Metal-oxide-semiconductor Heterostructure Field-effect Transistors Research Articles

Synthesis of thermally stable HfOxNy as gate dielectric for AlGaN/GaN heterostructure field-effect transistors**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0403000).

In this paper, we adopted thermally stable HfOxNy as gate dielectric for TiN/HfOxNy/AlGaN/GaN heterostructure field-effect transistors (HFETs) application. It demonstrated that the surface morphologies, composition, and optical properties of the HfOxNy films were dependent on oxygen flow rate in the O2/N2/Ar mixture sputtering ambient. The obtained metal–oxide–semiconductor heterostructure field-effect transistors by depositing HfO2 and HfOxNy dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900 °C, the device using HfOxNy dielectric operated normally with good pinch-off characteristics, while obvious degradation are observed for the HfO2 gated one at 600 °C. This result shows that the HfOxNy dielectric is a promising candidate for the self-aligned gate process.

Time-dependent dielectric breakdown of recessed AlGaN/GaN-on-Si MOS-HFETs with PECVD SiO2 gate oxide

This paper reports the first-time evaluation of the time-dependent dielectric breakdown of recessed AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with plasma enhanced chemical vapor deposition (PECVD) SiO2 gate oxide. The interface fixed charge density and oxide bulk charge density extracted from the flat-band voltage characteristics were 2.7 × 1011 ± 6.54 × 1010 cm−2 and -9.71 × 1017 ± 5.18 × 1016 cm−3, respectively. The time dependent dielectric breakdown (TDDB) characteristics exhibited longer lifetime estimation as the SiO2 thickness increased. The excellent reliability of the PECVD SiO2 film was validated for use as the gate oxide of recessed AlGaN/GaN MOS-HFET.

Characterization of interface states in AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with HfO2 gate dielectric grown by atomic layer deposition

The impact of oxidation agent and post-metallization annealing (PMA) on the quality of oxide-semiconductor interface in AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistors (MOS-HFETs) with HfO2 grown by atomic layer deposition (ALD) was investigated. About six orders of magnitude lower gate leakage current (∼10−7 mA/mm at −10 V) was observed for MOS-HFETs with HfO2 gate oxide grown by ALD using ozone oxidation agent (O-HfO2), compared with grown using water oxidation agent (T-HfO2). In addition, the output current frequency dispersion was effectively reduced for O-HfO2 by applying PMA performed at 400 °C in N2 ambient, where the current frequency dispersion measured using 100 ns-long pulses was reduced from 40% for as-deposited gate oxide to only 10% for devices after applied PMA. Frequency dispersion was found to be consistent with density of oxide/semiconductor interface states (Dit) determined near the semiconductor conduction and valence band edge using the capacitance-voltage curves measured at different temperatures (CV-T) and photo-assisted capacitance transient (photo-C-t) technique, respectively. MOS-HFET structures with O-HfO2 showed about one order of magnitude lower Dit near the semiconductor valence band compared with structures with T-HfO2 (2 × 1011 compared with 1012 eV−1 cm−2 at EC-E = 2.5 eV, respectively). Moreover, for devices with O-HfO2, Dit was further reduced in entire energy gap as a result of PMA performed at 400 °C. The increased Dit for T-HfO2 oxides was attributed to Hf-Hf bonds at the HfO2/GaN interface, as deduced X-ray photoelectron spectroscopy (XPS) analysis. In contrast, formation of HfHf bonds was negligible in as-deposited O-HfO2 oxide films, while PMA at 400 °C led to reduction of the hydroxyl group observed by XPS.

Comparative Study on Graded-Barrier AlxGa1−xN/AlN/GaN/Si Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistor by Using Ultrasonic Spray Pyrolysis Deposition Technique

Comparative study on a novel Al2O3-dielectric graded-barrier (GB) AlxGa1−xN/AlN/GaN/Si (x = 0.22 ∼ 0.3) metal-oxide-semiconductor heterostructure field-effect transistor (MOS-HFET) formed by using the ultrasonic spray pyrolysis deposition (USPD) technique has been made with respect to a conventional-barrier (CB) Al0.26Ga0.74N/AlN/GaN/Si MOS-HFET and the reference Schottky-gate HFET devices. The GB AlxGa1−xN was devised to improve the interfacial quality and enhance the Schottky barrier height at the same time. A cost-effective ultrasonic spray pyrolysis deposition (USPD) method was used to form the high-k Al2O3 gate dielectric and surface passivation on the AlGaN barrier of the present MOS-HFETs. Comprehensive device performances, including maximum extrinsic transconductance (gm,max), maximum drain-source current density (IDS,max), gate-voltage swing (GVS) linearity, breakdown voltages, subthreshold swing (SS), on/off current ratio (Ion/Ioff), high frequencies, and power performance are investigated.

