Seung Heon Shin

Radio Frequency Transistors and Circuits Based on CVD MoS2

We report on the gigahertz radio frequency (RF) performance of chemical vapor deposited (CVD) monolayer MoS2 field-effect transistors (FETs). Initial DC characterizations of fabricated MoS2 FETs yielded current densities exceeding 200 μA/μm and maximum transconductance of 38 μS/μm. A contact resistance corrected low-field mobility of 55 cm(2)/(V s) was achieved. Radio frequency FETs were fabricated in the ground-signal-ground (GSG) layout, and standard de-embedding techniques were applied. Operating at the peak transconductance, we obtain short-circuit current-gain intrinsic cutoff frequency, fT, of 6.7 GHz and maximum intrinsic oscillation frequency, fmax, of 5.3 GHz for a device with a gate length of 250 nm. The MoS2 device afforded an extrinsic voltage gain Av of 6 dB at 100 MHz with voltage amplification until 3 GHz. With the as-measured frequency performance of CVD MoS2, we provide the first demonstration of a common-source (CS) amplifier with voltage gain of 14 dB and an active frequency mixer with conversion gain of -15 dB. Our results of gigahertz frequency performance as well as analog circuit operation show that large area CVD MoS2 may be suitable for industrial-scale electronic applications.

Extremely High-Frequency Flexible Graphene Thin-Film Transistors

We have achieved 140-nm channel length graphene thin-film transistors (TFTs) on flexible glass with a 95-GHz intrinsic cutoff frequency and greater than 30-GHz intrinsic power frequency after standard de-embedding. The flexible glass substrate offers subnanometer surface smoothness as well as high thermal conductivity, 1 W/m · K, which can prevent thermomechanical failure, which is a limitation of plastic and rubber substrates. In addition, we developed a flexible 60-nm polyimide thin film as gate dielectric with low surface roughness less than 0.35 nm for optimal carrier transport and facilitate edge-injection contacts for low contact resistance. The maximum electron (hole) mobility is 4540 (1100) cm2/V · s, and the extracted contact resistance in the electron (hole) branch is 1140 (720) Ω · μm. The intrinsic cutoff frequency is 196% higher than our previous results on polymeric substrates. Importantly, the experimental saturation velocity of the graphene TFT is the highest for any flexible transistor on any material system reported so far.

Impact of H 2 High-Pressure Annealing Onto InGaAs Quantum-Well Metal–Oxide–Semiconductor Field-Effect Transistors With Al 2 O 3 /HfO 2 Gate-Stack

We report on the impact of H₂ high-pressure annealing (HPA) onto In_0.7Ga_0.3As MOSCAPs and quantum-well (QW) MOSFETs with Al₂O₃/HfO₂ gate-stack. After HPA with process condition of 300°C, H₂ ambient and pressure of 20 atm, we observed notable improvements of the capacitance-voltage (CV) characteristics in InGaAs MOSCAPs with Al₂O₃/HfO₂ gate-stack, such as reduction of equivalent-oxide-thickness and less frequency dispersion in the accumulation region. There was 20% improvement of the interfacial trap density (Dit). Then, we incorporated the HPA process into the fabrication of sub-100-nm In_0.7Ga_0.3As QW MOSFETs, to investigate the impact of HPA process. After HPA process, the device with L_g = 50 nm exhibits improved subthreshold-swing (SS) = 105 mV/decade, in comparison with SS = 130 mV/decade for the reference device without HPA process. Finally, we carried out reliability assessment under a constant-voltage-stress (CVS), and it turns out that the HPA process was effective in mitigating a shift of threshold voltage (ΔV_T) during the CVS. These are attributed to the effective passivation of oxide traps in the high-k dielectric layer and interfacial traps, after HPA process in the H₂ ambient.

A New Unified Mobility Extraction Technique of In 0.7 Ga 0.3 As QW MOSFETs

Conventional methods to extract an effective mobility (μeff) in a metal-oxide-semiconductor field-effect-transistor (MOSFET) tend to ignore the portions of parasitic components of the device. This can cause a substantial error in the extracted value of the effective mobility. In this letter, we have proposed a unified procedure that enables to accurately capture each portion of parasitic series resistance (RSD) and parasitic gate capacitance (Cg_par) components. Then, we have investigated the impact of the parasitic components on the extracted value of the effective mobility in a gate-last surface-channel In0.7Ga0.3As quantum-well MOSFET. We have found that the extracted effective mobility using our method turns out to be independent upon gate length (Lg) from 10 to 4 μm, which verifies the accuracy of the approach proposed in this letter.

Detection of current induced spin polarization in epitaxial Bi2Te3 thin film

We electrically detect charge current induced spin polarization on the surface of a molecular beam epitaxy grown Bi2Te3 thin film in a two-terminal device with a ferromagnetic MgO/Fe contact and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on the quantum kinetic theory. The order of magnitude and the sign of the hysteresis are consistent with the spin-polarized surface state of Bi2Te3.

Radio frequency transistors and circuit applications based on CVD MoS 2

Summary form only given. Molybdenum disulfide (MoS₂), a member of the transition metal dichalcogenide (TMD) family, is a 2D semiconductor with a direct bandgap of ~1.8 eV for single layers. Its bandgap allows for high I_on/I_off metal-oxide semiconducting field-effect transistors (FETs). Exfoliated monolayer MoS₂ FETs exhibit current saturation with on-state current densities of 300 μA/μm, as well as transconductances exceeding 40 μS/μm [1]. These properties make MoS₂ a desirable candidate for radio frequency (RF) applications [1, 2, 3]. However, in order for MoS₂ devices to move from laboratory experimentations to industrial scale applications, large area synthesis of high quality material is needed. Here we demonstrate gigahertz RF performance and circuits (amplifier and mixer) using chemical vapor deposited (CVD) monolayer MoS₂ FETs.

Damage free Ar ion plasma surface treatment on In0.53Ga0.47As-on-silicon metal-oxide-semiconductor device

In this paper, we investigated the effect of in-situ Ar ion plasma surface pre-treatment in order to improve the interface properties of In0.53Ga0.47As for high-κ top-gate oxide deposition. X-ray photoelectron spectroscopy (XPS) and metal-oxide-semiconductor capacitors (MOSCAPs) demonstrate that Ar ion treatment removes the native oxide on In0.53Ga0.47As. The XPS spectra of Ar treated In0.53Ga0.47As show a decrease in the AsOx and GaOx signal intensities, and the MOSCAPs show higher accumulation capacitance (Cacc), along with reduced frequency dispersion. In addition, Ar treatment is found to suppress the interface trap density (Dit), which thereby led to a reduction in the threshold voltage (Vth) degradation during constant voltage stress and relaxation. These results outline the potential of surface treatment for III-V channel metal-oxide-semiconductor devices and application to non-planar device process.