Specific features of proton interaction with transistor structures having a 2D AlGaN/GaN channel

It has been shown that the interaction of 1 MeV protons at doses of (0.5–2) × 10¹⁴ cm^–2 with transistor structures having a 2D AlGaN/GaN channel (AlGaN/GaN HEMTs) is accompanied not only by the generation of point defects, but also by the formation of local regions with a disordered nanomaterial. The degree of disorder of the nanomaterial was evaluated by multifractal analysis methods. An increase in the degree of disorder of the nanomaterial, manifested the most clearly at a proton dose of 2 × 10¹⁴ cm^–2, leads to several-fold changes in the mobility and electron density in the 2D channel of HEMT structures. In this case, the transistors show a decrease in the source–drain current and an order-of-magnitude increase in the gate leakage current. In HEMT structures having an enhanced disorder of the nanomaterial prior to exposure to protons, proton irradiation results in suppression of the 2D conductivity in the channel and failure of the transistors, even at a dose of 1 × 10¹⁴ cm^–2.     

The Effect of the Method by Which a High-Resistivity GaN Buffer Layer Is Formed on Properties of InAlN/GaN and AlGaN/GaN Heterostructures with 2D Electron Gas

AlGaN/AlN/GaN and InAlN/AlN/GaN structures with 2D electron gas have been grown on sapphire substrates by metal-organic vapor-phase epitaxy. The suppression of the parasitic conductivity of buffer GaN layers was provided either by intentionally raising the density of edge dislocations or by doping with iron (GaN:Fe). It was shown that using GaN buffer layers with a better crystal perfection and more planar surface results in the electron mobility in the 2D channel for carriers becoming 1.2–1.5 times higher.     

Multilayer AlN/AlGaN/GaN/AlGaN heterostructures for high-power field-effect transistors grown by ammonia MBE

The results of the optimization of the ammonia MBE technology of AlN/AlGaN/GaN/AlGaN heterostructures for high-power microwave field-effect transistors (FETs) are presented. The creation of technological systems of the EPN type for the deposition of group III nitrides by ammonia MBE, in combination with the development of optimum growth and postgrowth processes, make it possible to obtain AlN/AlGaN/GaN/AlGaN based heterostructures for high-power microwave FETs with the output static characteristics on the world best level. One of the main fields of application of the semiconductor heterostructures based on group III nitrides is the technology of high electron mobility transistors (HEMTs). Most investigations in this field have been devoted to the classical GaN/AlGaN structures with a single heterojunction. An alternative approach based on the use of double heterostructures with improved two-dimensional electron gas (2DEG) confinement offers a number of advantages, but such structures are usually characterized by a lower carrier mobility as compared to that in the single-junction structures. We succeeded in optimizing the double heterostructure parameters and growth conditions so as to obtain conducting channels with a 2DEG carrier mobility of 1450, 1350, and 1000 cm²/(V s) and a sheet electron density of 1.3 × 10¹³, 1.6 × 10¹³, and 2.0 × 10¹³ cm^?2, respectively. Experimental HEMTs with 1-μm-long gates based on the obtained multilayer heterostructure with a doped upper barrier layer exhibit stable current-voltage characteristics with maximum saturation current densities of about 1 A/mm and a transconductance of up to 180 mS/mm.     

Laser ablated ZnO layers for ALGaN/GaN HEMT passivation

ZnO layer in a role of passivation of the AlGaN/GaN-based high electron mobility transistors (HEMTs) is presented. The thin layer is deposited by pulsed laser deposition technique. It is fully compatible with the process technology of high electron mobility transistors prepared on AlGaN/GaN heterostructures due to its physical properties similar to the GaN. We have succeeded to (1) suppress the gate leakage current; (2) increase the maximum of the drain current and the electron drift mobility, and (3) ensure the threshold voltage to be unaltered by employment of the thin ZnO layer to the channel area of the HEMT.     

Impact of SF6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF₆ gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl₄ plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF₆ plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.     

Multilayer AIN/AlGaN/GaN/AlGaN heterostructures with quantum wells for high-power field-effect transistors grown by ammonia MBE

High-power field-effect transistors (FETs) are among the main applications of heterostructures based on group III metal nitrides, which in most cases implement the classical GaN/AlGaN structure with a single junction. An alternative approach based on the use of double heterostructures with imporved two-dimensional electron gas (2DEG) confinement offers a number of advantages, but such structures are usually characterized by a lower carrier mobility and density (in GaN layers of reduced thickness) as compared to the values in the single-junction structures. Optimization of the heterostructure design and ammonia MBE growth conditions allowed us to obtain multilayer AlN/AlGaN/GaN/AlGaN heterostructures with quantum wells, which are characterized by a 2DEG carrier mobility of 1100–1300 cm²/(V s) and a sheet electron density of (1.1–1.3) × 10¹³ cm^-2. Experimental FETs based on the obtained multilayer heterostructures in a static regime exhibit working current densities up to 0.6 A/mm at a transconductance of up to 150 mS/mm and a breakdown voltage above 100 V.     

