III-nitride High Electron Mobility Transistors Research Articles

Characteristics study of heterojunction III-nitride/β-Ga2O3 nano-HEMT for THz applications

In this research study, a recessed gate III-Nitride high electron mobility transistor (HEMT) grown on a lattice matched β-Ga2O3 substrate is designed. This research investigation aims to enhance DC and RF performance of AlGaN/GaN HEMT, and minimize the short-channel effects by incorporating an AlGaN back layer and field plate technique, which can enhances electron confinement in two-dimensional electron gas (2DEG). A precise comparison analysis is done on the proposed HEMT’s input characteristics, output characteristics, leakage current characteristics, breakdown voltage properties, and RF behaviour in presence and absence of AlGaN back layer in regards to field plate configuration. The inclusion of back barrier aids in raising the level of conduction band, which reduces leakage loss beneath the buffer, and aids in keeping the 2DEG to be confined to narrow channel. Furthermore, the field plate design offers an essential electric field drift between gate and drain, resulting to enhanced breakdown voltage characteristics.

High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al0.85Ga0.15N heterostructures on single-crystal AlN substrates

The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 ×1013/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 Ω/◻ is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors.

Ohmic contact morphology improvement with reduced resistance using Si/Au/Ti/Al/Ni/Au (AlGaN) and Si/Au/Ti/Al/Ni/Au (InAlN) stack layers in III-Nitride HEMTs

This article reports a Ti/Al-based ohmic contact utilizing a thin interfacial Au layer for improved morphology, edge acuity and low contact resistance for applications to III-Nitride high electron mobility transistors (HEMTs). Conventional Ti/Al contacts are based on a metal stack of the form Ti/Al/X/Au, where X is any suitable barrier layer such as Ni. The formation of ohmic contact in GaN and its alloy systems is governed by the formation of TiN during annealing; Au inter-diffusion during annealing also assists the formation of TiN islands and aids in reducing the contact resistance. Furthermore, inter-metallic phase formation between Ti and Al lowers the contact resistance. It is observed that the contact surface morphology strongly depends on the top Au layer thickness. Insertion of a very thin Au layer (2–3 nm) has drastically improved the surface quality with the achievement of contact resistance as low as 0.36 Ω mm and a specific contact resistance of 2.4 × 10−6 Ω cm2 on AlGaN/GaN HEMT. The reduction in contact resistance is due to the formation of more conducting intermetallic phases during annealing, which is assisted due to the thin Au layer insertion. Further reduction in the contact resistance to 0.13 Ω mm was also achieved with the introduction of Si as a dopant layer in the reported metal scheme. The measured rms surface roughness was reduced to ∼8 nm from 0.12 µm in comparison to the conventional ohmic contact.

SiNx/(Al,Ga)N interface barrier in N-polar III-nitride transistor structures studied by modulation spectroscopy

Contactless electroreflectance studies coupled with numerical calculations are performed on in-situ SiNx capped N-polar III-nitride high electron mobility transistor (HEMT) structures with a scaled channel thickness in order to analyse the built-in electric field in the GaN channel layer. The experimentally obtained field values are compared with the calculated field versus channel thickness curves. Furthermore, the experimental and theoretical sheet carrier densities, ns, are evaluated. While a gradual decrease in carrier concentration with decreasing channel thickness is expected for N-polar structures, experimentally a sudden drop in the ns values is observed for samples with very thin channels. The additional loss in charge was associated with a change in the SiNx/AlGaN interface Fermi level at very thin channel thicknesses.

A unified static-dynamic analytic model for ultra-scaled III-nitride high electron mobility transistors

