MESFET Research Articles

Demonstration of aluminum nitride metal oxide semiconductor field effect transistor on sapphire substrate

Abstract Aluminum nitride (AlN) is a promising ultrawide bandgap material with significant advantages for power electronics and optoelectronic applications due to its high breakdown voltage, mobility, and thermal conductivity. AlN Schottky barrier diodes (SBDs) and metal semiconductor field effect transistors (MESFETs) have shown potential but are limited by issues such as high off-state leakage current and complex structures to achieve ohmic contacts. To address these challenges, we report on the fabrication and characterization of an AlN metal oxide semiconductor field effect transistor (MOSFET) with a recessed gate structure. The source and drain contacts were fabricated on n-doped AlN epitaxy using Ti-based contacts with a Ti/Al/Ti/Au metal stack. To evaluate the performance of these contacts, a circular transmission line model (CTLM) was employed, and contacts were annealed at various temperatures ranging from 750 to 950 °C in a nitrogen ambient. Our results reveal that unannealed Ti-based contacts on AlN showed no current conduction. However, annealing these contacts at 950 °C for 30 s significantly reduced the specific contact resistance to 0.148 Ω·cm2, achieving an ~80% reduction compared to samples annealed at 750 °C. Utilizing these optimized contact conditions, we fabricated, to the best of our knowledge, the first AlN MOSFET. The fabricated AlN MOSFET exhibits a threshold voltage of −10.91 V, an effective mobility of 2.95 cm2/Vs, an on-off current ratio spanning two orders of magnitude, and a reverse breakdown voltage of approximately ~250 V in air without a field plate. 

Novel 4H–SiC MESFET with high ability in gate capacitances control for high frequency applications

The ability to control the gate capacitances is crucial for high-frequency applications, as it affects the device's frequency characteristics, gain and power handling capabilities. We present a 4H–SiC metal-semiconductor field-effect transistor (MESFET) with high gate capacitance control ability for high-frequency applications. The proposed structure (GCC-MESFET) consists of a step gate and a SiC well for adjusting the channel depletion layer and modifying the channel charges. Therefore, the gate capacitances will be controlled (GCC-MESFET). The proposed structure significantly improves the cut-off frequency (fT) and maximum oscillation frequency (fmax). The fT has increased from 23.5 GHz to 33 GHz and the fmax from 50.1 GHz to 54.4 GHz in the proposed structure compared to a conventional structure (C-MESFET). The results show that the DC maximum output power density (Pmax), DC transconductance (gm), cut-off frequency (fT) and maximum oscillation frequency (fmax) of GCC-MESFET improve in comparison with a conventional structure (C-MESFET). It is necessary to mention that the drain current and the breakdown voltage of the proposed structure increase by 48 % and 20 % respectively, compared with the C-MESFET structure due to modifying the channel charges and adjusting the electric field. So, the proposed structure can be used for high current, high voltage, high-power and high frequency applications.

High-voltage kV-class AlN metal-semiconductor field-effect transistors on single-crystal AlN substrates

This letter reports the demonstration and electrical characterization of high-voltage AlN metal-semiconductor field-effect transistors (MESFETs) on single-crystal AlN substrates. Compared with AlN MESFETs on foreign substrates, the AlN-on-AlN MESFETs showed high breakdown voltages of over 2 kV for drain-to-gate spacing of 15 μm and one of the highest average breakdown fields among reported AlN MESFETs. Additionally, the devices also exhibited decent drain saturation current and on/off ratio without complicated regrown or graded contact layers, which are several times higher than those of reported AlN-on-sapphire MESEFTs. This work is beneficial for the future development of ultrawide bandgap AlN power electronics.

Boltzmann Switching MoS2 Metal-Semiconductor Field-Effect Transistors Enabled by Monolithic-Oxide-Gapped Metal Gates at the Schottky-Mott Limit.

