GaN Metal Semiconductor Field-effect Transistor Research Articles

Temperature Dependent Analytical DC Model for Wide Bandgap MESFETs

In this paper, an analytical model has been developed to predict DC characteristics of wide bandgap metal semiconductor field effect transistors (MESFETs). The model evaluates potential distribution inside the channel of the device by dividing the Schottky barrier depletion layer into four distinct regions and predicts I - V characteristics both at the room as well as at elevated temperatures. It also considers selfheating effects caused by the high-drain current and predicts negative output conductance, usually exhibited by wide bandgap MESFETs. The validity of the proposed technique is ensured by applying it on GaN and SiC MESFETs. It has been shown that the developed technique offers ~50% and ~37% improved accuracy in predicting the output characteristics of the device at T = 300 K and T = 500 K, respectively, relative to the best reported model. Thus, the proposed technique can be employed in the device modeling software involving high-power MESFETs.

Design and function of GaN MESFET terahertz signal generator by finite difference method

The GaN based metal semiconductor field effect transistor (MESFET) devices have been designed by employing finite difference method in order to fit quite well for generation of terahertz signals. The equipotential and equi-electric field contours are broadly developed with variation of drain and gate voltages. The motion of Gunn domain with variation of voltage is constantly observed from source to drain in the channel area of MESFET. In fact, the domain is more clearly visible in the equi-electric field contours than that in the equipotential contours. The distribution of electron concentration (n) in the MESFET is also determined in the steady state mode by fitting right successive relaxation factor and electron relaxation time. The I-V curves of drain are tested by employing Gummel process in finite difference method. The MESFET device is designed in such way to obtain 26.5GHz output signal in the millimeter region by tailoring its physical parameters such as geometry of device, carrier concentration, mobilities etc.

High temperature electron transport properties in AlGaN/GaN heterostructures

Hall electron mobility (μH) and sheet concentration (ns) in AlGaN/GaN heterostructures have been measured from 77 to 1020 K. The effect of the deposited Al2O3 layer is also investigated with varying its thickness. It is found that μH decreases monotonously with the temperature (T) and its dependence is well approximated with the function of μH=4.5×103 exp(−0.004T) in the temperatures over 350 K. The function is different from the commonly used one of μH=AT−α (α∼1.5), which indicates that the mobility is not only governed by the polar optical phonon scattering but also the deformation potential scattering plays a role. The sheet electron concentration (ns) has a weak dependence on the temperature, that is, slightly decreases with temperature in 300–570 K and increases gradually up to 1020 K. The decrease is explained by considering the reduction in the polarization (probably both spontaneous and piezoelectric) charge and the increase seems to be due to the parallel conduction through the interface between GaN buffer layer and sapphire substrate. The dependence of sheet resistance (Rsh) in AlGaN/GaN is compared with that of n-GaN. In the low temperatures, AlGaN/GaN shows a lower Rsh due to its high mobility, however, at the temperatures higher than 350 K, Rsh of AlGaN/GaN becomes higher than that of n-GaN. This result implies that AlGaN/GaN high-electron-mobility-transistors are inferior to GaN metal-semiconductor field-effect transistors in terms of higher source, drain, and channel resistances at high temperature operations, although further investigations on other performances such as output power and reliability are needed. The Al2O3 deposited AlGaN/GaN shows about 15% higher ns than without Al2O3 layer for the whole temperatures. On the contrary, μH at 77 K shows a slight decrease with Al2O3 deposition, which degree is not affected by the layer thickness. In the temperatures higher than 400 K, μH is almost the same for with and without Al2O3 layer.

Electrothermal simulation of the self-heating effects in GaN-based field-effect transistors

We report results of the analytical and numerical investigation of self-heating effects in GaN-based high-power field-effect transistors. The problem of heat transfer in a transistor structure has been solved both analytically, using the method of images, and numerically. Two-dimensional electrothermal simulations of the GaN metal-semiconductor field-effect transistors on SiC and sapphire substrate have been performed in a framework of the drift-diffusion model. Using the physical-based simulations, we studied the dependence of the hot-spot temperature on the gate-to-gate pitch in the transistors with multiple gate fingers. Particular attention has been paid to comparison of self-heating effects in GaN transistors on SiC and sapphire substrates. The obtained results can be used for optimization of the thermal design of the GaN-based high-power field-effect transistors.

