A new model is presented for the simulation of the d.c. characteristics of three heterojunctions AlGaAs/GaAs Field Effect Transistors. The dependence of the carrier densities in the GaAs wells and in the doped AlGaAs layers is derived as a function of gate voltage by analytical resolution of Schrödinger equation and numerical resolution of Poisson's equation using Fermi-Dirac statistics. The simulated d.c. characteristics are obtained by numerical integration of the current density from source to drain and include the source and drain access resistances. The comparison between experimental measurements and calculated results for both long (L_g = 20 μm) and short (L_g = 2 μm) gate lengths MODFETs is excellent and demonstrates the validity of the model to optimize the device parameters. The influence of various parameters such as the AlGaAs layer thickness and electron velocity in the inverted well on the device performance is numerically simulated for a 1 μm gate length device. Their effect on the transistor characteristics such as transconductance and maximum drain current is discussed.     

Current—voltage characteristics of an AlGaAs/GaAs heterostructure FET for high gate voltages

Drain-to-source current for high gate voltages in AlGaAs/GaAs heterostructure FET's (HFET's) is found to depend on the electron saturation velocity in the AlGaAs layer. A simple model, which takes into account the current through the undepleted channel in the AlGaAs layer as a function of the electron saturation velocity in AlGaAs, is proposed for describing I-V characteristics of HFET's for high gate voltages. Using the model, effective electron saturation velocity in AlxGa1-xAs for different Al content levels has been obtained from the analysis of the present experimental results; 7 × 10⁶cm/s for x = 0.24 and 3 × 10⁶cm/s for x = 0.3 at a 4 × 10¹⁷cm^-3doping concentration.     

A noise model for high electron mobility transistors

A model to explain the noise properties for AlGaAs/GaAs HEMT's, AlGaAs/InGaAs/GaAs pseudomorphic HEMT's (P-HEMT's) and GaAs/AlGaAs inverted HEMT's (I-HEMT's) is presented. The model Is based on a self-consistent solution of Schrodinger and Poisson's equations. The influence of the drain-source current, frequency and device parameters on the minimum noise figure F_min and minimum noise temperature T_min, for different HEMT structures are presented. The study shows that P-HEMT's have a better noise performance than the normal and inverted HEMT's. The present model predicts that a long gate P-HEMT device will exhibit a better noise performance than a conventional HEMT. There is a range of doped epilayer thickness where minimum noise figure is a minimum for pseudomorphic HEMT's which is not observed in conventional and inverted HEMT's. The calculated noise properties are compared with experimental data and the results show excellent agreement for all devices     

Wide-bandwidth electron bolometric mixers: a 2DEG prototype andpotential for low-noise THz receivers

A new type of electron bolometric (hot electron) mixer is presented. The authors have demonstrated a three-order-of-magnitude improvement in the bandwidth compared with previously known types of electron bolometric mixers, by using the two-dimensional electron gas (2DEG) medium at the hetero-interface between AlGaAs and GaAs. Both in-house, MOCVD-grown material and MBE material were tested with similar results. The conversion loss (L_c) at 94 GHz is presently 18 dB for a mixer operating at 20 K, and calculations indicate that L_c can be decreased to about 10 dB in future devices. Calculated and measured curves of L_c versus P_Lo and I_DC, respectively, agree well. It is argued that there the several different known configurations of electron bolometric mixers will all show wide bandwidth, and that these devices are likely to become important as low-noise terahertz receivers in the future     

A charge control and current-voltage model for inverted MODFET's

An analytical model is used to investigate properties of the two-dimensional electron gas (2DEG) confined in a GaAs/AlGaAs quantum well (QW) formed in a inverted modulation doped field effect transistor (MODFET). The position of the Fermi level and the average distance of the carriers in the well have been calculated as a function of the 2DEG concentration, n_s. A charge control model is presented based on the self-consistent solution of Schrodinger and Poisson's equation. The results show a unique behavior of the average distance of the 2DEG which increases with n_s, a property unique to these type of structures. The analysis is extended to model current-voltage characteristics     

High/fmax collector-up AlGaAs/GaAsheterojunction bipolar transistors with a heavily carbon-doped basefabricated using oxygen-ion implantation

AlGaAs/GaAs collector-up heterojunction bipolar transistors (HBTs) with a heavily carbon-doped base layer were fabricated using oxygen-ion implantation and zinc diffusion. The high resistivity of the oxygen-ion-implanted AlGaAs layer in the external emitter region effectively suppressed electron injection from the emitter, allowing collector current densities to reach values above 10⁵ A/cm ². For a transistor with a 2-μm×10-μm collector, f_T was 70 GHz and f_max was as high as 128 GHz. It was demonstrated by on-wafer measurements that the first power performance of collector-up HBTs resulted in a maximum power-added efficiency of as high as 63.4% at 3 GHz     

Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolartransistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 μm². The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF₃. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5×2 μm² at submilliampere collector currents. The maximum f_T and f_max obtained from a 0.5×2 μm² emitter HBT are 46 and 42 GHz, respectively at I_C=1.5 and more than 20 GHz even at I_C=0.1 mA     

Enhancement-mode power heterojunction FET utilizingAl0.5Ga0.5As barrier layer with negligibleoperation gate current for digital cellular phones

We have developed a novel enhancement-mode double-doped AlGaAs/InGaAs/AlGaAs heterojunction FET (HJFET) with a 5 nm thick Al_0.5Ga_0.5As barrier layer inserted between an In _0.2Ga_0.8As channel layer and an upper Al_0.2 Ga_0.8As electron supply layer. The Al_0.5Ga _0.5As barrier layer reduces gate current under high forward gate bias voltage, resulting in a high forward gate turn-on voltage (V _F) of 0.87 V, which is 170 mV higher than that of an HJFET without the barrier layer. Suppression of gate current assisted by a parallel conduction path in the upper electron supply layer was found to be also important for achieving the high V_F. The developed device exhibited a high maximum drain current of 300 mA/mm with a threshold voltage of 0.17 V. A 950 MHz PDC power performance was evaluated under single 3.5 V operation. An HJFET with a 0.5 μm long gate exhibited 0.92 W output power and 63.6% power-added efficiency with 0.08 mA gate current (I_g) at -48 dBc adjacent channel leakage power at 50 kHz off-center frequency. This I_g is one-thirteenth to that of the HJFET without the barrier layer. These results indicate that the developed enhancement-mode HJFET is suitable for single low voltage operation power applications     

Relaxation study of AlGaAs cladding layers in InGaAs/GaAs (111)B lasers designed for 1.0-1.1 μm operation

InGaAs/GaAs-based lasers require thick AlGaAs cladding layers to provide optical confinement. Although the lattice mismatch between GaAs and AlGaAs is very low, relaxation may occur due to the thickness requirement for an AlGaAs waveguide of the order of microns. We have studied the relaxation of InGaAs/GaAs lasers with AlGaAs waveguides grown on GaAs (111)B substrates. We have observed by transmission electron microscopy (TEM) that certain AlGaAs layers show a high density of threading dislocations (TDs), whilst other AlGaAs layers remain essentially dislocation free. To explain the experimental results a model based on dislocation multiplication has been developed. TDs in the AlGaAs cladding layers are observed when the critical layer thickness (CLT) for dislocation multiplication has been overcome. Consequently, a design rule based on a modified CLT model for AlGaAs/GaAs (111)B is proposed.     

Energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells

We have studied electron energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells using mobility measurements as a function of electric field and temperature, in the range 3K to 300K. The results in the range 3 to 20K show a power loss rate which is dependent on (T_e − T_l), suggesting that the energy relaxation occurs through acoustic phonon scattering. At electron temperatures greater than 20K, the experimental results are modelled using a standard expression for polar optical phonons. This modelling yields 30meV and 31meV for the polar optical phonon energy in GaAs and InGaAs respectively.     

An analytical and computer-aided model of the AlGaAs/GaAs high electron mobility transistor

An analytical model for the AlGaAs/GaAs high electron mobility transistor (HEMT) or MODFET has been developed. This model uses a Trofimenkoff-type relation [1] for the electron velocity and electrical field and assumes that the electron velocity saturation inside the two-dimensional electron gas channel cause current saturation. It also takes into account the parasitic conduction in the AlGaAs layer by including a MESFET operation. Based on this model, analytical current-voltage equations suitable for computer simulation have been derived. Calculated results for sub-half-micrometer HEMT's show excellent agreement with measured characteristics.     

Epitaxial lift-off GaAs HEMT's

An extensive study of epitaxial lift-off (ELO) Al_0.3Ga _0.7As/GaAs modulation doped heterostructure high electron mobility field-effect transistors (HEMT's) is presented. Effects of ELO on electron transport properties of two-dimensional electron gas at AlGaAs/GaAs interface are investigated. An ELO HEMT with 1.5 μm gate length had a maximum extrinsic transconductance g_m-max=125 mS/mm, a unity current gain cut-off frequency f_t=10.5 GHz, and a maximum frequency of oscillation f_max=12 GHz. Statistical distributions of maximum intrinsic transconductance of ELO HEMT's are presented and compared with their on-wafer counterparts. Stability of the ELO HEMT's has also been evaluated by continuous operation at room temperature under dc bias     

Electron space-charge effects on high-frequency performance ofAlGaAs/GaAs HBTs under high-current-density operation

The high-frequency characteristics of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) under high-current-density biasing condition are investigated in conjunction with the electron space charge in the collector depletion layer. A significant increase in cutoff frequency f_t and maximum oscillation frequency f_max at the early stage of the base push-out was observed in HBTs with a lightly doped n-type collector structure, and is attributed to the collector depletion layer widening and the enhancement of the velocity overshoot effect caused by the increasing electron density     

