Fraunhofer develops high-speed CMOS sensor

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Scientists at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS) have developed an optoelectronic component that speeds up the readout of CMOS image sensors. Fraunhofer’s lateral drift field photodetector (LDPD) accelerates the sensor readout by a factor of up to a hundred compared to conventional pinned photodiodes (PPD).

PPDs are used to convert light signals into electrical pulses in CMOS sensors. However, the readout from PPDs is often not fast enough in sensors with larger pixels, in excess of 10µm, which are used for low-light applications such as fluorescence.

With a PPD, electrons simply diffuse to the exit; a comparatively slow process but which is sufficient for many applications. The LDPD, by comparison, has an internal electric field integrated into the photoactive region to accelerate this process.

To produce the component, the researchers improved upon the currently available CMOS chip manufacturing process based on the 0.35µm standard: ‘The additional LDPD component must not be allowed to impair the properties of the other components [in the sensor],’ said Werner Brockherde, head of department at the Fraunhofer IMS.

A prototype of the new high-speed CMOS image sensor is available. ‘We expect to get approval for series production next year,’ added Brockherde.

The high-speed CMOS sensors are ideal candidates for applications that require large pixels and a high readout speed, such as astronomy, spectroscopy or X-ray photography. The sensors are also ideally suited for use as 3D sensors based on the time-of-flight process, whereby light pulses reflecting from an object are used to create a 3D image. This technology is a compelling proposition for applications such as crash protection, as the sensors can precisely record their environment in three dimensions.

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