Low-resistance Ohmic contacts for high-power GaN field-effect transistors obtained by selective area growth using plasma-assisted molecular beam epitaxy

Selective area growth (SAG) has been demonstrated using plasma-assisted molecular beam epitaxy (PAMBE) resulting in vastly improved Ohmic contacts for GaN-based high-power field-effect transistors (FETs). A heavily doped n-GaN layer was grown only in the Ohmic contact region and the resulting nonalloyed Ti∕Al∕Ti∕Au metal contacts exhibited linear Ohmic behavior. Through rapid thermal annealing, very low specific contact resistivity (1.8×10−8Ωcm2) was obtained at 850°C. Furthermore, contact resistances below 0.8Ωmm were obtained by annealing at a wide range of temperatures (750–950°C). GaN metal-semiconductor FETs were fabricated to investigate the effect of the PAMBE-SAG on device performance, producing great improvement in the dc characteristics.

Selective-area growth and fabrication of recessed-gate GaN MESFET using plasma-assisted molecular beam epitaxy

For the first time, selective-area growth (SAG) technique has been developed using plasma-assisted molecular beam epitaxy (PAMBE), enabling fabrication of a recessed-gate structure for the metal-semiconductor field-effect transistor (MESFET) without etching. On patterned SiO 2 samples, polycrystalline GaN and single crystal n + -GaN were observed to grow in the masked and unmasked regions, respectively. The regrown layers were analyzed using AFM. Ohmic contact formed on the n + -GaN exhibited a vastly improved contact resistivity of 1.8 x 10 -8 Ω cm 2 , giving rise to excellent device characteristics including a peak drain current of 360 mA/mm and a maximum transconductance of 46 mS/mm. The advantages of SAG were further investigated by comparing the dc characteristics of recessed-gate and unrecessed MESFET.

Physics-based simulation of buffer-trapping effects on slow current transients and current collapse in GaN field effect transistors

Two-dimensional transient analyses of GaN metal-semiconductor field effect transistors (MESFETs) are performed in which a three level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor, and a deep acceptor are included. Quasipulsed current-voltage (I-V) curves are derived from the transient characteristics and are compared with steady-state I-V curves. It is shown that when the drain voltage VD is raised abruptly, the drain current ID overshoots the steady-state value, and when VD is lowered abruptly, ID remains at a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes quantitatively. The drain lag could be a major cause of current collapse, although some gate lag is also seen due to the buffer layer. The current collapse is shown to be more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because the change of ionized deep-donor density becomes larger and hence the trapping effects become more significant. It is suggested that to minimize the current collapse in GaN-based FETs, an acceptor density in a semi-insulating layer should be made low, although the current cutoff behavior may be degraded.

Double-Ion-Implanted GaN MESFETs with Extremely Low Source/Drain Resistance

AbstractIncorporation of Si ion implantation to GaN metal semiconductor field effect transistor (MESFET) processing has been demonstrated. The channel and source/drain regions formed using Si ion implantation into undoped GaN on sapphire substrate. In comparison with the conventional devices without ion implanted source/drain structures, the ion implanted devices showed excellent device performance. On-state resistance reduces from 210 Ω-mm to 105 Ω-mm. Saturation drain current and maximum transconductance increase from 36 mA/mm to 78 mA/mm and from 3.8 mS/mm to 10 mS/mm, respectively.

Response to “Comment on ‘Carrier trapping and current collapse mechanism in GaN metal-semiconductor field effect transistors’ ” [Appl. Phys. Lett. 86, 016101 (2005)

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Photoionization cross-section analysis for a deep trap contributing to current collapse in GaN field-effect transistors

