Sandeep G. Surya

A non-volatile resistive memory effect in 2,2′,6,6′-tetraphenyl-dipyranylidene thin films as observed in field-effect transistors and by conductive atomic force microscopy

The charge transport properties of 2,2′,6,6′-tetraphenyldipyranylidene (DIPO-Ph4), a large planar quinoid π-conjugated heterocycle, are investigated in field-effect transistor (FET) configuration and by conductive atomic force microscopy (c-AFM). The FET properties show a clear p-type behavior with a hole mobility up to 2 × 10−2 cm2 V−1 s−1 and on/off ratio of 104. The transfer characteristics Id/Vg present a clear hysteresis typical of a resistive memory effect. This memory effect is again observed by means of c-AFM in lateral mode using a nearby gold top-contact as the counter-electrode. The c-AFM current response recorded for variable distances d = 0.5–9.0 μm between the AFM tip and the top electrode shows a resistive switching behavior in the low-voltage 0.0–3.0 V region. Repeated “write-read-erase-read” cycles performed at low frequency reveal a non-volatile memory effect in the form of high-resistance and low-resistance states with a stable on/off ratio of 102 during cycling operation.

A low-cost, ultra sensitive hand-held system for explosive detection using piezo-resistive micro-cantilevers

This paper presents a hand-held explosive detector system based on piezo-resistive cantilever sensors. A novel signal conditioning scheme comprising current excitation method with modified half bridge configuration, is implemented to measure as low as 20 ppb variations in the sensor resistance. The proposed circuit is insensitive to thermoelectric and stray noise effects as well as to temperature changes with minimum SNR value of 20 dB which is better than previously reported values. The hand held system comprises micro-cantilever sensor, analog signal conditioning module, sigma-delta ADC, digital modules along with digital output displayed on the LCD. The micro-cantilevers feature a special coating like 6-Marcaptonicotonic acid (6-MNA) or 4-mercaptobenzoic acid (4-MBA) on one side of the cantilever. The selective reaction takes place with TNT and RDX molecules (the target analyte molecules) on the functionalized cantilever surface with special coating. This reaction changes the resistance of the piezo-resistive sensor. On-chip implementation of one of the techniques is presented while some other methods are realized using discrete components. The objective is to explore different techniques for converting cantilever deflection into a measurable quantity and challenges ahead of converting explosive detector system into a complete SoC. These challenges are also discussed in the paper.

A MEMS-based shifted membrane electrodynamic microsensor for microphone applications

In this paper we present a multidisciplinary modeling of a MEMS-based electrodynamic microsensor, when an additional vertical offset is defined, aiming acoustic applications field. The principle is based on the use of two planar inductors, fixed outer and suspended inner. When a DC current is made to flow through the outer inductor, a magnetic field is produced within the suspended inner one, located on a membrane top. In our modeling, the magnetic field curve, as a function of the vertical fluctuation magnitude, shows that the radial component was maximum and stationary for a specific vertical location. We demonstrate in this paper that the dynamic response of the electrodynamic microsensor was very appropriate for acting as a microphone when the membrane is shifted to a certain vertical position, which represents an improvement of the microsensors basic design. Thus, a proposed technological method to ensure this offset of the inner inductor, by using wafer bonding method, is discussed. On this basis, the mechanical and electrical modeling for the new microphone design was performed using both analytic and Finite Element Method. Firstly, the resonance frequency was set around 1.6 kHz, in the middle of the acoustic band (20 Hz – 20 kHz), then the optimal location of the inner average spiral was evaluated to be around 200µm away from the diaphragm edge. The overall dynamic sensitivity was evaluated by coupling the lumped elements from different domains interfering during the microphone function. Dynamic sensitivity was found to be 6.3 μV/Pa when using 100 µm for both gap and vertical offset. In conclusion, a bandwidth of 37.6 Hz to 26.5 kHz has been found which is wider compared to some conventional microphones.

Mechanical modeling and sensitivity evaluation of an electrodynamic MEMS microsensor

In this paper, we present the mechanical modeling of a MEMS electrodynamic microphone using finite element analysis. This new model aims to study the mechanical design of a microphone to predict its dynamic range performance. Two coaxial planar inductors, one external and the other is internal, are used in this microphone design. When the external inductor is flown by a current, it will produce a magnetic field within the internal suspended one located onto suspended membrane above a micromachined cavity. In the present study, the membrane is attached around its edges, to avoid opening in the top membrane surface which leads usually to an acoustic short path in low frequencies that can affect the microphone performance. So, both membrane resonant frequency and displacement have been determined according to the used technology in IIT Bombay university- India. The frequency was optimized around 1.6 KHz in the geometric mean of the acoustic band (20 Hz-20 kHz) and the harmonic displacement was around 8μm for the main resonant frequency. Finally, the sensitivity was evaluated by coupling different transducer domains involved in the microsensor principle and by using the lumped element model of the microphone. The ultimate sensitivity was found around 0.1V/Pa, which is considered to be quite good compared to previously published sensitivities.

Highly Sensitive ?R/R Measurement System for Nano-electro-Mechanical Cantilever Based Bio-sensors

Functionalized piezoresistive nano-electromechanical cantilevers are promising tools for sensor applications. This paper reports an integration of custom fabricated piezoresistive cantilever sensors and sensitive analog front end circuit for detection of bio-markers. The system operates on the principles of nano-meter deflection of cantilever sensors, due to antigen-antibody interactions, which in turn exhibits change in piezoresistance. The instrumentation hardware can measure resistance changes down to 14 parts per million (ppm) and maximum sensitivity is 2.134 V/ppm. Experiments have been performed with 90K? and 1M? base resistances. A wheat stone bridge connected resistors has been used where the functionalized cantilever forms one of the arms. The measured results using this battery operated system has been presented along with calibration technique. The size of this LCD (Liquid crystal display) based system was reduced to fit into a hand held point-of-care form factor. In addition, users can operate this measurement system with the help of computer to connect to the internet. Measurement results are presented with the introduction of bovine serum albumin (BSA) over the cantilever sensors. This demonstrates the possible application for detection of myocardial infarction in clinical settings.

Thikness dependence investigation of the mutual inductance link in concentric planar transformers

In this paper, we present a new analytic approach for the calculation of the mutual inductance between two concentric planar conductors. From this study, we aim to prove that coil thickness has a minor impact on mutual inductance value. This study will help to determine a simplified adapted expression of the induced voltage output at our electrodynamic microphone ends. This micromachined new sensor exploits the concentric transformer principle which relies on the use of two planar inductors: an outer fixed one and an inner suspended one. If the outer is biased, a mutual inductance link will be generated. This link strongly depends on inductors coil design geometry and the induced output voltage. The present work proves that for long conductors (large number of turns), coil thicknesses above 6μm will have no significant influence on mutual inductance values. This has been proven by an increase of only 0.01% if coil thickness reaches as high as 20 μm. Moreover, for thicknesses under 2μm, mutual inductance value is constant arround 7.2μH. This allows the use of much simpler expressions, with no thickness dependence, to evaluate mutual inductance values.

Organic field effect transistors for explosive and radiation dosimetry applications

This paper presents a brief overview of an Organic field effect transistor (OFET) based sensor applications. It highlights recent progress in the field of gas sensing and ionizing radiation sensing using OFETs. Besides step-by-step improvement of conventional devices, novel materials and novel approaches for OFET based gas and radiation sensing are discussed.