Saungeun Park

Black Phosphorus Flexible Thin Film Transistors at Gighertz Frequencies

Black phosphorus (BP) has attracted rapidly growing attention for high speed and low power nanoelectronics owing to its compelling combination of tunable bandgap (0.3 to 2 eV) and high carrier mobility (up to ∼1000 cm(2)/V·s) at room temperature. In this work, we report the first radio frequency (RF) flexible top-gated (TG) BP thin-film transistors on highly bendable polyimide substrate for GHz nanoelectronic applications. Enhanced p-type charge transport with low-field mobility ∼233 cm(2)/V·s and current density of ∼100 μA/μm at VDS = -2 V were obtained from flexible BP transistor at a channel length L = 0.5 μm. Importantly, with optimized dielectric coating for air-stability during microfabrication, flexible BP RF transistors afforded intrinsic maximum oscillation frequency fMAX ∼ 14.5 GHz and unity current gain cutoff frequency fT ∼ 17.5 GHz at a channel length of 0.5 μm. Notably, the experimental fT achieved here is at least 45% higher than prior results on rigid substrate, which is attributed to the improved air-stability of fabricated BP devices. In addition, the high-frequency performance was investigated through mechanical bending test up to ∼1.5% tensile strain, which is ultimately limited by the inorganic dielectric film rather than the 2D material. Comparison of BP RF devices to other 2D semiconductors clearly indicates that BP offers the highest saturation velocity, an important metric for high-speed and RF flexible nanosystems.

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.

Inkjet printing of high performance transistors with micron order chemically set gaps

This paper reports a 100% inkjet printed transistor with a short channel of approximately 1 µm with an operating speed up to 18.21 GHz. Printed electronics are a burgeoning area in electronics development, but are often stymied by the large minimum feature size. To combat this, techniques were developed to allow for the printings of much shorter transistor channels. The small gap size is achieved through the use of silver inks with different chemical properties to prevent mixing. The combination of the short channel and semiconducting carbon nanotubes (CNT) allows for an exceptional experimentally measured on/off ratio of 106. This all inkjet printed transistor allows for the fabrication of devices using roll-to-roll methodologies with no additional overhead compared to current state of the art production methods.

High-frequency prospects of 2D nanomaterials for flexible nanoelectronics from baseband to sub-THz devices

We report on the state of the art sub-μm length (L) flexible two dimensional radio frequency thin film transistors operating in the velocity saturation regime for achieving maximum carrier transport or under high-field. We realize large-area monolayer MoS₂ on flexible polyimide with 5 GHz cut-off frequency (f_T), a record value for flexible synthesized transitional metal dichalcogenides (TMDs). For higher frequency devices, flexible black phosphorus (BP) RF TFT is demonstrated for the first time with f_T ~ 17.5 GHz for L = 0.5 μm, yielding v_sat ~ 5.5 × 10⁶ cm/s. In addition, for flexible sub-THz nanosystem front-ends, we have achieved record 100 GHz graphene TFTs (v_sat ~ 8.8 × 106 cm/s) on flexible glass, 56% higher than that of graphene TFTs on polymeric substrates.

Quantitative scanning thermal microscopy of graphene devices on flexible polyimide substrates

A triple-scan scanning thermal microscopy (SThM) method and a zero-heat flux laser-heated SThM technique are investigated for quantitative thermal imaging of flexible graphene devices. A similar local tip-sample thermal resistance is observed on both the graphene and metal areas of the sample, and is attributed to the presence of a polymer residue layer on the sample surface and a liquid meniscus at the tip-sample junction. In addition, it is found that the tip-sample thermal resistance is insensitive to the temperature until it begins to increase as the temperature increases to 80 °C and exhibits an abrupt increase at 110 °C because of evaporation of the liquid meniscus at the tip-sample junction. Moreover, the variation in the tip-sample thermal resistance due to surface roughness is within the experimental tolerance except at areas with roughness height exceeding tens of nanometers. Because of the low thermal conductivity of the flexible polyimide substrate, the SThM measurements have found that the te...

Flexible 2D electronics using nanoscale transparent polyimide gate dielectric

We report NPI as a flexible dielectric for transistors based on 2D atomic sheets such as graphene and MoS2, which features high mechanical flexibility, stable electrical performances and low roughness. NPI offers the realistic prospects for highly flexible electronics beyond the typical 2% limitation of high-κ or ceramic gate dielectrics.

