Semiconductor Surface Potential Research Articles

Fully Printed Negative-Capacitance Field-Effect Transistors with Ultralow Subthreshold Swing and High Inverter Signal Gain.

The switching of conventional field-effect transistors (FETs) is limited by the Boltzmann barrier of thermionic emission, which prevents the realization of low-power electronics. In order to overcome this limitation, among others, unconventional device geometry with a ferroelectric/dielectric insulator stack has been proposed to demonstrate stable negative-capacitance behavior. Here, the switching of the ferroelectric layer behaves like a step-up amplifier and results in a body factor less than 1. This implies a larger change in the semiconductor surface potential compared to the applied gate voltage variation. The transistors with such ferroelectric/dielectric stack are known as negative-capacitance field-effect transistors (nc-FETs), and can demonstrate a subthreshold slope lower than the Boltzmann's limit (60 mV/decade). While nc-FETs have typically been realized with high-vacuum-deposition processes, here we show fully printed nc-FETs with amorphous indium-gallium-zinc oxide (a-IGZO) as the semiconductor material, Al2O3 as the dielectric, and poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) as the polymer ferroelectric. The printed nc-FETs demonstrate an extremely low subthreshold slope of ∼2.3 mV/decade at room temperature, which remains below the Boltzmann's limit for over 5 orders of magnitude of drain currents. Furthermore, the unipolar depletion-load-type inverters fabricated using n-type nc-FETs have demonstrated an extraordinary signal gain of 2691.

Effect and mechanism of cyanide degradation and Cu/Zn recovery by photoelectro-catalytic oxidation

• The photoelectrocatalytic system can effectively degrade cyanide wastewater. • The bending of the TiO 2 energy band occurs to improve the catalytic performance. • The catalyst and degradation products can be separated from the electrolyte. • The catalytic mechanism of total cyanide, Cu and Zn cyanide complexes was revealed. Cyanide wastewater contains highly toxic substances such as cyanide and metals. A simple and efficient photoelectro-catalytic oxidation method was proposed. Effect and mechanism of cyanide degradation and Cu/Zn recovery by photoelectro-catalytic oxidation were investigated. The experimental results show the degradation efficiency of total cyanide and removal efficiencies of copper and zinc reached 96.90%, 99.76% and 90.56%, respectively, under the optimum conditions of 2.5 V external bias voltage and 4.0 h with air and lighting. An external electric field can significantly improve the migration and separation of photo-generated electrons ( e – ) and holes ( h + ), thereby increasing the photocatalytic efficiency. Simultaneously, the surface potential of semiconductor is lower than the body under the action of bias voltage and leads to energy band bending. XPS analysis indicates that cyanide is finally degraded into carbon oxide and nitrogen. Cu and Zn are mainly recovered as elementary substance.

Time-resolved simulation of the negative capacitance stage emerging at the ferroelectric/semiconductor hetero-junction

Recently, a number of papers have demonstrated sub-60 mV/decade switching by using the negative capacitance (NC) effect in ferroelectric-gate FETs. However, the physical picture is not yet understood. In this paper, an alternative physical picture for emerging NC is proposed and the development of the NC stage at the ferroelectric/semiconductor hetero-junction is described. Proposed physical picture is based on two factors, 1. “decrease in an additional voltage originated from the depolarization field by surface potential of semiconductor” and 2. “Change in the distribution ratio of gate voltage (VG) to voltage applied to the ferroelectric layer (VF) and surface potential of the semiconductor (ψS) due to the capacitance change of semiconductor.” With considering these two essential phenomena, time-resolved simulations of the NC stage emerging at the ferroelectric/semiconductor hetero-junction were carried out. This NC phenomena expressed by the negative differential of the DF for the VF, i.e. (∂DF/∂VF<0), emerging in the MFS (metal/ferroelectric/semiconductor) capacitor without inserting dielectric layer, are dynamically simulated to discuss the proposed NC process. The simulation results clearly reveal that the NC stage is originated from the existence of additional voltage caused by the depolarization field by surface potential of semiconductor originated from the existence of remanent polarization of ferroelectric layer, and change in the capacitance of the semiconductor during polarization switching. The different physical picture from steady-state NC and transient NC can be clearly shown.

