Carrier Concentration In Channel Research Articles

High-Performance Top-Gated and Double-Gated Oxide–Semiconductor Ferroelectric Field-Effect Transistor Enabled by Channel Defect Self-Compensation Effect

In this article, we demonstrate a low-thermal budget defect-engineered process to achieve top-gated (TG) oxide–semiconductor ferroelectric field-effect transistors (FeFETs). The demonstrated TG FeFETs, with the channel length scaled down to 40 nm, exhibit a highly stabilized ferroelectric memory window (MW) of 2 V and a high current ON/ OFF ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{6}}$ </tex-math></inline-formula> . This is achieved by an engineered InGaZnOx (IGZO) and InSnOx (ITO) heterojunction channel that produces the defect self-compensation effect to passivate the intrinsic oxygen-deficient defects, existing in the indium-gallium-zinc-oxide (IGZO) channel interface and bulk. Effective interface/bulk defects passivation with good control of defect-induced channel carrier concentration has been notoriously difficult to achieve. Hence, realizing performant TG oxide-based FeFETs with back-end-of-line (BEOL) thermal budget constraints remains a fundamental challenge. Our study shows that heterojunction channel engineering on FETs and FeFETs can be a reliable solution to overcome this challenge. With such a technique, we can now enable double-gated (DG) ITO–IGZO FeFET and FETs. Such devices can enable BEOL-compatible reconfigurable nonvolatile logic switches that provide extremely low off-state leakage, high switch conductance ratio, and memory read-write disturb-free features.

Gate-Modulated High-Response Field-Effect Transistor-Type Gas Sensor Based on the MoS2/Metal-Organic Framework Heterostructure.

The high surface-to-volume ratio and decent material properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) make them advantageous as an active channel in field-effect transistor (FET)-type gas sensing devices. However, most existing TMD gas sensors are based on a two-terminal resistance-type structure and suffer from low responsivity and slow response, which has urged materials optimization as well as device engineering. Metal-organic frameworks (MOFs) have a large number of ordered binding sites in the pores that can specifically bind to gas molecules and can be decorated on TMD surfaces to enhance gas sensing capabilities. In this work, we successfully realize the FET-type gas sensor with MoS2-MOF as the channel. The fabricated gas sensor exhibits enhanced NH3 sensing performance (22.475 times higher in responsivity) as compared to the device with a bare MoS2 channel. In addition, the FET-type gas sensor geometry enables effective tuning of sensitivity through electrical gating based on the modulation over the channel carrier concentration. Furthermore, the dependence of responsivity on the MoS2 thickness is investigated as well to achieve an in-depth understanding of the electrical modulation mechanism of the MOF-decorated MoS2 gas sensors. The demonstrated results can pave an attractive pathway toward the realization of advanced high-response and tunable TMD-based gas sensing devices.

Non-Maxwell–Boltzmann dependence of channel carrier concentration in a quasi one dimensional charge density wave channel in the ballistic transport regime

With the aid of a coherent transport model utilizing the non-equilibrium Green function approach, a three terminal device with metallic gate, source and drain and a quasi one dimensional charge density wave (CDW) channel is simulated focussing on the transistor behaviour brought about by a sweep of the channel potential or equivalently the chemical potential in the channel. The channel is strongly insulating only at half-filling and moving to lower and higher carrier concentrations both incur a mean field phase transition to a conducting state. With the aid of conductance calculations for a pinned CDW condensate, we present calculations for the sub-threshold slope in terms of the hopping parameter or equivalently the width of the tight-binding chain. The effects of source to drain bias and length are examined. The conductance profiles are analyed in relation to transmission profiles. The observed CDW profiles are explained in terms of filling and Fermi surface nesting. Boundary conditions, gap equations and response functions are shown to reveal the commensurability conditions and size of the transport gap. The channel carrier concentration is modulated in an athermal (non-Maxwellian–Boltzmann) fashion, thereby making it an interesting prospect for steep transistors.

