Modulation-doped Field-effect Transistors Research Articles

InGaP/InGaAs/GaAs camel-gate p -channel pseudomorphic modulation-doped field effect transistor

A novel InGaP/InGaAs/GaAs p-channel pseudomorphic modulation-doped field effect transistor based on p+-GaAs/n+-InGaP/p-InGaP camel-like gate structure is reported for the first time. Owing to the p–n depletion of the camel-like gate and the presence of relatively large ΔEv at InGaP/InGaAs heterostructure, an extremely large gate turn-on voltage, greater than 2 V, is measured. For a 1×100 µm2 device, the experimental results show a maximum saturation current density of −345 mA/mm and a widely broad gate voltage swing, greater than 4 V, with 80% maximum transconductance. Furthermore, the fT and fmax values are 3.1 and 4.8 GHz, respectively.

Effect of temperature on the transfer characteristic of a 0.5 μm-gate Si:SiGe depletion-mode n-MODFET

An investigation of the low-temperature operation of a 0.5 μm-gate Si:SiGe depletion-mode n-type modulation-doped field-effect transistor is presented. The investigated temperatures range from T=300 to 180 K. The benefits of cryogenic operation are discussed. Experimental indications of parallel conduction in the device are presented, as well as their dependence on operating temperature. Measured data are compared with two-dimensional device simulations in MEDICI™ carried out using mobility values from Monte Carlo material calculations.

AlGaN/GaN Modulation-Doped Field-Effect Transistors with An Mg-doped Carrier Confinement Layer

AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with and without the Mg-doped semi-insulating carrier confinement layer were prepared and a detailed study on the electrical properties of these samples was performed. The addition of the Mg-doped semi-insulating carrier confinement layer resulted in the degradation of crystal quality. Thus, higher values of gm, IDS and the gate voltage swing are achieved by conventional MODFETs. The source-to-drain (S–D) leakage current of conventional MODFETs was also higher, since carriers could flow through the thick unintentionally doped GaN layer. However, the S–D leakage current was reduced from 50 mA/mm to 6 mA/mm with the insertion of the Mg-doped semi-insulating carrier confinement layer.

Generation–recombination noise in gallium nitride-based quantum well structures

Electronic noise has been investigated in AlxGa1−xN/GaN modulation-doped field-effect transistors of submicron dimensions, grown by molecular beam epitaxy techniques. Some 20 devices were grown on a sapphire substrate. Conduction takes place in the quasi-two-dimensional (2D) layer of the junction (xy plane) which is perpendicular to the triangular quantum well (z direction). A nondoped intrinsic buffer layer separates the Si-doped donors in the AlxGa1−xN layer from the 2D transistor plane. Since all contacts must reach through the AlxGa1−xN layer to connect internally to the 2D plane, parallel conduction through this layer is a feature of all modulation-doped devices. The excess noise has been analyzed as a sum of Lorentzian spectra and 1/fα noise. The Lorentzian noise is ascribed to trapping of the carriers in the AlxGa1−xN layer. The trap depths have been obtained from Arrhenius plots of log(τT 2) versus 1000/T. Comparison with previous noise results for GaAs devices shows that: (a) many more trapping levels are present in these nitride-based devices and (b) the traps are deeper (farther below the conduction band) than for GaAs, as expected for higher band-gap materials. Furthermore, the magnitude of the noise is strongly dependent on the level of depletion of the AlxGa1−xN donor layer. We also note that the trap-measured energies are in good agreement with the energies obtained by deep level transient spectroscopy.