A gate stack that facilitates a high-quality interface and tight electrostatic control is crucial for realizing high-performance and low-power field-effect transistors (FETs). However, when constructing conventional metal-oxide-semiconductor structures with two-dimensional (2D) transition metal dichalcogenide channels, achieving these requirements becomes challenging due to inherent difficulties in obtaining high-quality gate dielectrics through native oxidation or film deposition. Here, a gate-dielectric-less device architecture of van der Waals Schottky gated metal-semiconductor FETs (vdW-SG MESFETs) using a molybdenum disulfide (MoS2) channel and surface-oxidized metal gates such as nickel and copper is reported. Benefiting from the strong SG coupling, these MESFETs operate at remarkably low gate voltages, <0.5V. Notably, they also exhibit Boltzmann-limited switching behavior featured by a subthreshold swing of ≈60mVdec-1 and negligible hysteresis. These ideal FET characteristics are attributed to the formation of a Fermi-level (EF) pinning-free gate stack at the Schottky-Mott limit. Furthermore, authors experimentally and theoretically confirm that EF depinning can be achieved by suppressing both metal-induced and disorder-induced gap states at the interface between the monolithic-oxide-gapped metal gate and the MoS2 channel. This work paves a new route for designing high-performance and energy-efficient 2D electronics.

An energy-band modulated p-GaN/InGaN/AlN p-channel MESFET with high ION/IOFF ratio and steep subthreshold swing

In this work, we report on the high-performance p-GaN/InGaN/AlN multi-heterostructure p-channel metal–semiconductor field effect transistors (MESFETs) with energy-band modulated quantum well-like InGaN channel and low work function metal tungsten (W) as the gate material. A negative threshold voltage (VTH) of −0.35 V is achieved by precisely controlling the self-aligned etching depth at the active region. Benefiting from the enhanced hole confinement, the ION/IOFF ratio and subthreshold swing of the fabricated-channel MESFET are extracted to be 1.2 × 107 and 66 mV/dec, respectively, at room temperature. The idealized Schottky interface with TMAH and post-gate-annealing treatment shows an ultra-low voltage hysteresis of 0.08 V extracted at subthreshold area in the dual-sweep transfer curves.

Anisotropic Electron Mobility and Contact Resistance of β-Ga2O3 Obtained via Radio Frequency Transmission Line Methods on Schottky Devices.

Monoclinic semiconducting β-Ga2O3 has drawn attention, particularly because its thin film could be achieved via mechanical exfoliation from bulk crystals, which is analogous to van der Waals materials' behavior. For the transistor devices with exfoliated β-Ga2O3, the channel direction becomes [010] for in-plane electron transport, which changes to vertical [100] near the source/drain (S/D) contact. Hence, anisotropic transport behavior is certainly worth to study but rarely reported. Here we achieve the vertical [100] direction electron mobility of 4.18 cm2/(V s) from Pt/β-Ga2O3 Schottky diodes with various thickness via radio frequency-transmission line method (RF-TLM), which is recently developed. The specific contact resistivity (ρc) could also be estimated from RF-TLM, to be 4.72 × 10-5 Ω cm2, which is quite similar to the value (5.25 × 10-5 Ω cm2) from conventional TLM proving the validity of RF-TLM. We also fabricate metal-semiconductor field-effect transistors (MESFETs) to study anisotropic transport behavior and contact resistance (RC). RC-free [010] in-plane mobility appears as high as maximum ∼67 cm2/(V s), extracted from total resistance in MESFETs.

