News

Oct 31, 2023

In Lieu of PIR Detectors, ST’s New IR Sensor Uses Thermal Transistors

STMicroelectronics recently announced a new IR sensor using thermal MOSFETs (TMOS) in the place of lenses and passive IR pixels. ST claims the space savings afforded by the TMOS technology may help

STMicroelectronics recently announced a new IR sensor using thermal MOSFETs (TMOS) in the place of lenses and passive IR pixels. ST claims the space savings afforded by the TMOS technology may help designers improve performance and conserve board real estate for densely-integrated designs.

Modern IR detectors have existed since the 1960s, holding a popular place among hobbyists for their simple implementation as motion sensors. While traditional PIR technology requires motion to detect targets, TMOS-based sensors can reportedly accomplish presence detection even when targets are stationary.

This article dives into the technical details of ST's new TMOS IR sensor, the STHS34PF80, and how it can be used in new applications.

At the heart of the STHS34PF80 is multiple TMOS transistors for IR detection. Each transistor (built on an SOI process) operates in the subthreshold region, making the drain-source current extremely temperature dependent. As such, any change in the amount of IR radiation absorbed at the gate of the transistor (produced by any warm object) can determine a target's presence or motion quite reliably.

The subthreshold operation of the TMOS transistors creates a low drain-source current (compared to other operation regions) that conserves overall power and improves sensitivity. Typical PIR sensors require a Fresnel lens to focus sufficient IR radiation and create a measurable response. The STHS34PF80, however, can operate without a lens, demonstrating its sensitivity.

In addition to the IR detection hardware, digital readout circuitry is built-in using ST’s SOI CMOS technology, making for an extremely small overall package size. This dense integration, combined with the lens-free operation, makes the sensor a valuable tool to shrink sensors further.

ST has identified several applications that may benefit from the new sensor, such as security alarms, occupancy monitoring, and intelligent lighting and appliances.

Without a lens, the sensor's 80-degree field-of-view (FOV) achieves a 4 m range. An optical band-pass filter at the input ensures that ambient light conditions will not impact detection ability.

If designers need an even greater range or FOV, they can integrate custom or commercial Fresnel lenses to boost performance. ST has launched production on the sensor alongside a compatible evaluation board designers can use to test the sensor’s performance in application-specific scenarios.