Modelling the threshold voltage of p‐channel enhancement‐mode GaN heterostructure field‐effect transistors

p-Channel gallium nitride (GaN) metal–oxide–semiconductor heterostructure field-effect transistors utilising a polarisation induced two-dimensional hole gas operate inherently in depletion mode. The condition for their conversion to enhancement-mode operation is examined via analytical expressions for the threshold voltage and verified via technology computer-aided design (TCAD) simulations. Between the two heterostructures: (i) conventional GaN/aluminium GaN (AlGaN)/GaN and (ii) alternate AlGaN/GaN/AlGaN/GaN examined in this work, the authors demonstrate at higher threshold voltage (), the alternate heterostructure can potentially achieve a higher on-current by a factor of 2 of (), without degradation in the on–off-current ratio, expected ideally to be of the order of .

Al2O3-Dielectric In0.18Al0.82N/AlN/GaN/Si Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors With Backside Substrate Metal-Trench Structure

This paper investigates novel Al2O3-dielectric In0.18Al0.82N/AlN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with backside metal-trench structure grown by using a non-vacuum ultrasonic spray pyrolysis deposition technique. 3- $\mu \text{m}$ deep metal trenches coated with 150-nm thick Ni were formed on the backside of the Si substrate to improve the heat dissipation efficiency. The present In0.18Al0.82N/AlN/GaN MOS-HFET (Schottky-gate HFET) has demonstrated improved maximum drain–source current density ( $I_{\mathrm{ DS, \max }}$ ) of 1.08 (0.86) A/mm at $\textit{V}_{\mathrm{ DS}}=$ 8 V, gate-voltage swing of 4 (2) V, on/off-current ratio ( $I_{\mathrm{ on}} /I_{\mathrm{ off}} $ ) of $8.9\times 10^{8}$ ( $7.4 \times 10^{4} $ ), subthreshold swing of 140 (244) mV/dec, two-terminal off-state gate-drain breakdown voltage (BVGD) of −191.1 (−173.8) V, turn-on voltage ( $V_{\mathrm{ on}} $ ) of 4.2 (1.2) V, and three-terminal on-state drain-source breakdown voltage (BV $_{DS}$ ) of 155.9 (98.5) V. Enhanced power performances, including saturated output power ( $\textit{P}_{out}$ ) of 27.9 (21.5) dBm, power gain ( $\textit{G}_{a}$ ) of 20.3 (15.5) dB, and power-added efficiency (PAE) of 44.3% (34.8%), are achieved.

Al2O3-Dielectric InAlN/AlN/GaN ${\Gamma}$ -Gate MOS-HFETs With Composite Al2O3/TiO2 Passivation Oxides

Novel Al2O3-dielectric InAlN/AlN/GaN ${\Gamma }$ -Gate metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with composite Al2O3/TiO2 passivation oxides formed by using ultrasonic spray pyrolysis deposition/RF sputtering, respectively, are investigated. The ${\Gamma }$ -gate includes a 1- ${\mu }\text{m}$ long active gate on the Al2O3 dielectric and a 1- ${\mu }\text{m}$ long field-plate on the composite Al2O3/TiO2 oxides. The present ${\Gamma }$ -Gate MOS-HFET has demonstrated excellent on/off current ratio ( $I_{\mathrm{ on}}/I_{\mathrm{ off}}$ ) of $8.2 \times 10^{10}$ , subthreshold swing of 102.3 mV/dec, maximum extrinsic transconductance of ( $g_{m,{\mathrm{ max}}}$ ) of 210.1 mS/mm, maximum drain-source saturation current density ( $I_{\mathrm{ DS,max}}$ ) of 868.3 mA/mm, two-terminal off-state gate-drain breakdown voltage (BVGD) of −311.2 V, three-terminal drain-source breakdown voltage (BVDS) of 237 V at $V_{\mathrm{ GS}}= -10$ V, and power-added efficiency of 39.9% at 2.4 GHz. A conventional Schottky-gate HFET and TiO2-dielectric MOS-HFET were also prepared in comparison. The present design has shown superior dc/RF device performance. It is suitable for high-power RF circuit applications.

