An analytical threshold voltage and subthreshold current model forshort-channel AlGaAs/GaAs MODFETs

Short-channel effects on the subthreshold behavior are modeled in self-aligned gate AlGaAs/GaAs MODFETs through an analytical solution of the two-dimensional Poisson equation in the subthreshold region. Based on the resultant potential solution, simple and accurate analytical expressions for short-channel threshold voltage, subthreshold swing, and subthreshold drain current are derived. These are then used to develop an expression for minimum acceptable channel length. A comparative study of short-channel effects in enhancement-mode MODFETs with and without i-AlGaAs spacer layers indicates that channel lengths will be limited to 0.18-0.25 μm by subthreshold conduction. Minimum gate lengths for MODFETs with a spacer layer are notably larger than those without a spacer layer. Besides offering insights into the physics of short-channel effects in MODFETs, the model provides a useful basis for efficient design, analysis, and simulation of small geometry AlGaAs/GaAs MODFET digital circuits     

Experimental and theoretical investigation of the DC andhigh-frequency characteristics of the negative differential resistancein pseudomorphic AlGaAs/InGaAs/GaAs MODFET's

The authors investigated the negative differential resistance (NDR) in the I-V characteristics of pseudomorphic AlGaAs/InGaAs/GaAs modulation doped field-effect transistors (MODFETs) with gate lengths of 0.3 μm. They experimentally verified the existence of abrupt multiple NDR in both the input circuit and the output circuit. The NDR occurs over a short range of drain voltage (less than 200 mV) and gate voltage (less than 5 mV) for NDR induced by thermionic emission. The authors provide a general interpretation of the measured DC results based on tunneling real-space transfer (TRST) which occurs because of the formation of hybrid excited states across the InGaAs channel and AlGaAs donor layer. The existence of stable reflection is verified in both the input and output circuits with stable broadband frequency response in the output circuit to at least 49 GHz. These results show that NDR via TRST in pseudomorphic MODFETs can provide wideband frequency response not limited by the electron transit time from source to drain     

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     

Reduced short-channel effects in submicron N-HIGFET technologyusing sidewalls

In this letter, 0.35 μm gate length pseudomorphic AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect-transistors (HIGFETs) have been fabricated on GaAs. The short-channel effects have been reduced by using a sidewall technology. A high current density and a high transconductance were obtained, reflectively, 510 mA/mm and 550 mS/mm, in addition to a high value of extrinsic current gain cutoff frequency F_T=44 GHz. The dependencies of subthreshold current, threshold voltage, and output conductance on gate length have been emphasised     

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     

Double-side planar-doped AlGaAs/InGaAs/AlGaAs MODFET with currentdensity of 1 A/mm

AlGaAs/InGaAs/AlGaAs double-side planar-doped (DSPD) pseudomorphic MODFETs of 0.3-μm gate length with both excellent DC and RF performances are reported. A maximum unilateral gain cutoff frequency of 170 GHz and a maximum current gain cutoff frequency of 60 GHz are achieved. The devices exhibit a maximum transconductance of 500 mS/mm and an extremely high current density of 1 A/mm. These are the highest frequencies reported so far for MODFET devices capable of driving 1-A/mm current density. This current density is the highest ever reported with this type of layer structure     

Characterization of ultra-high-speed pseudomorphic AlGaAs/InGaAs(on GaAs) MODFETs

The authors report a detailed characterization of ultrahigh-speed pseudomorphic AlGaAs/InGaAs (on GaAs) modulation-doped field-effect transistors (MODFETs) with emphasis on the device switching characteristics. The nominal 0.1-μm gate-length device exhibit a current gain cutoff frequency (f_t) as high as 152 GHz. This value of f_t corresponds to a total delay of approximately 1.0 ps and is attributed to the optimization of layer structure, device layout, and fabrication process. It is shown that the electron transit time in these very short gate-length devices still accounts for approximately 60% of the total delay, and, as a result, significant improvements in switching speed are possible with further reductions of gate length. The results reported clearly demonstrate the potential of the pseudomorphic AlGaAs/InGaAs MODFET as an ultrahigh-speed device. Its excellent switching characteristics are attributed to the high saturation velocity (~2×10⁷ cm/s), 2DEG sheet density (2.5×10¹² cm^-2), and current drive capability (>200 mA/mm at the peak transconductance)     

