CMOS bandgap references and temperature sensors and their applications

Abstract

Two main parts have been presented in this thesis: device characterization and circuit. In integrated bandgap references and temperature sensors, the IC(VBE, characteristics of bipolar transistors are used to generate the basic signals with high accuracy. To investigate the possibilities to fabricate high-precision bandgap references and temperature sensors in low-cost CMOS technology, the electrical characteristics of substrate bipolar pnp transistors have been investigated over a wide temperature range. The measurement results of the IC(VBE, characteristics show that at a moderate current range, a good exponential relation between the base-emitter voltage and the collector current exists. Moreover, the temperature behavior of the base-emitter voltage can be well modeled with the well-known Gummel-Poon model. Even the negative correlation between the extracted parameters Vgo and is similar to that earlier reported for bipolar technology. Non-ideal effects, for instance the low forward common-emitter current gain (BF), high base resistance, low and high level injections and stress effect, etc. have also been analysed. Methods and suggestions have been given to get rid of the non-ideal effects in order to improve the performances of the bandgap references and temperature sensors. In order to design high-performance temperature sensors or bandgap references in CMOS technology, in addition to exploiting the best characteristics of the bipolar transistors, we needed to apply advanced circuit techniques. In this thesis, special advanced circuit techniques for application in low-speed sensor systems have been described. For instance, three signal auto-calibration can be applied to eliminate the additive and multiplexitive uncertainties of the transfer circuit. Chopping technique has been applied to reduce the low-frequency interferences and 1/f noise. By applying dynamic element matching (DEM), errors due to component mismatching can be reduced to second order. In the described sensor systems, indirect A/D converters (modulators) have been applied, because of their simplicity, high accuracy and high resolution. To reduce the effects of low-frequency interference and 1/f noise, a chopping technique was applied. To increase the input range, pre-amplifier and divider are applied for small and large signals respectively. By applying DEM technique the errors due to component mismatching can be significantly reduced. Accurate pre-amplifier and divider can be realized in this way. A circuit design for the thermocouple interface has been presented. On-chip bandgap reference and temperature sensor have been included for auto-calibration and the compensation of the reference junction temperature. Research on the characteristics of bipolar transistors fabricated in CMOS technology has been applied for designing high performance bandgap reference and temperature sensor. A switch-capacitor input circuit has been applied which allows the thermocouple voltage to have a rail-to-rail common-mode voltage. Furthermore, advanced circuit techniques such as three-signal auto-calibration, DEM and chopping mentioned above have been applied too. The interface circuit has been designed and implemented in 0.7-ï­m CMOS technology. The interface has been tested and the test results have been presented in this thesis. The design and test results of a DEM SC instrumentation amplifier have also been presented in this thesis