This paper presents an analytic model to calculate nodal charges and their corresponding internodal capacitances in ultra-scaled III-nitride high electron mobility transistors (HEMTs) in which carrier transport is expected to be quasiballistic. The dynamic model is combined with our previously published static model [K. Li and S. Rakheja, J. Appl. Phys. 123, 184501 (2018)] to provide a unified physical description of the HEMT. In the dynamic model, the position-dependent channel charge is obtained using a realistic potential profile in the channel, obtained from the solution of the Poisson equation. The effect of electric field lines penetrating from the ungated access regions into the channel (gated region) is included. Fringing electric field lines from the gate to the channel, which are especially critical in the off-region of the device, are included in the charge description. The channel charges are obtained self-consistently with the transport model and introduce 16 additional input parameters, which are necessary to explain the bias dependence of internodal capacitances in the off-state and in the off-to-on transition region of the device. Using the model, we elucidate the difference in the capacitance-voltage behavior of drift-diffusive and quasiballistic devices. The sensitivity of channel charges to the specific formulation of the potential profile is also examined. The unified model is applied to the experimental capacitance-voltage data of 42-nm and 105-nm gate-length InAlN/GaN HEMTs with an InGaN backbarrier. Additionally, the model is validated against hydrodynamic simulations of a 50-nm gate-length AlGaN/GaN HEMT with significant Joule heating. The model yields an excellent agreement with the measured and simulated data sets over a broad range of bias and temperature conditions. The model is also used to elucidate the role of contact resistance, Joule heating, and nodal capacitances on the RF performance, i.e., cut-off frequency, third-order intermodulation current, and the input third-order intercept point, of scaled III-nitride HEMTs.

An analytic current-voltage model for quasi-ballistic III-nitride high electron mobility transistors

We present an analytic model to describe the DC current-voltage (I-V) relationship in scaled III-nitride high electron mobility transistors (HEMTs) in which transport within the channel is quasi-ballistic in nature. Following Landauer's transport theory and charge calculation based on two-dimensional electrostatics that incorporates negative momenta states from the drain terminal, an analytic expression for current as a function of terminal voltages is developed. The model interprets the non-linearity of access regions in non-self-aligned HEMTs. Effects of Joule heating with temperature-dependent thermal conductivity are incorporated in the model in a self-consistent manner. With a total of 26 input parameters, the analytic model offers reduced empiricism compared to existing GaN HEMT models. To verify the model, experimental I-V data of InAlN/GaN with InGaN back-barrier HEMTs with channel lengths of 42 and 105 nm are considered. Additionally, the model is validated against numerical I-V data obtained from DC hydrodynamic simulations of an unintentionally doped AlGaN-on-GaN HEMT with 50-nm gate length. The model is also verified against pulsed I-V measurements of a 150-nm T-gate GaN HEMT. Excellent agreement between the model and experimental and numerical results for output current, transconductance, and output conductance is demonstrated over a broad range of bias and temperature conditions.

Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm2

We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast process that goes beyond the limitations of current III-Nitride high electron mobility transistors.

Analytic Modeling of Nonlinear Current Conduction in Access Regions of III-Nitride HEMTs

AbstractThe transconductance degradation caused by the non-linear resistance of access regions in III-nitride high electron-mobility transistors (HEMTs) is mainly responsible for limiting the RF linearity of the transistor. In this paper, we use Landauer’s transmission theory to develop an analytic electrothermal current-voltage (I-V) model of access regions in III-nitride HEMTs. With only 12 parameters, most of which have a physical origin and can be obtained through experimental calibration, the model is able to correctly predict the I-V behavior in access regions from the drift-diffusive to the quasi-ballistic transport regimes. Model accuracy is demonstrated by comparing the results against experimental and numerical hydrodynamic simulations of ungated transmission line structures with length scales ranging from few 10’s of nanometers to 10’s of micrometers.

(Invited) Electrothermal Performance Optimization of III-Nitride HEMTs Capped with Nanocrystalline Diamond

AlGaN/GaN high electron mobility transistors (HEMT) capped with nanocrystalline diamond (NCD) have been demonstrated in the past to outperform electrically and thermally their SiN-passivated counterparts. However, a major process limitation for the integration of a diamond heat spreader has been the O2-plasma damage in the gate opening associated with etching the diamond cap. A sacrificial gate (SG) process for plasma damage-free integration of top-side NCD capping layers is thus developed. On HEMTs with a SG, the addition of a NCD cap did not cause any significant degradation in mobility, carrier density, or sheet resistance. Hall characterization showed minimal (~6%) reduction in sheet carrier density and commensurate increase in sheet resistance, while maintaining mobility and on-state drain current density. Pulsed IDS and on-resistance were improved, indicating that a 10 nm SiN/500 nm NCD could offer improved AlGaN surface passivation compared to a more conventional 100 nm thick PECVD SiN film.

