Low-Voltage CMOS Temperature Sensor Design Using Schottky Diode-Based References
Type of Culminating Activity
Master of Science in Electrical Engineering
Electrical and Computer Engineering
R. Jacob Baker
Thermal management circuits have been used for many years in systems such as air conditioners, ovens, and engines. Today temperature sensors are often integrated onto the same chip as microprocessors, memory circuits, and other ICs to help control system temperature. In most commonly-used integrated CMOS temperature sensors, bias circuits that utilize a PN junction diode (or diode-connected PNP bipolar transistor) are used. This is due to the well-defined I-V temperature characteristics of the semiconductor PN junction. The forward bias voltage of this junction is approximately 0.7 V.
As CMOS device geometries continue to shrink, so does the supply voltage. As the supply voltage decreases, this 0.7 V drop can be a limiting factor. The need for a device with a similar well-defined temperature characteristic and a lower forward-bias voltage becomes obvious. The design of a temperature sensor using the Schottky metal- semiconductor (MS) junction diode as a replacement for the tradition PN junction diode is presented in this work. The voltage required to forward-bias a Schottky diode is approximately half that of a PN junction diode, which allows for lower voltage operation.
This research explores various temperature sensor topologies used for low-voltage temperature sensing. The topology used for the finished product is that of a fully differential sigma-delta temperature sensor. This topology was chosen for its excellent noise performance and good low-voltage operation. This sensor was designed and fabricated using the AMI 0.5um process through the MOSIS fabrication organization. The chip performance has been evaluated and compared to the simulated results to verify accurate low voltage operation over a wide temperature range. The final design achieves an effective resolution of 0.7 °C and consumes an average current of less than 1 μA at a rate of 20 temperature readings per second. Silicon results also confirmed that the fully differential sigma-delta sensor also shows better noise performance than a similar single- ended sigma-delta sensor. The Schottky-based current references used in the sensor achieve over 300 mV of additional low-voltage margin when compared to PN junction diode-based current references.
Cahoon, Curtis Wayne, "Low-Voltage CMOS Temperature Sensor Design Using Schottky Diode-Based References" (2008). Boise State University Theses and Dissertations. 494.