Fraunhofer researchers create faster optoelectronic sensor

Researchers in Germany have developed a way around the limits of current low-cost optical sensors for very sensitive applications.

Work at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS) has led to an optoelectronic component that can provide higher processing speeds when producing images for low-light applications such as X-ray detection and astronomy.

Modern digital camera equipment often makes use of complementary metal-oxide-semiconductor (CMOS) image sensors, which are cheaper to produce than other technology and use less power.

But while other electronic components keep getting smaller, optical CMOS chips are limited in areas where minimal light is available because they need sufficiently large pixels to capture enough information.

Components called pinned photodiodes (PPD) are typically used to convert the light signals into electrical pulses and to process the image, but are too slow to cope with certain-sized pixels and low-light applications.

The researchers have instead developed a new component called a lateral-drift field photodetector (LDPD), which captures electrons in a different way to produce a much faster readout.

‘In this component, the charge carriers generated by the incident light move at high speed to the readout node,’ said Werner Brockherde, head of department at the IMS.

Electrons in the PPD diffuse to readout point by a comparatively slow process that is sufficient for many applications, he said.

‘But by integrating an internal electric field into the photoactive region of the component, we have managed to accelerate this process by a factor of up to a hundred.’

The researchers have produced a prototype of a new high-speed CMOS image sensor using the technology and are now developing the manufacturing process further.

As well as astronomy, spectroscopy and X-ray photography, applications could include 3D sensors that can precisely record their environment, based on the time-of-flight process for areas such as crash protection.