Characterization of deep levels in high electron mobility transistor by conductance deep level transient spectroscopy

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor (HEMT) on silicon (111) substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas (2DEG) channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 77 to 600 K. With this method, known as Conductance Deep Level Transient Spectroscopy (CDLTS), majority deep levels with activation energy of 61 meV as well as minority carrier traps at 74 meV and capture cross-section respectively 2.56 × 10^− 15 cm², 2.1 × 10^− 15 cm² are identified. Finally, the correlation between the anomalies observed on the output characteristics and defects is discussed.     

改进的AlGaN/GaN HEMT小信号参数提取算法

制作了截止频率ft和最高震荡频率fmax分别为46.2和107.8 GHz的AlGaN/GaN高电子迁移率晶体管,并针对该器件建立了包含微分电阻Rfd和Rfs在内的18元件小信号等效电路模型;在传统的冷场条件提取器件寄生参数的基础上,通过对不同栅压偏置下冷场Z参数进行线性插值运算,可消除沟道分布电阻和栅极泄漏电流对寄生电阻的影响;再利用热场S参数对寄生参数部分进行去嵌,可提取得到本征参数。分析表明,此模型和算法提高了模型拟合精度,S参数的仿真结果与测试数据在200MHz到40GHz的频率范围内均符合很好,误差不到2%。     

Comparison of wurtzite and zinc blende III–V nitrides field effect transistors: a 2D Monte Carlo device simulation

An ensemble Monte Carlo method is used to compare the potentialities of zinc blende and wurtzite GaN for field effect transistor applications. First, bulk material electron transport properties are compared and we find that mobility, steady state velocity and velocity overshoot are at the advantage of zinc blende GaN. Then, zinc blende GaN and wurtzite GaN MESFET with very short gate length (L_g=0.12 μm) are investigated using a 2D Monte Carlo device simulation. A 50% gain in performance is obtained for the zinc blende GaN MESFET as compared with the wurtzite one. A zinc blende AlGaN/GaN HEMT is also simulated and exhibits a current density of 900 mA mm⁻¹, a transconductance of 480 mS mm⁻¹ and a cut-off frequency of 180 GHz.     

Increase in electron mobility of InGaAs/InP composite channel high electron mobility transistor structure due to SiN passivation

The influence of silicon nitride passivation on electron mobility of InGaAs/InP composite channel high electron mobility transistor structure has been studied. Different from the structures with single InGaAs channel, an increase in effective mobility μ_e with a negligible change of sheet carrier density n_s after SiN deposition is clearly observed in the composite channel structures. The enhancement of μ_e could be explained under the framework of electrons transferring from the InP sub-channel into InGaAs channel region due to the energy band bending at the surface region caused by SiN passivation, which is further confirmed by low temperature photoluminescence measurements.     

Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures

A non-polar AlGaN/GaN structure is a strong candidate for the high-voltage device that can operate in enhancement-mode compared to the depletion-mode operation that is practically unavoidable for a standard polar AlGaN/GaN structure. Growth of non-polar GaN is non-trivial and a two-step nucleation scheme was developed to produce high-quality non-polar a-plane AlGaN/GaN structures on r-plane sapphire. The anisotropic nature of non-polar GaN requires a modification to a typical polar GaN-based transistor fabrication process. A KOH wet etch proceeded by a dramatically different mechanism compared to the standard polar c-face AlGaN/GaN structure. This device with Pt/Au Schottky gate displayed a barrier height of 0.76 eV and an ideality factor of 4 at 20 °C.     

Performance projection of graphene nanomesh and nanoroad transistors

We examine the performance limits of field-effect transistors (FETs) with chemically modified graphene as the channel materials. Graphene nanoroad (XNR) and graphene nanomesh (XNM) can be created through selective chemical modification by an X adsorbate (either H or F) on graphene, which generates a bandgap while conserving the continuous two-dimensional (2D) atomistic layer. We adopt a ballistic transistor model, where the band structures were calculated using ab initio simulations to assess the performance of graphene nanoroad and nanomesh transistors. It is shown that arrays of graphene nanoroads, defined by hydrogenation or fluorination of atomically narrow dimer lines in a 2D graphene, are most ideal for transistor channel materials in terms of delivering a large ON-current, and significantly outperform Si metal-oxide-semiconductor field-effect transistors (MOSFETs). Alternatively, comparable performance to silicon can be achieved by careful design of a graphene nanomesh through patterned hydrogenation or fluorination. Both hydrogenation and fluorination lead to similar transistor performance, with fluorination more preferred in terms of chemical energetics.      

Determination of Spin-Orbit Interaction in InAs Heterostructure

Spin-orbit interaction (SOI) gives a useful tool to control spin precession in the semiconductor without external magnetic field. The Rashba effect induced by spin-orbit interaction enables to imagine the spin field effect transistor in which the resistance modulation is achieved by precession of spins moving in a channel. The oscillatory magnetoresistance was measured to determine SOI parameter of inverted type high electron mobility transistor structure where InAs quantum well is inserted to InAlAs/InGaAs barrier layer. The band structure and electron charge distribution of the structure was calculated using WinGreen simulator. Observed SOI parameters are large enough to produce high Rashba field of about a few Tesla. The magnitude of the SOI parameter is subject to change with the InAs quantum-well thickness.      

