Heterostructure Field-effect Transistor Structures Research Articles

Delta-doped AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors with high breakdown voltages

The fabrication and dc characteristics of AlGaN/GaN-based heterostructure field-effect transistors (HFETs) by employing the δ-doped barrier and the SiO2 insulated gate are reported. The device grown on sapphire substrate has a high drain-current-driving and gate-control capabilities as well as a very high gate-drain breakdown voltage of 200 V for a gate length of 1 μm and a source-drain distance of 3 μm. The incorporation of the SiO2 insulated gate and the δ-doped barrier into HFET structures reduces the gate leakage and improves the two-dimensional channel carrier mobility, and thereby allows one to take the inherent advantage of AlGaN/GaN HFETs with relatively high Al contents—the device structure is capable to deliver higher electron density (or drain current density) yet ensures an excellent pinch-off property as well as small gate leakage current. These characteristics indicate a great potential of this structure for high-power-microwave applications.

Comparison of two different Ti/Al/Ti/Au ohmic metallization schemes for AlGaN/GaN

Two different ohmic metallization schemes for AlGaN/GaN heterostructure field-effect transistor structures are investigated. Each consists of Ti/Al/Ti/Au layers, with the Ti contacting the AlGaN surface, and they differ only in the relative thicknesses of Ti and Al. The “Cornell” metallization consists of 20/100/45/55 nm thicknesses, respectively, while the “NRC” metallization consists of 30/80/120/55 nm thicknesses for the layers. Thus, the Cornell scheme is much richer in Al. The two schemes were compared on the same AlGaN/GaN growth wafer which, because of nonuniformity, provides a variety of contact resistances in the final annealed devices. The Cornell scheme formed good quality ohmic contacts at short anneal times, but clearly contained excess, unalloyed Al, as it showed visible, gas-evolving corrosion in a cleaning solution consisting of 1:1 HCl:H2O. The surface morphology was rough and the edge acuity was relatively poor. Auger depth profiling showed a single, almost uniformly mixed layer. The NRC scheme required a longer anneal time, and the contact resistance remained high for larger grained AlGaN/GaN material. It was stable in the HCl:H2O cleaning solution, and exhibited good edge acuity and a smooth surface morphology. Auger electron depth profiling of the NRC scheme showed two layers present in the annealed metal. The Auger results are consistent with transmission electron microscope results presented elsewhere. The results suggest that the Cornell scheme would be the choice for materials that are more difficult to contact.

Ammonia-molecular-beam epitaxial growth and optical properties of GaN/AlGaN quantum wells

GaN/AlGaN quantum wells on GaN templates have been grown on (0001) sapphire substrates using the ammonia-molecular-beam epitaxy technique. The GaN template layers were of the type used previously for growing high-mobility, heterostructure field-effect transistor structures. The photoluminescence properties of the quantum wells showed strong quantum-confined Stark effect in good agreement with theoretical calculations, as well as evidence of carrier localization due to in-plane well width fluctuation. At low temperature, the quantum well emissions were dominated by two or more localized exciton levels. Carrier thermalization to the lower localized level was observed as the temperature was raised. Evidence of interwell transfer of photocarriers was observed, with the holes appearing to transfer in the direction of the internal field within the AlGaN barrier.

Microstructural evidence on electrical properties of Ta/Ti/Al and Ti/Ta/Al ohmic contacts to n-AlGaN/GaN

Electrical properties and microstructures of Ta/Ti/Al and Ti/Ta/Al contacts to n-AlGaN/GaN heterostructure field-effect transistor structures were investigated using the transmission line method and transmission electron microscopy. The specific resistivity (5.3×10−7 Ω cm2) of Ta/Ti/Al contacts is much lower than that (5.1×10−4 Ω cm2) of Ti/Ta/Al contacts, for the same heterostructure and similar metallization. The contact resistivity was found to depend on the thickness of the AlGaN layer, interfacial phase, and interface roughness. The formation of interfacial phases by solid-state reactions with the metal layer during annealing appears to be essential for ohmic behavior on n-III-nitrides suggesting a tunneling contact mechanism.

