Conventional CMOS image sensors, which are the preferred choice for digital photography in both professional and consumer devices thanks to their low cost and low power consumption, are not suitable for low-light applications such as X-ray or astronomical photography because the large pixel cells necessary to compensate for low light levels do not allow high readout speeds. A new, patented optoelectronic component developed by researchers at the Fraunhofer Institute for Microelectronics eliminates this problem.
Conventional CMOS image sensors use pinned photodiodes (PPDs) to convert the light into electrical signals. However, with pixels above a certain size they cannot support the readout speeds typically needed in low-light applications. To solve this problem, the Fraunhofer researchers developed a new optoelectronic device called a lateral drift field photodetector (LDPD), in which charge carriers are driven to the collection electrode by an electric field at speeds up to 100 times the diffusion rate of charge carriers in PPDs.
To produce the new device, the Fraunhofer researchers improved the current CMOS fabrication process using 0.35 µm technology. A prototype of the new high-speed CMOS image sensors is already available, and approval for regular production is expected to come this year.
The high-speed CMOS sensors are ideal for applications that require large pixels and high readout speed, such as astronomy, spectroscopy and advanced X-ray photography. They are also good candidates for use in 3D sensors based on travel time (not time travel), in which a light source emits short pulses that are reflected by the sensed objects. The travel time of the reflected light is detected by the sensor and processed to create a full 3D image.
Image: Fraunhofer Institute for Microelectronics