Modulation-doped Field-effect Transistor Structures Research Articles

Improved two-dimensional electron mobility in asymmetric barrier delta-doped GaAs/AlGaAs modulation-doped field-effect transistor structures

We study the enhancement of electron mobility μ in barrier delta-doped GaAs/AlGaAs quantum-well-based modulation-doped field-effect transistor (MODFET) structures. We asymmetrically vary the doping concentrations Nd1 and Nd2 in the barriers on the substrate and surface sides, respectively, to obtain a nonlinear enhancement of μ as a function of the well width w through multi-subband effects. We show that an increase in doping concentration increases the surface electron density Ns, which in turn enhances μ. Interchanging Nd1 and Nd2 leads to no change in Ns but rather, an enhancement of μ as a function of w for Nd2 > Nd1 owing to asymmetric variation of subband wave functions, thereby implying a higher channel conductivity in a surface-doped structure than in an inverted doped structure. By keeping (Nd1 + Nd2) unchanged, the conductivity of a single-channel MODFET, Nd1 (Nd2) ≠ 0 and Nd2 (Nd1) = 0, can be enhanced by considering a MODFET based on an asymmetrically doped (Nd1 ≠ Nd2 ≠ 0) quantum well structure. We show that the highest Ns and μ product for these structures occurs almost before the onset of the occupation of the second subband. Our analysis of the effect of asymmetric doping profiles on channel conductivity can be utilized for the performance improvement of MODFET-like devices.

3 ns single-shot read-out in a quantum dot-based memory structure

Fast read-out of two to six charges per dot from the ground and first excited state in a quantum dot (QD)-based memory is demonstrated using a two-dimensional electron gas. Single-shot measurements on modulation-doped field-effect transistor structures with embedded InAs/GaAs QDs show read-out times as short as 3 ns. At low temperature (T = 4.2 K) this read-out time is still limited by the parasitics of the setup and the device structure. Faster read-out times and a larger read-out signal are expected for an improved setup and device structure.

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (IDmax), the threshold voltage (Vth), the peak dc trans-conductance (gm), and unity current gain cut-off frequency (fT). The calculated values of fT (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.

High photoresponsivity of a p-channel InGaP/GaAs/InGaAs double heterojunction pseudomorphic modulation-doped field effect transistor

In this letter, we report the electrical and optical characteristics of p-channel In0.13Ga0.87As double heterojunction pseudomorphic modulation-doped field effect transistor (MODFET) structure grown by gas source molecular beam epitaxy. The Hall mobility and the density of 2-DHGs (two-dimensional hole gases) in the pseudomorphic In0.13Ga0.87As channel were measured to be 250 cm2/V s and 1.9×1012 cm−2 at 300 K, and 5800 cm2/V s and 1.5×1012 cm−2 at 23 K, respectively. The fabricated p-channel MODFET shows a good mobility property which is due to high valence band discontinuity of InGaP/GaAs/InGaAs double barriers. The peak energy in the photoluminescence spectrum from the p-channel pseudomorphic MODFET structure was found to be 1.4 eV (λ=881 nm). The photoresponsivity with this modified pseudomorphic MODFET structure shows outstandingly better than that of a pin photodiode, particularly at low incident optical power.

Gas source molecular beam epitaxy growth and characterization of modulation-doped field-effect transistor structures

Lattice-matched /GaAs and strained modulation-doped field-effect transistor structures were grown by gas source molecular beam epitaxy by using Si as dopant. Detailed electrical characterization results are presented. The As/GaAs sample yielded dark two-dimensional electron gas densities of (300 K) and (77 K) which are comparable to the highest sheet electron densities reported in AlGaAs/InGaAs/GaAs and InAlAs/InGaAs/InP modulation-doped heterostructures. Persistent photoconductivity was observed in the strained samples only. A 0.797 eV deep level has been detected in the undoped GaInP layers of the structures. Another level, with DLTS peak height dependent on the filling pulse width, has been detected at the interface of the strained samples. Based on the DLTS and Hall effect measurement results, this level, which seems to be the origin of persistent photoconductivity, can be attributed to the strain relaxation related defects.

High-lying thermally excited subbands of two-dimensional electron gases in GaAs/AlGaAs heterojunction for modulation-doped field-effect transistor

The cyclotron resonance of two-dimensional electron gases was directly measured on a modulation-doped field-effect transistor structure by the gate voltage ratio spectroscopy. The intersubband energies between the lowest subband and the subbands with quantum index up to i=5 were determined by resonant subband-Landau-level coupling (RSLC) under relatively low magnetic field. The experimental results were in good agreement with the self-consistent calculations including the depolarization shift and exciton-like effect corrections. The quantum index dependence of the level splitting due to the RSLC was discussed.