Hybrid multimodule DC–DC converters accelerated by wide bandgap devices for electric vehicle systems

In response to the growing demand for fast-charging electric vehicles (EVs), this study presents a novel hybrid multimodule DC–DC converter based on the dual-active bridge (DAB) topology. The converter comprises eight modules divided into two groups: four Insulated-Gate Bipolar Transistor (IGBT) modules and four Metal–Semiconductor Field-Effect Transistor (MESFET) modules. The former handles high power with a low switching frequency, while the latter caters to lower power with a high switching frequency. This configuration leverages the strengths of both types of semiconductors, enhancing the converter’s power efficiency and density. To investigate the converter’s performance, a small-signal model is developed, alongside a control strategy to ensure uniform power sharing among the modules. The model is evaluated through simulation using MATLAB, which confirms the uniformity of the charging current provided to EV batteries. The results show an impressive power efficiency of 99.25% and a power density of 10.99 kW/L, achieved through the utilization of fast-switching MESFETs and the DAB topology. This research suggests that the hybrid multimodule DC–DC converter is a promising solution for fast-charging EVs, providing high efficiency, power density, and switching speed. Future studies could explore the incorporation of advanced wide bandgap devices to handle even larger power fractions.

Manipulation of bilayer MoS2-based MESFET with flexoelectric polarization field

We propose a bilayer MoS2-based metal-semiconductor field-effect transistor (MESFET) with the characteristic of mechanical modulation of its performance through flexoelectricity. The MESFET sample is fabricated with the bilayer MoS2 and the Si/SiO2 substrate as well as the gate electrode of Pt and the source/drain electrode of Ag/Au. The current-voltage relationships of the fabricated MESFET under the action of different gate voltages and tip forces as well as their combinations are measured with AFM. The tip force produces a strain gradient induced flexoelectric polarization field and thus changes the effective barrier height of Schottky contact. As a result, it can serve as an equivalent gate voltage, playing a significant tuning role in the performance such as transconductance and carrier mobility of the proposed MESFET. Within the drain-source voltage (VDS) range of 0–2 V, the maximum carrier mobility of the fabricated MESFET under the combined action of traditional gate voltage (VGS) and tip force reach 470 cm−2/(V∙s) significantly higher than that under VGS alone. The study provides valuable insights into the application of flexoelectricity in MESFET devices based on two-dimensional materials.

Low-power MoS2 metal–semiconductor field effect transistors (MESFETs) based on standard metal–semiconductor contact

With the popularization of electronic devices and the demand for portability, low-power consumption has become crucial for integrated circuit chips. Two-dimensional (2D) semiconductors offer significant potential in constructing low-power devices due to their ultrathin thickness, enabling fully depletion operation. However, fabricating these 2D low-power devices, such as negative-capacitance transistors or tunneling transistors, often requires multiple layers of gate dielectrics or channel band engineering, adding complexity to the manufacturing process and posing challenges for their integration with silicon technology. In this work, we have developed low-power MoS2 metal–semiconductor field effect transistors utilizing a standard metal–semiconductor contact, which eliminates the need for gate dielectrics and semiconductor heterojunctions. It demonstrates a sharp subthreshold slope (SS ∼ 64 mV/dec), a minimum operating gate voltage range (−0.5 ∼ 1 V), a minimum current hysteresis (3.69 mV), and a stable threshold voltage close to 0 V (Vth ∼ −0.27 V). Moreover, we implemented an inverter circuit with a high voltage gain of 47.

