Heterostructure Field-effect Transistors Research Articles

Charge-Inhomogeneity-Mediated Low-Frequency Noise in One-Dimensional Edge-Contacted Graphene Heterostructure Field Effect Transistors

Charge-Inhomogeneity-Mediated Low-Frequency Noise in One-Dimensional Edge-Contacted Graphene Heterostructure Field Effect Transistors

Reduced temperature in lateral (AlxGa1−x)2O3/Ga2O3 heterojunction field effect transistor capped with nanocrystalline diamond

The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current).

Enhancement mode β-(Al0.19Ga0.81)2O3/Ga2O3 HFETs with superlattice back-barrier layer

In this study, we numerically investigate the improvement in maximum drain current of an enhancement-mode β-(Al0.19Ga0.81)2O3/Ga2O3 heterostructure field-effect transistor (HFET) that employs a superlattice back-barrier layer. The measured transfer characteristics of a recently reported enhancement-mode β-(Al0.19Ga0.81)2O3/Ga2O3 HFET are reproduced using a device simulation and modeling tool. Subsequently, the transfer, output, radio frequency (RF), and off-state breakdown characteristics of the proposed device are revealed. The calculated off-state breakdown voltage is drastically enhanced from 26 V to 93 V for a gate length (LG) of 195 nm and a gate-to-drain separation (LGD) of 190 nm because of the superlattice back-barrier, which is one of the highest breakdown voltages reported for submicron Beta-Gallium Oxide (β-Ga2O3) transistors. Moreover, the insertion of the superlattice back-barrier layer improves subthreshold characteristics, resulting in a positive shift in the threshold voltage. The results indicate that using the superlattice back-barrier can dramatically enhance both the off-state current characteristics and RF characteristics of the enhancement mode in highly scaled β-Ga2O3 HFETs.

(Invited) Switching Characteristics of GaN Power Transistors

A shorter switching time is one of the key advantages of GaN power devices over Si IGBTs. [1] In this paper, we report on the simulation of GaN power transistor switching and investigate the effect on the switching time novel design approaches ranging from the gate edge engineering (beyond just using field plates [2] for optimizing the voltage distribution in the drain-to-gate spacing to using the quantum well channel designs [3] that lead to the electron wave function penetration into wide band gap cladding layers with the commensurate increase in the breakdown voltage while keeping the advantage of high mobility in the device channel. We also report on the effect of a low conducting passivation for smoothing or even eliminating the sharp maximum of the electric field in the vicinity of the gate and field plate edges on switching time.[4] Additional contacts in the drain-to-gate spacing for the field control and variable doping implants could also decrease the switching time. Further optimization could be achieved with the perforated channel designs [5] decreasing the parasitic series resistance and improving the heat dissipation.[1] A. Tüysüz, R. Bosshard, and J. W. Kolar, erformance Comparison of a GaN GIT and a Si IGBT for High-Speed Drive Applications, Proceedings of the International Power Electronics Conference - ECCE Asia (IPEC 2014), Hiroshima, Japan, May 18-21, 2014[2] S. Karmalkar, M. S. Shur, G. Simin and M. A. Khan, Field-Plate Engineering for Heterostructure Field Effect Transistors, IEEE Trans. ED, 52, 12, pp. 2516- 2532, December (2005)[3] M. Dyakonov and M. S. Shur, Consequences of Space Dependence of Effective Mass in Heterostructures, J. Appl. Phys. Vol. 84, No. 7, pp. 3726-3730, October 1 (1998)[4] G. Simin, M. Shur and R. Gaska, SETI-0070: Semiconductor device with low-conducting field-controlling element App. No 13/396,059, November 19, 2013 as US Patent No. 8,586,997[5] G. Simin, M. Gaevski, M. Shur, R. Gaska, Perforated Channel Field Effect Transistor, US patent 9,467,105, October 11, 2016

Study of electrical transport properties in split-gate AlGaN/GaN heterostructure field-effect transistors

In this study, we fabricated two similar split-gate AlGaN/GaN heterostructure field-effect transistors (SG AlGaN/GaN HFETs). However, minor variations in structural design resulted in substantial differences in the current–voltage characteristics. The mechanism of current modulation in the open region of the HFET can be explained by the fringe electric field (FEF) effect and the polarization Coulomb field (PCF) scattering for the applied biased split-gate. In particular, the effects of FEF and PCF scattering on the two-dimensional electron gas in the open region were analyzed separately when only the open region was on. Experimental results and TCAD simulation reveal the role of PCF scattering in SG AlGaN/GaN HFETs and provide evidence for the universality of PCF scattering in electronic devices with the AlGaN/GaN heterostructure. This study is meaningful for the structural optimization and electrical performance enhancement of SG AlGaN/GaN HFETs.