Sub-100 nm Gate Technologies for Si/SiGe-Buried-Channel RF Devices

A novel fabrication process of sub-100 nm self-aligned T-gates for heterostructure field-effect transistors (HFETs) using optical contact lithography is presented. A 500-nm-wide polyimide fin is used as an implantation mask, shrunk by dry etching and subsequently replaced by a gate metal. A low-resistive gate head to form a T-shape is independently defined by wet chemical etching. Using this method, Si/SiGe modulation-doped field-effect transistors (MODFETs) have been prepared, having a gate length of 90 nm. The self-alignment enables the realization of very small source/gate and gate/drain spacings of 200 nm. This yields, together with an optimized salicide (self-aligned silicide) ohmic contact, a much lower access resistance compared to conventional gates defined by e-beam lithography. A record transit frequency fT of 90 GHz and a very high transconductance of 570 mS/mm have been achieved for MODFETs.

Static frequency divider circuit using 0.15 µm gate length Si 0.2 Ge 0.8 /Si 0.7 Ge 0.3 p -MODFETs

The first demonstration of an integrated circuit using high mobility p-channel modulation doped field effect transistors (MODFETs) is reported. A static frequency divider was fabricated on Si/sub 0.7/Ge/sub 0.3//Si/sub 0.2/Ge/sub 0.8//Si/sub 0.7/Ge/sub 0.3/ strained heterostructures grown epitaxially on Si using ultra-high vacuum chemical vapour deposition; divide-by-two operation was achieved for clock frequencies of up to 2.9 GHz.

The characteristics of InGaAs/GaAs modulation-doped photodetectors with two absorption bands

Modulation-doped photodetectors incorporated the low-temperature growth of GaAs and InGaAs as the absorption layers, respectively, are described. The photodetectors, which behave as a normal-off modulation-doped field effect transistor, have a low dark current of 16 nA at a bias of 5 V and can be turned on by illuminating light. A high responsivity of 19 A/W biased at 10 V is presented under a 0.8-μm light illumination and the spectral response exhibits two main stronger absorption peaks at 870 and 910 nm, which are the band-edge transitions of GaAs and InGaAs, respectively.

High Electron Mobility SiGe/Si Transistor Structures on Sapphire Substrates

ABSTRACTSiGe/Si n-type modulation doped field effect structures and transistors (n-MODFETs) have been fabricated on r-plane sapphire substrates. The structures were deposited using molecular beam epitaxy, and antimony dopants were incorporated via a delta doping process. Secondary ion mass spectroscopy (SIMS) indicates that the peak antimony concentration was approximately 4×1019 cm−3. The electron mobility was over 1,200 and 13,000 cm2/V-sec at room temperature and 0.25 K, respectively. At these two temperatures, the electron carrier densities were 1.6 and 1.33×1012 cm−2, thus demonstrating that carrier confinement was excellent. Shubnikov-de Haas oscillations were observed at 0.25 K, thus confirming the two-dimensional nature of the carriers. Transistors, with gate lengths varying from 1 micron to 5 microns, were fabricated using these structures and dc characterization was performed at room temperature. The saturated drain current region extended over a wide source-to-drain voltage (VDS) range, with VDS knee voltages of approximately 0.5 V and increased leakage starting at voltages slightly higher than 4 V.

Characterization of 1.8-MeV proton-irradiated AlGaN/GaN field-effect transistor structures by nanoscale depth-resolved luminescence spectroscopy

We have used depth-resolved cathodoluminescence spectroscopy to examine AlGaN/GaN modulation-doped field-effect transistors that display degraded source-drain current characteristics after 1.8-MeV proton irradiation, along with bulk heterojunction field-effect transistor material after similar proton irradiation. For both cases, we have observed distinct changes in spectral emission features due to decreased internal electric-field strength and new point defects within different layers of the device structure with nanometer-scale depth resolution. These changes can account for the degraded electrical characteristics.