AlGaN/GaN-based diodes and gateless HEMTs for gas and chemical sensing

The characteristics of Pt/GaN Schottky diodes and Sc/sub 2/O/sub 3//AlGaN/GaN metal-oxide semiconductor (MOS) diodes as hydrogen and ethylene gas sensors and of gateless AlGaN/GaN high-electron mobility transistors (HEMTs) as polar liquid sensors are reported. At 25/spl deg/C, a change in forward current of /spl sim/6 mA at a bias of 2 V was obtained in the MOS diodes in response to a change in ambient from pure N/sub 2/ to 10% H/sub 2// 90% N/sub 2/. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25/spl deg/C. Gateless AlGaN/GaN HEMT structures exhibit large changes in source-drain current upon exposing the gate region to various polar liquids, including block co-polymer solutions. The polar nature of some of these polymer chains lead to a change of surface charges in gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. The nitride sensors appear to be promising for a wide range of chemicals, combustion gases and liquids.     

Isolated photosystem I reaction centers on a functionalized gated high electron mobility transistor

In oxygenic plants, photons are captured with high quantum efficiency by two specialized reaction centers (RC) called Photosystem I (PS I) and Photosystem II (PS II). The captured photon triggers rapid charge separation and the photon energy is converted into an electrostatic potential across the nanometer-scale (~6 nm) reaction centers. The exogenous photovoltages from a single PS I RC have been previously measured using the technique of Kelvin force probe microscopy (KFM). However, biomolecular photovoltaic applications require two-terminal devices. This paper presents for the first time, a micro-device for detection and characterization of isolated PS I RCs. The device is based on an AlGaN/GaN high electron mobility transistor (HEMT) structure. AlGaN/GaN HEMTs show high current throughputs and greater sensitivity to surface charges compared to other field-effect devices. PS I complexes immobilized on the floating gate of AlGaN/GaN HEMTs resulted in significant changes in the device characteristics under illumination. An analytical model has been developed to estimate the RCs of a major orientation on the functionalized gate surface of the HEMTs.     

Flexible Gallium Nitride for High‐Performance,Strainable Radio‐Frequency Devices

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio‐frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high‐frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical‐free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state‐of‐the‐art values for electrical performance, with electron mobility exceeding 2000 cm² V^?1 s^?1 and sheet carrier density above 1.07 × 10¹³ cm^?2. The influence of strain on the RF performance of flexible GaN high‐electron‐mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.     

Highly Flexible and Transparent Multilayer MoS2 Transistors with Graphene Electrodes

A highly flexible and transparent transistor is developed based on an exfoliated MoS₂ channel and CVD‐grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (～74%), and current on/off ratio (>10⁴) with an average field effect mobility of ～4.7 cm² V^?1 s^?1, all of which cannot be achieved by other transistors consisting of a MoS₂ active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (～22 meV) forms at the MoS₂/graphene interface, which is comparable to the MoS₂/metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.     

Impact of Device Technology Processes on the Surface Properties and Biocompatibility of Group III Nitride Based Sensors

In this work we have investigated the impact of typical device processing steps on the surface properties (roughness, chemical composition, contact angle to water) of group III‐nitride based chemical sensors with emphasis on the electrical performance of the sensor and the biocompatibility. Basic sensing device is an AlGaN/GaN high electron mobility transistor. The widely distributed mammalian cell cultures HEK 293FT and CHO‐K1 served as biological model systems. The processing of the devices had only little influence on the cell growth onto the sensor. In all cases it was superior to silicon surfaces. Fluorine dry etching smoothes the surface and forms an oxide, which improves the electrical properties of the AlGaN/GaN sensor. In contrast, autoclave treatment enhances the carbon contamination with negative impact on the sensor properties and increased the contact angle to water. For all other treatments the contact angle recaptures a value of about 50 ± 5° after exposure to air or water droplets for some hours due to the contamination by hydrocarbons.     

Control of carrier density by self-assembled monolayers in organic field-effect transistors

Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO(2) gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO(2) gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.     

High-Performance Poly-Si Nanowire NMOS Transistors

A novel field-effect transistor with Si nanowire (NW) channels is developed and characterized. To enhance the film crystallinity, metal-induced lateral crystallization (MILC) and/or rapid thermal annealing (RTA) techniques are adopted in the fabrication. In the implementation of MILC process, it is shown that the arrangement of seeding window plays an important role in affecting the resulting film structure. In this regard, asymmetric window arrangement, i.e., with the window locating on only one of the two channel sides is preferred. When MILC and RTA techniques are combined, it is found that single-crystal-like NWs are achieved, leading to significant performance improvement as compared with the control with channels made up of fine-grain structures by the conventional solid-phase crystallized (SPC) approach. Field-effect mobility up to 550 cm²/V-s is recorded in this study