Study on the frequency characteristics of split-gate AlGaN/GaN HFETs

In this study, we report on the direct-current (DC) and frequency characteristics of split-gate heterostructure field-effect transistors (HFETs) with different split-gate lengths. The split-gate HFET contains two distinct operating parts, the large and the small transconductance parts. The split-gate HFET has a current cutoff frequency ([Formula: see text]) of 814[Formula: see text]MHz and a maximum oscillation frequency ([Formula: see text]) of 18.8[Formula: see text]GHz in the large transconductance part. Meanwhile, in the small transconductance part, there is a higher unilateral power gain of [Formula: see text][Formula: see text]GHz. Also, the split-gate HFET exhibits a voltage gain greater than 1 (0[Formula: see text]dB) at both DC and frequencies. Additionally, compared to the large transconductance part, the small transconductance part of the split-gate HFET presents a 53.5% reduction in DC power consumption and a larger input signal voltage range. Split-gate HFETs can be used as conventional power amplifiers in the large transconductance part and are also suitable as low-power voltage amplifiers in the small transconductance part.

Effect of device size on conduction channel effective width and polarization Coulomb field scattering intensity of split-gate AlGaN/GaN heterostructure field-effect transistors

In this study, the ungated AlGaN/GaN devices with special mesa structures were designed. The hypothesis of effective width expansion was tested experimentally by using ungated samples with different sizes, and an empirical expression for calculating the conduction channel effective width was given according to the experimental results. Finally, split-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with different gate lengths were prepared, and the channel electron mobility of split-gate samples was calculated by using the empirical expression and polarization Coulomb field (PCF) scattering theoretical model. The results showed that, for split-gate AlGaN/GaN HFETs, with the increase in the gate length, the total amount of additional polarization charges underneath the gate increases, resulting in the enhancement of the PCF scattering intensity.

Optimized photoelectric performance of MoS2/graphene heterostructure device induced by swift heavy ion irradiation

Two-dimensional materials with exceptional electronic and photoelectric properties are expected to develop a new-generation of ultrathin, high efficiency, broadband, and flexible photodetectors. For the potential application towards outer space exploration, the harsh irradiation environment must be taken into consideration. In this work, irradiation effects of MoS2/Graphene heterostructure field effect transistors (MoS2/G FETs) induced by swift heavy ions (SHIs) were explored. After irradiation, a new Raman peak denoted as D peak emerged, which demonstrated that SHI irradiation induced damage in graphene. Photoluminescence investigation of MoS2 indicated that SHI irradiation activated the excitation conversion of trion A− to excitation A0. Latent tracks were observed on MoS2/G/SiO2 by using atomic force microscope. The decreased resistance and increased carrier mobilities were detected at fluence lower than 5 × 1010 ions/cm2, which could be ascribed to the localized annealing of graphene during irradiation. The increased photocurrent and responsivity at fluence lower than 1 × 1010 ions/cm2 could be interpreted as the enhanced photoinduced gate voltage resulting from the SHI irradiation induced defects in MoS2/G FET. The devices were deteriorated at fluence of 1 × 1011 ions/cm2. The optimized and degraded properties of the devices could be ascribed to competition among doping, local annealing and defect scattering.

Non-linear pH responses of passivated graphene-based field-effect transistors

Graphene-based field-effect transistors (FETs) are suitable for pH sensors due to their outstanding surface chemical properties and its biocompatibility. To improve the devices' stability and pH sensitivity, different sets of dielectric passivation layers composed of monolayer hexagonal boron nitride with and without aluminum oxide layers were evaluated. Non-linearities of the pH response were observed. Heterostructure FETs were derived from subtractive manufacturing of commercially transferred two-dimensional materials on four-inch SiO2/Si wafers via stainless steel and polypropylene masking. Phosphate solutions (10 mM) of varying pH were incubated on bare devices, whereby liquid-gating elucidated linear changes in the Dirac voltage of hBN/graphene (−40 mV/pH) that was smaller than a device consisting only of monolayer graphene (−47 mV/pH). Graphene-based FETs were passivated with aluminum oxide nanofilms via electron beam or atomic layer deposition and were observed to have distinct Raman spectral properties and atomic force microscopy topologies corroborating the hypothesis that morphological differences of the deposited aluminum oxide influence the pH-dependent electrical properties. Atomic layer deposition of aluminum oxide on the 2D sensing areas resulted in non-linear shifting of the Dirac voltage with respect to pH that evolved as a function of deposition thickness and was distinct between graphene with and without hexagonal boron nitride as a capping monolayer. The non-linear response of varying thickness of AlxOy on graphene-based FETs was progressively reduced upon basic wet etching of the AlxOy. Overall, passivated graphene-based transistors exhibit deposition-dependent pH responses.