Embedded gate CVD MoS 2 microwave FETs

Recent studies have increased the cut off frequencies achievable by exfoliated MoS2 by employing a combination of channel length scaling and geometry modification. However, for industrial scale applications, the mechanical cleavage process is not scalable but, thus far, the same device improvements have not been realized on chemical vapor deposited MoS2. Here we use a gate-first process flow with an embedded gate geometry to fabricate short channel chemical vapor deposited MoS2 radio frequency transistors with a notable fT of 20 GHz and fmax of 11.4 GHz, and the largest high-field saturation velocity, vsat = 1.88 × 106 cm/s, in MoS2 reported so far. The gate-first approach, facilitated by cm-scale chemical vapor deposited MoS2, offers enhancement mode operation, ION/IOFF ratio of 108, and a transconductance (gm) of 70 μS/μm. The intrinsic fT (fmax) obtained here is 3X (2X) greater than previously reported top-gated chemical vapor deposited MoS2 radio frequency field-effect transistors. With as-measured S-parameters, we demonstrate the design of a GHz MoS2-based radio frequency amplifier. This amplifier has gain greater then 15 dB at 1.2 GHz, input return loss  > 10 dB, bandwidth  > 200 MHz, and DC power consumption of ~10 mW.High-frequency electronics: embedded gates boost MoS 2 radio frequency transistors2D materials enable radio frequency transistors, yet the absence of a bandgap in graphene limits its maximum oscillation frequency. A team lead by Sanjay Kumar Banerjee at the University of Texas at Austin fabricated radio frequency field-effect transistors using monolayer MoS2 grown by chemical vapor deposition. The devices feature an embedded gate structure which ensures optimal gate control over the conducting channel and improves the channel-dielectric interface, whilst requiring a reduced number of fabrication steps. As a result, the device exhibits a maximum oscillation frequency as high as 11.4 GHz, an ION/IOFF current ratio of 108, and a remarkable transconductance of 70 μS/μm, among the highest achieved so far for MoS2 devices fabricated by means of chemical vapor deposition. These results advance the state-of-the-art performance of atomically thin radio frequency transistors.

Graphene based GHz flexible nanoelectronics and radio receiver systems (Invited)

It has been more than a decade since the early papers on graphene devices. Today, much progress has been achieved including material synthesis, device physics, and integrated circuits. All of this progress is particularly beneficial for flexible nanoelectronics where the high mobility of graphene can enable RF and microwave devices operating in the GHz regime. Here, we review the progress of flexible graphene transistors and circuits over the past decade. Recently, state of the art mobilities and GHz radio receiver systems have been demonstrated on plastic substrates.

Large Reduction of Hot Spot Temperature in Graphene Electronic Devices with Heat-Spreading Hexagonal Boron Nitride

Scanning thermal microscopy measurements reveal a significant thermal benefit of including a high thermal conductivity hexagonal boron nitride (h-BN) heat-spreading layer between graphene and either a SiO2/Si substrate or a 100 μm thick Corning flexible Willow glass (WG) substrate. At the same power density, an 80 nm thick h-BN layer on the silicon substrate can yield a factor of 2.2 reduction of the hot spot temperature, whereas a 35 nm thick h-BN layer on the WG substrate is sufficient to obtain a factor of 4.1 reduction. The larger effect of the h-BN heat spreader on WG than on SiO2/Si is attributed to a smaller effective heat transfer coefficient per unit area for three-dimensional heat conduction into the thick, low-thermal conductivity WG substrate than for one-dimensional heat conduction through the thin oxide layer on silicon. Consequently, the h-BN lateral heat-spreading length is much larger on WG than on SiO2/Si, resulting in a larger degree of temperature reduction.

Record fT, fmax, and GHz amplification in 2dimensional CVD MoS2 embedded gate fets

We report on chemical vapor deposited (CVD) M0S2 radio frequency (RF) transistors with a record fT of 20 GHz and fmax of 11.4 GHz, and the largest high-field saturation velocity, Vsat = 1.88 × 106 cm/s, in MoS2 reported so far. The gate-first approach, facilitated by cm-scale CVD MoS2, offers enhancement mode operation, Ion/Ioff ratio of 108, and the highest reported transconductance (gm) of 70 μS/μm. The intrinsic ft (fmax) obtained here is 3X (2X) greater than previously reported top-gated CVD MoS2 RF FETs. With as-measured S-parameters, we demonstrate the design of a GHz MoS2-based RF amplifier. This amplifier has gain greater than 15 dB at 1.2 GHz, input return loss > 10 dB, bandwidth > 200 MHz, and DC power consumption of ∼10 mW.