A hybrid approach for obtaining normally off high electron mobility transistors (HEMTs) combining fluorine treatment, recess etch techniques, and AlGaN buffer was studied. The effects of process variations (recess etch depth and fluorine treatment duration) and epitaxial differences (AlGaN and carbon doped GaN buffers) on the DC characteristics of the normally off HEMTs were investigated. Two different epitaxial structures and three different process variations were compared. Epitaxial structures prepared with an AlGaN buffer showed a higher threshold voltage (V th = +3.59 V) than those prepared with a GaN buffer (V th = +1.85 V).

In this paper, we report an angstrom-thick atomic layer deposited (ALD) aluminum oxide (Al 2 O 3) dielectric passivation layer for an AlGaN/GaN high electron mobility transistor (HEMT). Our results show a 55% improvement in the gate lag performance of the design and a decrease by half in interface state density upon coating with two cycles of ALD Al 2 O 3. DC characteristics such as current density, threshold voltage, and leakage currents were maintained. ALD Al 2 O 3 passivation layers with thicknesses up to 10 nm were investigated. XPS analyses reveal that the first ALD cycles are sufficient to passivate GaN surface traps. This study demonstrates that efficient passivation can be achieved in atomic-scale with dimensions much thinner than commonly used bulk layers.

In this study, an enhancement-mode (E-mode) GaN high electron mobility transistor (HEMT) with lateral tri-gate structure field effect transistor (FinFET) is proposed. To passivate the fin width, while keeping the normally-off performance of the FinFET intact, an ultrathin aluminium-oxide/sapphire (Al2O3) gate dielectric is proposed (in a basic single-finger 0.125 mm device). Later, the DC and radio frequency (RF) performances of the proposed FinFET designs (with optimized fin width and Al2O3 thickness) are compared with that of conventional planar HEMT. DC and RF measurements are performed using power transistors in 10-fingers configuration, with a total gate periphery of 2.5 mm. The effect of Fin structure and Al2O3 thickness on the electrical performance of HEMTs, including threshold voltage (Vth) shift, transconductance (gm) linearity, small-signal gain, cut off frequency (ft), output power (Pout), and power-added efficiency (PAE) are investigated. Based on our findings, FinFET configuration imposes normally-off functionality with a Vth= 0.2 V, while the planar architecture has a Vth= -3.7 V. Originating from passivation property of the alumina layer, the FinFET design exhibits two orders of magnitude smaller drain and gate leakage currents compared to the planar case. Moreover, large signal RF measurements reveals an improved Pout density by over 50% compared to planar device, attributed to reduced thermal resistance in FinFETs stemming from additional lateral heat spreading of sidewall gates. Owing to its superior DC and RF performance, the proposed FinFET design with ultrathin gate dielectric could bear the potential of reliable operating for microwave power applications, by further scaling of the gate length.

The effects of gate post-metal annealing (PMA) on the DC and RF characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) were investigated. The unannealed and post gate-metal annealed AlGaN/GaN HEMTs were fully fabricated in-house using 0.5 µm gate length technology. PMA was performed at 450 ºC for 10 minutes in nitrogen ambient for one of the wafers. The main focus was the effect of PMA on the electrical performance of HEMTs, including gate resistivity, transconductance, small-signal gain, output power (Pout), and threshold voltage (Vth) shift. It is achieved that HEMT with PMA has a gain of 18.5 dB, while HEMT without PMA shows a small-signal gain of 21.8 dB, as the PMA process increases the gate resistance (Rg) and decreases the transconductance (gm). The large-signal performance of the sample with PMA is better than the one without PMA, having an increase of 1.4 W/mm in Pout from 20.9 W to 24.4 W. The transistor with PMA also demonstrates a reliable gate performance and stable Vth, and the wafer exhibits better uniformity.