Xiuli Gao

Enhanced Sensing of Nucleic Acids with Silicon Nanowire Field Effect Transistor Biosensors

Silicon nanowire (SiNW) field effect transistors (FETs) have emerged as powerful sensors for ultrasensitive, direct electrical readout, and label-free biological/chemical detection. The sensing mechanism of SiNW-FET can be understood in terms of the change in charge density at the SiNW surface after hybridization. So far, there have been limited systematic studies on fundamental factors related to device sensitivity to further make clear the overall effect on sensing sensitivity. Here, we present an analytical result for our triangle cross-section wire for predicting the sensitivity of nanowire surface-charge sensors. It was confirmed through sensing experiments that the back-gated SiNW-FET sensor had the highest percentage current response in the subthreshold regime and the sensor performance could be optimized in low buffer ionic strength and at moderate probe concentration. The optimized SiNW-FET nanosensor revealed ultrahigh sensitivity for rapid and reliable detection of target DNA with a detection limit of 0.1 fM and high specificity for single-nucleotide polymorphism discrimination. In our work, enhanced sensing of biological species by optimization of operating parameters and fundamental understanding for SiNW FET detection limit was obtained.

Development of a Reliable Micro-Hotplate With Low Power Consumption

In this paper, a reliable micro-hotplate with low power consumption is demonstrated by optimizing its structure and materials. Its power consumption is successfully decreased by adopting a structure with two high length-to-width slender beams. High reliability is achieved by improving its mechanical strength and long-term stability. A stacked membrane with residual stress controlled and corner compensations improve its mechanical strength. Pt heater element and Al2O3 adhesive layer deposited by atomic layer deposition (ALD) improve its long-term stability. Test results indicate that the micro-hotplate consumes only 18 mW when heated up to 400°C with a heating response time<;5 ms and cooling response time<;3 ms. It can sustain at least 842400 circles pulsed-mode operation with ultra-low resistance drift and can continuously work stably at around 450°C for at least 500 h.

A High-Performance Three-Dimensional Microheater-Based Catalytic Gas Sensor

We report a high-performance catalytic gas sensor based on a 3-D microheater. Two catalytic gas sensors were fabricated by sol-gel process, respectively based on a 3-D microheater with a concave active membrane and a comparative 2-D microheater with a rectangular active membrane, introducing Pd-Pt as catalytic material. Test results of the sensor response to methane indicate that the output signal (ΔV), sensitivity, and signal-to-noise ratio of the 3-D microheater-based gas sensor were more than twice of those of the 2-D microheater-based gas sensor. Moreover, it had a higher output signal-to-power ratio (ΔV/P).

Ultra-sensitive nucleic acids detection with electrical nanosensors based on CMOS-compatible silicon nanowire field-effect transistors

Silicon nanowire field-effect transistors (SiNW-FETs) have recently emerged as a type of powerful nanoelectronic biosensors due to their ultrahigh sensitivity, selectivity, label-free and real-time detection capabilities. Here, we present a protocol as well as guidelines for detecting DNA with complementary metal oxide semiconductor (CMOS) compatible SiNW-FET sensors. SiNWs with high surface-to-volume ratio and controllable sizes were fabricated with an anisotropic self-stop etching technique. Probe DNA molecules specific for the target DNA were covalently modified onto the surface of the SiNWs. The SiNW-FET nanosensors exhibited an ultrahigh sensitivity for detecting the target DNA as low as 1 fM and good selectivity for discrimination from one-base mismatched DNA.

CMOS MEMS-based thermoelectric generator with an efficient heat dissipation path

This paper presents a CMOS MEMS-based thermoelectric energy generator (TEG) device with an efficient heat dissipation path. For present CMOS MEMS-based thermoelectric generator devices, the output performance is greatly limited by the high thermal-contact resistance in the system. For the device proposed in the work, the silicon substrate is etched into two comb-shaped blocks thermally isolated from each other, which form the hot and cold sides. Thin-film-based thermal legs are densely located between the two blocks along the winding split line. Low internal thermal-contact resistance is achieved with the symmetrical thermal structure. When the TEG device is embedded between the heat source and heat sink, the heat loss can be well controlled with flat thermal-contact pads of the device. For a full device with 900 n/p-polysilicon thermocouples, the measured open-circuit voltage reaches as high as 146 mV K−1, and the power factor reaches almost five times higher value compared to the previously reported results. A test system integrated with a single device presents an open-circuit voltage of 110 mV K−1 when forcibly cooled by a Peltier cooler, or 26 mV K−1 when cooled by ambient air.

