T.-H. Joubert

Active-bootstrapped gain-enhancement technique for low-voltage circuits

A gain-enhancement technique based on active bootstrapping is proposed which ideally increases the low-frequency gain by two gain stages with no speed penalty and without adding to the phase shift. Therefore, high-frequency performance and feedback stability are not compromised. In practice, the gain enhancement will be limited by nonidealities. The technique is particularly suited for minimum supply voltages since no cascodes are used and, although the discussion pertains to CMOS, can also be applied in bipolar and BiCMOS technologies. A complete analysis, including the effects of nonidealities, is performed. A generally applicable design methodology is formulated.

Electroluminescence From Two Junction Punch Through Structures in Silicon Nanowires

Hot carrier electroluminescence in two junction devices under punch through conditions manufactured in silicon on insulator nanowires are investigated. Of interest is the spectral content of the light emission, as well as the external power efficiency and the light extraction efficiency. An order of magnitude improvement in external power efficiency was achieved relative to a bulk silicon p-n junction in avalanche.

A CMOS reduced-area SRAM cell

In CMOS, an SRAM cell containing six transistors and five routing wires is generally used. If a smaller number of transistors and/or fewer connection lines were possible, the packing density of SRAM chips may be improved. In this paper, a four-transistor SRAM cell for implementation in a standard digital CMOS process is proposed. The implementation of the smallest area cell in a 1.2 /spl mu/m n-well CMOS process shows successful cell operation. Two memory block architectures are also discussed, as well as the specific cell configurations for correct operation. Depending on the memory block architecture, the four-transistor cell area reduction, when compared to a six-transistor cell, is either 14.7% or 37.3%.

A CMOS microdisplay with integrated controller utilizing improved silicon hot carrier luminescent light sources

Microdisplay technology, the miniaturization and integration of small displays for various applications, is predominantly based on OLED and LCoS technologies. Silicon light emission from hot carrier electroluminescence has been shown to emit light visibly perceptible without the aid of any additional intensification, although the electrical to optical conversion efficiency is not as high as the technologies mentioned above. For some applications, this drawback may be traded off against the major cost advantage and superior integration opportunities offered by CMOS microdisplays using integrated silicon light sources. This work introduces an improved version of our previously published microdisplay by making use of new efficiency enhanced CMOS light emitting structures and an increased display resolution. Silicon hot carrier luminescence is often created when reverse biased pn-junctions enter the breakdown regime where impact ionization results in carrier transport across the junction. Avalanche breakdown is typically unwanted in modern CMOS processes. Design rules and process design are generally tailored to prevent breakdown, while the voltages associated with breakdown are too high to directly interact with the rest of the CMOS standard library. This work shows that it is possible to lower the operating voltage of CMOS light sources without compromising the optical output power. This results in more efficient light sources with improved interaction with other standard library components. This work proves that it is possible to create a reasonably high resolution microdisplay while integrating the active matrix controller and drivers on the same integrated circuit die without additional modifications, in a standard CMOS process.

Inkjet-printed conductive features for rapid integration of electronic circuits in centrifugal microfluidics

This work investigates the properties of conductive circuits inkjet-printed onto the polycarbonate discs used in CD-based centrifugal microfluidics, contributing towards rapidly prototyped electronic systems in smart ubiquitous biosensors, which require sensitive and robust signal readout at low power and cost, and with wireless connectivity. A protocol for inkjet-printing electronic networks on CD substrates is developed. The circuit modeling of conductive tracks is discussed, and validated against experimental results. A design procedure is presented for reliably printing conductive networks with feature dimensions between 150 μm and 2 mm on CDs, and yielding electronic circuits operating with a bandwidth of 1 MHz.

CMOS in-pixel optical pulse frequency modulator

This paper covers the design of a complementary metal oxide semiconductor (CMOS) pixel readout circuit with a built-in frequency conversion feature. A single pixel was designed containing all of the signal-to-frequency conversion circuitry, after which an array of 64 pixels was designed. The on-chip frequency modulation technique is preferred over analog-to-digital conversion (ADC) techniques due to its smaller size and the possibility of a higher dynamic range. Considerations are made regarding the size of the components, as various characteristics of CMOS devices are limited by decreasing the scale of the components [1]. All components of the pixel are designed from first principles to meet the requirements of a small pixel size of 30 × 30 square micrometers and an output resolution greater than that of an 8-bit ADC. The photodetector is realized by an n+-p+/p-substrate diode with a parasitic capacitance of 3.3 fF. The analog front-end was designed around a Schmitt-trigger circuit. The photo current is integrated on an integration capacitor of 200 fF. The circuit schematic and layout were designed using Cadence Virtuoso and the process used was the ams CMOS 350 nm process. The results were confirmed to comply with the required size and resolution specifications and the layout passed all verification checks. The dynamic range achieved was 58.828 dB with an output frequency range between 12.341 kHz and 10.783 MHz. It was also found that the output frequency in this range has a linear relationship to the photocurrent generated by the photodiode.