Device Technologies of GaN-on-Si for Power Electronics: Enhancement-Mode Hybrid MOS-HFET and Lateral Diode

In this paper, a high performance enhancement-mode (E-mode) Al2O3/GaN hybrid MOS-HFET and a novel AlGaN/GaN lateral diode are experimentally demonstrated. Based on the proposed approach to engineer the dielectric/GaN positive interface fixed charges ( Q it+) by postdielectric annealing, the E-mode metal–oxide–semiconductor heterojunction field-effect transistor (MOS-HFET) obtains a better tradeoff between the threshold voltage ( V TH) control and drain current drive capability due to the significant suppression of Q it+ and the associated remote scattering effect. Owing to the uniquely high V TH and gate swing, the E-mode MOS-HFET exhibits respectable higher faulty turn on immunity that is prone to improve the reliability of the device in power switching applications and simplify the gate driver design. On the other hand, by featuring a new turn-on mechanism of MIS-gate-controlled 2-D electron gas (2DEG) channel, the diode exhibits ultralow turn-on voltage and reverse leakage current, which are beneficial to lowering the conduction loss and off-state power consumption of GaN-on-Si power diodes. Moreover, the diode is compatible with E-mode hybrid MOS-HFET, which is promising for realizing full-GaN single-chip power ICs.

Comparative studies on AlGaN/GaN/Si MOS-HFETs with Al2O3/TiO2 stacked dielectrics by using an ultrasonic spray pyrolysis deposition technique

Al0.26Ga0.74N/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) grown on a Si substrate with Al2O3/TiO2 stacked gate dielectrics formed by using non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique are investigated. High permittivity (k) values of Al2O3 and TiO2 were characterized to be 9 and 46.1, respectively, with an equal layer thickness of 10 nm. The present MOS-HFET (Schottky-gate HFET) design has demonstrated enhanced device characteristics at 300 K, including maximum drain–source current density (IDS,max) of 725 (530) mA/mm, IDS at VGS = 0 V (IDSS0) of 471 (383) mA/mm, gate-voltage swing (GVS) of 2.5 (1.6) V, two-terminal gate-drain breakdown voltage (BVGD) of −182 (−121) V, turn-on voltage (Von) of 4.9 (3.2) V, three-terminal off-state drain-source breakdown voltage (BVDS) of 174 (103) V, on/off current ratio (Ion/Ioff) of 5.6 × 107 (3.7 × 103), unity-gain cut-off frequency (fT) of 10.3 (6.8) GHz, maximum oscillation frequency (fmax) of 14.8 (8.6) GHz, and power-added efficiency (P.A.E.) of 38.5% (31.7%) at 2.4 GHz. High temperature device characteristics up to 450 K are also discussed.

Normally‐off AlGaN/GaN‐on‐Si MOS‐HFET with a monolithically integrated single‐stage inverter as a gate driver

A normally-off AlGaN/GaN-on-Si metal–oxide–semiconductor-heterojunction field-effect transistor (MOS-HFET) with an integrated single-stage inverter is developed. The integrated single-stage GaN inverter consisted of an AlGaN/GaN driver MOS-HFET and a resistive load. With the monolithically integrated gate driver, the gate charging current was boosted from 10 to 170 mA, which reduced the charging and discharging times from 626 to 107 ns and 553 to 196 ns, respectively.

Degradation Characteristics of AlGaN/GaN MOS-Heterostructure FETs by Alpha-Particle Irradiation

AlGaN/GaN metal-oxide-semiconductor heterostructure field effect transistors (MOS-HFETs) and schottky HFETs were irradiated with 18 MeV alpha-particles. Irradiated devices showed degradation in current-voltage (I-V) and capacitance-voltage (C-V) characteristics. After irradiation, both devices exhibited the positive shift of threshold voltage (Vth) and the increase of on-resistance (Ron). We also observed the increase of contact (RC) and sheet resistance (Rsh) as irradiation fluence increased. Post-irradiation characteristics were attributed to irradiation-induced displacement damage with strong dependence on fluence. Hall pattern measurement confirmed significant degradation of carrier transport property by alpha-particle irradiation. Extracted density of oxide interface states was also increased after irradiation.