(Invited) Leading-Edge Diamond FET, MEMS, and Photodetector Devices

Diamond is a candidate material for next-generation power electronics, micro-electro mechanical systems (MEMS), and solar-blind deep-ultraviolet (DUV) photodetector devices with excellent thermal stability and radiation hardness, which operate under extreme environment. In order to use an advantage of high-density hole channel of hydrogenated diamond (H-diamond) surface, we have developed the high-k stack gate dielectrics and AlN heterojuction gate for H-diamond FETs, such as HfO2/HfO2, LaAlO3/Al2O3 Ta2O5/Al2O3, and ZrO2/Al2O3, AlN/Al2O3 prepared by a combination of sputter-deposition (SD) and atomic layer deposition (ALD) techniques [1,2]. We also demonstrated the artificial diamond Fin-FETs with high-current level [3] and the nanolaminate insulator gate metal-oxide-gate FETs (MOSFETs) with k value as high as 100 [4], and the new transistor concept named by metal-insulator-metal-semiconductor field-effect transistor (MIMS-FET) to achieve normally-off operation by combining the advantages of MOSFET and metal-semiconductor FET [5]. In addition, we developed the routine ion-implantation process for preparing the diamond cantilever with a resonant frequency quality factor as high as one million [6,7]. We have also developed thermally stable Schottky barrier photodiode (SPD), metal-semiconductor-metal photodetector (MSMPD), and MSM-type SPD (IDF-SPD) with a large photoconductivity gain in DUV wavelength and a large discrimination ratio between DUV/visible light responsivity [8,9]. In this presentation, we will review the comprehensive our work on the diamond FET, MEMS, and photodetector devices. Acknowledgements: This work was in collaboration with J-W. Liu, M. Imura, M-Y. Liao, J. Alvarez in NIMS and A. Ouchiero and E. Obaldia in University of Taxes, Dallas, and partly supported by JSPS KAKENHI Grant Number 20H00313.

Design, fabrication and measurement of metal-semiconductor field effect transistor based on zinc oxide material

The paper mainly focuses on the design, fabrication and measurement on Zinc Oxide (ZnO)-based Metal-Semiconductor Field Effect Transistor. The research problem in this study is difficulty on observing the electronic properties of ZnO materials to fabricate the high performance transistor design with non-toxic semiconductor materials. Even though the wide band gap materials of Group III and V possess high performance properties for fabricating the power electronics devices, the harmful impacts could not be reduced. The research solution for the problem statement in this study is emphasized on the non-toxic materials of Group II and VI-based high performance power electronics devices fabrication. The experimental studies of the device fabrication were conducted by Pulse Laser Deposition (PLD) process in standard laboratory. The step-by-step procedures for MSFET device fabrication were discussed and the confirmation of developed device fabrication was completed. The approaches on all measurement were completed based on band diagram condition, quantum interference on metal-semiconductor materials, and current-voltage characteristics. The step by step measurement for fabricated device for the proposed structure could be confirmed by standard measurement techniques. The proposed design has been validated for the utilization of high performance applications. The physical properties and physical characteristics for measurement results were confirmed by the theoretical analyses. The numerical analyses have been completed with the help of MATLAB. All results have been proved by recent research works.

Improved SOI-MESFET structures for enhanced efficiency and optimized DC/RF characteristics

ABSTRACT In this paper, two new silicon on insulator metal-semiconductor field effect transistor (SOI-MESFET) structures are presented. Two parallel layers of oxide and aluminium are added at the gate edge of these structures. Also, in the buried oxide part of the Aluminium Edge and Silicon-Well MESFET (AESW-MESFET) structure, a silicon well and two aluminium layers are added. Moreover, and to improve the DC and RF characteristics, as compared to the Conventional MESFET (C-MESFET) structure, a silicon well and a silicon layer are added in the box oxide section in the Silicon Edge and Silicon-Well MESFET (SESW-MESFET) structure. By these changes, the value of the breakdown voltage in the normal structure has increased from 15.8 V to 33.1 V and 30.9 V in the proposed AESW-MESFET and SESW-MESFET structures, respectively. In addition, the maximum output power has been associated with a significant increase of 4.44 and 5.24 times, respectively. Compared to the C-MESFET, the proposed structures reduce gate-source and gate-drain capacitors and significantly increases conductivity. The cut-off frequency values are increased from 19.3 GHz (the normal structure) to 37.3 GHz and 35 GHz (the proposed structures), and the maximum oscillation frequencies are increased from 80 GHz to 154 GHz and 102.3 GHz. Therefore, the results show that the proposed structures have good performance and the ability to work at high power and high frequency.