Current-gain cutoff frequency comparison of InGaAs HEMTs

The current-gain cutoff frequency performance of pseudomorphic InGaAs/AlGaAs (20% InAs composition) high-electron-mobility transistors (HEMTs) on GaAs is compared to that of lattice-matched InGaAs/InAlAs HEMTs on InP. The current-gain cutoff frequency (f_t) characteristics as a function of gate length (L_g) indicate that the f_t-L_g product of ~26 GHz-μm in InGaAs/InAlAs HEMTs is 23% higher than that of ~21 GHz-μm in InGaAs/AlGaAs HEMTs. The performance of InGaAs/AlGaAs HEMTs is also 46% higher than that of conventional GaAs/AlGaAs HEMTs (~18 GHz-μm). These data are very useful in improving the device performance of HEMTs for a given gate length     

Recessed-gate AlGaAs-InGaAs-GaAs pseudomorphic HEMT withSi-planar-doped etch stop layer

An improved slot etch technique based on an Si planar doped layer has been applied to gate recessing in the fabrication of AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs). The devices exhibited comparable g_m with much better breakdown and leakage behaviour than conventional pseudomorphic HEMT devices     

Accurate modeling for parasitic source resistance intwo-dimensional electron gas field-effect transistors

An accurate two-layer model has been developed for parasitic source resistance in two-dimensional electron gas field-effect transistors (2DEGFETs). In this model, the 2DEG concentration-voltage and current density-voltage relations at the cap/barrier/2DEG junction are taken into account, based on the self-consistent charge control model and effective mass tunneling theory. Empirical 2DEG velocity field characteristics are also included. To show the feasibility of this method, the source resistance in conventional GaAs/AlGaAs 2DEGFETs was analyzed and the AlGaAs thickness dependence was discussed. For comparison, the lattice-matched GaInAs/AlInAs 2DEGFET and pseudomorphic GaInAs/AlGaAs 2DEGFET were examined. It was shown that introducing a highly doped cap layer leads to a drastic reduction in source resistance for pseudomorphic 2DEGFETs but has a very small effect for GaInAs/AlGaAs 2DEGFETs     

Utilization of an electron beam resist process to examine theeffects of asymmetric gate recess on the device characteristics ofAlGaAs/InGaAs PHEMTs

The DC and microwave characteristics of two sets of AlGaAs/InGaAs PHEMTs having a gate length of 0.2 μm are compared. The first set is composed of devices fabricated using a trilayer electron beam resist process for T-gate recess and metallization. The second set is composed of devices fabricated using a new four-layer electron beam resist process which enables the asymmetric placement of a T-gate in a wide recess trench. Devices fabricated using the four-layer resist process showed improved breakdown voltage, lower gate-drain feedback capacitance, lower output conductance, and higher f_max with only slight reduction of drain current and transconductance. For example, the off-state drain-source breakdown voltage increased from 5.2 to 12.5 V, and the f_max, increased from 133 to 158 GHz as the drain side cap recess, L_ud, was increased from 0 to 0.55 μm     

AlInGaAs/AlGaAs应变量子阱增益特性研究

采用Shu Lien Chuang方法计算了AlInGaAs/AlGaAs应变引起价带中重、轻空穴能量变化曲线,在Harrison模型的基础上详细地计算了AlInGaAs/AlGaAs和GaAs/AlGaAs量子阱电子、空穴子能级分布并且进一步研究了这两种材料在不同注入条件下的线性光增益.进一步计算比较可以得出AlInGaAs/AlGaAs应变量子阱光增益特性要优于GaAs/AlGaAs非应变量子阱增益特性,因此AlInGaAs/AlGaAs应变量子阱半导体材料应用于半导体激光器比传统GaAs/AlGaAs材料更具优势.     

A comparative study of temperature sensitivity of InGaAsP andAlGaAs MQW lasers using numerical simulations

We used numerical simulation to compare the temperature sensitivity of an InGaAsP MQW laser emitting at 1.55 μm and an AlGaAs MQW laser at 0.82 μm. By artificially changing the InGaAsP laser gradually into a structure similar to the AlGaAs laser, we gained quantitative insight into how each material or structural parameter causes the relatively low T₀ of the InGaAsP MQW laser. Using a typical MQW structure we demonstrated the relative importance of parameters involving Auger recombination, current leakage over the quantum barrier, optical confinement and band offset. We found that if these parameters were made the same as the AlGaAs laser, the T₀ of the InGaAsP laser was even better than that of the AlGaAs laser. Our numerical simulation confirmed that the Auger recombination is the main cause of low T₀ in MQW InGaAsP lasers. We also discovered that thermal current leakage over the barrier and Auger recombinations are correlated with each other and both factors must be improved to increase the T₀ of InGaAsP lasers to that of AlGaAs lasers