Germanium wafers for strained quantum wells with low disorder

We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading dislocation density of (6±1)×105 cm−2, nearly an order of magnitude improvement compared to control strain-relaxed buffers on Si wafers. The associated reduction in short-range scattering allows for a drastic improvement of the disorder properties of the two-dimensional hole gas, measured in several Ge/SiGe heterostructure field-effect transistors. We measure an average low percolation density of (1.22±0.03)×1010 cm−2 and an average maximum mobility of (3.4±0.1)×106 cm2/Vs and quantum mobility of (8.4±0.5)×104 cm2/Vs when the hole density in the quantum well is saturated to (1.65±0.02)×1011 cm−2. We anticipate immediate application of these heterostructures for next-generation, higher-performance Ge spin-qubits, and their integration into larger quantum processors.

(Invited) Optical Properties and Device Application of One-Dimensional Vdw Heterostructures Based on Single-Walled Carbon Nanotubes

A typical one-dimensional (1D) van der Waals (vdW) heterostructure consists of SWCNT, boron nitride nanotube (BNNT), and molybdenum disulfide nanotube (MoS2NT), grown coaxially by successive chemical vapor deposition (CVD) steps [1, 2]. The coaxially nested structure based on SWCNTs, which has diverse electronic properties (metallic or semiconducting), can expand the board application possibilities of 1D vdW heterostructures [3]. By comparing the optical properties of films of BNNT@MoS2NT and SWCNT@BNNT@MoS2NT, we found strong photoluminescence (PL) from monolayer MoS2NT and quenching of PL by coupling to SWCNT through thin BNNT [4]. The inter-tube excitons are demonstrated by ultrafast optical spectroscopy [5]. The inter-tube excitons, the counterpart of inter-layer excitons for 2D heterostructures, suggest a possibility of efficient photovoltaic applications of the 1D heterostructure [6]. In addition to MoS2 nanotubes, we will discuss the growth control of WS2 and NbS2 nanotubes. A general strategy we can tune the CVD condition from 2D flake to 1D tube is proposed. The junctions of MoS2 and WS2 nanotube are also realized as the outermost layer. More specific optical properties are studied for semiconductor SWCNTs and chirality-sorted SWCNTs [7,8]. Semiconductor SWCNT or chirality-sorted SWCNT wrapped with BNNT can be regarded as the ideal building blocks of field-effect transistors (FET) [9]. We have demonstrated the radial semiconductor–insulator–semiconductor (S-I-S) tunneling heterojunction diode by using a micrometer long 1D vdW heterostructure SWCNT@BNNT@MoS2NT [10]. We will discuss a more recent array of 1D heterostructure FET devices.Part of this work was supported by JSPS KAKENHI Grant Number JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan.

Investigation of Electrical Properties of N‐Polar AlGaN/AlN Heterostructure Field‐Effect Transistors

AlN‐based field‐effect transistors (FETs) enable high‐breakdown voltage, high drain current, and high‐temperature operation. To realize high‐frequency devices, N‐polar AlGaN/AlN heterostructure FETs are focused on. N‐polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN FET is fabricated using metal–organic vapor‐phase epitaxy, and its electrical characteristics are evaluated. An N‐polar AlN layer is grown on a sapphire substrate with a misorientation angle of 2.0° toward m‐axis, on which a 20 nm thick Al0.9Ga0.1N base layer and a 20 nm Al0.1Ga0.9N channel layer are grown. The static FET operation is confirmed to exhibit an n‐channel and pinch‐off. Normally, during operation with a turn‐on voltage of −3.2 V, a high operating breakdown voltage of 620 V and high operating temperature of 280 °C are also confirmed.

Detection of HER-3 with an AlGaN/GaN-Based Ion-Sensitive Heterostructure Field Effect Transistor Biosensor.