(Invited) Electrothermal Performance Optimization of III-Nitride HEMTs Capped with Nanocrystalline Diamond

AlGaN/GaN high electron mobility transistors (HEMT) capped with nanocrystalline diamond (NCD) have been demonstrated in the past to outperform electrically and thermally their SiN-passivated counterparts [1, 2]. However, a major process limitation for the integration of a diamond heat spreader has been the O2-plasma damage in the gate opening associated with etching the diamond cap. To reduce plasma damage, Anderson et al. have demonstrated a two-step diamond etch process, where the plasma power is reduced towards the end of the etch to minimize damage to the AlGaN surface [3]. While this process has enabled improved electrical performance of NCD-capped HEMTs, optimal passive heat spreading requires NCD films thicker than 0.5 µm which challenges the two-step etch approach [4]. Further complications such as on-wafer variations in NCD thickness have motivated the search for a more robust gate process. Recently, we have proposed the use of a sacrificial gate (SG) prior to diamond growth [5, 6]. The SG stack consisted of a 10 nm thick ALD-grown Al2O3 layer followed by a 100 nm thick PECVD-grown SiNx layer. The SiNx layer served as an etch stop for the NCD layer, and the Al2O3 served as an etch stop for the SiNx layer. The Al2O3/SiNx stack was patterned with the dimensions of the gate and a blanket 500 nm thick CVD NCD film was conformally-grown over the feature. The gate feature was then realigned and a high power O2-plasma was used to clear the NCD film in the gate region. This resulted in a much more reliable gate opening since the window for NCD over-etch was much larger. The sacrificial SiNx finger was subsequently etched away using a SF6-ICP process. The Al2O3 layer was removed in dilute HF solution with no measureable degradation to the rest of the device. Finally, the Ni/Au gate metal stack was lifted off. On HEMTs with a SG, the addition of a NCD cap did not cause any significant degradation in mobility, carrier density, or sheet resistance. By contrast, when a SG process was not used to protect the AlGaN surface, the NCD-capped HEMT experienced a decrease in mobility and an increase in sheet resistance. For the SG NCD-capped HEMT, the off-state current increased by less than an order of magnitude and threshold voltage shifted by about 1 V. However, on-state drain current levels remained comparable and degradation in dynamic on resistance (RON,DYN) upon an off-state drain bias stress was improved, indicating potential improvement in AlGaN surface passivation by the NCD film [2, 3]. Such improvement is also corroborated by the improvement in breakdown voltage in NCD-capped HEMTs with a conventional gate opening design. This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices. We will also review several novel NCD concepts, such as in-plane control of NCD thermal conductivity, as well as a diamond-gated AlGaN/GaN HEMT [7, 8]. [1] M.J. Tadjer, et al., IEEE Electron Dev. Lett., vol. 33, no. 1, pp. 23-25, 2012. [2] D.J. Meyer, et al., IEEE Electr. Dev. Lett., vol. 35, no. 10, pp. 1013, 2014. [3] T.J. Anderson et al., 70th Dev. Research Conf. Proc., pp. 155-156, 2012. [4] A. Wang, M.J. Tadjer, and F. Calle, Semicond. Sci. Technol., vol. 28, pp. 055010 (2013). [5] T.J. Anderson et al., CS Mantech Conf. Digest, pp. 205-208, 2013. [6] M.J. Tadjer, et al., Phys. Status Solidi A, 2015, in press. [7] T.J. Anderson, et al., IEEE Electr. Dev. Lett., vol. 34, no. 11, pp. 1382, 2013. [8] J. Anaya et al., Acta Materialia 103 (2016) 141-152.

Investigation of Surface- and Buffer-Induced Current Collapse in GaN High-Electron Mobility Transistors Using a Soft Switched Pulsed \(I-V\) Measurement

In this letter, we investigated the behaviors of surface- and buffer-induced current collapse in AlGaN/GaN high-electron mobility transistors (HEMTs) using a soft-switched pulsed \(I-V\) measurement with different quiescent bias points. It is found that the surface- and buffer-related current collapse have different relationship with the gate and drain biases ( V \(_{\mathrm { {\!GS0,}}}\) V \(_{\mathrm { {\!DS0}}}\) ) during quiescent bias stress. The surface-induced current collapse in devices without passivation monotonically increases with the negative V \(_{\mathrm { {\!GS0}}}\) , suggesting that an electron injection to the surface from gate leakage is the dominant mechanism and the Si3N4 passivation could effectively eliminate such current collapse. The buffer-induced current collapse in devices with intentionally carbon-doped buffer layer exhibits a different relationship with V \(_{\mathrm { {\!GS0}}}\) after surface passivation. The buffer-related current collapse shows a bell-shaped behavior with V \(_{\mathrm { {\!GS0}}}\) , suggesting that a hot electron trapping in the buffer is the dominant mechanism. The soft-switched pulsed \(I-V\) measurement provides an effective method to distinguish between the surface- and buffer-related current collapse in group III-nitride HEMTs.