Numerical modelling of Chemisorption of oxygen gas molecules on the surface of semiconductor for gas sensors applications

Abstract Charge transfer is the main mechanism for the detection of gas interaction on semiconductor materials surfaces. We have used the Wolkenstien theory of adsorption instead of the conventional Langmuir isotherm for oxygen gas adsorption has been used in this paper. A numerical model of chemisorption of oxygen gas at the surface of semiconductor gas sensor is presented in this paper. Using this method, we can quantitatively calculate the effect of oxygen adsorption on various electronic properties of the n-type CdS semiconductor such as chemisorbed induced surface potential and work function. In the presented work we have numerically calculated the effect of ambient oxygen gas pressure on the surface potential of n-type CdS semiconductor. The simulation of surface coverage of chemisorbed species has been simulated as a function of oxygen gas pressure, Temperature and bulk doping level of n-type CdS.

Evaluation of C-V characteristic of MFS capacitor considering effects of interface traps

An improved model combining the quantum mechanical model, dipole switching theory with silicon physics of MOS capacitor was proposed to investigate the effects of interface traps on the surface potential of semiconductor, C-V characteristic and memory window of MFS capacitor. The simulated results show that both -V and C-V curves shift toward the positive-voltage direction and memory window becomes worse as the density of interface trap states increases. It is expected that this work can give some guidance to the experimental researchers for the design and characteristic improvement of MFS devices.

Saturation Photo-Voltage Methodology for Semiconductor/Insulator Interface Trap Spectroscopy

The presence of large densities of electrically active defects is still an unsolved issue for future high-mobility/high-k CMOS device technologies. This relates to degraded device performance and reliability. Regrettably, conventional admittance-based characterization techniques often fail when applied to non-Si based devices. Among others, enhanced generation of minority carriers and much longer defect time constants make their results inaccurate. Rather than of seeking to adapt commonly-used techniques, we instead aim at direct measuring the semiconductor surface potential by means of the Saturation surface PhotoVoltage (SPV) technique. This approach allows for a DIT estimation which is not limited by the trap response time or hindered by minority carrier generation. Moreover, the DIT can be estimated over the whole bandgap regardless of sample doping type. We here report several case studies in support of the proposed approach. We will also show that SPV can be applied for the characterization of multi-layered Ge and III-V devices incorporating high-k insulators.

A carrier-based analytical theory for negative capacitance symmetric double-gate field effect transistors and its simulation verification

A carrier-based analytical drain current model for negative capacitance symmetric double-gate field effect transistors (NC-SDG FETs) is proposed by solving the differential equation of the carrier, the Pao-Sah current formulation, and the Landau–Khalatnikov equation. The carrier equation is derived from Poisson’s equation and the Boltzmann distribution law. According to the model, an amplified semiconductor surface potential and a steeper subthreshold slope could be obtained with suitable thicknesses of the ferroelectric film and insulator layer at room temperature. Results predicted by the analytical model agree well with those of the numerical simulation from a 2D simulator without any fitting parameters. The analytical model is valid for all operation regions and captures the transitions between them without any auxiliary variables or functions. This model can be used to explore the operating mechanisms of NC-SDG FETs and to optimize device performance.

Use of negative capacitance to simulate the electrical characteristics in double-gate ferroelectric field-effect transistors

The surface potential and drain current of double-gate metal-ferroelectric-insulator-semiconductor (MFIS) field-effect transistor were investigated by using the ferroelectric negative capacitance. The derived results demonstrated that the up-converted semiconductor surface potential and low subthreshold swing S = 34 (<60 mV/dec) can be realized with appropriate thicknesses of ferroelectric thin film and insulator layer at room temperature. What's more, a reduction gate voltage about 260 mV can be reached if the ON-state current is fixed to 600 μA/μm. It is expected that the derived results can offer useful guidelines for the application of low power dissipation in ongoing scaling of FETs.