Growth of high mobility GaN and AlGaN/GaN high electron mobility transistor structures on 4H-SiC by ammonia molecular-beam epitaxy

Ammonia molecular-beam epitaxy has been used to grow high-quality epilayers of GaN and AlGaN/GaN heterostructure field-effect transistor (HFET) structures on insulating 4H-SiC. The growth process, which used a magnetron sputter epitaxy deposited buffer layer of AlN, has been described previously. Ex situ pretreatment of the SiC substrate was found to be unnecessary. For a single 2.0 μm thick silicon doped epilayer, a room temperature (RT) electron mobility of 500 cm2/Vs was measured at a carrier density of 6.6×1016 cm−3. For the HFET structure, a room temperature mobility of 1300 cm2/Vs at a sheet carrier density of 3.3×1012 cm−2 was observed, increasing to 11 000 cm2/Vs at 77 K. The surface morphology of the layers indicated a coalesced mesa structure similar to what we observed for growth on sapphire, but with a lower overall defect density and correspondingly larger grain size. The observation of well-resolved Shubnikov de Haas oscillations at fields as low as 3 T indicated a relatively smooth interface.

Ta-based interface ohmic contacts to AlGaN/GaN heterostructures

Al/Ti based metallization is commonly used for ohmic contacts to n-GaN and related compounds. We have previously reported an ohmic contact scheme specially designed for AlGaN/GaN heterostructure field-effect transistors (HFETs) [D. Qiao et al., Appl. Phys. Lett. 74, 2652 (1999)]. This scheme, referred to as the “advancing interface” contact, takes advantage of the interfacial reactions between the metal layers and the AlGaN barrier layer in the HFET structure. These reactions consume a portion of the barrier, thus facilitating carrier tunneling from the source/drain regions to the channel region. The advancing interface approach has led to consistently low contact resistance on Al0.25Ga0.75N/GaN HFETs. There are two drawbacks of the Al/Ti based advancing interface scheme, (i) it requires a capping layer for the ohmic formation annealing since Ti is too reactive and is easily oxidized when annealing is performed in pure N2 or even in forming gas, and (ii) the atomic number of Al and that of Ti are too low to yield efficient backscattered electron emission for e-beam lithographic alignment purposes. In this work, we investigated a Ta based advancing interface contact scheme for the HFET structures. We found that the presence of Ta in this ohmic scheme leads to (1) a specific contact resistivity as low as 5×10−7 Ω cm2, (2) efficient electron emission for e-beam lithographic alignment, and (3) elimination of the capping layer for the ohmic annealing.

Photoluminescence properties of δ-doped barrier layers in modulation-doped InAlAs/InGaAs field-effect transistor structures

A photoluminescence peak from the Si δ-doped InAlAs barrier layers (δ peak) is found around the InAlAs exciton peak from the buffer layer in 15 modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structure wafers. The δ peak position ordinarily shifts to shorter wavelength with increasing δ doping concentration. The intensity reduction of the δ peak due to raising the temperature from 6 K is considerably slower than that of the InAlAs exciton peak. The excitation spectrum of the δ peak is clearly different from that of the InAlAs exciton peak and seems to reflect the optical absorption dominated by the potential slope in the upper side of the barrier layer and the quasi-Fermi level. The δ peak is not detected from HFET structures without the contact layers. The δ peak is attributed to the recombination of electrons from the δ-doped layer and photogenerated holes weakly confined in the upper side of the barrier layer.

Frequency Response of Trap States in an AlxGa1−xN/GaN Heterostructure Field-Effect Transistor Measured at the Nanoscale by dC/dV Spectroscopy

ABSTRACTScanning capacitance spectroscopy has been used to characterize, at the nanoscale, the frequency-dependent response of surface charge and of charge in the two-dimensional electron gas of an AlxGa1−xN/GaN heterostructure field-effect transistor structure. dC/dV spectra were measured in a scanning capacitance microscope with a voltage signal consisting of a triangle wave at frequencies of 1 – 50 Hz applied to the sample. The spectra were obtained in the dark (except for 600 nm laser light from the scanning capacitance apparatus) and under illumination. Measurements were performed in the vicinity of and away from charged threading dislocations visible in scanning capacitance images. In the absence of illumination, the dC/dV data indicate that electrons are trapped at or near the AlxGa1−xN surface, consistent with suggestions in the literature that a high density of surface states exists on the free AlxGa1−xN surface. Frequency-dependent measurements show that emission times for these traps can be as long as several hundred ms. In the presence of illumination, reduced electron trapping is observed. The nature and behavior of trap states in the vicinity of threading dislocations is found to differ significantly from that in regions between dislocations for measurements in the dark, and suggest that the electrostatic potential due to the charged threading dislocation is negligible at the surface.