Analytic solution of the velocity-saturated MOSFET/MODFET waveequation and its application to the prediction of the microwavecharacteristics of MODFETs

An AC model for the saturated MODFET is reported. The MOSFET/MODFET wave equation accounting for velocity saturation and channel length modulation is derived. An exact solution of the wave equation is obtained in terms of Bessel functions. A frequency power series is used to derive analytic expressions for the intrinsic Y -parameters. This AC model is applied to the prediction of the microwave characteristics of 1-μm AlGaAs/GaAs MODFETs and GaAlAs/InGaAs/GaAs pseudomorphic MODFETs. The parameters used by the AC model are extracted by fitting the I-V characteristics. The parasitics are either estimated or measured. A good prediction of the scattering parameters measured from 2 to 18.4 GHz is achieved for different biases. The deviation of the calculated unilateral power gain from the measured values was on average 1.25 and 2.18 dB for the conventional MODFET and the pseudomorphic MODFET, respectively     

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     

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     

A study of hot-electron degradation effects in pseudomorphic HEMTs

In this work we report on hot-electron stress experiments performed on commercial AlGaAs/InGaAs/GaAs pseudomorphic HEMTs. The stress cycles induce a permanent increase of the gate-drain breakdown voltage (breakdown walkout). Different characterization techniques are applied to thedevices under test, consistently indicating that the physical mechanism underlying walkout is the build-up of negative charge in the region between the gate and drain.     

Gate-controlled negative differential resistance in drain currentcharacteristics of AlGaAs/InGaAs/GaAs pseudomorphic MODFETs

The observation of negative differential resistance (NDR) and negative transconductance at high drain and gate fields in depletion-mode AlGaAs/InGaAs/GaAs MODFETs with gate lengths L _g ~0.25 μm is discussed. It is shown that under high bias voltage conditions, V_ds>2.5 V and V_gs>0 V, the device drain current characteristic switches from a high current state to a low current state, resulting in reflection gain in the drain circuit of the MODFET. The decrease in the drain current of the device corresponds to a sudden increase in the gate current. It is shown that the device can be operated in two regions: (1) standard MODFET operation for V_gs<0 V resulting in f_max values of >120 GHz, and (2) a NDR region which yields operation as a reflection gain amplifier for V_gs >0 V and V_ds>2.5 V, resulting in 2 dB of reflection gain at 26.5 GHz. The NDR is attributed to the redistribution of charge and voltage in the channel caused by electrons crossing the heterobarrier under high-field conditions. The NDR gain regime, which is controllable by gate and drain voltages, is a new operating mode for MODFETs under high bias conditions     

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.     

Relationships between common source, common gate, and common drain FETs

This paper comprehensively analyzes the relationship between common source (CS), common gate (CG), and common drain (CD) field-effect transistors (FETs). The signal and noise parameters of the CG and CD configuration can be obtained directly by using a simple set of formulas from CS signal and noise parameters. All the relationships provide a bi-directional bridge for the transformation between CS, CG, and CD FETs. This technique is based on the combination of an equivalent-circuit model and conventional two-port network signal/noise correlation matrix technique. The derived relationships have universal validity, but they have been verified at 2/spl times/40 /spl mu/m gatewidth (number of gate fingers /spl times/ unit gatewidth) double-heterojunction /spl delta/-doped AlGaAs/InGaAs/GaAs pseudomorphic high electron-mobility transistor with 0.25-/spl mu/m gate length. Good agreement has been obtained between calculated and measured results.     

