MODFET Structures Research Articles

AlGaN/GaN self-aligned MODFET with metal oxide gate for millimeter wave application

As promising candidates for future microwave power devices, GaN-based high-electron mobility transistors (HEMTs) have attracted much research interest. An investigation of the operation of AlGaN/GaN n type self-aligned MOSFET with modulation doped GaN channels is presented. Liquid phase deposited (LPD) SiO2 is used as the insulating material. An analytical model based on modified charge control equations is developed. The investigated critical parameters of the proposed device are the maximum drain current (IDmax), the threshold voltage (Vth), the peak DC trans-conductance (gm), break down voltage (Vbr) and unity current gain cut-off frequency (fT). The typical DC characteristics for a gate length of 1μm with 100μm gate width are following: Imax=800mA/mm, Vbreak-down=50V, gm_extrinsic=200mS/mm, Vpinchoff=−10V. The analysis and simulation results on the transport characteristics of the MOS gate MODFET structure is compared with the previously measured experimental data. The calculated values of fT (20–130GHz) suggest that the operation of the proposed device effectively, has sufficiently high current gain cutoff frequencies over a wide range of drain voltage, which is essential for high-power performance at microwave frequencies. The proposed device offers lower on-state resistance. The results so obtained are in close agreement with the experimental data.

Kinetics of hydride disintegration in a 2D Si channel formation by the Si–GeH4 MBE and demonstration of a Si/SiGe interface blurring in electrical characteristics of heterostructures

In the present activity the properties of the Si/SiGe MODFET structure with a Si electronic transport channel in SiGe layer are studied. Larger attention is given to the interfaces of a channel and their influence on the electrical characteristics of a structure. Is shown, that in actual structures the kinetics of molecules disintegration not only determine a structure profile near interfaces, but also can promote the origin of nanostructural compositions in these areas. It can exhibit in a formation of arrays of quantum dots from one of component of solid solution on the layer boundary, resulting to a lot of effects as in longitudinal and transversal conductivity of the epitaxial structure.

High electron mobility in SiGe/Si n -MODFET structures on sapphire substrates

For the first time, SiGe/Si n-modulation doped field effect transistor (n-MODFET) structures have been grown on sapphire substrates. A room temperature electron mobility value of 1271 cm2/V-s at an electron carrier density (ne) of 1.6×1012 cm−2 was obtained. At 250 mK, the mobility increases to 13 313 cm2/V-s (ne=1.33×1012 cm−2) and Shubnikov-de Haas oscillations appear, showing excellent confinement of the two-dimensional electron gas.

Self-Aligned SiGe MOS-Gate FET with Modulation-Doped Quantum Wire Channel for Millimeter Wave Application

This paper describes a self-aligned SiGe MOS-gate field-effect transistor (FET) having a modulation-doped (MOD) quantum wire channel. An analytical model based on modified charge control equations accounting for the quantum wire channel, is presented predicting the transport characteristics of the MOS-gate MODFET structure. In particular, transport characteristics of devices having strained SiGe layers, realized on Si or Ge substrates, are computed. The transconductance gm and unity-current gain cutoff frequency (fT) are also computed as a function of the gate voltage VG. The calculated values of fT suggest the operation of one-dimensional SiGe MODFETs to be around 200 GHz range at 77°K, and 120 GHz at 300°K.

High Al-content AlGaN/GaN MODFETs for ultrahigh performance

The use of an AlGaN layer with high Al mole-fraction is proposed to increase the equivalent figures of merit of the AlGaN/GaN MODFET structure. It is shown that the room temperature mobility has little degradation with increasing Al mole-fraction up to 50%. 0.7-/spl mu/m gate-length Al/sub 0.5/Ga/sub 0.5/N/GaN MODFETs by optical lithography exhibit a current density of 1 A/mm and three-terminal breakdown voltages up to 200 V. These devices on sapphire substrates without thermal management also show CW power densities of 2.84 and 2.57 W/mm at 8 and 10 GHz, respectively, representing a marked performance improvement for GaN-based FETs.

High performance (fT∼500GHz) In0.52Al0.48As/ In0.53Ga0.47As/InP quantum wire modfets employing asymmetric coupled-well channels

A coupled-well InAlAs/InGaAs quantum wire MODFET structure is proposed, for which simulations predict improved frequency performance (>500 GHz), over a wider range of Vg, as compared to well/wire devices with a standard MODFET heterointerface. A comparison of several transverse potential well profiles, obtained by varying the placement of a thin barrier within a 100 A finite well, is presented. In all cases, the quantum wires consist of a 0.1 μm long channel and a 150 A finite-square-well lateral profile. It has been found that the peak of the electron distribution for the first confined state, as measured from the heterointerface, changes dramatically depending on the location of the thin barrier. For quantum wire structures, realized in the lattice matched system of In0.52Al0.48As/In0.53Ga0.47As/InP, a change in the barrier location of 25 A is accompanied by a shift in the carrier peak of more than 40 A (~20 A closer to or farther from the spacer-well interface than in the standard MODFET profile). Implications of this are reflected in the current-voltage characteristics (Id-Vd) and frequency responses (fT-Vg) of the proposed structures.

