TMOS thermal sensor

TMOS is a new type of thermal sensor consisting in a micromachined thermally isolated transistor fabricated using CMOS - SOI (Silicon on Insulator) technology. A thermal sensor is a device able to detect the thermal radiation emitted by an object located in the FOV ( Field Of View ) of the sensor generating an electric signal proportional to the incident IR power. It has been developed in the last decade by the Technion - Israel Institute of technology.

Operating principle

The operating principle of TMOS is that the thermal IR radiation absorbed heats up the TMOS and this cause a variation in the TMOS temperature. The temperature variation produces a current or a voltage output signal proportional to the absorbed radiation. TMOS performance depends on the transistor operating region and configuration : two terminals component, diode like, or three terminals component. Subthreshold region is the prefeared one because avoids self heating effects and leads to higher sensitivity. Another reason to work in subthreshold region is that TMOS is an active device so requires a bias, however in this operating region the power consumption is lower than in other ones. From a circuit point of view the produced TMOS signal can be modelled as a temperature dependent current source i_sig in parallel with the gm V_gs generator for small signal equivalent circuit. The value of i_sig is directly proportional to the drain source current variation with respect to TMOS operating temperature and to the temperature variation induced on the TMOS by the radiation absorbed from target object. This temperature has a direct dependence on the absorbing efficiency, the incident radiation power and on the thermal conductance of the sensor. TMOS sensitivity depends if the device is working in current or voltage mode. In the first case sensitivity corresponds to the temperature coefficient of current TCC, that is defined as the inversly proportional to drain source current and directly proportional to the derivative of drain source current respect to the operating temperature. In the voltage mode the sensitivity is the temperature coefficient of voltage and is inversly proportional to the voltage bias and to the derivative of voltage respect to temperature for the considered operated temperature.

FABRICATION PROCESS

APPLICATIONS

Thermal sensors have a lot of different applications. They respond to thermal IR radiation so their main application is for thermal IR camera. The other possible qpplications regards gas analysis, human detection for autonoumous driving, presence detection, people counting, security system or thermal monitoring during fabrication process. Until now the TMOS thermal sensor has been used for motion and presence detection. It will rapresent a perfect candidate also for human body contactless temperature measurments.