Electrical properties of granular semiconductors : modelling and experiments on metal-oxide gas sensors

DC, AC, and the transient characteristics of granular n-type semiconductors are modelled using the drift-diffusion theory. The transient model describes the electrical large-signal response to both voltage and temperature changes. The analysis is based on the dynamic electrical model of the grain-boundary region and electronic trapping in the acceptor-type electronic interface states at the grain boundaries. The use of different approximations in the model derivation results in a simple fully analytical model and a semianalytical model, which requires numerical methods in the solution. The models are verified by performing numerical device simulations with SILVACO ATLAS. The models are also fitted to experimental data. They are in excellent agreement with ATLAS and the experimental data. Compared to ATLAS the transient calculations employing the semianalytical model are four orders of magnitude faster on a standard PC computer, yet having the same accuracy. The existence of electronic traps at grain boundaries results in nonlinear DC, extraordinary AC, and highly complex and nonlinear transient electrical characteristics. The current-voltage curves can be divided into four characteristic regions: linear, sublinear, superlinear, and series resistance limited regions. The electrical-equivalent-circuit presentations of the AC characteristics have, in addition to the common resistors and capacitors, special RL and RC circuit branches associated with the electronic trapping. These circuit branches have negative admittance. In the experimental part an atomic-layer-deposited SnO2 microhotplate gas sensor was designed and fabricated for the first time. The sensors exhibit good response and recovery to ethanol, acetone, and acrylonitrile vapours, and good stability. In addition, the developed model is extended to the case of n-type gas-sensitive surface-type metal oxides. The adsorption of gases is described by a surface-state model. The model is employed in the quantitative explanation of the new effects in metal-oxide gas sensors: the bias-dependent sensitivity and negative admittance effects, which were observed experimentally in commercial WO3 gas sensors. These effects can be used for increasing the selectivity of the gas sensors

Diffusion-emission theory of photon enhanced thermionic emission solar energy harvesters

Numerical and semi-analytical models are presented for photon-enhanced-thermionic-emission (PETE) devices. The models take diffusion of electrons, inhomogeneous photogeneration, and bulk and surface recombination into account. The efficiencies of PETE devices with silicon cathodes are calculated. Our model predicts significantly different electron affinity and temperature dependence for the device than the earlier model based on a rate-equation description of the cathode. We show that surface recombination can reduce the efficiency below 10% at the cathode temperature of 800 K and the concentration of 1000 suns, but operating the device at high injection levels can increase the efficiency to 15%.Comment: 5 pages, 4 figure

Thermoelectric bolometers based on ultra-thin heavily doped single-crystal silicon membranes

We present ultra-thin silicon membrane thermocouple bolometers suitable for fast and sensitive detection of low levels of thermal power and infrared radiation at room temperature. The devices are based on 40 nm-thick strain tuned single crystalline silicon membranes shaped into heater/absorber area and narrow n- and p-doped beams, which operate as the thermocouple. The electro-thermal characterization of the devices reveal noise equivalent power of 13 pW/rtHz and thermal time constant of 2.5 ms. The high sensitivity of the devices is due to the high Seebeck coefficient of 0.39 mV/K and reduction of thermal conductivity of the Si beams from the bulk value. The bolometers operate in the Johnson-Nyquist noise limit of the thermocouple, and the performance improvement towards the operation close to the temperature fluctuation limit is discussed.Comment: 11 pages, 3 figure

Glass Polarization Induced Drift in Microelectromechanical Capacitor

We present a quantitative physical model for glass substrate polarization and study the glasspolarization by measuring the capacitance drift from microelectromechanicalcapacitor test structure. The model consists of mobile and immobile charge species, which are related to alkali metals and non-bridging oxygen in glass. The model explains consistently our results and the previously observed non-homogeneous charging effect in a radio-frequency switch fabricated on a glass substrate. The results indicate that the bulk properties of the glass layer itself can be a significant source of drift. The modeling allows estimation of the drift behavior of the several kinds of device structures.Peer reviewe