Silicon nanowire hot electron electroluminescence

This paper investigates the avalanche electroluminescence characteristics of pn junctions formed in silicon nanowires fabricated in a silicon-on-insula*tor (SOI) technology. Since carriers are confined to the nanowires, it is possible to study the effect of electric field strength on device performance while the current density and carrier concentrations are kept constant. This is achieved by varying the nanowire length while keeping the bias current constant, eventually driving the pn junction into the reach-through bias condition. It is observed that photon emission for photon energies higher than 1.2 eV increases when the nanowire length is reduced, while photon emission with energies less than 1.2 eV decreases. The higher electric field in the nanowire at shorter nanowire lengths enhances the high-energy photon emission and attenuates the low energy photon emission.

Micro-incubator for bacterial biosensing applications

The presence of Escherichia coli (E. coli ) is a commonly used indicator micro-organism to determine whether water is safe for human consumption.1 This paper discusses the design of a micro-incubator that can be applied to concentrate bacteria prior to environmental water quality screening tests. High sensitivity and rapid test time is essential and there is a great need for these tests to be implemented on-site without the use of a laboratory infrastructure. In the light of these requirements, a mobile micro-incubator was designed, manufactured and characterised. A polydimethylsiloxane (PDMS) receptacle has been designed to house the 1-5 ml cell culture sample.2 A nano-silver printed electronics micro-heater has been designed to incubate the bacterial sample, with an array of temperature sensors implemented to accurately measure the sample temperature at various locations in the cell culture well. The micro-incubator limits the incubation temperature range to 37±3 °C in order to ensure near optimal growth of the bacteria at all times.3 The incubation time is adjustable between 30 minutes and 9 hours with a maximum rise time of 15 minutes to reach the set-point temperature. The surface area of the printed nano silver heating element is 500 mm2. Electrical and COMSOL Multiphysics simulations are included in order to give insight on micro-incubator temperature control. The design and characterization of this micro-incubator allows for further research in biosensing applications.

Handheld chemiresistive gas sensor readout system

Low-cost and non-invasive diabetes diagnosis is increasingly important [1], and this paper presents a handheld readout system for chemiresistive gas sensors in a breath acetone diagnostic application. The sensor contains reference and detection devices, used for the detection of gas concentration. Fabrication is by dropcasting a metaloxide nanowire solution onto gold interdigitated electrodes, which had been manufactured on silicon. The resulting layer is a wide bandgap n-type semiconductor material sensitive to acetone, producing a change in resistance between the electrode terminals [2]. Chemiresistive sensors typically require temperatures of 300-500 °C, while variation of sensing temperature is also employed for selective gas detection. The nano-structured functional material requires low temperatures due to large surface area, but heating is still required for acceptable recovery kinetics. Furthermore, UV illumination improves the sensor recovery [3], and is implemented in this system. Sensor resistances range from 100 Ω to 50 MΩ, while the sensor response time require a sampling frequency of 10Hz. Sensor resistance depends on temperature, humidity, and barometric pressure. The GE CC2A23 temperature sensor is used over a range of -10°C to 60°C, the Honeywell HIH5031 humidity sensor operates up to 85% over this temperature range, and the LPS331AP barometric pressure sensor measures up to 1.25 bar. Honeywell AWM43300V air flow sensors monitor the flow rate up to 1000 sccm. An LCD screen displays all the sensor data, as well as real time date and time, while all measurements are also logged in CSV-format. The system operates from a rechargeable battery.

Comparison of inkjet-printed silver conductors on different microsystem substrates

Applications for diagnostic and environmental point-of-need require processes and building blocks to add smart features to disposable biosensors on low-cost substrates. A novel method for producing such biosensors is printing electronics using additive technologies. This work contributes to the toolbox of processes, materials and components for printed electronics manufacturing - as well as rapid prototyping - of circuits. Printing protocols were developed to facilitate successful inkjet printing of nanosilver ink (Harima NPS-JL) onto different microsystem substrates using a functional printer (Dimatix DMP-3281). Photo paper is a standard inkjet substrate, which were compared with glass, polycarbonate (PC), plastic projector transparency foil, and polydimethylsiloxane (PDMS). Comparison attributes include physical and electrical properties. The layout design comprised dogbone elements of 8 mm length, and widths varying between 100 μm and 2 mm. All printed features were thermally cured for 1 hour at 120 °C. The physical characteristics were measured with a laser scanning microscope (Zeiss LSM-5) to determine the width, thickness and surface roughness of the printed features. An LCR meter (GW-Instek 8110) was used to measure the printed structures’ electrical characteristics (resistance, capacitance and inductance). A lumped element model and layout design rules were extracted to assist in standardized design procedures. The model incorporates prediction of the bandwidth attainable with these structures. The layer thickness on all substrates is larger than the 1 μm on photo paper, and varies between 1.6 μm (PC) and 7 μm (PDMS). The spreading for PDMS is similar to photo paper, but since for the other substrates it is between 5 (glass) and 10 (PC) times larger than for photo paper, the layout design rules require large spacing, leading to larger area networks. Electrical probing on the PDMS is not consistent and results are inconclusive. For the other substrates, the comparative dogbone resistance (100 μm width) is significantly larger than the 2 Ω standard, varying from 12.6 Ω (PC) to 19.3 Ω (glass). The bandwidth relative to photo paper is smaller by a factor of between 6 (PC) and 9.5 (glass).