We perform a more incisive numerical analysis of the photon energy dependence of the photoionization cross section of a prominent deep trap (conventionally labeled as Trap 1), which had been found [Klein et al., Appl. Phys. Lett. 75, 4016 (1999)] to act as a contributor to current collapse phenomena in GaN metal-semiconductor field-effect transistors. The analytical expression for the photoionization cross section of Trap 1 is taken in the form of a convolution of a temperature-independent electronic part with a thermally broadened Franck–Condon (FC) factor, which also applies to the relevant regime of large lattice relaxation. For a direct comparison with earlier results, we specialize the present analysis to an electronic cross-section part represented by the Lucovsky model in combination with the semiclassical (Gaussian) approximation for the FC factor. In qualitative accordance with an earlier estimation by Klein et al. we obtain a value of EO≈1.9 eV for the classical optical ionization energy in combination with a full width at half maximum of 0.64 eV. The latter implies, on the assumption for the average phonon energy, to be of order 50 meV, an apparently unusually large magnitude, D≈1.1 eV, for the Franck–Condon shift. This parameter constellation is equivalent to a thermal ionization (electron binding) energy, ET=EO−D, of about ET≈0.8 eV. Such a location of Trap 1 near the middle of the upper half of the fundamental gap of GaN, ET≈Eg/4, is at clear variance to the earlier suggestion by Klein et al. for Trap 1 to be a midgap level. The present estimation offers a chance for detecting the Trap 1 also by deep-level transient spectroscopy measurements. An eventual availability of photoionization cross-section data for different temperatures is seen to be the prerequisite for a decisive reduction of residual uncertainties concerning the configuration coordinate diagram.

Carrier trapping and current collapse mechanism in GaN metal–semiconductor field-effect transistors

A mechanism for current collapse in GaN metal–semiconductor field-effect transistors is proposed, which assumes the existence of acceptor traps with multiple states in the band gap. Current collapse has been experimentally observed in the current–voltage characteristic after the drain voltage sweep had exceeded the threshold for impact ionization in a previous measurement. In the proposed model, electrons generated by impact ionization are captured by neutral acceptor trap states in the substrate located above the valence band. The charged trap states move to an energy level located near midgap, creating a positively charged depletion region in the channel, and causing current collapse. With increasing drain bias, the quasi-Fermi level approaches the charged trap states at the drain end of the gate, initiating detrapping of the electrons and restoring the current. The calculated results show good agreement with published experimental data.

Properties of wurtzite GaN MESFETs studied by two-dimensional full band Monte Carlo approach

The properties of a GaN Metal–Semiconductor Field Effect Transistor were studied based on two-dimensional full band Monte Carlo simulations. The dependences of the distributions of the electric potential, electron concentration, electric field, drift velocity, and average electron energy on the gate–source voltage ( V GS) and drain–source voltage ( V DS) were obtained, then the characteristics of the drain current ( I DS) versus the drain–source voltage ( V DS) and the transconductance ( G m) versus V GS characteristics were determined. At V DS =15 V and V GS =0 V, I DS is 5.03 A/cm, which is higher value. The G m− V GS curve shows bell shaped and the maximum G m is 112 ms/mm at V DS =15 V and V GS =1.5 V. The current gain cutoff frequency ( f T) is 98 GHz at V DS =15 V and V GS =0 V.

Model for illumination-dependent trap occupancy in optically biased GaN metal-semiconductor field effect transistor for improved electrical characteristics

The effect of wavelength of illumination on the trap occupancy of an optically biased GaN metal-semiconductor field effect transistor (MESFET) for its improved electrical characteristics is presented. The model evaluates the I d to V d characteristics and transconductance of the device for the cases of light turning both on and off at a reference time t = 0. The photovoltaic effect across the Schottky junction and the depletion width modulation in the active layer are taken into account along with the gate-length modulation and parasitic source and drain resistances. It is shown that the trap-related degradation of the drain characteristics could be overcome when the device is used under illumination with light acting as a dual gate.

Drain current collapse in GaN metal–semiconductor field-effect transistors due to surface band-bending effects

The small gate length of high-frequency field-effect transistors (FETs) leads to situations where the role played by the parasitic access regions in their circuit properties becomes dominant. Due to surface band-bending effects present in wide bandgap semiconductors such as GaAs or GaN, two parasitic FETs having ungated surfaces as floating gates appear in series with the targeted one. They have to be considered properly under any electrical or optical stimulation applied. From the electrical point of view, the drain current set-up process itself leads to voltage drops along the channel that modify the surface charge of the above floating gates. By this mechanism, some surface states beside the drain terminal are charged, thus building a negatively-charged floating gate that strongly degrades the current transfer ability of the drain contact. These degraded output features are responsible for the drain current collapse observed in GaN metal–semiconductor field-effect transistors. Their recovery from this collapsed state by both electrical and optical excitations is also explained by the model.