Analysis of self-aligned MOSFETs with modulation-doped SiGe channels

An analysis of enhanced performance field-effect transistor (FET) structures, incorporating the self-aligned gate feature of conventional Si MOSFETs and the higher carrier-mobility-channel configuration of modulation-doped FETs, is presented. A modified charge-control model is developed to account for the incorporation of the MOS gate in a modulation-doped FET structure. Electrical characteristics are computed for these devices having strained SiGe layers on Si and/or Ge substrates. Theoretical results are computed using the ultra-high hole mobility reported in the literature for the modulation-doped SiGe-Ge-SiGe p-MOSFETs. It is shown that these devices will extend the use of SiGe based FETs to over the 80 GHz range for a 0.25 μm gate at 77 K, while maintaining the high packing density inherent in Si MOSFETs.

Effects of Reactive Ion Etching on Phonon-Electron Interactions in Inalas-Ingaas Modulation-Doped Field-Effect Transistor Structures Studied by Raman Scattering

Raman Scattering by coupled longitudinal optic phonons and two-dimensional electron gas electrons in In0.52Al0.48As-In0.53Ga0.47As δ-doped heterostructures provides a powerful probe of electronic properties in these In-based structures. The two highest frequency modes, of the three coupled electron-phonon modes expected in this system, were observed, with the highest frequency mode being identified for the first time in InGaAs-based systems. The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy. For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved. These measurements are particularly useful for heavily-doped structures for which room-temperature Hall measurements cannot distinguish channel electrons from those in parallel conduction paths. In addition HBr-based reactive ion etching has been performed on these structures, and the effects have been correlated with etch time and bias voltage for structures with different cap layer thicknesses.

Novel high mobility Ga0.51In0.49P/GaAs modulation-doped field-effect transistor structures grown using a gas source molecular beam epitaxy

A standard Ga0.51In0.49P/GaAs modulation-doped field-effect transistor (MODFET) structure and a novel Ga0.51In0.49P/GaAs MODFET structure where the Ga0.51In0.49P spacer layer was replaced by an undoped Al0.3Ga0.7As layer were grown using a gas source molecular beam epitaxy. The Hall mobility of the novel MODFET’s structures are 6600 and 36 400 cm2/V s at room temperature and 77 K, respectively, which are more than twice as high as that in the ordinary Ga0.51In0.49P/GaAs MODFETs structure. The mobility is attributed to better carrier confinement and smoother heterointerface. Furthermore, it is found that both ordinary and novel MODFET’s structures have small photo-persistant conductivity effects at low temperatures and that the FETs made in these materials had no threshold voltage shift at low temperatures after illumination.

Emission and absorption of phonons by the two-dimensional electron gas in a GaAs MODFET structure

The authors have observed both emission and absorption of phonons by the two-dimensional electron gas in a GaAs modulation-doped field effect transistor (MODFET). For the first time in this system directly emitted LA phonons were seen, in addition to the TA products of LO down-conversion. The angular dependence of the LA feature suggested that its origin was piezoelectric not deformation potential coupling. Direct absorption of phonons by the 2DEG was found to have a response time approximately ns, so that the transmission stop bands of a superlattice phonon fitter could be used as a dispersive spectrometer.

Analysis of pseudomorphic GaAs/InGaAs/AlGaAs modulation-doped field-effect transistor structures by secondary-ion mass spectrometry and ion channeling

Secondary-ion mass spectrometry has been used to analyze the Al and In depth profiles in pseudomorphic GaAs/InGaAs/AlGaAs modulation-doped field-effect transistor structures. Surface segregation of In was found on the upper interface of the strained InGaAs layer under a moderate growth temperature of 510 °C. The segregation effect was found to be suppressed when the As4 overpressure was doubled. Ion channeling measurements are capable of differentiating the coherency of the strained InGaAs layer in the samples studied. A correlation was found between the coherency of the layer and its electrical transport property.

Growth studies of pseudomorphic GaAs/InGaAs/AlGaAs modulation-doped field-effect transistor structures

Strained GaAs/InGaAs/AlGaAs quantum well structures have been grown by molecular beam epitaxy at substrate temperatures from 375 to 510 °C. The well layer thickness and In composition are analyzed by ion channeling and particle-induced x-ray emission as a function of growth temperature. Test structures for modulation-doped field-effect transistors grown at 375 and 510 °C under two different As4 overpressures were also characterized by Van der Pauw measurements and low-temperature photoluminescence. The observed differences in film quality can be explained by the influence of substrate temperature and As4 flux on the cation surface mobility during growth of the InGaAs layer.