Observation of Power MOSFET Composed of Silicon Carbide with a Planar Type in the Voltage Applying State Using a Scanning Probe Microscope

We have developed a scanning probe microscope to evaluate the device with high spatial resolution in a power semiconductor device with a voltage applied. Specifically, we observed the cross sectional structure exposed SiC planar MOS-FET using a multifunctional probe microscope, which combines atomic force microscopy, Kelvin probe force microscopy, and scanning capacitance force microscopy. By observing the channel formation and carrier concentration change in response to the applied voltage, we were able to visualize the internal state of the SiC power MOSFET.

Suppressing Interface Strain for Eliminating Double-Slope Behaviors: Towards Ideal Conformable Polymer Field-Effect Transistors.

High-mobility polymer field-effect transistors (PFETs) are being actively explored for applications in soft electronic skin and low-cost flexible displays because of their superior solution processability, mechanical flexibility, and stretchability. However, most of high-mobility PFETs often deviate from the idealized behavior with variable mobility, large threshold voltage, and high off-state current, which masks their intrinsic properties and significantly impedes their practical applications. Here, it is first revealed that interface strain between polymer thin film and rigid substrate plays a crucial role in determining the ideality of PFETs, and demonstrate that various ideal conformable PFETs can be successfully fabricated by releasing strain. It is found that strain in film can be released by one-step peeling strategy, which can reduce π-π stacking distance and suppress generation of oxygen doped carriers, thereby obtaining linearly injected charge carriers and decreased carrier concentration in channel, eventually realizing ideal PFETs. More impressively, the fabricated ideal conformable PFET array displays outstanding conformability to curved objects, and meanwhile showing excellent organic light-emitting display driving capability. The work clarifies the effect of the interface strain on the device ideality, and strain can be effectively released by a facile peeling strategy, thus offering useful guidance for the construction of ideal conformable PFETs.

Effect of PECVD SiNx deposition process parameters on electrical properties of SiNx/AlGaN/GaN structures

The effect of parameters of plasma enhanced chemical vapor deposition (PECVD) processes for SiNx film fabrication on the electrical parameters of dielectric/АlGaN/GaN structures has been studied. The effect of growing film composition, additional heterostructure surface treatment with nitrogen plasma before dielectric deposition and HF biasing during treatment on the parameters of the С–V and I–V curves of SiNx/АlGaN/GaN structures has been analyzed. We show that films with nitrogen to silicon concentration ratios of 60 and 40% and a high oxygen content exhibit a decrease in the positive fixed charge in the structures although the I–V curves of the structures exhibit current oscillations. Information has been reported on the effect of PECVD process mode on current oscillation parameters, e.g. period and amplitude, and length of I–V curve section in which oscillations occur. Possible explanation of these oscillations has been suggested. Additional nitrogen plasma treatment of heterostructure surface before monosilane supply to the chamber changes the magnitude and sign of fixed charge and reduces the free carrier concentration in the 2D gas channel of SiNx/АlGaN/GaN heterostructures. Experimental evidence has been provided for the effect of PECVD process parameters and surface preparation on the electrical parameters of the heterostructures grown.

Improvement in Schottky barrier inhomogeneities of Ni/AlGaN/GaN Schottky diodes after cumulative γ-ray irradiation

This article reports the effect of gamma (γ)-ray irradiation on barrier inhomogeneities that leads towards improvement in diode parameters in Ni-AlGaN/GaN Schottky diodes. The Schottky diodes were subjected to a cumulative γ-ray dose up to 15 kGy and their current–voltage (I–V) and capacitance–voltage (C–V) characteristics were measured simultaneously at different temperatures during the pristine stage and after each radiation dose. The Schottky barrier height (Φ b) had an increase of 10% to 20% in the temperature range greater than 250 K. Whereas, the change in the ideality factor (η) was found to be prevalent at lower temperatures (<250 K). More linearity in the behavior of η variation with temperature was found post γ-irradiation showing an improvement in homogeneity of the metal/semiconductor interface. Post γ-ray exposure, barrier inhomogeneities at the metal/semiconductor interface were found to reduce due to annealing effects that also led towards an increase in the contribution of thermionic emission current flow. Further, a decrease of 16% in the standard deviation of the Gaussian distribution of Φ b around the mean Φ b was obtained. A decrease in contact resistance (R C) was deduced using a circular transmission line method, which was also due to the partial annealing effect of γ-ray radiation. Finally, the channel carrier concentration (n s), extracted using C–V analysis, was found to remain unaltered.

