Effective suppression of current collapse in both E- and D-mode AlGaN/GaN HEMTs on Si by [(NH4)2Sx] passivation

An effective suppression of drain current collapse was realized in both Enhancement (E)-mode and Depletion (D)-mode AlGaN/GaN High-electron-mobility-transistors (HEMTs) on 4-inch Silicon (111) by ammonium sulfide [(NH4)2Sx] passivation. The current collapse was studied using the pulsed current-voltage characteristics with the pulse width of 200 ns and pulse period of 1 ms. With reference to the AlGaN/GaN HEMTs without sulfur passivation, about 30% of the drain current collapse was suppressed for drain quiescent biases of 25 to 30 V. Obtaining low current collapse is essential to demonstrate high power GaN HEMTs

High Johnson’s figure of merit (8.32 THz·V) in 0.15-µm conventional T-gate AlGaN/GaN HEMTs on silicon

AlGaN/GaN high-electron-mobility transistors (HEMTs) with a 0.15-µm gate were fabricated on a Si substrate with an 8-nm-thick AlGaN barrier. The device exhibited a unity current gain cutoff frequency fT of 63 GHz and maximum oscillation frequency fmax of 124 GHz. Its three-terminal OFF-state breakdown voltage BVgd is as high as 132 V. The estimated Johnson's figure of merit (=BVgd × fT) is 8.32 × 1012 V/s (8.32 THz·V), which is the highest value ever reported for a conventional SiN-passivated T-gate AlGaN/GaN HEMTs on a Si substrate without an additional field plate or gamma gate

Demonstration of AlGaN/GaN MISHEMT on Si with low-temperature epitaxy grown AlN dielectric gate

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMT) with a low-temperature epitaxy (LTE)-grown single crystalline AlN gate dielectric were demonstrated for the first time and the post-gate annealing effects at 400 °C were studied. The as-deposited LTE-AlN MISHEMT showed a maximum drain current (I Dmax) of 708 mA/mm at a gate bias of 4 V and a maximum extrinsic transconductance (g mmax) of 129 mS/mm. The 400 °C annealed MISHEMT exhibited an increase of 15% in g mmax, an order of magnitude reduction in reverse gate leakage and about a 3% suppression of drain current (I D) collapse. The increase of g mmax by post-gate annealing is consistent with the increase of 2DEG mobility. The suppression of I D collapse and the reduction of gate leakage current is attributed to the reduction of interface state density (5.0 × 10¹¹ cm⁻²eV⁻¹) between the AlN/GaN interface after post-gate annealing at 400 °C. This study demonstrates that LTE grown AlN is a promising alternate material as gate dielectric for GaN-based MISHEMT application.Published versio

A Compact Model for Generic MIS-HEMTs Based on the Unified 2DEG Density Expression

In this paper, the 2-D electron gas density (ns) and Fermi level (Ef) analytical expressions as an explicit function of the terminal biases that covers the strong- and moderate-inversion and subthreshold regions and scalable with physical parameters are developed. It is validated by the comparison with the (exact) numerical solutions for different device parameters, in which the device operating region may encompass one or two lowest sub-bands ( E0 and E1) in the triangular well. With the unified Ef model, a surface-potential (φs) based drain-current (Ids) model for the metal-insulator-semiconductor (MIS) high electron-mobility transistor (HEMT) is developed. Nonlinear source/drain access region resistances ( Rs and Rd) can also be modeled via a subcircuit, including an empirical Rs model for capturing the current-collapse effect. The compact drain-current model is shown to match the experimental data of MIS HEMTs very well in both subthreshold and strong-inversion regions, with smooth and symmetric behaviors and including the (dc) self-heating effect. It also models the corresponding MIS diode C-V using the same set of physical and minimum fitting parameters.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore

Reduction of current collapse in AlGaN/GaN MISHEMT with bilayer SiN/Al2O3 dielectric gate stack

We have studied the drain current dispersion characteristics of conventional AlGaN/GaN HEMTs and SiN/Al2O3/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MISHEMT) fabricated on silicon substrate. The fabricated MISHEMT exhibited an IDmaxof >1000 mA/mm and gmmax of 241 mS/mm. Compared to conventional AlGaN/GaN HEMTs, about an order of magnitude lower gate leakage current and a ∼ 60% reduction in drain current (ID) collapse was observed in the MISHEMTs. The observation of low ID collapse is due to the occurrence of low interface state density (6.39 × 1010 eV-1 cm-2) between the bilayer dielectric (SiN/Al2O3) and GaN surface which is confirmed through the AC-conductance method