Human epidermal growth factor receptor-3 (HER-3) plays a key role in the growth and metastasis of cancer cells. The detection of HER-3 is very important for early screening and treatment of cancer. The AlGaN/GaN-based ion-sensitive heterostructure field effect transistor (ISHFET) is sensitive to surface charges. This makes it a promising candidate for the detection of HER-3. In this paper, we developed a biosensor for the detection of HER-3 with AlGaN/GaN-based ISHFET. The AlGaN/GaN-based ISHFET biosensor exhibits a sensitivity of 0.53 ± 0.04 mA/dec in 0.01 M phosphate buffer saline (1× PBS) (pH = 7.4) solution with 4% bovine serum albumin (BSA) at a source and drain voltage of 2 V. The detection limit is 2 ng/mL. A higher sensitivity (2.20 ± 0.15 mA/dec) can be achieved in 1× PBS buffer solution at a source and drain voltage of 2 V. The AlGaN/GaN-based ISHFET biosensor can be used for micro-liter (5 μL) solution measurements and the measurement can be performed after incubation of 5 min.

Sub-bandgap photon-assisted electron trapping and detrapping in AlGaN/GaN heterostructure field-effect transistors

We have investigated photon-assisted trapping and detrapping of electrons injected from the gate under negative bias in a heterostructure field-effect transistor (HFET). The electron injection rate from the gate was found to be dramatically affected by sub-bandgap laser illumination. The trapped electrons reduced the two-dimensional electron gas (2DEG) density at the AlGaN/GaN heterointerface but could also be emitted from their trap states by sub-bandgap photons, leading to a recovery of 2DEG density. The trapping and detrapping dynamics were found to be strongly dependent on the wavelength and focal position of the laser, as well as the gate bias stress time prior to illumination of the HFET. Applying this phenomenon of trapping and detrapping assisted by sub-bandgap photons, red, green, and purple lasers were used to demonstrate photo-assisted dynamic switching operations by manipulation of trapped carriers at the surface of an AlGaN/GaN HFET. A physical model based on band diagrams, explaining the trapping and detrapping behavior of electrons, has been presented.

Undoped Strained Ge Quantum Well with Ultrahigh Mobility of Two Million.

We develop a method to fabricate an undoped Ge quantum well (QW) under a 32 nm relaxed Si0.2Ge0.8 shallow barrier. The bottom barrier contains Si0.2Ge0.8 (650 °C) and Si0.1Ge0.9 (800 °C) such that variation of Ge content forms a sharp interface that can suppress the threading dislocation density (TDD) penetrating into the undoped Ge quantum well. The SiGe barrier introduces enough in-plane parallel strain (ε∥ strain -0.41%) in the Ge quantum well. The heterostructure field-effect transistors with a shallow buried channel obtain an ultrahigh two-dimensional hole gas (2DHG) mobility over 2 × 106 cm2/(V s) and a very low percolation density of (5.689 ± 0.062) × 1010 cm-2. The fractional indication is also observed at high density and high magnetic fields. This strained germanium as a noise mitigation material provides a platform for integration of quantum computation with a long coherence time and fast all-electrical manipulation.

Novel GaN-based double-channel p-heterostructure field-effect transistors with a p-GaN insertion layer

GaN-based p-channel heterostructure field-effect transistors (p-HFETs) face significant constraints on on-state currents compared with n-channel high electron mobility transistors. In this work, we propose a novel double heterostructure which introduces an additional p-GaN insertion layer into traditional p-HFETs. The impact of the device structure on the hole densities and valence band energies of both the upper and lower channels is analyzed by using Silvaco TACD simulations, including the thickness of the upper AlGaN layer and the doping impurities and concentration in the GaN buffer layer, as well as the thickness and Mg-doping concentration in the p-GaN insertion layer. With the help of the p-GaN insertion layer, the C-doping concentration in the GaN buffer layer can be reduced, while the density of the two-dimensional hole gas in the lower channel is enhanced at the same time. This work suggests that a double heterostructure with a p-GaN insertion layer is a better approach to improve p-HFETs compared with those devices with C-doped buffer layer alone.

Gate-controlled hysteresis curves and dual-channel conductivity in an undoped Si/SiGe 2DEG structure

Exploring the cryogenic transport properties of two-dimensional electron gas in semiconductor heterostructures is always a focus of fundamental research on Si-based gate-controlled quantum devices. In this work, based on the electrical and magnetic transport characteristics of Si/SiGe heterostructure Hall bar-shaped field effect transistors (FETs) at 10 K and 1.6 K, we study the effects of electron tunneling, which occurs in the heterostructure and populates the oxide/semiconductor interface, on its transport properties. The initial position of dual-channel conduction is determined by the gate-controlled electrical hysteresis curves. Furthermore, we discover that there exist different tunneling mechanisms of electrons in a Si quantum well under the action of gate voltage, and the electron tunneling can well explain the two drain current plateaus appearing in direct-current transfer characteristics. Combining the power-law exponent of electron mobility versus density curve and the gate-related Dingle ratio, we clarify the dominant scattering mechanism, and the result can be supported by different tunneling mechanisms. Our work demonstrates the gate-dependent electronic transport performance in undoped Si/SiGe heterostructure FETs, which has an implication for the development of Si/SiGe heterostructure gate-defined quantum dot quantum computation.

