Aapo Varpula

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%.

Harvesting Vibrational Energy Using Material Work Functions

Vibration energy harvesters scavenge energy from mechanical vibrations to energise low power electronic devices. In this work, we report on vibration energy harvesting scheme based on the charging phenomenon occurring naturally between two bodies with different work functions. Such work function energy harvester (WFEH) is similar to electrostatic energy harvester with the fundamental distinction that neither external power supplies nor electrets are needed. A theoretical model and description of different operation modes of WFEHs are presented. The WFEH concept is tested with macroscopic experiments, which agree well with the model. The feasibility of miniaturizing WFEHs is shown by simulating a realistic MEMS device. The WFEH can be operated as a charge pump that pushes charge and energy into an energy storage element. We show that such an operation mode is highly desirable for applications and that it can be realised with either a charge shuttle or with switches. The WFEH is shown to give equal or better output power in comparison to traditional electrostatic harvesters. Our findings indicate that WFEH has great potential in energy harvesting applications.

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.

Modelling of dc characteristics for granular semiconductors

The dc characteristics of granular n-type semiconductors are calculated analytically with the drift-diffusion theory. Electronic trapping at the grain boundaries (GBs) is taken into account. The use of quadratic and linear GB potential profiles in the calculation is compared. The analytical model is verified with numerical simulation performed by SILVACO ATLAS. The agreement between the analytical and numerical results is excellent in a large voltage range. The results show that electronic trapping at the GBs has a remarkable effect on the highly nonlinear I–V characteristics of the material.

Modeling of transient electrical characteristics for granular semiconductors

Modeling of the electrical large-signal response of granular n-type semiconductors is carried out at following three different levels: (i) simple fully analytical model, (ii) semianalytical numerical model, and (iii) numerical device simulation. The electrical transients induced by both voltage and temperature changes are calculated. The analysis is based on the dynamic electrical model of the grain-boundary (GB) region, the drift-diffusion theory, and electronic trapping in the acceptor-type electronic interface states at the GBs. The electronic trapping is described using the standard rate equation. The models are verified by performing numerical device simulations using SILVACO ATLAS. The agreement between the proposed semianalytical model and ATLAS results is excellent during the whole transient and up to rather high electric fields. Compared to ATLAS, the calculations performed with the present semianalytical model are four orders of magnitude faster on a standard PC computer. The approximative analy...

Negative admittance in resistive metal oxide gas sensors

The negative admittance effect is observed in a WO3-based resistive gas sensor MOS1 from Environics Oy. The effect is caused by electron trapping (i.e. oxygen ionization) at the grain boundary. The results show that the current component related to the modulation of the grain-boundary barrier dominates in dry clean air and the charging or discharging current dominates in humid air conditions. An equivalent electrical circuit model for the sensor response is presented.

Small-signal analysis of granular semiconductors

The small-signal ac response of granular n-type semiconductors is calculated analytically using the drift-diffusion theory when electronic trapping at grain boundaries is present. An electrical equivalent circuit (EEC) model of a granular n-type semiconductor is presented. The analytical model is verified with numerical simulation performed by SILVACO ATLAS. The agreement between the analytical and numerical results is very good in a broad frequency range at low dc bias voltages.

Characterization of epiready n + -GaAs (100) surfaces by SPV-transient

Surface photovoltage (SPV) transient provides a non-destructive, contact-free method for characterization of semiconductor surfaces. Here we study SPV-transients of differently cleaned, heavily doped epiready GaAs wafers. After a rapid initial part the transient shows a very slow decay taking place in 100 - 1000 s time scale. Chemical NH4OH:H2O2:H2O cleaning and atomic hydrogen UHV cleaning are applied. SPV-transients are measured by Kelvin probe in normal atmospheric conditions. A large signal surface trapping model is developed which includes both majority and minority carrier processes and covers the whole light on, steady state, light off sequence. Model fitting allows band bending, energy and density of surface states as well as electron and hole capture cross-sections to be extracted. The results show that the traps are electronic states in thin oxide layer covering the samples. This conclusion is based on the finding that the capture cross-sections are very small, in the range 10-19 - 10-26 cm2, which calls tunneling for explanation. This indicates that after cleaning the oxide layer is rapidly re-grown in laboratory atmosphere in less than 30 min. Typical band bendings are 0.6 - 0.8 eV, trap energies are slightly above the mid-gap and the density of occupied trap states is around 5×1012 cm-2 at thermal equilibrium.

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

We present thermal detectors based on 40 nm-thick strain tuned single crystalline silicon membranes shaped into a heater area supported by narrow n- and p-doped beams, which also operate as a thermocouple. The electro-thermal characterization of the devices reveals a noise equivalent power of 13 pW/Hz1/2 and a 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 performance enables fast and sensitive detection of low levels of thermal power and infrared radiation at room temperature. The devices 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.

Modeling the effect of mobile ion contamination on the stability of a microelectromechanical resonator

We present a theoretical model for mobile ion contamination in a silicon microelectromechanical resonator. In the model both drift and diffusion of the mobile charge in dielectric films are taken into account. The model is verified through a comparison to existing experimental data. We show that the model can describe the frequency drift of resonators in a wide temperature range.