Novel graphene electrochemical transistor with ZrO2/rGO nanocomposites functionalized gate electrode for ultrasensitive recognition of methyl parathion

A novel graphene electrochemical transistor (GECT) based on zirconia/reduced graphene oxide (ZrO2/rGO) nanocomposites functionalized gate electrode and graphene as channel was successfully designed and fabricated for highly sensitive detection of methyl parathion (MP). ZrO2/rGO nanocomposites were co-electrodeposited on a glassy carbon electrode considering the special affinity of ZrO2 toward phosphate groups and high specific surface area of rGO. The sensing mechanism is attributed to the change of effective gate voltage applied to GECT induced by the electrochemical reactions of MP at the functionalized gate electrode, thus tuning the channel carrier concentration and the channel current. Due to the excellent electrocatalytic activity of ZrO2/rGO on the gate and the high carrier mobility of graphene in the channel, the developed sensor exhibited excellent sensing performances with ultra-low detection limit (10 pg/mL) and ultra-wide linear range from 10 pg/mL to 10 μg/mL, which especially stand out of the reported devices. The GECT sensor also showed good selectivity toward usual interferents and strong practicality through testing MP in Chinese cabbage samples. The developed GECT devices provided not only the novel platform for pesticide detection with high sensitivity but the new method of enhancing the sensing performance through specifically functionalized gate electrode.

(Invited) Device Scalability of InGaZnO TFTs for Next-Generation Displays

Amorphous oxide semiconductor thin-film transistors (TFTs) are used as the backplane for large-screen, high-resolution OLED and LCD displays. Amorphous InGaZnO (a-IGZO) TFTs are widely used in commercialized TVs, tablets, and wearable displays for its good device performance, stability, and process controllability. Also, due to its extremely low off-current, a-IGZO TFTs are used for low refresh rate and low-power displays. Furthermore, a-IGZO TFTs have a low process temperature, enabling fabrication on various substrates (including flexible films), and also hybrid integration with other backplane technologies, such as LTPS or Si CMOS. By taking advantage of the low off-current and low process temperature, a-IGZO can be monolithically integrated in the back-end of Si CMOS for low-power applications, as well as embedded memories for long retention and non-volatile operation. There are many incentives to shorten the channel length of IGZO TFTs. First, shorter channels can boost the current without incorporating high-mobility oxides, which usually suffer from reliability issues and small process margin. When gate drivers are integrated directly onto the panel, larger currents allow smaller device footprint, thinner bezel width, and less dead space. Second, transistors with sub-micron pitch are desired to bridge the pixel gap between smartphone displays and micro-displays, and to be used for consumer AR/VR displays with > 1000 ppi. Third, reduced device length is necessary for fine-grained integration with nanoscale CMOS. However, short-channel TFTs suffer from short-channel effects (SCE). For example, threshold voltage (Vt) roll-off, where the Vt reduces for short channel lengths, causes device variability and hence, worse process tolerance. Therefore understanding and control of SCE is critical for designing and fabricating short-channel TFTs.In this paper, we study the device scalability of short-channel IGZO TFTs down to 1μm. The electrical channel length (Leff) is not the same as the gate length (Lg), and is determined by the carrier profile at the gate edges. Leff ( = Lg - ΔL) is obtained by the transmission line method (TLM), or shift-and-ratio method. Accurate ΔL extraction is important in determining the exact device mobility. Since scalability is related to the carrier concentration profile, it is strongly dependent on the process, where oxygen and hydrogen content varies with process conditions, and profiles change during thermal treatments. Devices with higher carrier concentration generally result in larger ΔL and worse SCE. Device simulations using TCAD shows the dependence on overlap (Lov), slope of carrier Gaussian decay (Xc), channel carrier concentration (Nch) on the Leff and SCE. TCAD simulation results are matched with experimental measurements for verification of the simulation models and parameters used. We define a critical carrier concentration (Ncrit) which defines the Leff. Changing the carrier profile while maintaining Ncrit results in little difference in ΔL. For devices with an underlap, similar ΔL values result in similar SCE. However, for devices with an overlap, even with the similar ΔL values, SCE is more sensitive to carrier profile at the channel edges. This study shows that SCE may be controlled through shaping of the carrier concentration profile through optimization of the device structure and process conditions allowing fabrication of short-channel TFTs with good scalability.