Enhanced thermoelectric performance via quantum confinement in a metal oxide semiconductor field effect transistor for thermal management

The performance of thermoelectric devices is gauged by the dimensionless figure of merit {{{{{boldsymbol{ZT}}}}}}. Improving {{{{{boldsymbol{ZT}}}}}} has proven to be a formidable challenge given the interdependence of its constitutive quantities, namely Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature. Here, we use quantum confinement to decouple Seebeck coefficient and electrical conductivity to demonstrate an order of magnitude quantum-based enhancement to the thermoelectric figure of merit {{{{{boldsymbol{ZT}}}}}} in complimentary metal-oxide-semiconductor field-effect transistors. While most quantum-based enhancement is done through physical confinement using two-dimensional materials, our approach uses electrical confinement. Because of this, our device is more robust than the two-dimensional materials currently used. We further articulate that improvement by as much as a factor of 50 could be achieved in a practical setting. Our approach further provides a path for monolithic integration of on-chip coolers and energy scavengers with virtually no deviation from the fabrication flow of standard electronics.

Multifunctional Magnetic Oxide-MoS2 Heterostructures on Silicon.

Correlated oxides and related heterostructures are intriguing for developing future multifunctional devices by exploiting their exotic properties, but their integration with other materials, especially on Si-based platforms, is challenging. Here, van der Waals heterostructures of La0.7 Sr0.3 MnO3 (LSMO) , a correlated manganite perovskite, and MoS2 are demonstrated on Si substrates with multiple functions. To overcome the problems due to the incompatible growth process, technologies involving freestanding LSMO membranes and van der Waals force-mediated transfer are used to fabricate the LSMO-MoS2 heterostructures. The LSMO-MoS2 heterostructures exhibit a gate-tunable rectifying behavior, based on which metal-semiconductor field-effect transistors (MESFETs) with on-off ratios of over 104 can be achieved. The LSMO-MoS2 heterostructures can function as photodiodes displaying considerable open-circuit voltages and photocurrents. In addition, the colossal magnetoresistance of LSMO endows the LSMO-MoS2 heterostructures with an electrically tunable magnetoresponse at room temperature. This work not only proves the applicability of the LSMO-MoS2 heterostructure devices on Si-based platform but also demonstrates a paradigm to create multifunctional heterostructures from materials with disparate properties.

2D semimetal with ultrahigh work function for sub-0.1 V threshold voltage operation of metal-semiconductor field-effect transistors

Metal-semiconductor field-effect transistors (MESFETs) offer the advantages of efficient gate control and low power consumption due to the large junction capacitance. However, the strong Fermi-level pinning caused by the metal-induced gap states makes it a great challenge to build a high-quality Schottky junction with a large energy barrier and low leakage current. Moreover, the upper limit of the work function of conventional metals is around 5 eV, which prevents the formation of a wide depletion region and the reduction of power consumption. In this work, 2D semimetals with high work function have been used as the gate to fabricate 2D van der Waals MESFETs. Compared with PtSe2 and PdSe2, TiS2 with ultrahigh work function of 6.6 eV forms the largest Schottky barrier height with MoS2. And TiS2/MoS2 MESFETs display a sub-0.1 V threshold voltage owing to the large built-in potential and exhibit a small subthreshold swing of 73 mV/dec. The tuning of the channel carrier density from the back gate can further optimize the device performance and provide a strategy to build AND logic gate in a dual-gated transistor. Our work successfully demonstrates the use of high-work-function 2D semimetal to realize high-performance MESFET with low power consumption.

Temperature dependence of electrical characteristics of Si-implanted AlN layers on sapphire substrates

AlN with a large bandgap energy is one of the most attractive materials for high-temperature applications. However, performance of AlN devices at high temperatures has been limited by technical problems with electrical characterization systems. Here, we show that Schottky-barrier diodes (SBDs) and metal-semiconductor field-effect transistors with Si-implanted AlN channels can operate at 1100 K and 1000 K, respectively. The breakdown voltage and barrier height of the AlN SBD were 610 V and 3.5 eV, respectively. We found that the high barrier height and thermal stability of the Ni contact on AlN greatly contributed to high-temperature operation of the devices.

