"...when the pixels exceed a certain size, the PPDs have a speed problem", explains Werner Brockherde, head of department at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS. Low-light applications tend to call for high image rates. "But the readout speed using PPD is too low", says Brockherde.
The Fraunhofer researchers have developed a new photodiode, the lateral drift field photodetector (LDPD), that is said to be unique and has already been patented. "In this component, the charge carriers generated by the incident light move at high speed to the readout node," explains the researcher. With the PPD the electrons are said to diffuse to the exit; a comparatively slow process but which is sufficient for many applications. "But by integrating an internal electric fi eld into the photoactive region of the component, we have managed to accelerate this process by a factor of up to a hundred."
To produce the new LDPD, the Fraunhofer researchers improved upon the currently available CMOS chip manufacturing process based on the 0.35 μm standard: "The additional LDPD component must not be allowed to impair the properties of the other components," says Brockherde. The prototype of the new high-speed CMOS image sensors is already available. "We expect to get approval for series production next year," says Brockherde.