Influence of oxygen adsorption on the surface potential of a metal oxide semiconductor

Analytical expressions describing dependences of the surface density of adsorbed oxygen ions and energy band bending in the subsurface region of a metal oxide semiconductor on the oxygen concentration that consider not only the process of neutral gas particle adsorption, but also their charge transfer at the expense of electron capture from the conduction band are presented. It is demonstrated that the heat of oxygen ion absorption is equal to the sum of the heat of neutral particle adsorption and the energy gap between the Fermi level and the level of the oxygen ion on the semiconductor surface. When the adsorption equilibrium is established, an analytical expression describing the time dependence of the energy band bending can be obtained only for small change of the oxygen concentration in the gas mixture.

Measurement of semiconductor surface potential using the scanning electron microscope

We calibrate the secondary electron signal from a standard scanning electron microscope to voltage, yielding an image of the surface or near-surface potential. Data on both atomically abrupt heterojunction GaInP/GaAs and diffused homojunction Si solar cell devices clearly show the expected variation in potential with position and applied bias, giving depletion widths and locating metallurgical junctions to an accuracy better than 10 nm. In some images, distortion near the p-n junction is observed, seemingly consistent with the effects of lateral electric fields (patch fields). Reducing the tube bias removes this distortion. This approach results in rapid and straightforward collection of near-surface potential data using a standard scanning electron microscope.

Semiconductor surface potential relaxation in the MIS structure in the presence of convective currents in insulator and through its boundaries

It is shown that the Berglund method for determining the metal-insulator-semiconductor surface potential relaxation as the structure voltage changes is valid only in the absence of convective currents in the insulator. A method for determining the potential relaxation in the presence of convective currents is suggested. The currents induced in the structure due to changes in the contact potential difference during temperature variations are also considered.

Metal–Insulator–Semiconductor Diode Characterization on n-GaN by Capacitance–Voltage Measurement at 150 °C

GaN metal–insulator–semiconductor (MIS) diodes (MISDs) were evaluated with capacitance–voltage (C–V) measurement at 150 °C and under UV light illumination. From C–V scan at 150 °C, a high density of SiO2/GaN interface traps was found to pin the semiconductor surface potential (ΦS) at 0.7–0.9 eV. At -20 V, transient capacitance measurement was performed at 150 °C under dark condition or after 5 min UV light illumination. From the transient measurements, ΦS of the steady status was measured to be 4.2 eV, which is much larger than the band gap of GaN, indicating that no thermal equilibrium was achieved at the steady status. The reason for the absence of thermal equilibrium was discussed. It was suggested that the leakage through the oxide is too large compared with the electron–hole generation rate in this wide-band-gap semiconductor. The insulative properties of conventional insulators should be reevaluated for wide-band-gap semiconductors.

The nonlinear carrier transport in a bipolar semiconductor sample

A theory of formation of the voltage across a bipolar semiconductor sample due to thecurrent flow accounting for the energy band bending near the semiconductor surfaces ispresented. The non-equilibrium space charge layers near the sample surfaces and theboundary conditions in the real metal–semiconductor junction have been taken intoaccount. It is shown that the voltage–current relation of a thin sample at weak injectiondiffers essentially from the classical Ohm’s law and becomes nonlinear for certainsemiconductor surface parameters. Complex voltage–current relations and thephoto-induced electromotive force measurements allow determining the surfacerecombination rate in the real metal–semiconductor junction and the semiconductor surfacepotential.

