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GaN-based Tri-gate high electron mobility transistors

: Ture, Erdin
: Ambacher, Oliver; Bolognesi, Colombo R.; Palacios, Tomas

Freiburg/Brsg., 2017, XII, 158 S.
Freiburg/Brsg., Univ., Diss., 2017
Fraunhofer IAF ()

The rapidly-growing data throughput rates in a wide range of wireless communication applications are pushing the established semiconductor device technologies to their limits. Considerably higher levels of solidstate output power will therefore be needed to meet the demand in the next generation satellite communications as well as the RADAR systems. Owing to their superior material properties such as high breakdown fields and peak electron velocities, GaN-based high electron mobility transistors (HEMTs) have recently prevailed in high-power systems operating in the microwave frequency bands. On the other hand, technologies based on GaAs or InP still make up a large portion of the state-of-the-art devices and circuits at the millimetre-wave (MMW) and sub-MMW frequencies. Due to the strong dependence of the intrinsic device parameters on the applied bias point, highly-scaled GaN HEMTs are prone to experiencing deteriorated high frequency characteristics which severely limit the highpower performance as well. Here, the relatively poor control of the gate electrode is known to be the underlying root of the performance drop. In an attempt to overcome this by means of reinforcing the gate-control, 3-dimensional GaN HEMT devices featuring the Tri-gate topology are developed in this work, exhibiting enhanced performance in terms of both off- and on-state figures of merit. One of the first obstacles is establishing a Tri-gate process scheme that is fully compatible with the planar-gate GaN HEMT process, allowing for a straightforward integration. On that account, electron-beam-defined mesa etching is found to be the most applicable method for patterning the nano-channels which constitute the Tri-gate FETs with promising device characteristics. It is then shown that a thorough optimisation of the fin geometry and a 3-D passivation approach help improving the on-state DC performance as well as suppressing the short channel effects (SCE) in the off-state. By taking advantage of the superior gate-control, normally-off AlGaN/GaN FinFETs are subsequently presented with as high as +0.2 V of threshold and 60 V of breakdown voltages, as opposed to -1.7 V of threshold and 28 V of breakdown voltages achieved by the conventional GaN FETs. In order to further improve the on-state performance, advanced heterostructures with InAlGaN and AlN barriers are employed which result in up to 3.8 A/mm of saturation drain current density, being one of the highest recorded values among GaN-based Tri-gate devices. Following the DC-improvements, both small- and large-signal parameters of the Tri-gate HEMTs are then optimised towards a more linear behaviour with respect to the bias point. Once again, with the help of alternative barrier layers and the reduction of the parasitic gate capacitances, a flatter RFtransconductance behaviour is achieved, leading to a bias-independent current-gain cut-off frequency of higher than 60 GHz. It is also revealed by the large-signal load-pull investigation that a maximum RF output power density of 3.7 W/mm (compared to 2.5 W/mm of the planar FETs) can be reached by the AlN/GaN FinFETs while showing a simultaneous ft of 80 GHz. Finally, the presented W-band power amplifier with 30 dBm of saturated output power and logic inverter designs for the first time demonstrate the excellent circuit-level performance of the GaNbased Tri-gate devices. In the end, the developed GaN Tri-gate technology is proven to be a viable candidate for achieving significantly higher RF output power without undergoing a cut-off frequency degradation. The demonstrated results of the fabricated monolithic microwave integrated circuits (MMICs) with Watt-level output power up to 90 GHz also promote the great potential of Tri-gate FETs for both MMW power amplifier and high-speed logic applications.