Effect of gate leakage current on noise properties of AlGaN/GaN field effect transistors

The effect of the gate leakage current fluctuations on noise properties of AlGaN/GaN heterostructure field effect transistors (HFETs) has been studied in conventional HFET structures and in AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors (MOS-HFETs). The comparison of the noise properties of conventional AlGaN/GaN HFETs and AlGaN/GaN MOS-HFETs fabricated on the same wafer, allowed us to estimate the contribution of the gate current noise to the HFET’s output noise. The effect of the gate current fluctuations on output noise properties of HFETs depends on the level of noise in the AlGaN/GaN HFETs. For the transistors with a relatively high magnitude of the Hooge parameter α∼10−3, even a relatively large leakage current Ig (Ig/Id∼10−3–10−2, where Id is the drain current) does not contribute much to the output noise. In HFETs with a relatively small values of α (α∼10−5–10−4), the contribution of the leakage current to output noise can be significant even at Ig/Id∼10−4–10−3. For such transistors, a very rapid increase of the 1/f noise with gate bias was observed. The differences in the noise behavior can be linked to the material quality of the AlGaN and GaN layers in different types of HFETs.

Depth-resolved spectroscopy of interface defects in AlGaN/GaN heterostructure field effect transistors device structures

We report depth-resolved low energy electron excited nanometer spectroscopy from AlGaN/GaN heterostructure field effect transistors structures with AlGaN thicknesses as thin as 20 nm. By varying the voltage of a low energy electron beam in ultrahigh vacuum, we can determine whether defect induced luminescence is being emitted from the GaN buffer layer, the interfacial region where the two-dimensional electron gas (2DEG) resides, and the AlGaN barrier layer. By increasing the GaN buffer thickness, known to enhance the electron concentration of the 2DEG by reducing the dislocation density in the active region, we observed an enhancement in AlGaN luminescence, and a 20% reduction in the full width at half maximum of the near band edge peak. When a similar structure with no 2DEG is measured, we find a factor of 8 increase in midgap yellow luminescence relative to the GaN buffer emission. Taken together, these findings indicate that differences in buffer layer thickness and electrical quality can affect not only dislocation density and point defect densities, but also the optical properties of the AlGaN barrier layer and its 2DEG interface.

Photoluminescence spectra from δ-doped barrier layers in modulation-doped InAlAs/InGaAs field-effect transistor structures

The photoluminescence (PL) spectra from the Si δ-doped InAlAs barrier layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with contact and recess-etch-stopper layers are investigated. A PL peak associated with the δ-doped layer (δ peak) is found. The δ-peak position shifts to shorter wavelength with increasing δ-doping concentration. The intensity reduction of the δ peak due to raising the temperature from 6 K is considerably slower than that of the exciton peak from the InAlAs buffer layer at about 8100 Å, to which confined electrons and holes probably contribute. As a result, the δ-peak overwhelms the exciton peak and becomes the main peak of the InAlAs PL at temperatures higher than 40 K. The δ peak is not detected from HFET structures without the contact and recess-etch-stopper layers. The δ peak is attributed to the recombination of electrons from the δ-doped layer and photogenerated holes confined in the upper side of the barrier layer.

Nondestructive evaluation of channel carrier concentration in modulation-doped InAlAs/InGaAs field-effect transistor structures by room-temperature photoluminescence

The sheet carrier concentration (Ns) of channel layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with n+InGaAs contact layers has been successfully and nondestructively determined using the room-temperature photoluminescence (PL) method. It is found that the spectral energy width between the maximum position of the main PL peak around 0.8 eV and the half-maximum position on the higher energy side has a good positive linear correlation with the Ns of the channel measured by the van der Pauw method. The scattering of the data is less than ±3×1011 cm−2. The determination of Ns is effective even if the HFET structures have not only n+InGaAs contact layers but also layers for InAlAs Schottky level-shift diodes. From a comparison with low-temperature PL spectra, the main PL peak is attributed to the e2h transition in the quantum well of the channel. It is considered that the slope of the peak stretches further to the high energy as the Fermi energy in the channel become higher, i.e., as the Ns becomes larger.

Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the “advancing” Al/Ti metallization

The ohmic contact formation of Al/Ti on AlGaN/GaN heterostructure field effect transistors (HFETs) with and without Si implantation was investigated. Direct implantation and implantation through an AlN capping layer were studied. Compared to implantation through AlN, direct implantation is more effective in reducing the contact resistance. An Al(200 Å)/Ti(1500 Å) bilayer structure, called the “advancing” metallization, was used in this investigation to take advantage of consuming nearly all the top AlGaN layers for easy carrier access to the GaN layer underneath. Combining the direct implantation and the advancing metallization, low contact resistance of the order of 0.25 Ω mm (∼5.6×10−6 Ω cm2) can be readily obtained on HFET structures with an AlGaN layer about 340 Å thick and with an Al fraction of at least 22%.