Deep levels studies of AlGaN/GaN superlattices

The results of microcathodoluminescence (MCL) and deep levels transient spectroscopy (DLTS) studies of AlGaN/GaN modulation doped field effect transistor (MODFET) structures and modulation doped and undoped AlGaN/GaN superlattices (SLs) grown by metalorganic chemical vapor deposition are presented. A strong blue shift in the energy position of the band-edge MCL peak in modulation doped SLs compared to undoped SLs is observed and attributed to screening of the built-in piezoelectric field in AlGaN/GaN SLs by the high density of two-dimensional electron gas in the GaN quantum wells. The dominant traps in AlGaN/GaN MODFETs and doped SLs with Al composition in the barrier close to 28% were found to be the electron traps with activation energy 0.9 eV located near the AlGaN/GaN interface. In undoped SLs similar traps were observed and their energy position was found to strongly depend on the Al composition in the barrier and on the strength of the electric field in space charge region of the Schottky diode used for DLTS measurements.

Detection of single photons using a field effect transistor with a layer of quantumdots

We demonstrate single-photon detection with a GaAs/AlGaAs modulation-dopedfield effect transistor. High sensitivity of the transistor to light is obtained byincorporating in its structure a layer of self-organized InAs quantum dots in closeproximity to the conducting channel but separated by a thin AlGaAs barrier. Weshow that capture of a single photoexcited carrier by a quantum dot results in asizeable change in the source–drain current of the transistor, allowing thedetection of a single photon.

Si/SiGe n -channel modulation-doped field effect transistor on air

Si/SiGe n-channel modulation-doped field effect transistors (MODFETs) have been fabricated on a 10 µm-thick membrane by removal of the Si substrate and SiGe virtual substrate under the device layers. The membrane devices, surrounded by air, were characterised after thinning and compared to the unthinned characteristics. A large reduction of the off-currents of the MODFETs on air, due to an increase in substrate resistance, has been measured, making them more suitable for low-power applications.

Simulations of Si:SiGe MODFET analogue applications

We present simulations of different analogue building blocks using n-type depletion-mode Si:SiGe modulation doped field effect transistors (MODFETs). The device parameters used are extracted from the dc measurements of the SiGe MODFETs by using the Si MOSFET model in HSPICE. The limits of the validity of the Si MOSFET model for SiGe MODFET simulations are discussed. This fast-track approach is compared to both a more elaborate MEDICI–AURORA–SPICE approach and experimental data. The simulations of an inverting current mirror compare qualitatively well to the experimental results within a restricted gate voltage swing. The simulations demonstrate the potential of n-type Si:SiGe MODFET circuit design based on III–V design techniques and Si MOSFET models in SPICE

Si/SiGe n-MODFETs on thin SiGe virtual substrates prepared by means of He implantation

Si/SiGe n-type modulation-doped field-effect transistors grown on a very thin strain-relieved Si/sub 0.69/Ge/sub 0.31/ buffer on top of a Si(100) substrate were fabricated and characterized. This novel type of virtual substrate has been created by means of a high dose He ion implantation localized beneath a 95-nm-thick pseudomorphic SiGe layer on Si followed by a strain relaxing annealing step at 850/spl deg/C. The layers were grown by molecular beam epitaxy. Electron mobilities of 1415 cm/sup 2//Vs and 5270 cm/sup 2//Vs were measured at room temperature and 77 K, respectively, at a sheet carrier density of about 3/spl times/10/sup 12//cm/sup 2/. The fabricated transistors with Pt-Schottky gates showed good dc characteristics with a drain current of 330 mA/mm and a transconductance of 200 mS/mm. Cutoff frequencies of f/sub t/=49 GHz and f/sub max/=95 GHz at 100 nm gate length were obtained which are quite close to the figures of merit of a control sample grown on a conventional, thick Si/sub 0.7/Ge/sub 0.3/ buffer.

Monolithically integrated bacteriorhodopsin–GaAs field-effect transistor photoreceiver

We have applied the large photovoltage developed across a layer of selectively deposited bacteriorhodopsin to the gate terminal of a monolithically integrated GaAs-based modulation-doped field-effect transistor, which delivers an amplified photoinduced current signal. The integrated biophotoreceiver device exhibits a responsivity of 3.8 A/W. The optoelectronic integrated circuit is achieved by molecular-beam epitaxy of the field-effect transistor's heterostructure, photolithography, and selective-area bacteriorhodopsin electrodeposition.