Highly sensitive methyl parathion sensor based on Au-ZrO2 nanocomposites modified graphene electrochemical transistor

The excessive use of pesticides does great harm to environment and human health, a high performance sensor is urgently needed for the management of pesticides contamination. Herein, a novel graphene electrochemical transistor (GECT) for the detection of methyl parathion (MP) was successfully fabricated for the first time. A single layer of graphene was covered between the source and drain electrodes by the wet transfer method to act as channel for the transistor. Au-ZrO2 nanocomposites were electrodeposited at graphene channel of the device, considering the special affinity of ZrO2 nanoparticles toward phosphate group and electrocatalytic ability of Au nanoparticles. The sensing mechanism was attributed to the change of carrier concentration in channel induced by the electrochemical reaction of MP at the graphene channel, which built up the mathematic relationship between the change of channel current and the concentration of MP. Different modified materials and electrodeposition conditions were compared and optimized. Due to the high carrier mobility of graphene and excellent catalytic performance of Au-ZrO2 nanocomposites in the channel, the developed sensor showed a limit of detection as low as 0.1 ng/mL and ultra-wide linear range from 0.4 ng/mL to 7 μg/mL, which are comparable or superior to those using the large-scale instruments. Meanwhile, the device exhibited good selectivity toward usual interferents and high accuracy for detection of MP in Chinese cabbage samples, demonstrating the promising practicality of the devices. It is expected that GECT can act as a desirable transducer platform for highly sensitive and selective detection of pesticides.

The influence of low indium composition ratio on sol–gel solution-deposited amorphous zinc oxide thin film transistors

In this work, we examined the influence of In doping on the morphological and structural characteristics of ZnO active layers grown by the spin coating and the electrical performance of ZnO-based thin film transistors. XRD results indicated that the active layers were amorphous due to the absence of any sharp peaks. XPS analysis was carried out to determine indium amounts as an atomic percentage in ZnO and oxidation state of In. AFM images indicated that the roughness of the active layers decreased with increasing indium concentration in ZnO. The indium doping has dramatically improved the electrical parameters of ZnO-based transistors. The field-effect mobility of undoped TFT increased ~ 157 times by adding %3 In content. The highest field-effect mobility (μsat) of 12.9 cm2V−1 s−1 was obtained for %3 In-doped ZnO TFT (IZO3). Also IZO3 has a 6.96 V threshold voltage (Vth), 106Ion/Ioff ratio, 1.98 V/dec subthreshold slope (SS), and a high on-current of 4.6 mA. We ascribed the performance enhancement of devices with In doping due to increasing carrier concentration of channel. These results show that the low concentration of indium incorporation is very crucial to change the morphological properties of ZnO active layers and to obtain high-performance solution-processed TFTs.

Effects of interfacial dielectric layers on the charge transport characteristics in sol-gel based amorphous metal-oxide thin-film transistors

We report the effects of interfacial dielectric layers on the field-effect mobility of thin-film transistors (TFTs) with sol-gel based amorphous indium-gallium-zinc oxide (IGZO) films on thermally-grown silicon dioxide (SiO2) and sol-gel based zirconium dioxide (ZrO2) dielectrics. When the field-effect mobility as a function of the channel carrier concentration was extracted, the SiO2 based IGZO-TFT exhibited higher mobility than the ZrO2 based IGZO-TFT at a low carrier concentration. However, the field-effect mobility of the ZrO2 based IGZO-TFT increased as the gate charges increasing due to the gate bias applied. We also investigate the effects of interfacial dielectric layers on the charge transport characteristics in IGZO-TFTs by conducting temperature-dependent field-effect mobility measurements in the temperature range of 120 – 300 K. By analyzing an Arrhenius plot of the results, we confirm that the charge transport of IGZO-TFTs is dominant by the multiple-trapping-and-release model and discuss the effective mobility, which is significantly affected by the interfacial trap level and channel carrier concentration. Finally, we investigate the figure of merit and the activation energy of IGZO-TFTs with ZrO2 dielectrics fabricated by the optimized solution-process developed in this study through comparisons with other sol-gel based amorphous metal-oxide TFTs reported in the literature.

