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Electrical properties of atomic layer deposited aluminum oxide on gallium nitride

We report on our investigation of the electrical properties of metal/Al2O3/GaN metal-insulator-semiconductor capacitors. We determined the conduction band offset and interface charge density of the alumina/GaN interface by analyzing the capacitance-voltage characteristics of atomic layer deposited Al2O3 films on GaN substrates. The conduction band offset at the Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band conditions in the GaN was inferred, which we attribute to a fixed net positive charge density of magnitude 4.60 × 1012 cm−2 at the Al2O3/GaN interface. We provide hypotheses to explain the origin of this charge by analyzing the energy band line-up.

Interface charge engineering at atomic layer deposited dielectric/III-nitride interfaces

Interface charges at atomic layer deposited Al2O3/III-nitride interfaces were investigated for III-nitride layers of different polarity. A large positive sheet charge density is induced at the Al2O3/III-nitride interface on all the orientations of GaN and Ga-polar AlGaN, and this sheet charge can be significantly altered using post-metallization anneals. It is proposed that the charges are caused by interfacial defects that can be passivated and neutralized through a H2 based anneal. Tailoring of the interface charge density described here can be used to improve critical device characteristics such as gate leakage and electron transport, and for lateral electrostatic engineering.

Correlation between DC and rf degradation due to deep levels in AlGaN/GaN HEMTs

We investigate the role of the 0.5 eV traps in determining GaN HEMT degradation by means of DC and rf testing, and 2D numerical simulation. We demonstrate that generation of deep levels, having an activation energy of 0.5 eV, is responsible for the degradation observed during rf aging; we show that the occurrence of trap-induced degradation depends on rf driving conditions. We also show that degradation can be explained by the generation of a damaged region within the AlGaN layer at the gate-drain edge, and that the DC and pulsed device degradation effects have a different dependence on the width and depth of the damaged region.

Analytical Model for Power Switching GaN-Based HEMT Design

The GaN high-electron mobility transistor (HEMT) structure has been widely investigated, particularly for radio-frequency applications. This structure is suitable for high-frequency switching applications in power electronics because of the high breakdown field of GaN and the high mobility of the channel. In this paper, a physical model for predicting the power-switching operation of GaN-based HEMTs as a function of material and device parameters is proposed. Analytical equations for total losses and power dissipation density are derived and discussed both qualitatively and quantitatively.

Polarization engineered 1-dimensional electron gas arrays

One-dimensional electron gas based devices are of great interest due to their promise in high-performance electronics and future device applications. However, synthesis and patterning of arrays of nanowires is a challenge in all material systems. Here we demonstrate a novel system based on vicinal AlGaN/GaN heterostructures that enables direct electrostatic tuning of the dimensionality of electrons from 1 D to 2 D. Our approach, based on polarization engineering, enables top-down fabrication of dense arrays of pure 1-dimensional electron channels with carrier confinement equivalent to 90 meV, that are capable of carrying technologically relevant current densities up to 130 mA/mm. A direction-dependent small-signal capacitance-voltage profiling to probe the Fermi occupation function of electron gas was used to demonstrate distinct signatures of 1-dimensional density of states and transport in these structures at room temperature. The system discussed here is based on polarization-induced anisotropic charge...

Characterization of a dielectric/GaN system using atom probe tomography

Characterization of Al2O3 gate dielectric on GaN using pulsed laser Atom Probe Tomography is reported. Atomic layer deposition was used to grow 5 nm of aluminum oxide on Ga-polar GaN. No oxidation of the surface of the semiconductor was observed and the interface was found to be non-abrupt. A significant amount of carbon impurities (1019/cm3) were detected in the dielectric film that matches well with the estimated bulk trap density from C–V measurements. Our experiments suggest possible correlation between trap-related electrical hysteresis and the observed impurity concentration in these films.

Characterization and Numerical Simulations of High Power Field-Plated pHEMTs

This paper presents the results obtained both by experimental measurements and numerical simulations carried out on state-of-the-art Field-Plated GaAs-based pHEMTs. The effect of field-plate length on DC and RF operation of pHEMTs will be discussed showing that the adoption of an optimal field-plate structure can significantly boost the device RF power performance, resulting in power density up to 2 W/mm measured under continuous wave RF signals at 2 GHz. The physical origin of the DC-to-RF dispersion in the fabricated devices has been associated with a hole-trap located at 0.65 eV from the valence band as obtained from current-DLTS measurements. The experimental results will also be supported and validated by numerical simulations. It will be shown that the beneficial effects arising from the adoption of the field-plate structure lie in its control on the trapped charge population responsible for the DC-to-RF dispersion mechanism.

Comparison of Cu-gate and Ni/Au-gate GaN HEMTs large signal characteristics

In this paper a complete comparison between Copper (Cu) gate and Nickel-Gold (Ni/Au) gate passivated AlGaN/GaN High Electron Mobility Transistors (HEMTs) is presented. DC and Radio Frequency (RF) performance was compared in order to evaluate the behaviour of the two Schottky contacts in the standard HEMT structure. From the obtained data a critical drain current collapse was observed in the Cu-gate devices, with detrimental effects on the RF performance, while the Ni/Au-gate performed nicely both during pulsed I–V and RF measurements. An investigation on the drain current transients and on ID – VGS characteristics, obtained by pulsed signals showed that an acceptor trap at the Cu/AlGaN interface, with activation energy of about 0.43 eV, could be responsible for the Cu-gate HEMT poorer performance. The results suggest that a detailed investigation on surface treatments, gate metal quality and deposition methods is needed in order to fabricate Cu-gate GaN HEMTs.

Study of GaN HEMTs electrical degradation by means of numerical simulations

In this paper, we investigate the effects of dc stress on GaN high-electron mobility transistors (HEMTs) by means of numerical simulations. Following stress tests showing a degradation of static characteristics (dc), the formation of an electron trap in the AlGaN barrier layer was related to the observed degradation according to the results obtained from numerical simulations carried out by introducing a trapping region underneath the gate edge. The worsening of the device dc performance is evaluated by changing the extension of the degraded region and the trap concentration while studying the variation of parameters like the saturated drain current IDSS, the output conductance gO, and the device transconductance gM. An increase in the trap concentration induces a worsening of any of the abovementioned parameters; an increase in the extension of the degraded region induces a degradation of IDSS and gM, but can reduce gO.