STMicroelectronics recently introduced a new IR sensor that utilizes thermo-mosfets (TMOS) to replace lens or active IR pixels. ST claims that the space-saving that is provided by the TMOS technology could assist designers in improving their efficiency and saving board real estate for designs that are densely integrated.
ST’s most recent IR sensor has a totally different sensing system and does not have the need for an IR lens. Image (modified) taken by permission of STMicroelectronics
The latest IR detectors have been around since the 1960s and have held the attention of hobbyists due to their easy implementation for motion sensors. In contrast to traditional PIR, technology requires movement to identify targets, TMOS-based sensors are able to achieve detection of presence even when targets are stationary.
This article delved into the technical aspects of ST’s latest sensors for IR based on TMOS, The STHS34PF80, and the ways it can be used in new ways.
Ultra-sensitive Temperature Dependence
The core of STHS34PF80 is a number of transistors with TMOS that are used to enable IR detection. Each transistor (built using an SOI process) operates within the subthreshold area and makes the drain-source current highly temperature-dependent. Therefore, any variation in the amount of IR radiation absorbed by its gate (produced by any object that is warm) can detect a movement or presence of a target precisely.
Subthreshold conduction creates a low but temperature-dependent current to isolate the temperature and IR radiation the sensor receives. Image taken with permission from UC Berkeley
The subthreshold operation of TMOS transistors results in a lower charge-source voltage (compared to other regions of operation) which conserves power overall and enhances the sensitivity. Common sensors for PIR require the use of a Fresnel lens to concentrate enough IR radiation to produce an observable response. The STHS34PF80 is able to operate without a lens. This demonstrates its sensitivity.
ST created TMOS devices along with analog and digital circuitry in order to create a compact but high-performance IR detector that includes presence and motion detection. Image courtesy of STMicroelectronics.
Alongside the IR detection device, the reading circuitry for digital is integrated by the ST SOI CMOS technology, resulting in a very compact overall size. This incredibly dense integration, coupled with a lens-free operation, makes the sensor an ideal instrument to further shrink sensors.
New Vision for Smarter Devices
ST has discovered numerous applications which could be benefited by the sensor’s new technology, including burglar alarms as well as intelligent lighting and appliances.
The lens-free FOV sensor allows for many different applications within smaller package dimensions. Image courtesy of STMicroelectronics.
Without a lens installed, the sensor’s angular field-of-view (FOV) can be achieved with an area of 4 meters. A band-pass filter optically located in the input assures that lighting conditions outside do not affect the ability to detect.
The ratio of the input transmittance demonstrates the sensor’s ability to function independently of visible light, making it more effective for designers to design sense. Image courtesy of STMicroelectronics.
If designers require more range or FOV, they may incorporate commercial or custom Fresnel lenses to increase performance. ST has started production on the sensor, along with a compatible evaluation board that designers can utilize to evaluate the sensor’s performance in situations that are specific to the application.