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In this work, recessed gate AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with double AlGaN barrier designs are fabricated and investigated. Two different recessed depths are designed, leading to a 5 nm and a 3 nm remaining bottom AlGaN barrier under the gate region, and two different Al% (15% and 20%) in the bottom AlGaN barriers are designed. First of all, a double hump trans-conductance (gm)−gate voltage (VG) characteristic is observed in a recessed gate AlGaN/GaN MIS-HEMT with a 5 nm remaining bottom Al0.2Ga0.8N barrier under the gate region. Secondly, a physical model is proposed to explain this double channel characteristic by means of a formation of a top channel below the gate dielectric under a positive VG. Finally, the impacts of Al% content (15% and 20%) in the bottom AlGaN barrier and 5 nm/3 nm remaining bottom AlGaN barriers under the gate region are studied in detail, indicating that lowering Al% content in the bottom can increase the threshold voltage (VTH) toward an enhancement-mode characteristic.

This paper addresses scaling issues in AIGaN/GaN heterojunction field-effect transistor's (HFETs) using ensemble Monte Carlo techniques. For gate lengths below 0.25 μm, ƒT values are known not to scale linearly with the inverse gate length. The authors' simulations show this to be due to an increasing difference between the lithographic gate length and the effective gate length as the devices shrink. The results for AIGaN/GaN are compared with In0.52Al0.48-In0.53Ga0.47As-InP devices, and the authors found that the limiting role of velocity overshoot and depletion region spread causes the GaN HFETs to have a peak ƒT of ∼ 220 GHz compared to ∼ 500 GHz for InGaAs devices. [ABSTRACT FROM AUTHOR]

A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 mΩ ∙ cm² and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 mΩ ∙ cm² for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to + 1 V. [ABSTRACT FROM AUTHOR]

In this letter, a novel process for recessed-gate AIGaN/GaN high-electron-mobility transistors using an Al2O3/ SiNx, dielectric has been developed. The Al2 03/SiN,1, dielectric bilayer was used as a recess etch-mask for short-gatefootprint definition. Recessed-gate devices with a gate length of 70 nm have been fabricated on a molecular-beam-epitaxygrown layer structure using this process. After the removal of the dielectric layers, excellent dc and small-signal results, a high drain-current density of 1.5 A/mm, a unity gain cutoff frequency of 160 GHz, and a maximum frequency of oscillation of 200 GHz were obtained. [ABSTRACT FROM AUTHOR]

In this work, recessed gate AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with double AlGaN barrier designs are fabricated and investigated. Two different recessed depths are designed, leading to a 5 nm and a 3 nm remaining bottom AlGaN barrier under the gate region, and two different Al% (15% and 20%) in the bottom AlGaN barriers are designed. First of all, a double hump trans-conductance (gm)–gate voltage (VG) characteristic is observed in a recessed gate AlGaN/GaN MIS-HEMT with a 5 nm remaining bottom Al0.2Ga0.8N barrier under the gate region. Secondly, a physical model is proposed to explain this double channel characteristic by means of a formation of a top channel below the gate dielectric under a positive VG. Finally, the impacts of Al% content (15% and 20%) in the bottom AlGaN barrier and 5 nm/3 nm remaining bottom AlGaN barriers under the gate region are studied in detail, indicating that lowering Al% content in the bottom can increase the threshold voltage (VTH) toward an enhancement-mode characteristic. [ABSTRACT FROM AUTHOR]