Organometallic vapor phase epitaxial growth of GaAs-based pseudomorphic modulation-doped field-effect transistor structures

We report the preparation of InGaAs/AlGaAs strained-layer(pseudomorphic) modulation-doped field-effect transistor (MODFET) structures by organometallic vapor phase epitaxy (OMVPE). Devices fabricated from these structures with 0.9 μm gate lengths had dc extrinsic transconductances up to 340 mS/mm. Microwave testing up to 40 GHz showed current gain cutoff frequencies ( fT ) of 22 GHz and estimated maximum frequency of oscillation ( fmax ) of 70 GHz. This is the first report to our knowledge of the use of OMVPE material in the fabrication of pseudomorphic MODFETs.

Calculation of the electron wave function in a graded-channel double-heterojunction modulation-doped field-effect transistor

We investigate theoretically three double-heterojunction modulation-doped field-effect transistor structures with different channel composition. All of these transistors have an InxGa1−xAs channel sandwiched between two doped Al0.3Ga0.7As barriers with undoped spacer layers. In one of the structures, x varies from 0 from either heterojunction to 0.15 at the center of the channel quadratically and in the other two, constant values of x of 0 and 0.15 are used. We solve Poisson’s and Schrödinger’s equations self-consistently for the electron wave function for all three cases. The results showed that the two-dimensional electron gas (2DEG) concentration in the channel of the quadratically graded structure is higher than the x=0 one and slightly lower than the x=0.15 one, and the mean distance of the 2DEG is closer to the center of the channel for this transistor than the other two. These two effects have important implications on the electron mobility in the channel.

Magnetoresistance method to determine GaAs and AlxGa1−xAs mobilities in AlxGa1−xAs/GaAs modulation-doped field-effect transistor structures

Charge carrier mobilities are conveniently measured in simple, homostructure field-effect transistors (FET’s) by means of the geometric magnetoresistance (GMR) technique. Heterostructure FET’s, however, are more complicated because of multiple conducting regions, as well as multiple conducting bands within a given region. We apply a multilayer GMR mobility model to a frequently used heterostructure FET design, namely, the Al0.3Ga0.7As/GaAs modulation-doped FET (MODFET). By analyzing the results at different magnetic fields, we can separate the contributions of the various GaAs subbands and the Al0.3Ga0.7As conduction band. In the particular MODFET structure studied here, the lowest GaAs subband mobility ranges from 5.7×103 cm2/Vs at threshold to 6.9×103 cm2/Vs at saturation, while the Al0.3Ga0.7As mobility is about 5×102 cm2/Vs. This is the first time that the various mobilities in MODFET structures have been separately measured.

Extremely low contact resistances for AlGaAs/GaAs modulation-doped field-effect transistor structures

We report contact resistances as low as 0.035 Ω mm between the ohmic contact metal and the two-dimensional electron gas channel in an AlGaAs-GaAs modulation-doped field-effect transistor structure. The contact resistances are achieved by transient annealing of AuGe/Ni/Au, and are to our knowledge the smallest values reported to date for these structures at room temperature. Assuming no change in the semiconductor sheet resistivity under the metal contact, the calculated specific contact resistivity would be as low as 5×10−8 Ω cm2, however, the true figure is not known. Details of our contact study include varying temperature-time cycles and heterojunction structures. Current-voltage measurements at 77 K show ohmic behavior, revealing the destruction of the heterojunction barrier in the contact region. Preliminary Auger energy spectroscopy profiles support an intermixing of the metal and semiconductor constituents down through the original AlGaAs-GaAs interface. We, therefore, propose a model where the current flows laterally from grains (formed during annealing) into the two-dimensional electron gas.

Strained-quantum-well, modulation-doped, field-effect transistor

A new modulation-doped field-effect transistor structure which employs a thin (8 nm) strained layer of In 0.25 Ga 0.75 As as the channel is described. The prototype device with a 2.9 μm gate length had a peak, room-temperature, unilluminated extrinsic transconductance of 91 mS/mm (VDS = 1 V). If device performance scaled linearly with gate length to 1 μm, as do (Al, Ga)As/GaAs, single-interface, modulation-doped FETs, this is the best room-temperature performance reported to date for an (In, Ga)As channel, modulation-doped transistor. Furthermore, the transconductance of the strained-quantum-well transistor proved to be almost constant with gate voltage, varying less than 13% over a 0.5 V range at 77 K. This will allow the fabrication of extremely linear microwave amplifiers and supports the assertion that the current conduction path is the (In, Ga)As channel.