Metal-insulator-semiconductor Sensors Research Articles

Errors of Integrated Hydrogen Sensors Based on FETs with Structure Pd (Ag)–Ta2O5 –SiO2 –Si

Introduction There are a lot of hydrogen sensors commercially available or in development, which can be used in the environmental and fire-explosion safety monitoring devices [1]. Among solid-state gas sensitive elements, the capacitor and transistor elements based on metal-insulator-semiconductor (MIS) structures possess the best compatibility with the integrated circuits’ elements. Therefore, such sensitive elements seem promising to develop the integrated hydrogen sensors and gas analysis microsystems-on-chip. The sensors based on MIS-devices (MIS-capacitors and field-effect transistors called as FETs) have been studied by many investigators. A great contribution to the developments of gas-sensitive MIS-devices has been made by the researchers at Linköping University [2] since their pioneering work [3]. The researchers at NRNU MEPhI developed the MIS sensors based on structures Pd (or Pt)-SiO2-Si, Pd/Ti-SiO2-Si, Pd (or Pt)-Ta2O5-SiO2-Si. The best characteristics belong to the integrated cells containing on silicon chip two gas-sensing elements (Pd-resistor and FET with structure Pd(Ag)-Ta2O5-SiO2-Si), temperature sensor and heater. Effects of electrical modes, chip temperature and radiation on the performance of these sensors were studied in previous works (e.g., [4] – [6]). However, sensors’ errors of were not investigated. The motivation of this work is to estimate errors and its components of FET-based hydrogen sensors, using the experimental data and the models proposed for calculation of sensors’ errors. Experimental technique and results The layout of sensor chip, simplified wiring diagrams of elements and parameters of FET are presented in Figure 1 and in Table 1. The n-channel FET element based on Pd(Ag)-Ta2O5-SiO2-Si (1) structure was fabricated on single chip together heater-resistor (2) and with temperature sensor (3) by means of conventional n-MOS-technology using laser evaporation Pd(Ag)-films. The Pd(Ag)-resistor was not investigated because it was previously shown that it has worse characteristics than FET. The measuring circuit provides the constant drain current ID (0.1 mA) and source-drain voltage VD (0.2 V), and the output voltage V is the gate voltage VG . The chip temperature T being equal to (130 ± 2 °С) was supported by the temperature-stabilization circuitry using on-chip thermo-sensor and heater. Five sensors with similar characteristics were selected (dispersion of initial output voltage V 0 is not more than 10%). There were repeatedly measured hydrogen responses of sensors for different concentrations Ci (e.g., ordinary j-cycle in Figure 2a). The model’s parameters ΔVCM and k of the function ΔVC (C) based on the approximation of the averaged values of the experimental points ΔVCi were determined as in [7] (Figure 2b). Averaged values of FET’s parameters in Table 1, values of Vai (Ci ), values of ΔVCi (Ci ) and variation indices θ Vi (Ci ), presented in Table 2, were calculated using Equations (1). Modeling of errors and discussion To estimate errors of FET-based sensors there were proposed general and numerical models of V(D,C), hydrogen sensitivities Si and Sd , absolute ΔC and relative errors δC, the maximum absolute error Δ(V) of the output voltage V, which are given by Equations (2) – (7). According to (3), the component V 0 of model V (D,C) depend on primary voltage V 00, the initial time drift ΔV 0t (t) and the drift ΔV 0(D) associated with the total hydrogen dose D = ∫C(t)dt, and as errors of “zero”, can be compensated by calibrating the sensor measuring circuit. Value V 00 depends on VT 0, ID and b (3). According to models the measurement errors depend on the instrumental errors of etalon concentration ∆C 0 and of voltage measurement ΔVV , the random errors ΔVai associated with the voltage V variations of various sensors, fluctuations of chip temperature ΔVT [4], of electrical circuits’ parameters ΔVID [5] and additional errors ΔVZ due to the influence of other gases and radiation [6]. Besides, errors depend on hydrogen concentration, sensitivity Sd and total hydrogen dose D. Approximated dependences of relative errors on the concentration for different conditions (e.g., Figure 3) can be used to estimate operating concentration ranges for given δC, and hydrogen sensitivity threshold CT (e.g., as in Table 3). Conclusions The proposed methodology of estimation errors of sensors based on FET with structure Pd(Ag)-Ta2O5-SiO2-Si, using numerical models with parameters determined from experimental hydrogen responses, can be applied to estimate errors of integrated sensors with other types of FETs and measuring circuits.