Neutron-induced displacement damage effect and mechanism of AlGaN/GaN high electron mobility transistor

In this paper, neutron-induced displacement damage effects of AlGaN/GaN High electron mobility ransistor (HEMT) device and heterostructure on the Xi’an pulse reactor are studied. The equivalent 1 MeV neutron fluence is 1 × 10&lt;sup&gt;14&lt;/sup&gt; n/cm&lt;sup&gt;2&lt;/sup&gt;. The direct-current characteristics and low frequency noise characteristics of the HEMT deviceare used to characterize the performances before and after being irradiated by the neutrons, and then the experimental results are analyzed theoretically. The analysis results showed that the displacement damage effect caused by the neutron irradiation will introduce the bulk defects into the device. The bulk defects at the channel cause the electrical performance of the device to degrade through trapping electrons and scattering electrons, which are mainly manifested as the drift of positive threshold voltage, the decrease of output saturation drain current, and the increase of gate leakage current. In order to further confirm the effect of neutron irradiation on the defect density of the device, we implement the low-frequency noise test and the calculation of the device, and the results show that the defect density at the channel of the device increases from 1.78 × 10&lt;sup&gt;12&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt;·eV&lt;sup&gt;–1&lt;/sup&gt; to 1.66 × 10&lt;sup&gt;14&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt;·eV&lt;sup&gt;–1&lt;/sup&gt;, which is consistent with the results of the electrical characteristics test, indicating that the new defects introduced by neutron irradiation do degrade the electrical performance of the device. At the same time, the capacitor-voltage test is also carried out to analyze the Schottky heterojunctions before and after neutron irradiation. It is found that the channel carrier concentration is significantly reduced after irradiation, and the flat band voltage also drifts positively. The analysis shows that after irradiating the device with neutrons, a large number of defects will be generated in the channel, and these defects will affect the concentration and mobility of the channel carriers, which in turn will influence the electrical performance of the device. These experimental results can be used for designing the AlGaN/GaN high electron mobility transistor for radiationhard reinforcement.

Photo-Controlled Quantum Capacitors in Gated Graphene-Insulator-Graphene for Terahertz Frequency and Phase Modulations

This paper proposes a theoretical model of photo-controlled quantum capacitors in double-gated graphene-insulator-graphene (DGGIG) structure for terahertz (THz) frequency and phase modulations. In the model, the current-voltage characteristics of each segment of the DGGIG structure are controlled by different gate voltages. Using the different voltages, the DGGIG structure can be used to either generate plasma oscillation waves or shift phase in the terahertz band. Not only has the DGGIG structure various electrical properties, but also the optical properties are also excellent. Photoexcitation alters carrier concentration in DGGIG channel, and thus causes the change of the quantum capacitors. We propose a photo-controlled quantum capacitor model to quantify this phenomenon. The model shows that the quantum capacitors and photoexcitation have a sublinear dependence. An example of a device based on the model is given to describe the application of photo-controlled quantum capacitors in terahertz frequency and phase modulations. The modelled device shows that photoexcitation can change the frequency and phase of the plasma oscillation waves.

Low voltage operation of GaN vertical nanowire MOSFET

• GaN vertical nanowire MOSFET is fabricated by top-down approach. • Both threshold voltage and saturation voltage below 1 V are due to the small diameter and carrier concentration of channel. • GaN vertical nanowire MOSFET show small current collapse due to high gate controllability. • GaN vertical nanowire MOSFET is also promising for low voltage logic applications.

N-Type Ionic-Organic Electronic Ratchets for Energy Harvesting.

Ionic-organic ratchets are three-terminal electronic devices with asymmetric conductivity of the active layer. These devices are capable of generating useful direct current electrical power by converting electromagnetic noise signals available in any environment. In this work, we demonstrate for the first time an n-type ionic-organic ratchet which can generate a current of up to 7.29 μA and power up to 12.5 μW that exceed the values reported for many of the presently state-of-the-art, p-type organic electronic ratchets. We show that n-type ratchets require elimination of electron traps at the SiO2 surface, which is not required in p-type devices. This can be achieved by using a trap-free passivation layer such as benzocyclobutene, where the traditional silane treatment is insufficient. Chemical doping is employed to further fill electron traps in the channel and increase carrier concentration and mobilities. Scanning Kelvin probe force microscopy studies provide evidence of a pn-like rectifying junction in the n-type ratchets fabricated in this work, which inherently differs from the rectification mechanism of previous ionic-organic p-type ratchets.

