A Study on Characterization of Low Frequency Noise in GaN-Based Power Devices

Abstract

This study deals with fabrication and low frequency noise characterization of GaN-based metal oxide semiconductor field effect transistors (MOSFETs) and GaN-based fin-shaped field effect transistors (FinFETs) with nanochannel arrays for next generation power switching devices. In the GaN-based MOSFETs, the gate dielectrics decrease the gate leakage current and permit a substantial gate voltage swing. The nanochannel arrays of GaN-based FinFETs exhibit remarkably enhanced performance such as improved short-channel effect, high ON current, low gate leakage current, and excellent subthreshold behavior due to different channels in the AlGaN/GaN FinFETs: a 2DEG channel formed at the upper region of the nanochannel and two lateral MOS channels on the sidewall of the GaN fin. Firstly, normally-off Al2O3/GaN MOSFETs are fabricated using a simple tetramethylammonium hydroxide (TMAH) treatment as post-gate recess etch avoiding any plasma etch damage for decreasing gate leakage current. The effects of the TMAH treatment on the etched GaN surface are investigated by comparing the device performance and using low frequency noise (1/f) and capacitance-voltage (C-V) measurements for Al2O3/GaN interface condition. The surface treated devices improve the maximum transconductance (gm), the maximum drain current (Idmax), and off-state breakdown voltage (BV). For a quantitative comparison with conventional devices, the oxide trap density (Not) is extracted using the unified 1/f noise model, while the interface trap density (Dit) is extracted using a high-low frequency C-V (HLCV) method. After the TMAH surface treatment, Not is found to have decreased from 5.40 × 1019 to 2.50 × 1019 eV-1cm-3 while Dit is decreased from 2.8 × 1012 to 1.1 × 1011 eV-1cm-2, as compared with conventional devices. The surface treatment is thus shown to lower trap density in the Al2O3/GaN MOSFETs by smoothing the surface and suppressing plasma damage in the recessed GaN surfaces. Secondly, the AlGaN/GaN FinFETs with different gate-to-drain lengths (Lgd) are fabricated by fully covering the nanochannel region with a metal gate. The AlGaN/GaN FinFETs with fully covered gate nanochannels significantly enhance the device performance because the access series resistance can be notably. The Lgd in GaN-based power devices is an important physical dimension for the BV, which increases linearly with the Lgd spacing; however, devices with a longer Lgd can inhibit some aspects of device performance due to higher ON resistance. The effects of the Lgd on fully covered gate nanochannel AlGaN/GaN FinFETs are investigated using low frequency noise (LFN) measurements. It shows that the Lgd can affect on the LFN of the AlGaN/GaN FinFETs. The FinFETs with larger Lgd exhibit smaller Idmax and gmmax because the larger Lgd has a high ON-resistance and the FinFETs with a smaller Lgd exhibit slightly smaller gate leakage current due to small carrier velocity in the channel. The minimum LFN is observed at the Lgd = 3.5 μm in the FinFET. In comparison, the devices with longer Lgd have larger LFN values. The small Lgd devices exhibit less dependence on the carrier number fluctuations with correlated mobility fluctuation. The mobility fluctuation due to reduction of the vertical electric field of 2DEG and MOS channels and the relatively low electric field in the drain-side channel caused by the drain bias, as determined by the unified 1/f noise model and mobility fluctuation theory.