Normally-Off MOS-HFET on AlGaN/GaN-on-Si(110) Grown by NH3MBE

Due to the difficulty of GaN epitaxy growth on Si(001) substrate, GaN-on-Si wafers are generally grown on Si(111) substrates. Because of the poor electrical characteristics of Si(111) orientation, monolithic integration between CMOS ICs and GaN devices cannot be implemented on GaN-on-Si(111) wafers. At this point of view, Si(110) substrate will be an alternative choice for GaN-on-Si epitaxy growth, because it has good atomic arrangement with AlN seed layer and an advantage of excellent hole mobility characteristics over Si(001). We have developed high quality GaN epitaxy growth technique on Si(110) substrate using NH3 MBE and demonstrated normally-off AlGaN/GaN-on-Si(110) metal–oxide–semiconductor-heterojunction field-effect transistors with an OFF-state breakdown voltage of 610 V. The fabricated device exhibited promising characteristics comparable with those achieved on conventional GaN-on-Si(111) wafers.

High quality PECVD SiO2 process for recessed MOS-gate of AlGaN/GaN-on-Si metal–oxide–semiconductor heterostructure field-effect transistors

A high quality SiO2 deposition process using a plasma enhanced chemical vapor deposition system has been developed for the gate insulator process of normally-off recessed-gate AlGaN/GaN metal–oxide–semiconductor-heterostructure field-effect transistors (MOS-HFETs). SiO2 films were deposited by using SiH4 and N2O mixtures as reactant gases. The breakdown field increased with increasing the N2O flow rate. The optimum SiH4/N2O ratio was 0.05, which resulted in a maximum breakdown field of 11MV/cm for the SiO2 film deposited on recessed GaN surface. The deposition conditions were optimized as follows; a gas flow rate of SiH4/N2O (=27/540sccm), a source RF power of 100W, a pressure of 2Torr, and a deposition temperature of 350°C. A fabricated normally-off MOS-HFET exhibited a threshold voltage of 3.2V, a specific on-resistance of 4.46mΩcm2, and a breakdown voltage of 810V.

Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature**Project supported by the International Science and Technology Collaboration Program of China (Grant No. 2012DFG52260).

In this paper, TiN/AlOx gated AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistors (MOS-HFETs) were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600 °C with the contact resistance approximately 1.6 Ω·mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/AlOx gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate AlGaN/GaN MOS-HFETs.

Ti0.5Al0.5O-Dielectric AlGaN/GaN/Si Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors by Using Non-Vacuum Ultrasonic Spray Pyrolysis Deposition

This work investigates a Ti0.5Al0.5O-dielectric Al0.26Ga0.74N/GaN metal-oxide-semiconductor heterostructure field-effect transistor (MOS-HFET) grown on a Si substrate, prepared by using a non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. High dielectric constant (k) was characterized to be 29.1 for a 20-nm thick Ti0.5Al0.5O. Hooge coefficient (αH), low-frequency noise spectra (1/f), high/low frequency capacitance-voltage (C-V), pulse I-V measurements were also performed to investigate the improved passivation and insulation of the MOS-structure. The present MOS-HFET design has exhibited superior improvements of 23% in maximum drain-source current density (IDS, max), 25.1% in drain-source saturation current density at VGS = 0 V (IDSS0), 46.3% in two-terminal off-state gate-drain breakdown voltage (BVGD), and 65% in three-terminal drain-source breakdown voltage (BVDS), as compared to a reference Schottky-gate HFET device. Enhanced high-frequency, power, and high-temperature performances up to 450 K are also studied.