Improved breakdown characteristics of monolithically integrated III-nitride HEMT–LED devices using carbon doping

We report selective growth of AlGaN/GaN high electron mobility transistors (HEMTs) on InGaN/GaN light emitting diodes (LEDs) for monolithic integration of III-nitride HEMT and LED devices (HEMT–LED). To improve the breakdown characteristics of the integrated HEMT–LED devices, carbon doping was introduced in the HEMT buffer by controlling the growth pressure and V/III ratio. The breakdown voltage of the fabricated HEMTs grown on LEDs was enhanced, without degradation of the HEMT DC performance. The improved breakdown characteristics can be attributed to better isolation of the HEMT from the underlying conductive p-GaN layer of the LED structure.

Applications of TEM imaging, analysis and electron holography to III-nitride HEMT devices

This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ∼2.1 × 10 13/cm 2 have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.

Turn-on voltage engineering and enhancement mode operation of AlGaN/GaN high electron mobility transistor using multiple heterointerfaces

A novel device design for enhancement mode operation of III-nitride high electron mobility transistor (HEMT) structure has been proposed and demonstrated. The proposed HEMT device structure consists of a multi-heterojunction (SiO 2/Al x Ga 1− x N/GaN/Al y Ga 1− y N) design in contrast to a single AlGaN/GaN heterojunction commonly used in conventional III-nitride HEMT designs. Normally OFF operation of the proposed HEMT design is expected to take place for thicker GaN and the top Al x Ga 1− x N layers than previously allowed. The effects of design parameters on the value of the device turn ON voltage have been studied extensively using simulations. Various device structures have been developed based on the simulations results and epitaxially grown by MOCVD. I– V measurements support the effects of design parameters on the turn ON voltage predicted by the simulations and have confirmed the enhancement mode operation.

Direct immobilization and hybridization of DNA on group III nitride semiconductors

A key concern for group III-nitride high electron mobility transistor (HEMT) biosensors is the anchoring of specific capture molecules onto the gate surface. To this end, a direct immobilization strategy was developed to attach single-stranded DNA (ssDNA) to AlGaN surfaces using simple printing techniques without the need for cross-linking agents or complex surface pre-functionalization procedures. Immobilized DNA molecules were stably attached to the AlGaN surfaces and were able to withstand a range of pH and ionic strength conditions. The biological activity of surface-immobilized probe DNA was also retained, as demonstrated by sequence-specific hybridization experiments. Probe hybridization with target ssDNA could be detected by PicoGreen fluorescent dye labeling with a minimum detection limit of 2 nM. These experiments demonstrate a simple and effective immobilization approach for attaching nucleic acids to AlGaN surfaces which can further be used for the development of HEMT-based DNA biosensors.

High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates

Record performance of high-power GaN/Al/sub 0.14/-Ga/sub 0.86/N high-electron mobility transistors (HEMTs) fabricated on semi-insulating (SI) 4H-SiC substrates is reported. Devices of 0.125-0.25 mm gate periphery show high CW power densities between 5.3 and 6.9 W/mm, with a typical power-added efficiency (PAE) of 35.4% and an associated gain of 9.2 dB at 10 GHz. High-electron mobility transistors with 1.5-mm gate widths (12×125 μm), measured on-wafer, exhibit a total output power of 3.9 W CW (2.6 W/mm) at 10 GHz with a PAE of 29% and an associated gain of 10 dB at the -2 dB compression point. A 3-mm HEMT, packaged with a hybrid matching circuit, demonstrated 9.1 W CW at 7.4 GHz with a PAE of 29.6% and a gain of 7.1 dB. These data represent the highest power density, total power, and associated gain demonstrated for a III-nitride HEMT under RF drive.