Ultra low noise characteristics of AlGaAs/InGaAs/GaAs pseudomorphicHEMT's with wide head T-shaped gate

The fully passivated low noise AlGaAs/InGaAs/GaAs pseudomorphic (PM) HEMT with 0.13 μm T-shaped gate was fabricated using dose split electron beam lithography method (DSM). This device exhibited low noise figures of 0.31 and 0.45 dB at 12 and 18 GHz, respectively. These noise figures are the lowest value ever reported for the GaAs based HEMT's. These results are attributed to the extremely low gate resistance which results from wide head T-shaped gate having the higher ratio more than 10 of gate head length to gate footprint     

Reverse modeling of E/D logic submicrometer MODFETs and predictionof maximum extrinsic MODFET current gain cutoff frequency

A method for estimating the source resistance, fringe capacitance, gate length, and effective saturation velocity from the microwave Y -parameters of MODFETs with known vertical structure is discussed. The scheme is applied to a variety of MODFETs fabricated on molecular-beam-epitaxial (MBE) material using a submicrometer enhancement/depletion- (E/D-) mode IC process. More than 100 MODFETs were measured and analyzed. Both the values and variances of the extracted parameters are very physical. In particular, it is found that the extracted saturation velocity (1) is independent of the gate length in the regime studied (0.25-0.91 μm); (2) is rather independent of process and threshold voltage variations; (3) is marginally higher when the Al mole fraction is increased from 20% to 28%; (4) is not significantly higher in pseudomorphic InGaAs than in GaAs; and (5) is quite a bit higher than is often assumed or extracted, with a value close to the stationary peak velocity in undoped GaAs. There is little sign of overshoot above this limit. Using the extracted peak velocity and a simple analytical MODFET model, the extrinsic current gain cutoff frequency (f_Tx) is predicted well in the gate-length regime studied     

Quantum-well p-channel AlGaAs/InGaAs/GaAs heterostructureinsulated-gate field-effect transistors with very high transconductance

Quantum-well p-channel pseudomorphic AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors with enhanced hole mobility are described. The devices exhibit room-temperature transconductance, transconductance parameter, and maximum drain current as high as 113 mS/mm, 305 mS/V/mm, and 94 mA/mm, respectively, in 0.8-μm-gate devices. Transconductance, transconductance parameter, and maximum drain current as high as 175 mS/mm, 800 mS/V/mm, and 180 mA/mm, respectively were obtained in 1-μm p-channel devices at 77 K. From the device data hole field-effect mobilities of 860 cm²/V-s at 300 K and 2815 cm²/V-s at 77 K have been deduced. The gate current causes the transconductance to drop (and even to change sign) at large voltage swings. Further improvement of the device characteristics may be obtained by minimizing the gate current. To this end, a type of device structure called the dipole heterostructure insulated-gate field-effect transistor is proposed     

High-frequency noise of InyGa1?yAs/AlxGa1?xAs MODFETs and comparison to GaAs/AlxGa1?xAs MODFETs

Noise parameter measurements for recently developed 1 ?m gate InyGa1?yAs/Al0.15Ga0.85As MODFETs have been performed at 8 GHz at room and cryogenic temperatures. Owing to the relatively small Cgs/?gm ratio in these devices compared to identical conventional GaAs/AlGaAs MODFETs, both room- and cryogenic temperature noise figures have been reduced. In addition, the light sensitivity and drift in noise figure at cryogenic temperatures observed in conventional GaAs/AlGaAs MODFETs have been sub stantially reduced.     

Elevated temperature performance of pseudomorphic AlGaAs/InGaAsMODFETs

Parametric DC measurements on pseudomorphic AlGaAs/InGaAs modulation-doped field-effect transistors (MODFETs) were carried out over the 300-405 K temperature range. A gradual channel device model was developed to simulate the temperature dependent behavior and assist in the interpretation of the characteristics. The simulations are shown to provide good predictive ability and confirm the physical reasons why the zero temperature coefficient point of a MODFET occurs only for gate bias voltages below the threshold voltage     

Observation of high-frequency high-field instability inGaAs/InGaAs/AlGaAs DH-MODFETs at K band

The authors report the observation of high-field instability at room temperature with oscillation frequency as high as 24 GHz in GaAs/InGaAs/AlGaAs double-heterojunction-MODFETs (DH-MODFETs) of 1.2 μm gate length. Negative drain differential resistance was also observed in these devices under various forward gate biases. The nature of this instability is believed to be caused by the efficient removal of the real-space transferred hot two-dimensional electrons in the AlGaAs layer through the forward-biased Schottky gate. A tuned oscillator, with a fundamental oscillation frequency as high as 19.68 GHz, has also been demonstrated at a gate bias of 1.3 V