Short-channel Al 0.5 Ga 0.5 N/GaNMODFETs withpower density > 3 W/mm at 18 GHz

The authors have demonstrated 0.25 /spl mu/m gate-length Al/sub 0.5/Ga/sub 0.5/N/GaN MODFETs on sapphire substrates which exhibit CW output power densities >3 W/mm at 18 GHz, the highest reported to date for microwave FETs in the K band. This confirms the promise of the high Al-content AlGaN/GaN MODFET structure.

Low frequency noise sources in InAlAs/InGaAs MODFETs

Detailed analysis of the 1/f low-frequency noise (LFN) in In/sub 0.52/Al/sub 0.48/As/InGaAs MODFET structures is performed, for low drain bias (below pinch-off voltage), in order to identify the physical origin and the location of the noise sources responsible for drain current fluctuations in the frequency range 0.1 Hz-10/sup 5/ Hz. Experimental data were analyzed with the support of a general modeling of the 1/f LFN induced by traps distributed within the different layers and interfaces which constitute the heterostructures. Comparative noise measurements are performed on a variety of structures with different barrier (InAIAs, InP) and different channel (InGaAs lattice matched to InP, strained InGaAs, InP) materials. It is concluded that the dominant low frequency noise sources of InAlAs/InGaAs MODFET transistors in the ON state are generated by deep traps distributed within the bulk InAlAs barrier and buffer layers. For reverse gate bias, the gate current appears to be the dominant contribution to the channel LFN, whereas both the gate current and the drain and source ohmic contacts are the dominant sources of noise when the device is biased strongly in the ON state. Heterojunction FET's on InP substrate with InP barrier and buffer layers show significantly lower LFN and appear to be more suitable for applications such as nonlinear circuits that have noise upconversion.

Photoluminescence from modulation doped AlGaAs/ low-temperature molecular beam epitaxy-grown GaAs heterostructures

Photoluminescence (PL) results from a novel modulation doped AlGaAs/ low-temperature molecular beam epitaxially-grown-GaAs (LT-GaAs MODFET) heterostructure are reported. A new PL line at 1.65 eV is consistently observed in all the LT-GaAs MODFET structures investigated. A spatially indirect transition from a two-dimensional electron gas at the heterojunction interface to the holes in AlGaAs is believed to be responsible for the observed 1.65 eV PL line. LT-GaAs MODFET structures in which LT-GaAs region is grown at 350 °C show additional lines lying in the band edge region as well as deep inside the band gap region of LT-GaAs.

Photoluminescence From 2D Electron Gas and Many Body Effects as a Probe of the Crystalline Quality of Pseudomorphic GaAs/InGaAs/AlGaAs and GaAs/N(GaAs)10(N+l)(InAs)m/AlGaAs Modfet Structures

We have performed low temperature (5K) photoluminescence (PL) measurements in order to study the crystalline quality of pseudomorphic modulation-doped field effect transistors (MODFET's) grown by MBE. MODFET's based on GaAs/AlGaAs with either an InGaAs or a (GaAs)n(InAs)m short period superlattice (SPS) channel have been studied. On modulation doped structures, the presence of free carriers into the channel strongly affects the PL emission,. In this paper, we present a study of PL line shape of MODFET structures with different channel thicknesses and growth conditions. A correlation between the PL line shape and these parameters is clearly observed.

OMVPE-grown (Al xGa 1− x) 0.5In 0.5P/InGaAs MODFET structures: growth procedure and Hall properties