Characterization of GaN-based metal–semiconductor field-effect transistors by comparing electroluminescence, photoionization, and cathodoluminescence spectroscopies

We report on a methodological comparison between photocurrent (PC), electroluminescence (EL), and cathodoluminescence (CL) investigations on GaN metal–semiconductor field-effect transistors. The purpose of this work is to show the effectiveness and the complementarity of these experimental techniques and to investigate the presence and nature of electron traps which limit the performances of the devices. PC measurements reveal four distinct energy levels, located at 1.75, 2.32, 2.67, and 3.15 eV, responsible for current collapse. The 1.75 eV level has also been observed in low temperature EL curves. The 2.32 and 2.67 eV levels, on the basis of the comparison with CL and EL results, can be correlated with the so-called “yellow band,” located at 2.2 eV. The origin of 1.75 and 3.15 eV levels is at present unknown, however a nonradiative nature has been attributed to the 3.15 eV level, due to the absence of this signature in both CL and EL spectra. The luminescence measurements also reveal the presence of the donor–acceptor pair emission at 3.27 eV and the near-band-edge transition at 3.45 eV. EL measurements show a series of emission peaks in the energy range between 1 and 1.4 eV, while the CL spectra reveal a broadband at 2.8 eV, which arises mainly from the semi-insulating layer. This result has been obtained by increasing the energy of the CL electron beam, allowing us to investigate both the conduction channel and the layers underneath it.

GaN-based optoelectronic devices on sapphire and Si substrates

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm 2/V s with a sheet carrier density of 4.8×10 12 cm −2 was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.

Highly doped thin-channel GaN-metal–semiconductor field-effect transistors

We report on the influence of the channel doping on dc, high frequency, and noise performance of GaN metal–semiconductor field-effect transistors (MESFETs) grown on sapphire substrates. The devices with the channel thicknesses from 50 to 70 nm and doping levels up to 1.5×1018 cm−3 were investigated. An increase in the channel doping results in the improved dc characteristics, higher cutoff, and maximum oscillation frequencies, and reduced low frequency and microwave noise. The obtained results demonstrate that the dc and microwave performance characteristics of short-channel GaN MESFETs may be comparable to those for conventional AlGaN/GaN heterostructure FETs.

Photoionization spectroscopy of traps in GaN metal-semiconductor field-effect transistors

Measurements of the spectral and intensity dependences of the optically-induced reversal of current collapse in a GaN metal-semiconductor field-effect transistor (MESFET) have been compared to calculated results. The model assumes a net transfer of charge from the conducting channel to trapping states in the high-resistivity region of the device. The reversal, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built-in electric field associated with the trapped charge distribution. For a MESFET in which two distinct trapping centers have been spectrally resolved, the experimentally measured dependence upon light intensity was fitted using this model. The two traps were found to have very different photoionization cross-sections but comparable concentrations (4×1011 cm−2 and 6×1011 cm−2), suggesting that both traps contribute comparably to the observed current collapse.

Fabrication of a High-Power Gan Metal Semiconductor Field-Effect Transistor

ABSTRACTA high-power metal semiconductor field-effect transistor (MESFET) for operating at a very large-current using GaN is reported for the first time. GaN was grown by metalorganic chemical vapor deposition (MOCVD). Sapphire substrates were used for GaN growth. A GaN MESFET with a large size was fabricated. Multi-finger gates were used for large-current operation. The total gate width was 8 cm and the gate length was 2 νm. The electrode materials of the source and the drain were Al/Ti/Au and the schottky electrodes were Pt/Au. The gate, source, and drain were isolated using SiO2. An FET structure was fabricated using a dry-etching technique. Multi-electrode structures were also fabricated using SiO2 for isolating the source, drain, and gate electrodes, respectively. The FET was operated at a current of over 5 A. The breakdown voltage was over 500 V. The transconductance (gm) was about 12 mS/mm. The pinch-off voltage was about -8 V. We confirmed that this GaN MESFET can also be operated at a current of 10 A.

GaN MESFETs on (111) Si substrate grown by MOCVD

A GaN metal-semiconductor field-effect transistor (MESFET) has been grown on (111) Si substrate by metalorganic chemical vapour deposition using Al0.27Ga0.73N/AlN intermediate layers. The device showed a maximum extrinsic transconductance 25 mS/mm and a drain-source current 169 mA/mm with a complete pinch-off for the 2.5 µm gate-length. The Al0.27Ga0.73N/AlN intermediate layers were effective in obtaining a mirror-like surface morphology and a high-resistive undoped GaN layer beneath a channel layer. The GaN MESFET on Si also exhibited no self-heating effects (drain-source current reduction) under high-power conditions, which results from the better thermal properties of Si than those of sapphire.