Jeng-Wei Yu

Short channel effects on gallium nitride/gallium oxide nanowire transistors

Gallium nitride/gallium oxide GaN/Ga2O3 nanowire metal-oxide-semiconductor field-effect-transistors are shown to operate at an average electron velocity of ∼1.24 × 107 cm/s and threshold-voltage roll-off of −0.2 V as the transistor gate length Lg reduced from 500 to 50 nm. Improvement of saturation current to 120 μA and unity current/power-gain cut-off frequency to 150/180 GHz is observed on Lg = 50 nm devices. Our study reveals the advantages of using (i) polarization-induced positive charges and high-k dielectric at the {11¯01¯}GaN/{002}Ga2O3 interface to provide carrier confinement and to shield the drain field, and (ii) polarization-induced negative charges at the (0001)GaN/sapphire interface to form a back-barrier to suppress leakage and improve the short-channel transport properties.

High mobility thin film transistors with indium oxide/gallium oxide bi-layer structures

We investigate the transport properties of thin-film transistors using indium oxide (In2O3)/gallium oxide (Ga2O3) bi-layer stacks as the channel material. At low gate bias, we observe the transistor field-effect mobility increases with the film resistivity to μFE = 51.3 cm2/Vs and ON/OFF current ratio to 108 due to combinatorial layer thickness modulation. With the Ga2O3 layer thickness ratio increase to R = 14.35%, these observations are accompanied with one-order-of-magnitude reduction in the transistor subthreshold swing to 0.38 V/decade and suggest a trap-limited conduction mechanism upon which the reduced scattering centers due to annihilation of subgap states improve the device electric characteristics without post-growth annealing.

Scaling performance of Ga2O3/GaN nanowire field effect transistor

A three-dimensional finite element solver is applied to investigate the performance of Ga2O3/GaN nanowire transistors. Experimental nanowire results of 50 nm gate length are provided to compare with the simulation, and they show good agreement. The performance of a shorter gate length (<50 nm) is studied and scaling issues of the short-channel effect are analyzed. With a better surrounding gate design and a recessed gate approach, the optimal conditions for a 20 nm gate length are explored in this paper.

Transport properties of gallium nitride nanowire metal-oxide-semiconductor transistor

We investigated the transport properties of [112¯0]-gallium nitride (GaN)/gallium oxide (Ga2O3) single nanowire metal-oxide-semiconductor field-effect-transistor grown on (0001) sapphire substrates. With 0.1 μm gate-length and 60 nm wire-size, the device exhibits maximum transconductance of 85 μS, saturation current of 105 μA, and unity current gain bandwidth ft at 95 GHz. From a three-dimensional (3D) diffusion-and-drift model analysis, polarization-induced negative space charge of −3 × 1012 cm−2 at the back interface of GaN/sapphire, positive space charge of 7 × 1012 cm−2 at the inclined semi-polar {11¯01¯} GaN/Ga2O3 interfaces with screening by two-dimensional electron gas to keep charge neutrality were found responsible for the high-speed transport characteristics.

100GHz depletion-mode Ga 2 O 3 /GaN single nanowire MOSFET by photo-enhanced chemical oxidation method

We reported transport characterization on [112̄0]GaN single nanowire (SNW)-MOSFET laterally- and directionally-grown on (0001) sapphire substrates. The 60nm-dia. Ga<inf>2</inf>O<inf>3</inf>/GaN SNW-MOSFET of 0.1µm gate length was shown to exhibit a saturation current of 160µA, current on/off ratio of 10⁵, swing of 85mV/dec, transconductance of 85µS, and unity current gain bandwidth f<inf>t</inf> at 95GHz. From a 3D diffusion and drift model analysis, it is shown that a polarization induced 2D electron gas (2DEG) density of 7×10¹² cm² with mobility of 1000cm²/V-sec confined at the interface of semi-polar {11̄01̄} GaN/Ga<inf>2</inf>O<inf>3</inf> was responsible for the high-speed transport characteristics.

Scaling of GaN single nanowire MOSFET with cut-off frequency 150GHz

We characterize the transport properties of [11-20] GaN/Ga2O3 nanowire (NW)-MOSFET epitaxially grown on (0001) sapphire substrates. When passivated with 10nm-thick Ga2O3 on the {1-10-1 }GaN triangular facets, the 50 nm-dia. Ga2O3/GaN NW-MOSFET with 50nm gate length exhibits a saturation current of 130 μA, transconductance of 64 μS, current on/off ratio of 104, subthreshold swing of 100mV/dec, and unity current (power) gain bandwidth fT (fMAX) at 150 (180)GHz. Using a 3D diffusion and drift model analysis, we found that the short channel effect in a Lg=50nm Ga2O3/GaN NW-MOSFET at an aspect ratio of 5 was suppressed due to contribution from polarization-induced negative space charge of -2.8×1012 cm2 at the abrupt crystalline interface between GaN NW and sapphire. The superior DC transport properties and good RF response can be ascribed the to polarization-induced 2D electron gas (2DEG) density of 7× 1012 cm2 with mobility of 1000cm2/V-sec confined at the semi-polar {1-10-1} GaN/Ga2O3 interfaces.

75GHz Ga2O3/GaN Single Nanowire Metal- Oxide-Semiconductor Field-Effect Transistors

We reported high-speed transport properties on gallium nitride (GaN) single nanowire (NW) transistors laterally grown on the (0001) sapphire substrates. Due to the preservation of surface stoichiometry and passivation effects by the facet growth of Ga2O3/GaN, the 60nm-dia. SNW-MOSFET device of 0.2μm gate length was shown to exhibit a saturation current of 145μA, , current on/off ratio of 10^5, sub-threshold swing of 85mV/dec, transconductance of 74μS, and unity current/power gain cut-off frequency fT/fmax of 75/96GHz. From a 3D diffusion and drift model analysis, these electric characteristics can be ascribed to a polarization induced 2D electron gas (2DEG) effect with a density of 7x10^12 cm^-2 confined at the semi-polar GaN/Ga2O3 interfaces. Immune to the post-growth processing induced damage and stress effects, our device performance can be characterized by a channel mobility of 1600cm^2/V-sec which approaches the intrinsic mobility value of bulk GaN.