Model-free determination of the dependence of charge density in accumulation and inversion semiconductor layers on the semiconductor surface potential from the voltage-capacitance characteristics of metal-insulator-semiconductor structures

A method for the analysis of the quasi-static voltage-capacitance characteristics of metal-insulator-semiconductor structures in the majority-carrier depletion region of the semiconductor surface is developed. The method provides for the quantitative evaluation of the doping concentration, the flat-band voltage, and the effective values of the capacitance and thickness of the gate insulator. The dependences of the surface potential and surface charge in the semiconductor on the gate-electrode potential are obtained in the absence of a priori data on the state of the electron gas during high enrichment or strong inversion. It is shown that these experimental dependences can be used as a criterion for the validity of the theory of the space-charge region in a semiconductor when the effects of degeneracy and dimensional quantization of electron gas are considered.

Fluctuation of semiconductor surface potential in Si-SiO2 structure after irradiation and subsequent annealing

Fluctuations of the semiconductor surface potential in a Si-SiO2 structure, caused by irradiation with different high-energy particles (50 MeV electrons and 40 keV arsenic ions) and subsequent annealing, have been studied by measuring the semiconductor interface state parameters. It is established that the fluctuation of the semiconductor surface potential decreases slightly, from 0.049 V to 0.044 V, after irradiation, while a considerable increase is observed after annealing. For the samples irradiated by arsenic ions, the increase in fluctuation is much larger (0.096 V) than that for the electron-irradiated samples (0.06 V).

Calculation of the Response of Field-Effect Transistors to Charged Biological Molecules

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Robust approximations are presented that allow for the simple calculation of the total charge and potential drop <formula formulatype="inline"><tex>$\psi_{0}$</tex> </formula> across the region of electrolyte containing charged biological macromolecules that are attached to the gate area of a field-effect transistor (FET). The attached macromolecules are modeled as an ion-permeable membrane in contact with the insulator surface, exchanging protons with the electrolyte as described by the site-binding model. The approximations are based on a new screening length involving the Donnan potential in the membrane and are validated by comparison to the results obtained by numerical solution of the one-dimensional Poisson–Boltzmann equation in the electrolyte and membrane. For gates covered with amphoteric materials such as SiO<formula formulatype="inline"> <tex>$_{2}$</tex></formula>, the high surface charge density <formula formulatype="inline"> <tex>$\sigma_{0}$</tex></formula> due to proton exchange at values of <emphasis emphasistype="boldital">p</emphasis>H far from the point-of-zero charge is a nonlinear function of <formula formulatype="inline"><tex>$\psi_{0}$</tex> </formula>, but <formula formulatype="inline"><tex>$\psi_{0}$</tex></formula> and <formula formulatype="inline"><tex>$\sigma_{0}$</tex></formula> are still linear functions of the semiconductor surface potential between the source and drain. Nonlinear expressions for the amphoteric site charge at the contacts can thus be applied effectively with the new approximations to calculate the current-voltage characteristics of the FETs using the strong inversion and charge-sheet models. </para>

Tunable hydroxyapatite wettability: Effect on adhesion of biological molecules

Surface wettability modifications of biocompatible materials and wettability patterning are attractive methods for directed biological cells immobilization for tissue engineering, drug delivery, gene transfer, etc. Hydroxyapatite is known as an implantable biomimetic material and a substrate for effective adhesion of biological cells of various origins. Here we report the use of a low-energy electron irradiation to achieve tunable wettability of the hydroxyapatite in a wide range of contact angles, from 10° to 100°, with accuracy of ±3°. The incident electrons generate electron/hole pairs resulting in significant variation of the surface potential of the hydroxyapatite semiconductor and give rise to pronounced wettability modification. Tailoring the gradually varied wettability state in the hydroxyapatite nanoceramics enabled the differential binding of biological materials with different surface properties, such as bovine serum albumin (BSA) and deoxyribonucleic acid (DNA).

