Faisal Razi Ahmad

Silicon Carbide Integrated Circuits With Stable Operation Over a Wide Temperature Range

In this letter, silicon carbide MOSFET-based integrated circuits have been designed, fabricated, and successfully tested from -193 °C (80 K) to 500 °C. Silicon carbide single MOSFETs remained fully operational over a 700-°C wide temperature range and exhibited stable I-V characteristics. The circuits that include operational amplifier (op-amp), 27-stage ring oscillator, and buffer were tested and shown to be functional up to 500 °C with relatively small performance variation between 300 °C and 500 °C. High-temperature evaluation of these circuits confirmed stable operation and survivability of both the ring oscillator and op-amp for more than 100 h at 500 °C.

Remote driven and read MEMS sensors for harsh environments.

The utilization of high accuracy sensors in harsh environments has been limited by the temperature constraints of the control electronics that must be co-located with the sensor. Several methods of remote interrogation for resonant sensors are presented in this paper which would allow these sensors to be extended to harsh environments. This work in particular demonstrates for the first time the ability to acoustically drive a silicon comb drive resonator into resonance and electromagnetically couple to the resonator to read its frequency. The performance of this system was studied as a function of standoff distance demonstrating the ability to excite and read the device from 22 cm when limited to drive powers of 30 mW. A feedback architecture was implemented that allowed the resonator to be driven into resonance from broadband noise and a standoff distance of 15 cm was demonstrated. It is emphasized that no junction-based electronic device was required to be co-located with the resonator, opening the door for the use of silicon-based, high accuracy MEMS devices in high temperature wireless applications.

Remote excitation and readout of a high Q silicon resonator

This paper presents the first published report of a totally passively driven actuation and readout of an electromechanical resonator using inductive coupling. The goal of this work is to remotely excite and read a MEMS comb drive resonator with a focus on low power operation while simultaneously maximizing standoff distance without the use of active electronics at the sensor location. Initial measurements focused on the determining the relationship of the received signal level with the drive parameters. Reading the resonator through integrated piezoresistors, the drive response showed the expected dependence of both the RF power and AM modulation depth and the coil separation matched a simple model of a 6 cm coil radius and B4 dependence. Measurements of an entirely wireless (read and driven) resonator were made to explore the standoff capability with a practical limit of ∼15 dBm RF power to both the drive and receive systems. A standoff of 9 cm was demonstrated limited by power input to the device. The effect of read and drive coil position was also studied.

On the development of CdS properties upon processing CdTe devices

CdS plays an important role in CdTe devices through both the thickness impact on Jsc and the less well understood effect on FF and Voc by impacting the minority carrier lifetime in the CdTe near the interface. In this study, two techniques are explored, chemical bath deposition and radio frequency sputtering, effectively exploring the role of film density. A multitude of characterization techniques have been exploited on CdS films after various stages in the process of device fabrication as the CdS properties change significantly upon processing of the films. For this purpose we compared the properties for as-deposited films, films that are exposed to a CdTe deposition heating profile without the actual CdTe deposition, as well as CdS films as they appear within completed devices. Especially the CdCl2 process turned out to result in huge changes within the CdS film. From this study it is concluded that significant changes occur to the CdS film depending on its starting density. Change in electrical properties and intermixing occur during the CdTe deposition and are thus temperature related, while a change in optical properties and carrier lifetime near the interface is established after CdCl2.