Use of MIS Sensors of Radiation in High-Field Electron Injection Modes

A model is proposed that describes the change in the charge state of metal-insulator-semiconductor (MIS) structures and sensors made from it under the conditions of the simultaneous effect of radiation ionization and high-field injection of electrons from a semiconductor. The proposed model takes into account the interaction of injected electrons with holes generated by radiation and high-field ionization and holes trapped at the interface of the SiO2 film with a semiconductor, as well as the generation of surface states during the annihilation of part of holes during their interaction with injected electrons. It is shown that the MIS sensor, where the high-field injection of electrons into the dielectric film takes place, can be used to control the intensity of radiation by determining the radiation ionization current from the time dependence of the voltage across the sensor using the proposed model. It has been established that in the high-field electron injection mode, a significant increase in the dose sensitivity of MIS sensors is possible, but at the same time, the operating time and dose range of the MIS sensor can be significantly reduced.

Prototype of Nitro Compound Vapor and Trace Detector Based on a Capacitive MIS Sensor

A prototype of a nitro compound vapor and trace detector, which uses the pyrolysis method and a capacitive gas sensor based on the metal–insulator–semiconductor (MIS) structure type Pd–SiO2–Si, was developed and manufactured. It was experimentally established that the detection limit of trinitrotoluene trace for the detector prototype is 1 × 10−9 g, which corresponds to concentration from 10−11 g/cm3 to 10−12 g/cm3. The prototype had a response time of no more than 30 s. The possibility of further improving the characteristics of the prototype detector by reducing the overall dimensions and increasing the sensitivity of the MIS sensors is shown.

Comprehensive Research on the Response of MIS Sensors of Pd‒SiO2‒Si and Pd‒Ta2O5‒SiO2‒Si Structures to Various Gases in Air

The response Pd–SiO2–Si and Pd–Ta2O5–SiO2–Si metal‒insulator‒semiconductor (MIS) sensors to H2, H2S, NO2, C2H5SH, NH3, and Cl2 in air was studied. The static and dynamic characteristics of the MIS sensors were obtained. The minimum detectable concentrations of the test gases were determined. It is shown that the MIS sensors are highly sensitive to the studied gases.

Nitride-based metal–insulator–semiconductor ultraviolet sensors with a sputtered lanthanum oxide (La2O3) insulator

This study reports the fabrication of GaN metal–insulator–semiconductor (MIS) ultraviolet (UV) sensors with an La2O3 insulating layer. With a 5V applied bias, the leakage current of the fabricated MIS sensors with La2O3 insulating layers may be low to 4.95×10−11A. The dark current was substantially reduced, and the UV-to-visible contrast ratio was enhanced by inserting the La2O3 layer. Furthermore, the noise equivalent power was substantially reduced, and detectivity was enhanced by using La2O3 insulating layers.