Modulation doped (Al x Ga 1- x ) 0.5In 0.5P/In y Ga 1- y As/GaAs single quantum well structures have been grown by low pre ssure metal organic vapour phase epitaxy and characterized by Hall measurements and cathodoluminescence. The high conduction band offset in this structure is favourable for the fabrication of MODFET structures with high two-dimensional electron gas densities in the In y Ga 1- y As channel. Electron sheet densities up to 2.75 x 10 12 cm -2 have been obtained for x = 0.4 and y = 0.3 with appropriate doping. The structures have excellent Hall properties at room temperature and at 77 K. The critical interface between the InGaAs channel layer and the AlGaInP barrier layer has been improved by placing a thin AlGaAs interlayer (1.8 nm thick) between these two layers. This arrangement provides an abrupt interface between quantum well and interlayer and suppresses the interaction of the two-dimensional electron gas in the quantum well with the non-ideal interface between the interlayer and the AlGaInP barrier layer. Cathodoluminiscence studies revealed that the mechanism of strain relaxation in these structures is distinctly different from the mechanism observed in AlGaAs/InGaAs/GaAs heterostructures grown by molecular beam epitaxy. The results of this first study on AlGaInP/InGaAs/GaAs heterostructures lend strong support to the hypothesis that structures of this type are capable to displace AlGaAs/InGaAs/GaAs heterostructures for the fabrication of high speed field effect transistors on GaAs substrates.

Photo-induced current measurement for the characterized of deep-level traps in lattice-matched MODFETs on InP substrate

This paper describes a measurement setup developed for the characterization of deep-level traps in field-effect transistors (FETs) using the photo-induced current spectroscopic technique (PICS) including the steady-state and pulsed drain biasing. This technique and the conventional DLTS technique have been applied to a GaAs MESFET, and two lattice-matched modulation-doped FETs based on InP substrate to demonstrate its usefulness. In the MESFET structure, the PICS technique identified electron emissions at 0.72 and 0.81 eV and hole emission at 0.45 eV, whereas the DLTS technique identified an acceptor level at 0.42 eV. In one of the MODFETs the PICS technique identified electron emissions at 0.69 and 0.71 eV, whereas the DLTS technique identified an acceptor level at 0.39 eV. In the second MODFET structure, the PICS technique identified electron emissions at 0.40 and 0.71 eV, and the DLTS technique identified donor traps at 0.37 and 0.40 eV and an acceptor trap at 0.22 eV. The PICS and DLTS results are interpreted using the so-called Frank-Condon shift that exists between the optical and thermal emissions.

Modeling of the source resistance in modulation-doped FETs

A distributed multiple layer transmission line (MTL) model has been developed for the planar source ohmic contacts as encountered in modulation-doped FETs. Detailed current and voltage distributions under the MTL system have been obtained by numerical solutions of the coupled differential equations. The results clearly demonstrate that source resistance R S cannot be estimated simply from knowledge of the behavior of series resistance R NG in non-gated TLM test structures. The effects of the heavily-doped cap layer and the variation of the gate-source spacing on R NG and R S have been studied. Differences between our results based on the MTL model and that obtained from the lumped element contact end-resistance (LECR) model are explained satisfactorily. Our results show that the LECR method could be used to calculate the series resistance of the MODFET structures provided appropriate end elements are first determined by the distributed MTL model. Our results clearly indicate that attempts to reduce R S by increasing the cap layer thickness or decreasing the gate-source spacing would have only limited effectiveness. However, reduction of the interfacial contact resistivities is more effective in the reduction of R S , as demonstrated in the case of pseudomorphic structures. Graphical results are presented that clearly show the dependence of R S on the structural parameters of representative conventional and pseudomorphic test MODFETs.

(InAs)1/(GaAs)n Superlattice Quantum Well Lasers

Strained layer In0.2Ga0.8As/GaAs quantum-well (QW) lasers have been used to pump Erbium Doped Fiber Amplifiers for use in opto-electronic communication systems. These InGaAs/GaAs Q-W lasers grown as graded index separate confinement heterostructures (GRINSCH) emit at 980 nm and have exhibited low threshold current densities, high electrical to optical power conversion efficiencies, low noise, and low temperature sensitivities [1–3]. Recently there has been much attention toward the growth and fabrication of devices containing short period superlattices of (InAs)m/(GaAs)m or (GaAs)m/(AlAs)n compositions [4–9]. Indeed the growth of inverted MODFET structures using a (GaAs)/(AlAs) superlattice barrier in place of an AlGaAs random alloy exhibited an increase in low temperature mobility due to reductions in the number of impurities and interface roughness [7]]. Can the optical properties of strained layer InGaAs/GaAs Q-W lasers also be improved by growing an (InAs)/(GaAs)4 short period superlattice as the active layer in place of the In0.2Ga0.8 As random alloy?This paper reports the fabrication of ridge waveguide lasers using a 6 period (InAs)1/(GaAs)4 superlattice active region in a GRINSCH laser structure. One period of the superlattice consists of 1 monolayer (ML) of InAs and 4 ML of GaAs. Thus the average In composition in the superlattice is 20%. This superlattice laser is an alternative to the In0.2Ga0.8As/GaAs random alloy quantum-well laser used as a pump source for Erbium Doped Fiber Amplifiers. In a single quantum well In0.2Ga0.8As/GaAs laser the active region is a thin layer (-70Å thick) sandwiched between GaAs barrier layers. The superlattice active laser reported here has 6 periods of (InAs)1/(GaAs)4 as the light emitting region of a total width comparable to a single quantum well of random alloy composition.