Improvements in the Quasi-static Capacitance–Voltage Characterization of Semiconductor–Insulator Interface States (Si/SiO2)

The quasi-static capacitance–voltage determination of the electron density of states as a function of energy at the semiconductor–insulator interface is addressed. The respective effects are analyzed of random and systematic errors in a measured capacitance–voltage characteristic on the interface-state distribution derived. The random errors show up as fluctuations that grow indefinitely in magnitude as the energy approaches either band edge. Systematic errors are manifested in under- and overshoots near the band edges. The most important are the systematic errors associated with the estimation of the insulator capacitance and the identification of the relationship between the semiconductor surface potential and the applied voltage. A method for minimizing the errors is proposed. It is noted that the method should enable one to substantially expand the accessible energy range and to significantly improve the accuracy to which the density of states is evaluated. These advantages are confirmed by an experiment on the Si/SiO2 interface. It is found that the energy range can be made as wide as about 0.9 eV and the accuracy of the semiconductor surface potential can be improved to about 0.1 meV, so that the integrated density of states can be determined to within about 5 × 107 cm–2. It is inferred from the experimental data that the interface states are concentrated near the conduction band edge and are due to positive oxide fixed ions rather than Pbcenters. The ions should act as electron traps involved in tunnel electron exchange with the conduction band of the silicon.

Dark currents in long wavelength infrared HgCdTe gated photodiodes

The fabrication of HgCdTe photodiodes using plasma-induced p-to-n type conversion for junction formation shows promise in improving array uniformity and device yields in comparison to more traditional fabrication technologies. Previously, characterization and analysis of the diode current-voltage (I–V) characteristics of fabricated devices have given indications that surface-leakage current mechanisms are limiting device performance. To further investigate the effectiveness of the surface passivation employed in the fabrication process, gated-diode structures have been fabricated. The gated-diode structure enables the semiconductor surface potential to be varied, thus allowing the characteristics of surface-leakage currents and their effect on device performance to be evaluated. The long wavelength infrared (LWIR) HgCdTe gated photodiodes used in this study have been characterized using I–V measurements for variable gate-bias voltage and variable temperature. Analysis of the experimental results indicates that plasma-induced type conversion produces an n (lightly doped)-on-p junction that is highly susceptible to a trapped positive charge in the passivation layer, which results in increased surface-tunneling currents. Modeling of the various dark-current mechanisms is used to show the effect on dark-current generation of the surface band bending induced by variations in surface potential. In addition, temperature-dependent I–V measurements and analysis have also been conducted.

Determination of the absolute value of the semiconductor surface potential by the quasi-static capacitance-voltage characteristics of an MIS structure

A simultaneous analysis of the derivatives CV′Vg of the experimental and ideal quasi-static capacitance-voltage characteristics (plotted as a function of the normalized differential capacitance of a metal-insulator-semiconductor (MIS) structure) allows identification of regions within the semiconductor band gap Eg, in which interface states are virtually aTSent and the relation between the surface potential ψS of the real semiconductor and the voltage Vg applied to the MIS structure may be readily ascertained. This allows an accurate enough determination of the additive constants ψS0(Vg0) necessary to calculate the dependence ψS(Vg) in the entire range of Vg by numerical integration of the experimental quasi-static C-V characteristic. The comparison of this dependence with the ideal one characterizes in detail the integral electronic properties of the semiconductor-insulator heterojunction: the Eg-averaged density of interface states, the qualitative pattern of their distribution over the band gap, and the flat-band voltage VFB and its components caused by a charge fixed in the undergate insulator and a charge localized at boundary states. A high accuracy of the VFB measurements allows detection of even a weak physical response of MIS structures to external factors or to variations in the heterojunction technology. Results of such an analysis for a typical SiO2/Si interface of an n-Si-MOS (metal-oxide-semiconductor) structure are considered. The application of CV′-CV diagrams for analyzing the high-frequency C-V characteristics is considered.