Microstructure of Ti/Al ohmic contacts for n-AlGaN

Transmission electron microscopy was employed to evaluate the microstructure of Al/Ti ohmic contacts to AlGaN/GaN heterostructure field-effect transistor structures. Contact resistance was found to depend on the structure and composition of the metal and AlGaN layers, and on atomic structure of the interface. A 15–25-nm-thick interfacial AlTi2N layer was observed at the contact-AlGaN interface. Formation of such nitrogen-containing layers appears to be essential for ohmic behavior on n-type III-nitride materials suggesting a tunneling contact mechanism. Contact resistivity was found to increase with Al fraction in the AlGaN layer.

Measurement of piezoelectrically induced charge in GaN/AlGaN heterostructure field-effect transistors

Electron concentration profiles have been obtained for AlxGa1−xN/GaN heterostructure field-effect transistor structures. Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics. Characterization of a nominally undoped Al0.15Ga0.85N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al0.15Ga0.85N/GaN interface. Measurements performed on an Al0.15Ga0.85N/GaN transistor structure with a buried Al0.15Ga0.85N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al0.15Ga0.85N isolation layer, both of which are attributable to piezoelectric effects.

Demonstration of the heterostructure field-effect transistor as an optical modulator

A new semiconductor waveguide absorption modulator is demonstrated utilizing the heterostructure field-effect transistor structure. The modulator of 300 μm length and 10 μm width achieves an extinction ratio of 8 for a gate voltage change of 2.5 V and an absorption change greater than 2300 cm−1. The transistor transconductance is 92 ms/mm for a 1 μm device and an identical structure has been reported as an edge-emitting laser providing an ideal combination for optoeletronic integration.

Monte Carlo simulation of semiconductor devices

This paper gives a review of applications of the Monte Carlo technique and recent literature on Monte Carlo modeling of semiconductor devices. The emphasis of the original research results reported in this paper is on self-consistent ensemble Monte Carlo simulation of GaAs/AlGaAs Heterostructure Field-Effect Transistors (HFETs) and novel HFET structures. We consider both electron and hole transport keeping in mind possible applications for complementary devices and circuits. Hole transport properties in GaAs are treated using corrected expressions for the scattering rates and a very accurate analytical description of the valence bands valid to about 1 eV. We review the corrected rates. Our simulations demonstrate superlinear behavior in the velocity-field relationship at low temperatures, and a temperature maximum is found in the low-field mobility between 40 K and 60 K at a doping density of 10 16 cm −3. We describe in detail our two-dimensional self-consistent Monte Carlo simulator and demostrrate the usefulness of Monte Carlo simulations for developing and validating simple device models utilized in computer-aided design tools for VLSI circuits. From this perspective, the unified charge-control model is also briefly outlined. Short-channel effects in self-aligned HFETs are shown to be caused by the injection of charge from the contact regions into the buffer beneath the device channel for gate lengths shorter than about 0.5 μm, given an adequately large aspect ratio. For possible incorporation in charge-control models, the threshold voltage shift as well as the output conductance in saturation are found to be approximately inversely proportional to the gate length under the same conditions. Our simulations show that a p-i-p + buffer structure improves the carrier confinement to the channel, reducing the output conductance by about 80% in a 0.3 μm device. Two device concepts recently proposed, aimed at increasing carrier velocities, are studied. The Variable Threshold HFET (VTHFET) as well as the Split-Gate HFET (SGHFET) utilize a gate voltage swing that is made to depend on lateral position. This position dependence raises the resistivity and thus the electric field and the velocity near the source contact. For a certain doping configuration, a p-type-like VTHFET is shown to exhibit a 78% increase in the current-gain cutoff frequency f T, a 59% increase in the maximum transconductance, and substantially higher K-factor compared with a conventional 0.5 μm GaAs/AlGaAs HFET. In n-type VTHFETs, the only pronounced improvement is broader and flatter high g m region. The much larger improvement using p-type VTHFETs is shown to be associated withthe lower hole mobility, the absence of satellite valleys, and the smaller velocity overshoot. The n-VTHFET also suffer from a big shift in overall threshold voltage. The remarkable modulation of velocity and energy profiles achieved could in its own right warrant applications in other n-type devices, such as real-space transfer devices. Guidelines for succesful utilization of the VTHFET concept in other materials and for other device geometries are given.