ALD gate dielectrics for improved threshold voltage stability in AlGaN/GaN MOS-HFETs for power applications

Dielectrics by atomic layer deposition (ALD) are sought after for fabricating AlGaN/GaN based metal oxide semiconductor heterojunction field effect transistors (MOS-HFETs) for power applications. The ideal gate dielectric is required to suppress gate leakage and minimize threshold voltage (VT) instability by hosting minimal interface traps. Additionally, with the need for an enhancement mode device, it is preferable if it minimizes VT shift in the negative direction. For the first time, we compare popular ALD dielectrics like SiO2, Al2O3, HfO2 and HfAlO with identical electrical thickness on AlGaN/GaN, thereby ensuring identical electrostatic conditions across different dielectrics. High-k ALD dielectrics (HfAlO, HfO2 and Al2O3) are found to suppress gate leakage but host a high density of interface traps with AlGaN, thereby resulting in significant VT instability. ALD SiO2 gate dielectric, annealed in N2 above 600 °C, is a promising gate dielectric candidate which provides the most stable and least negative shift in VT while also substantially suppressing gate leakage below that of an HFET.

Investigation of flat band voltage shift in recessed-gate GaN MOSHFETs with post-metallization-annealing in oxygen atmosphere

We have investigated the effects of post-metallization-annealing (PMA) in oxygen atmosphere on recessed-gate GaN metal-oxide-semiconductor heterostructure field effect transistors (MOSHFETs). The flat band voltage of MOS is a function of bulk and interface charges in the oxide, which strongly depends on a post-annealing process as well as deposition conditions. A positive threshold voltage shift enabling normally-off operation has been achieved by an O2 PMA process where the GaN MOSHFET employed an ICPCVD SiO2 gate oxide with a Ni/Au metal gate. According to the analysis using energy dispersive x-ray spectroscopy in transmission electron microscopy and x-ray photoelectron spectroscopy, it is suggested that the improved SiO2/GaN interface quality with an enhanced metallic-like Ga level was responsible for the positive shift in threshold voltage.

AlGaN/GaN Metal–Oxide–Semiconductor Heterojunction Field-Effect Transistor Integrated With Clamp Circuit to Enable Normally-Off Operation

We have developed an AlGaN/GaN-on-Si metal- oxide-semiconductor heterojunction field-effect transistor (MOSHFET) integrated with a clamp circuit to enable normally-off operation. A clamp circuit consisting of a multilayer MIM capacitor and a Schottky barrier diode was monolithically integrated with a normally ON AlGaN/GaN MOSHFET. The integrated clamp circuit shifted the input driving signal from (0, 20 V) to (-19.2, 0.8 V), allowing the normally ON AlGaN/GaN MOSHFET with a pinchoff voltage of -14 V to be operated as a normally-off device. The multichannel device with a 5-mm channel width exhibited a drain current density of ~470 mA/mm and a breakdown voltage of 900 V. In comparison with other conventional normally-off GaN-based FETs, a higher threshold voltage with high current density (i.e., low ON-resistance) can be achieved with easy device processing, no need for gate recess or complicated epitaxial growth.

Extraction of the interface trap density through the differential subthreshold ideality factor technique in normally-off AlGaN/GaN MOSHFETs

Normally-off AlGaN/GaN-on-Si metal-oxide-semiconductor heterojunction field-effect transistors (MOSHFETs) were fabricated combining a SiO2 gate insulator and recessed-gate etching. We have studied the extraction of the sub-bandgap interface trap densities of normally-off AlGaN/GaN MOSHFETs by using the differential subthreshold ideality factor (DIF) technique. This method allows threshold-voltage (V T )-independent extraction of interface states by using current-voltage data in the subthreshold region. The trap density is extracted to be D it (E) = 1 × 1012 ∼ 3 × 1013 [eV−1·cm−2] at the SiO2/GaN interface. An increase in the interface trap density was detected by the applying the DIF technique after the gate bias stress test.

Diode Bridge Embedded AlGaN/GaN Bidirectional Switch

We have developed an AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) based bidirectional switch with embedded diode bridges for power switching applications. Four Schottky barrier diodes were embedded in an AlGaN/GaN MOSHFET to minimize the parasitic elements and thus reduce the chip area. The fabricated device functioned as a normally OFF, bidirectional switch, where the gate threshold voltage was ~1 V in both forward and reverses modes. The maximum drain current density in forward and reverse operation was ~120 mA/mm with the gate voltage of 12 V. The forward and reverse OFF-state breakdown voltages were 861 and 946 V, respectively.