High-performance silicon-based nano-thermoelectric bolometers for uncooled infrared sensing

Infrared (IR) sensors and photodetector arrays are employed in various imaging applications (such as night vision), remote temperature measurement, and chemical analysis. These applications are in space and environmental sensing, transport, health and medicine, safety, security, defense, industry, agriculture, etc. Optical chemical analysis employs IR absorption spectroscopy which enables the identification and quantification of gases, liquids, and materials based on their unique absorption spectra which are feature-rich in the IR region. State-of-the-art (SoA) quantum photodetectors utilize either photoconductivity or the photovoltaic effect. Commercial quantum photodetectors are widely available in the spectral range from UV to short-wave infrared (SWIR), but in mid-wave IR (MWIR) and long-wave IR (LWIR), they require exotic materials and cooling to maintain high sensitivity. Thermal detectors (bolometers) are a competing technology that can reach high sensitivities in IR without the need for cooling and can be manufactured using widely available semiconductor technologies. SoA bolometers include resistive bolometers, diode- or transistor-based bolometers, and thermoelectric bolometers. By utilizing nanomaterials and integrated design, we have minimized the thermal mass and demonstrated fast and sensitive nano-thermoelectric IR bolometers with high thermoelectric efficiency. We review the application and development of the silicon-based nano-thermoelectric infrared bolometers: modelling, design, fabrication, and electro-optical characteristics. The enabling materials, silicon nanomembranes, are also discussed, and the first devices used to test the potential of these nanomembranes, the electro-thermal devices, are reviewed and new experimental results are presented.</p

Nano-thermoelectric infrared bolometers

Infrared (IR) radiation detectors are used in numerous applications from thermal imaging to spectroscopic gas sensing. Obtaining high speed and sensitivity, low-power operation and cost-effectiveness with a single technology remains to be a challenge in the field of IR sensors. By combining nano-thermoelectric transduction and nanomembrane photonic absorbers, we demonstrate uncooled IR bolometer technology that is material-compatible with large-scale CMOS fabrication and provides fast and high sensitivity response to long-wavelength IR (LWIR) around 10 $\mu$m. The fast operation speed stems from the low heat capacity metal layer grid absorber connecting the sub-100 nm-thick n- and p-type Si nano-thermoelectric support beams, which convert the radiation induced temperature rise into voltage. The nano-thermoelectric transducer-support approach benefits from enhanced phonon surface scattering in the beams leading to reduction in thermal conductivity, which enhances the sensitivity. We demonstrate different size nano-thermoelectric bolometric photodetector pixels with LWIR responsitivities, specific detectivities and time constants in the ranges 179-2930 V/W, 0.15-3.1$\cdot10^{8}$ cmHz$^{1/2}$/W and 66-3600 $\mu$s, respectively. We benchmark the technology against different LWIR detector solutions and show how nano-thermoelectric detector technology can reach the fundamental sensitivity limits posed by phonon and photon thermal fluctuation noise.Comment: 20 pages, 4 figures, 1 tabl

Single-Walled Carbon Nanotube Network Field Effect Transistor as a Humidity Sensor

Single-walled carbon nanotube network field effect transistors were fabricated and studied as humidity sensors. Sensing responses were altered by changing the gate voltage. At the open channel state (negative gate voltage), humidity pulse resulted in the decrease of the source-drain current, and, vice versa, the increase in the source-drain current was observed at the positive gate voltage. This effect was explained by the electron-donating nature of water molecules. The operation speed and signal intensity was found to be dependent on the gate voltage polarity. The positive or negative change in current with humidity pulse at zero-gate voltage was found to depend on the previous state of the gate electrode (positive or negative voltage, respectively). Those characteristics were explained by the charge traps in the gate dielectric altering the effective gate voltage, which influenced the operation of field effect transistor.Peer reviewe