Experimental and modeling studies of acetylene detection in hydrogen/acetylene mixtures on PdM bimetallic metal–insulator–semiconductor devices

The effect of the composition of the metal gate on acetylene response in hydrogen/acetylene mixtures was tested for a number of metal–insulator–semiconductor (MIS) devices with bimetallic gates. The motivation for this study was that bimetallic catalysts are employed in the commercial acetylene hydrogenation process because of superior performance compared to the pure metals. A variety of metal compositions and operating temperatures were tested, with the largest reproducible acetylene response observed for a 15% Ag/Pd sensor at 398 K. Kinetic modeling of the relevant surface reactions on Pd and bimetallic PdAg provided insights into how temperature, feed concentration, and Ag content affected response. The model predicted that significant coverages of carbon species formed on the surface, mainly CH(s) and C(s) on Pd and CCH(s) on PdAg, and that these species strongly influenced sensor responses. The dynamic changes in the surface coverages of carbonaceous intermediates after acetylene introduction were correlated with a response overshoot seen experimentally. The model indicated that the superior performance of Ag/Pd sensors relative to Pd sensors could be explained in terms of a higher hydrogen consumption rate. These results indicate a strong connection between the high reaction rates observed for industrial Ag/Pd acetylene hydrogenation catalysts and acetylene response in Ag/Pd MIS sensors.

MIS sensors for hydrogen content measurements in 10−4–102 volume percent range

New types of MIS (metal-insulator-semiconductor) sensors enabling direct measurements of hydrogen content in a wider range (up to 10 vol %) are developed. Using these sensors with special sampling techniques allows to measure hydrogen content up to 100 vol %. New class of hydrogen analyzers for various applications in hydrogen power engineering could be designed on the basis of new MIS sensors.

Profiling of hydrogen in metal-insulator-semiconductor sensors using neutron reflectivity

Although interfacial hydrogen has long been accepted as the species responsible for H2 detection in metal-insulator-semiconductor (MIS) sensors, direct observation of this species has been elusive. In this work, the use of neutron reflectivity (NR) to quantify the hydrogen concentration in regions of an MIS device is reported. The presence of multiple states at both the metal-insulator interface and within the insulator that may contribute to the hydrogen response is suggested by the scattering length density profiles obtained from NR. This technique provides a great promise for direct study of the detection mechanisms for MIS sensors.

10.1007/s11700-008-1014-0

The morphological features of palladium thin films deposited on different substrates are described. Film deposition has been performed by means of the pulsed laser evaporation method. It is shown that the grain structure of palladium films is formed independently of the substrate roughness. Particular emphasis is placed on the correlation between gas-sensitive metal-insulator-semiconductor (MIS) sensor properties and the nanostructure of palladium films used as metal electrodes in these sensors. It is concluded that a change in the morphology of palladium films has no direct influence on the degradation of the hydrogen sulphide sensitivity of MIS sensors that arises after sensor annealing in air enriched with hydrogen.

Role of the Oxide Layer on the Performances of a-Si:H Schottky Structures Applied to PDS Fabrication

AbstractIn this work we present results of studies performed on Schottky and MIS (metal-insulator – semiconductor) PSD (position sensitive detectors) structures: substrate (glass)/ Cr (300 nm) / a-Si:H [n] (37 nm) / a-Si:H [i] (600 nm) / SiO2 (1.5 nm – for the MIS) / Au (7 nm). The effect of the interfacial oxide layer between Au and a-Si:H, for the MIS structures, was studied and compared with the Schottky, in order to determine how beneficial it could be for device performances and time degradation. For doing so, the Au thickness of 70Å was deposited by thermal evaporation on an oxide free (Schottky) and oxidized (20Å) (MIS) a-Si:H surfaces. These structures were characterized by SIMS, RBS, SEM and AFM in order to correlate the obtained diffusion profile of Au at the interface and the topography with the presence of the oxide at the interface. The results show that the Au inter-diffuses very easily in the oxide free a-Si:H surface, even at room temperature, degrading the devices performance. On the other hand, the MIS structures, with their interfacial oxide present no structural changes after annealing and the PSD produced are stable. We believe that this effect is associated with the barrier effect of the interfacial oxide that prevents the Au diffusion. The optimized 1D MIS sensors are stable and exhibit a linearity error as low as 0.8 % and sensitivities of 33 mV/cm for a 5 mW spot beam intensity at a wavelength of 532 nm, while the Schottky sensors showed a time degradation of their characteristics.