Fabrication of sub-100nm dual-gate MODFETS with enhanced performance

A novel dual-gate MODFET structure with two gates (<50 nm long) separated by 50 nm or less has been fabricated on a GaAs/AlGaAs modulation-doped substrate, using standard MODFET fabrication steps and high-resolution electron-beam lithography for all levels. Measurements show that the two gates cannot be treated independently, that the threshold voltage of the device can be tuned by proper biasing the first gate, high transconductance and cutoff frequency can be achieved by tailoring the field distribution under the two gates.

Spatially resolved photoluminescence using spectral correlation

A technique is demonstrated for dramatically reducing the amount of time required to perform spatially resolved photoluminescence measurements of peak wavelength and intensity without use of an optical multichannel analyzer. A high-resolution reference spectrum is collected at one location on the wafer and the peak wavelength and intensity are determined for that location. Comparison spectra at only 3 to 5 wavelengths are then collected at a mesh of locations across the wafer. A correlation algorithm is used to extract the peak wavelength and intensity at all of the comparison locations. Examples are provided from AlGaAs layers and from AlGaAs and InGaAs layers in pseudomorphic MODFET structures.

Subthreshold current in undoped AlGaAs/GaAs MODFET structures

The subthreshold current in undoped AlGaAs/GaAs MODFET structures can have a negative gradient with respect to the gate voltage, whereas in doped structures it has normally either a positive gradient or is constant. A two-dimensional finite-difference analysis is used to clarify this difference. It is shown that the behavior of the threshold current in an undoped structure is due to the effect of gate bias on the leakage between the drain and the gate. This leakage occurs near the top surface of the AlGaAs layer, and is strongly affected by the lateral straggle of the n/sup +/ implants and by the surface states in AlGaAs. The charge-control model, which is conventionally applied to the doped case, does not describe the behavior of the subthreshold current in the undoped case because it neglects leakage currents.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Very-high-performance highly strained Ga/sub 0.23/In/sub 0.77/As channel MODFETs

Summary form only given. The authors report a transconductance of 843 mS/mm and a cutoff frequency 45.3 GHz for 1.2- mu m FETs and 33 GHz for 1.9- mu m FETs obtained by using a MODFET structure containing a highly strained Ga/sub 0.23/In/sub 0.77/As channel on InP substrate. It is important to either keep the gate recess quite shallow ( approximately 7 nm total) or control the process with a stop-etch layer for the achievement of these exceptionally high f/sub T/*L/sub g/ products. In device processing, three different gate-recess processes were used. Each is described. Results show that the combination of Ga/sub 0.23/In/sub 0.77/As and selective etch gate recess is an excellent technology for realizing fast FETs. >

MOVPE grown high performance 0.25 μm AlGaAs/InGaAs/GaAs pseudomorphic MODFETs

Abstract We report the growth by MOVPE of Al x Ga 1− x As/In y Ga 1- y As/GaAs strained layer, or pseudomorphic, MODFET structures with x ⩽ 0.28 and y ⩽ 0.20. The metalorganics used were TMGa, TMAl, TMIn; the group V used was arsine at 100% concentration, while silane and disilane were used for doping. Hall measurements on these structures showed 2DEG sheet carrier concentrations up to 2×1012 cm -2 , and excellent room temperature and 77 K mobilities. Devices fabricated from this material with E-beam defined 0.25 μm gate lengths had DC extrinsic transconductances of 300 mS mm -1 . RF testing on wafers up to 40 GHz showed that these MODFETs exhibited good microwave properties, with an extrapolated current gain cut-off frequency ( f T ) of 64 GHz and an estimated maximum frequency of oscillation ( f max ) of 135 GHz.

Charge Control in SiGe Quantum-Well MOSFETs and MODFETs

ABSTRACTA one-dimensional charge control model based on the self-consistent numerical solution of the Schroedinger and Poisson equation has been developed for n- and p-channel quantum-well (QW) MOSFETs and MODFETs in the strained-layer SiGe heterostructure material system. Results are presented for prototype QW n-channel MODFET and MOSFET structures experimentally demonstrated in the literature. Side channel formation limits the maximum usable QW channel densities and voltage swings. The optimization of the charge control characteristics of the p-channel QW-MOSFET is achieved by maximizing the Ge content of the QW SiGe layer and minimizing the Si spacer thickness to the oxide. Channel densities on the order of 1×1013cm22 are feasible.