Improved multi-recessed p-buffer 4H–SiC metal-semiconductor field-effect transistor with high power added efficiency

Based on the Multi-Recessed p-Buffer (MRB) Metal-Semiconductor Field-Effect Transistor (MESFET), this paper presents an Improved MRB 4H–SiC MESFET (IMRB MESFET) with high Power-Added Efficiency (PAE). The proposed structure has optimized recessed buffer layer from gate to drain. While maintaining excellent Radio Frequency performance, the maximum power density and PAE are greatly improved. The simulation results show that compared with MRB MESFET, the saturated drain current of IMRB MESFET is increased by 91.1%, the transconductance is increased by 15.5%, the Pmax is increased by 32.4%, and the PAE is increased by 40.29%. In summary, the IMRB MESFET is not only simple to manufacture, but also increases the energy conversion efficiency.

Revealing the adhesion, stability, and electronic structure of SiC/M (M=Au, Pt) interface: A first-principles study

The interface bonding and electronic properties between SiC and metal are the key factors affecting the physical transport phenomena of metal-semiconductor field-effect transistors (MESFETs). The work of adhesion, interface energy and electronic structure of SiC/M (M = Au, Pt) interface are explored by first-principles method. Forty possible interface models were constructed for different surfaces and stacking positions, and the four most stable interfaces were screened based on the work of adhesion and interface energy. Moreover, the interfacial bonding behavior and strength of SiC/M were quantitatively and qualitatively described through partial density of states (PDOS), crystal orbital Hamilton population curves (COHP), and charge density distribution (CDD). The PDOS demonstrated covalent bond characteristics between M − Si and M − C, which are primarily bonded by M-d and Si-p or C-p orbital hybridization. Further, the COHP and CDD analyses clearly manifested that the Pt–C bond at the interface exhibits more pronounced electron aggregation and greater bonding strength than the Au–C bond. This study offers a comprehensive understanding of the interfacial bonding strength between SiC and Au or Pt and demonstrate that SiC/Pt contact is the more favorable option for electronic device materials.

Silicon-doped β-Ga2O3 films grown at 1 µm/h by suboxide molecular-beam epitaxy

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 1016 to 1019 cm−3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature (Tsub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of &amp;lt;2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 1017 cm−3 reveal a room-temperature mobility of 124 cm2 V−1 s−1 that increases to 627 cm2 V−1 s−1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga2O3 films grown by S-MBE at growth rates of ∼1 µm/h.

High-performance β-Ga2O3 Schottky barrier diodes and metal-semiconductor field-effect transistors on a high doping level epitaxial layer

High-performance β-Ga2O3-based Schottky barrier diodes (SBDs) and metal-semiconductor field-effect transistors (MESFETs) are fabricated on a non-delta-doped high doping level (>1018 cm−3) epitaxial wafer. Their electrical properties and stabilities after annealing at 200–400 °C are investigated. The ON/OFF ratios for all the SBDs are over 108. The ideality factors and Schottky barrier heights for the Au/β-Ga2O3 SBDs with different annealing temperatures range from 1.5 to 2.5 and from 0.8 eV to 1.0 eV, respectively. There are obvious pinch-off and saturation characteristics for the β-Ga2O3 MESFETs. The maximum drain current (ID,max) for the as-fabricated β-Ga2O3 MESFET is 46.8 mA/mm at anode voltage of 2.0 V. They are much higher than the previously reported values of non-delta-doped and nanobelt β-Ga2O3 MESFETs. Threshold voltages for the β-Ga2O3 MESFETs shift to the negative direction with the increase of annealing temperature. This study is meaningful to push forward the development of β-Ga2O3-based electronic devices for practical applications.