Effects of a polyimide coating on the hydrogen selectivity of MIS sensors

To enhance the selectivity of metal–insulator–semiconductor (MIS) sensors for hydrogen detection in mixed-hydrocarbon streams, a palladium MIS sensor has been modified by depositing a polyimide (PI) layer above the palladium surface. Response measurements indicate that PI-coated sensors inhibit the effect of gases, such as carbon monoxide, acetylene, ethylene, and methane on H2 response, dramatically improving selectivity to hydrogen. The effect of coated layer thickness on sensor response is also evaluated, and it is found that the sensor response rate strongly depends on PI film thickness. The modified performance of the PI coated sensor to hydrogen in gas mixtures is explained by both the permselective properties of the membrane and metal surface modification resulting from the PI coating.

The response of palladium metal-insulator-semiconductor devices to hydrogen–oxygen mixtures: comparisons between kinetic models and experiment

The operation of hydrogen-sensitive metal-insulator-semiconductor (MIS) devices in the presence of oxygen is described using a detailed model of the surface and interface kinetics. The solution methods developed here build on existing models by considering adsorbed oxygenated species in the interaction between atomic hydrogen at the metal-semiconductor interface and the external surface. The net effect of the adsorbed oxygenated species is to increase the amount of interfacial hydrogen predicted to exist within the structure at equilibrium. These theoretical predictions are compared to computed results from a previously existing model; furthermore, both mechanistic models are analyzed in light of new and previously published experimental response trends for MIS devices. Although the two models considered in this work are each found to be useful in understanding some aspects of the response, elementary reaction mechanisms appear to be inadequate for prediction of response curves. The results of these comparisons suggest that the kinetics for operation of MIS sensors in hydrogen–oxygen mixtures are quite complex, and may be strongly morphology-dependent.

Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology

We have investigated the effects of carbon monoxide coadsorption on the performance of hydrogen-sensitive palladium metal–insulator–semiconductor (MIS) sensors, and have found that device performance is highly dependent on the morphology of the metal film. On smooth, continuous films, experiments utilizing H2/CO mixtures show that CO induces a time-delayed increase in response to H2. Based on a kinetic model of CO and hydrogen adsorption on the Pd film, this increase is interpreted as being due to trapping of H atoms at the Pd–SiO2 interface by CO adsorbed on the metal surface. However, MIS sensors fabricated so that a large concentration of voids exist in the metal film respond in a markedly different fashion: on these devices, the addition of CO causes a time-delayed decrease in the sensor response. Possible explanations for this attenuating CO effect are discussed. The experimental and modeling results presented here suggest that sensor behavior in the presence of competitively adsorbing mixtures can be quite complex, with film imperfections playing an important role in determining the response.

Fast Gas Sensing with Metal-Phthalocynaine Films in MIS Structures

The NH3gas sensing characteristics of vanadyl-phthalocyanine (VOPc) films in metal-insulator-semiconductor (MIS) hetero structures were investigated. The MIS device showed a rectification behavior and an increase in conductivity at the presence of NH3 gas. The response was very fast and reversible. The conductivity change observed for the MIS sensors was found to be originating from the change of the barrier height between the junctions.

MIS hydrogen sensors containing LB film insulator layers

Langmuir-Blodgett (LB) solid ultrathin films have been incorporated as the insulator layer in Pd/insulator/n-Si Schottky barrier diode metal-insulator-semiconductor (MIS) hydrogen sensors. The effects on device response and recovery of M- and I-layer thickness, temperature, hydrogen concentration and ambient atmosphere are examined. Optimized devices, containing organic LB films, show better responses than conventional oxide-containing MIS sensors and are capable of withstanding extended exposure to temperatures of up to 180 °C. Sensitivities of 0.5 ppm or better are achievable with good long-term operating stability.