Nandita DasGupta

Low temperature (LT) and high pressure oxidized (HPO) Al2O3 is investigated as a gate dielectric for AlInN/GaN MIS-HEMTs. The time and temperature of the oxidation process was optimized for best performance. X-ray photoelectron spectroscopic (XPS) studies confirmed the near complete oxidation of Al to form Al2O3. MIS-HEMTs with 7 nm thick LT-HPO Al2O3 showed six orders reduction in gate leakage current and five orders improvement in ID,ON/ID,OFF ratio compared to the reference HEMT. Also, these MIS-HEMTs proved to be significantly better than HEMTs in terms of maximum drain current, subthreshold slope and off-state breakdown voltage. Reliability studies under constant voltage stress conditions show that the threshold voltage variation is within acceptable limits. Interface trap charges were estimated with the help of dynamic capacitance dispersion technique. The improved current collapse in MIS-HEMTs over HEMTs indicates the good quality of the interface between the dielectric and barrier layer.

In this paper, we have reported high pressure oxidized thin aluminium layer as a gate dielectric for GaN-based MIS-HEMTs and studied the interface traps at Al2O3/III-Nitride interface using the capacitance-conductance method. Effect of oxygen plasma treatment prior to aluminium layer deposition has also been investigated. Significant reduction in gate leakage current has been observed in all fabricated MIS-HEMTs compared to reference HEMT in both reverse and forward bias conditions. Forward bias swing is also larger for MIS-HEMTs. Significant reduction in interface trap density was found for MIS-HEMTs with oxygen plasma treatment.

In this paper, Aluminium Oxide (Al2O3) grown by thermal oxidation of AlInN/GaN heterostructure is investigated as a gate dielectric for metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). Thermal oxidation of the hetero structure was carried out under high pressure oxygen ambient as well as by rapid thermal oxidation with an aim to reduce the process temperature and time. Post gate annealing in forming gas ambient has also been optimized to achieve minimum gate leakage current density. Compared with the reference schottky barrier device, significant reduction in gate leakage current density has been obtained in both forward and reverse region of operation. The linear dependence of the resulting oxide thickness on square root of oxidation time signifies diffusion-limited oxidation mechanism, similar to oxidation of Silicon.

A physics-based simple and accurate compact model of drain current for GaN-based high electron mobility transistors (HEMTs) is presented. The model is developed using analytical relations for charges in the 2-D electron gas and barrier layer. For the first time, a simple charge linearization approach has been used for GaN-based HEMTs. The access regions are accurately modeled using transistors. The model is rigorously validated over a wide range of geometries and parameters for AlGaN/GaN and AlInN/GaN HEMTs. The model also passes the DC Gummel symmetry test.

AlInN/GaN metal insulator semiconductor high electron mobility transistor (MIS-HEMT) with high pressure oxidized aluminium as gate dielectric is investigated in this paper. The fabricated MIS-HEMT shows more than six orders of reduction in the gate leakage current in reverse bias and more than three orders of reduction in forward bias compared to the reference HEMT devices also fabricated on same substrates. A maximum drain current of 750 mA/mm was achieved due to improvement in the gate swing for MIS-HEMT. The MIS-HEMT devices also showed good improvement in the subthreshold slope and ID,ON/ID,OFF ratio compared to HEMT devices.

Dependence of threshold voltage (VTh) on oxide thickness (tox) for GaN-based metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) using sputter-deposited Al2O3 as gate dielectric is studied in detail. Different III-nitride (III-N) heterostructures (AlGaN/GaN and AlInN/GaN) with/without GaN cap layer were used for fabricating these MIS devices. Interestingly, for all the sets of devices, a positive shift in VTh was observed initially with a increase in tox, followed by a negative shift of the same. A comprehensive analytical model has been proposed to explain the variation of VTh with tox and has been shown to match the experimental data for MIS-HEMTs fabricated on different heterostructures and with different values of tox. This model allows the extraction of different charge components in the oxide or at oxide/III-N interface. Normally OFF AlInN/GaN MIS-HEMTs with VTh of +0.67 V have been demonstrated with the optimized tox of sputtered Al2O3.

AlInN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) have been fabricated with reactive-ion-sputtered (RIS) Al2O3 as a gate dielectric. Significant reduction in the gate leakage current is achieved upon insertion of RIS-Al2O3. MIS-HEMTs also show better transconductance, drain characteristics, and ION/IOFF ratio. Most interestingly, a positive shift in threshold voltage is observed for MIS-HEMTs indicating the presence of net negative charge at oxide-semiconductor interface. The origin and stability of the negative charge at the interface is discussed in this letter.

Gate leakage mechanisms in AlInN/GaN and AlGaN/GaN metal insulator semiconductor high-electron-mobility transistors (MIS-HEMTs) with SiNx as gate dielectric have been investigated. It is found that the conduction in the reverse gate bias is due to Poole-Frenkel emission for both MIS-HEMTs. The dominant conduction mechanism in low to medium forward bias is trap-assisted tunneling while it is Fowler-Nordheim tunneling at high forward bias. However, conduction near zero gate bias is dominated by defect-assisted tunneling for both sets of MIS-HEMTs. The gate leakage current is primarily dependent on the properties of the gate dielectric material and dielectric/semiconductor interface rather than the barrier layer. A model is proposed for the gate leakage current in GaN-based MIS-HEMTs, and the method to extract the related model parameters is also presented in this paper. The proposed gate current model matches well with the experimental results for both AlInN/GaN and AlGaN/GaN MIS-HEMTs over a wide range of gate bias and measurement temperature.