Suspended CNTs/MoS2 heterostructure field effect transistor for high performance biosensor and its application for serum PSA detection

Field effect transistor (FET) biosensors provide highly sensitive and rapid detection of bioactive proteins and biomarkers. However, the interface scattering may degrade device performance and the supported substrate also limits the exposed area to biological analytes. Here, a suspended heterostructure field effect transistor (FET) was fabricated for the detection of prostate specific antigen (PSA) in clinical serum samples from patients with prostate cancer. First, we tested the performance of the biosensor by β-actin detection, and the suspended CNTs/MoS2 heterostructure FET showed higher mobility, conductivity and sensitivity with enhanced stability compared to the conventional supported MoS2 FET. Furthermore, the suspended device showed ultrasensitive detection of PSA with the limit of detection (LOD) as low as 10-13 μg/μL and large detection range from 10-13 to 10-4 μg/μL. The device also showed rapid response and higher stability. For clinical application, multiple serum samples were analyzed by the suspended CNTs/MoS2 FET biosensors. The PSA concentration measured by the device was highly consistent with those obtained by the enzyme-linked immunosorbent assays (ELISA) method. Taken together, the suspended CNTs/MoS2 FET showed sensitive and reliable detection capability with great potential for rapid clinical examination and biomarker monitoring.

Depletion- and Enhancement-Mode p-Channel MISHFET Based on GaN/AlGaN Single Heterostructures on Sapphire Substrates

We report on p-channel metal-insulator-semiconductor heterostructure field-effect transistors (MISHFET) based on p-GaN/uid-GaN/Al0.29Ga0.71N single heterostructures on sapphire substrates, grown by metalorganic vapor phase epitaxy (MOVPE). The impact of p-GaN layer removal and channel layer thickness adjustment by dry-etching on the characteristics of the MISHFET are investigated. Depending on the remaining GaN thickness (tGaN), the fabricated MISHFET show either depletion-mode (d-mode) operation with a threshold voltage Vth of 3.8 V and an on-current |ID,on| of 9.5 mA/mm (tGaN = 21 nm) or enhancement-mode (e-mode) operation with Vth of -2.3 V and |ID,on| of 1.5 mA/mm (tGaN = 12 nm). Independent of the etching depth, all devices exhibit a very low off-state drain current |ID,off| 10-8 mA/mm and a steep subthreshold swing (SS) between 80 and 89 mV/dec. Similar to n-channel devices, a Vth instability caused by charge trapping at the dielectric/semiconductor interface is found, emphasizing that careful interface engineering is required for good device performance.

Investigations on Wide-Gap Al0.21Ga0.79N Channel MOS-HFETs With In0.12Al0.76Ga0.12N Barrier/Buffer and Drain Field-Plate

This work investigates, for the first time, wide-gap Al0.21Ga0.79N channel metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with In0.12Al0.76Ga0.12N barrier/buffer and drain field-plate (DFP) designs. High-k and wide-gap Al2O3 was grown as the gate oxide and surface passivation by using non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. A control device having the same epitaxial layers, except with In0.12Al0.88N barrier/buffer was studied in comparison. Enhanced spontaneous polarization effect, improved interfacial quality, and enhanced carrier confinement have been achieved by using the In0.12Al0.76Ga0.12N barrier/buffer design, which has successfully resulted in improved carrier transport, increased electron concentration, and high current densities. The present In0.12Al0.76Ga0.12N/AlN/Al0.21Ga0.79N MOS-HFET design with (without) DFP design has demonstrated superior maximum drain-source current density (IDS, max) of >1 (>1) A/mm at VDS = 20 V, high saturated drain-source current density at VGS = 0 V (IDSS0) of 791.1 (755) mA/mm, and low specific on-resistance (Ron, sp) of 2.83 (2.81) mΩ/cm2. High device figure-of-merit (FOM) on BVDS2/Ron, sp of 93.7 (75.4) MW/cm2 was also obtained with the three-terminal on-state drain-source breakdown voltage (BVDS) of 515 (460) V. The present design is promisingly advantageous to high-current and high-voltage power-switching circuit applications.