Full Band Monte Carlo Simulation of Wurtzite AlGaN/GaN MODFETs

We present full band Monte Carlo simulations of a wurtzite Al0.15Ga0.85N/GaN modulation-doped field-effect transistor (MODFET). We found that without inclusion of the piezoelectric effect, the electron concentrations in the channel are much lower than obtained from experimental data. The calculated Ids-Vds curves show a strong negative differential resistance, which is a feature observed in experimental devices. Self-heating effects are usually believed to be the main cause of the negative differential resistance. Our simulations do not include self-heating, and this would indicate that at least part of what is observed is also caused by the drift-velocity behavior vs. electric field of the narrow conduction channel. For a 0.2 μm gate MODFET, the simulations yield a maximum trans-conductance Gm ≈ 250 mS/mm with VG = 1.0 V and Vds = 5.0 V. When VG = 0.0 V and Vds = 8.0 V, we obtain a maximum cutoff frequency fT = 180 GHz with Id = 1159 mA/mm.

New quantum dot transistor

Modulation-dopedN-AlGaAs/GaAs/InAs/GaAs/InAs/GaAs heterostructures with InAs quantum dots (QDs) in the device channel have been grown andinvestigated. Their photoluminescence spectra and electrontransport properties in both low and high electric fields werestudied. Using these structures, modulation-doped field effecttransistors havebeen fabricated and analysed. It was demonstrated that the QD field effect transistors present a new type of hot-electrondevice, promising for high-speed applications.

De-embedded ultra-low noise 0.1 µm gate lengthGe/Si 0.4 Ge 0.6 p -MODFET

De-embedded high frequency (HF) noise parameters of Ge p-channel modulation doped field effect transistors (MODFETs) on a Si0.4Ge0.6 virtual substrate have been determined for the first time. The 0.1 × 100 µm devices have a minimum noise figure NFmin = 0.3 dB (0.5 dB) ± 0.2 dB, a noise resistance Rn = 95 Ω (90 Ω) ± 5 Ω, an optimum reflection coefficient Γopt = 0.7∠6° and an associated gain Gass = 14 dB (13 dB) at 1.2 GHz (2.5 GHz), at a bias corresponding to the optimum unity current-gain frequency fT = 55 GHz and the maximum oscillation frequency fMAX = 135 GHz. Noise results are given in the 1.2–12 GHz frequency range and 20–160 mA/mm current range.

High electric field transport in modulation-doped InAs self-assembled quantum dots for high-frequency applications

Modulation-doped N–AlGaAs (GaAs/InAs/GaAs/InAs)/GaAs-heterostructures with InAs-quantum dots (QDs) have been grown and investigated. Using these structures, modulation-doped field-effect transistors (MODFETs) have been fabricated and analyzed. It has been observed that they have the anomalous two-step shape in the current–voltage curve in contrast to the conventional curve with saturation. The saturation current Idss for the second step practically does not depend on the gate bias UG, showing that the concentration of electrons participating in the current flow becomes independent of UG. It has been demonstrated that the QD-MODFETs present a promising type of the hot electron devices for high frequency applications.

Absorption and Emission of Acoustic Phonons by a Two‐Dimensional Electron Gas

We have observed acoustic phonons, absorbed as nanosecond heat pulses directly by the two-dimensional electron gas (2DEG) of a GaAs-Al 0.3 Ga 0.7 As Modulation Doped Field Effect Transistor (MODFET) structure. The effect of phonons was to increase the electron temperature of the 2DEG and also the mobility. From a comparison with the effect of electrical heating, it was possible to determine the absolute level of the phonon power observed. Finally by comparison with data on TA and LA phonon absorption by the 2DEG, it could be concluded that the deformation potential was the dominant electron-phonon coupling mechanism in GaAs.