Vertical GaN-on-GaN Schottky diodes as α-particle radiation sensors

Among the different semiconductors, GaN provides advantages over Si, SiC and GaAs in radiation hardness, resulting in researchers exploring the development of GaN-based radiation sensors to be used in particle physics, astronomic and nuclear science applications. Several reports have demonstrated the usefulness of GaN as an α-particle detector. Work in developing GaN-based radiation sensors are still evolving and GaN sensors have successfully detected α-particles, neutrons, ultraviolet rays, x-rays, electrons and γ-rays. This review elaborates on the design of a good radiation detector along with the state-of-the-art α-particle detectors using GaN. Successful improvement in the growth of GaN drift layers (DL) with 2 order of magnitude lower in charge carrier density (CCD) (7.6 × 1014/cm3) on low threading dislocation density (3.1 × 106/cm2) hydride vapor phase epitaxy (HVPE) grown free-standing GaN substrate, which helped ~3 orders of magnitude lower reverse leakage current (IR) with 3-times increase of reverse breakdown voltages. The highest reverse breakdown voltage of -2400 V was also realized from Schottky barrier diodes (SBDs) on a free-standing GaN substrate with 30 μm DL. The formation of thick depletion width (DW) with low CCD resulted in improving high-energy (5.48 MeV) α-particle detection with the charge collection efficiency (CCE) of 62% even at lower bias voltages (-20 V). The detectors also detected 5.48 MeV α-particle with CCE of 100% from SBDs with 30-μm DL at -750 V.Published versio

High vertical breakdown strength in with low specific on-resistance AlGaN/AlN/GaN HEMTs on silicon

Off-state and vertical breakdown characteristics of AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) on high-resistivity (HR)-Si substrate were investigated and analysed. Three-terminal off-state breakdown (BVgd) was measured as a function of gate–drain spacing (Lgd). The saturation of BVgd with Lgd is because of increased gate leakage current. HEMTs with Lgd = 6 µm exhibited a specific on-resistance RDS[ON] of 0.45 mΩ cm2. The figure of merit (FOM = BVgd2/RDS[ON]) is as high as 2.0 × 108 V2 Ω–1 cm–2, the highest among the reported values for GaN HEMTs on Si substrate. A vertical breakdown of ∼1000 V was observed on 1.2 µm thick buffer GaN/AlN grown on Si substrate. The occurrence of high breakdown voltage is due to the high quality of GaN/AlN buffer layers on Si substrate with reduced threading dislocations which has been confirmed by transmission electron microscopy (TEM). This indicates that the AlGaN/AlN/GaN HEMT with 1.2 µm thick GaN/AlN buffer on Si substrate is promising candidate for high-power and high-speed switching device applications

Enhancing the piezoelectric modulus of wurtzite AlN by ion beam strain engineering

The piezoelectric modulus of wurtzite aluminum nitride (AlN) is a critical material parameter for electrical components, ultimately contributing to the energy efficiency and achievable bandwidth of modern communication devices. Here, we demonstrate that the introduction of metallic point-defects (Ti, Zr, Hf) improves the piezoelectric modulus of as-received, unstrained, epitaxially grown AlN. The metals are incorporated by ion implantation with an acceleration energy of 30 keV to a fluence of 1015 at cm-2, which causes an elongation along the wurtzite c-axis. The stored internal strain energy increases the piezoelectric polarization of the thin AlN layer. This can equivalently be described by an enhancement of the piezoelectric modulus d33. The incorporation of 0.1 at. % Ti enhances the piezoelectric modulus by ∼30%; significantly exceeding gains obtained by alloying with the same amount of Sc

Investigation of gate leakage current mechanism in AlGaN/GaN high-electron-mobility transistors with sputtered TiN

The gate leakage current mechanism of AlGaN/GaN Schottky barrier diodes (SBDs) and high-electron-mobility transistors (HEMTs) with sputtered TiN is systematically investigated. The reverse leakage current (JR) of TiN SBDs increases exponentially with the increase of reverse voltage (VR) from 0 to −3.2 V (Reg. I). This conduction behavior is dominated by Poole-Frenkel emission from TiN through an interface state of 0.53 eV to the conductive dislocation-related continuum states. The obtained interface state of 0.53 eV may be due to the plasma damage to the surface of the AlGaN/GaN HEMT structure during the TiN sputtering. When the TiN SBDs are biased with −20 < VR < −3.2 V, JR saturated due to the depletion of the 2-dimensional electron gas (2DEG) channel (Reg. II). This conduction behavior is dominated by the trap-assisted tunneling through the interface state at ∼0.115 eV above the Fermi level. The three terminal OFF-state gate leakage current of AlGaN/GaN HEMTs exhibited an activation energy of 0.159 eV, which is in close agreement with the obtained interface state of ∼0.115 eV from saturated JR (Reg. II) of the SBDs. The observation of the negative temperature coefficient (−1.75 V/K) from the OFF-state breakdown voltage (at 1 μA/mm) of AlGaN/GaN HEMTs is due to the trap-assisted tunneling mechanism, which is also well correlated with the conduction mechanism realized from the reverse leakage current of the SBDs.Published versio

Band alignment between GaN and ZrO2 formed by atomic layer deposition

The band alignment between Ga-face GaN and atomic-layer-deposited ZrO2 was investigated using X-ray photoelectron spectroscopy (XPS). The dependence of Ga 3d and Zr 3d core-level positions on the take-off angles indicated upward band bending at GaN surface and potential gradient in ZrO2 layer. Based on angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔE V of 1 ± 0.2 eV and conduction band discontinuity ΔE C of 1.2 ± 0.2 eV at ZrO2/GaN interface were determined by taking GaN surface band bending and potential gradient in ZrO2 layer into account.Published versio