Fully transparent high performance thin film transistors with bilayer ITO/Al-Sn-Zn-O channel structures fabricated on glass substrate

In this work, fully transparent high performance double-channel indium-tin-oxide/Al–Sn–Zn–O thin-film transistors (ITO/ATZO TFTs) are successfully fabricated on glass by radio frequency (RF) magnetron sputtering. The ITO layer acts as the bottom channel layer to increase the channel carrier concentration. The top ATZO channel layer, which is deposited via high oxygen partial pressure in the sputtering process, is useful to control the minimum off-state current. After annealing, the ITO/ATZO TFT demonstrates outstanding electrical performances, including a high ON/OFF current ratio (Ion/Ioff) of 3.5 × 108, a steep threshold swing (SS) of 142.2 mV/decade, a superior saturation mobility (μsat) of 246.0 cm2/Vs, and a threshold voltage VT of 0.5 V. The operation mechanisms for double-channel structures are also clarified.

Effect of carrier number in density of states and quantum capacitance of with and without disorder GNR-FET on drain current

In this paper with and without disorder grapheme nanoribbon (GNR) channel is being considered for different electrical parameters (Current, Number of carriers at steady state, Density of states, Quantum Capacitance). Due to increase in drain potential from 0 V to 0.7 V, the carrier concentration changes in channel region. Because of the dependence of channel potential on carrier concentration, the constant value carrier concentration cannot be considered to calculate drain current. Self-consistent field (SCF) method is utilized to calculate channel potential for the proposed model. The change in carrier concentration of channel is related to quantum capacitance. Quantum capacitance has a significant effect when drain to source bias is low. Density of states (DOS), dependent current expression is derived for with and without disorder grapheme nanoribbon (GNR) channel based field effect transistor (FET). Energy broadening due to the contact between grapheme nanoribbon (GNR) and metal leads is also considered to show effective carrier collection. The model also presents the short channel effect of device. The simulation of current-voltage, Density of states (DOS) using MATlab 2014a software is carried out.

Trap Kinetics in Solution-Processed Organic Thin-Film Transistors

The role of traps in controlling source–drain current in organic thin-film transistors (OTFTs) is not well understood and has so far been studied only in structures with silicon dioxide as the dielectric. We study trap kinetics in solution-processed OTFT with a polymer gate dielectric and 6,13-bis(triisopropylsilylethynyl) pentacene as the organic semiconductor. The trapping kinetics has been studied at different temperatures using time analyzed transient spectroscopy, which enables the detailed line shape analysis of electrical transients. We show unambiguously that the non-exponential turn-ON current transients are not due to the capture of majority holes, but rather are due to the emission of electrons modulating the channel carrier concentration through field. Two dominant electron traps with the activation energies of 0.14 and 0.20 eV are identified of which the former is discrete and the latter continuously distributed.

Electrical properties of BaTiO3 based – MFIS heterostructure: Role of semiconductor channel carrier concentration

Effect of semiconductor channel carrier concentration on the modifications in the electrical properties of Ag/BaTiO3/SrTiO3/ZnO Metal-Ferroelectric-Insulator-Semiconductor (MFIS) heterostructure has been investigated. Under 4 V applied voltage, low leakage current density ∼3.2 × 10−6 A/cm2, has been observed in ZnO based MFIS heterostructure, which becomes ∼5.0 × 10−6 A/cm2 for MFIS with Al:ZnO channel. Observation of counterclockwise butterfly shaped C-V behavior confirms that, hysteresis in C-V is due to spontaneous ferroelectric polarization and field effect. A device with ZnO semiconductor exhibit ∼2700% modulation which decreases to ∼800% for Al: ZnO channel with good retention behavior. Pulse induced write/erase repeatability of source/drain current confirms the usefulness of the presently studied devices for non-volatile switching memory application.