Behaviour of a catalytic combustion methane gas sensor working on pulse mode

A catalytic combustion type methane gas sensor with Pd-Pt/ ³-Al2O3 catalyst working on pulse voltage mode is investigated in this paper, after fabricated by micromachining and sol-gel process on a silicon substrate. In order to achieve low power consumption as well as high sensitivity, pulse voltage was applied to a bridge circuit consisted of the sensor, and the ratio of the gas sensitivity to power consumption was considered as an optimizing factor when selecting high level voltage. Outputs from four low level voltages indicate that the response time of the sensor decreases with the increasing low level voltage, while the sensitivity is almost 1.8 times of the value working on constant voltage in some cases, which path a way to improve the performance of the sensor.

A high heating efficiency two-beam microhotplate for catalytic gas sensors

This paper presents a suspended-membrane-type microhotplate with high heating efficiency for catalytic gas sensors. This microhotplate has only two supporting beams through which heat loss via conduction can be effectively reduced. By isolating heat in a rectangular active membrane, high heating efficiency can be achieved. Power per active area (PPAA) of the microhotplate is only about 30% in comparison with current microhotplates. Based on the two-beam microhotplate, a catalytic gas sensor was fabricated by sol-gel process introducing Pd-Pt as the catalytic material. Test results indicate that this catalytic gas is sensitive to methane with a high sensitivity. And the sensor output has a fairly linear relation to methane concentration.

Thermocapillary Actuation of Droplets on a Microfluidic Chip

Abstract In this paper, we report a digital liquid transporting chip which can manipulate droplets precisely based on thermal Marangoni force induced by thermal gradient. Detailed mathematical discussion shows that threshold force for mobilization is a function of droplet size and liquid parameters, and droplet velocity after depinning is a function of applied thermal gradient, droplet size and liquid parameters. The device with Ti as micro-heater, Au film as pad, Cr/Au as controlling electrode arrays, high quality SiO2 film as dielectric layer on glass substrate is fabricated by silicon bulk process and employs fluorocarbon polymer as hydrophobic layer to functionalize the surface to be hydrophobic so as to facilitate droplet transportation. Test results show that transportation velocity of 3 μl DI water and silicone oil can reach 0.1 mm/s and 1 mm/s respectively at a voltage of 7 V. Driving voltage, transportation velocity of droplets and effect of contact angle hysteresis are discussed and the results are in agreement with theoretical results. The results provide some practical guidelines for the design of microfluidic chips based on thermocapillary actuation. In this study, we demonstrated that thermocapillary actuation of droplets is tunable and the chip can find great application in lab-on-a-chip due to its simple structure and low-cost fabrication process.

A low power catalytic combustion gas sensor based on a suspended membrane microhotplate

A microhotplate based catalytic combustion gas sensor with low power consumption employing Pd-Pt/ γ-Al2O3 is presented in this paper. The microhotplate was fabricated on a silicon substrate by micromachining process. And the sensitive material Pd-Pt and compensation material γ-Al2O3 were prepared by sol-gel process. This sensor has been tested under constant working voltage to methane gas and achieved good performance. With the increasing of working voltage, both the output signal and power consumption of the sensor increased. The ratio of the sensitivity to power consumption was considered as an optimizing factor for selecting a rational working voltage. The sensor output signal showed a fairly linear relationship to the methane gas concentration. It achieved a sensitivity of 2mV/%CH4 to 50% LEL methane gas at a low power of 52mW.

Confirmation on the size-dependence of Young's modulus of single crystal silicon from the TEM tensile tests

With the developed process, four MEMS tensile testing chips of <110>-oriented single crystal silicon (SCS) nanobeams were achieved in one SOI wafer with thickness from 45 to 100nm. Mounting the chips onto a custom made TEM sample holder, which integrated also comb drives and force sensor beam, in-situ TEM tensile tests were carried out. The measured Youngs modulus (from 74Gpa to 178Gpa) is in agreement well with our previous results of tensile tests in SEM. A simple model was constructed to explain the behaviors of the Youngs modulus by tensile tests, resonance tests and pull-in tests since all datas approach to ∼75 GPa when the nanobeams thickness close to 40nm.