ARRI publishes an explanation of image sensor technology used in its cameras:

"Although the science behind the breakthrough performance of ALEXA's custom designed CMOS sensor is complex, the use of large photosites and a Dual Gain Architecture are its two main principles.

By employing unusually large photosites (in today's world of tiny cell phone sensors), ALEXA's sensor exhibits high dynamic range, high sensitivity and low crosstalk. The larger a photosite is, the more light it can capture and the lower the noise.

The Dual Gain Architecture simultaneously provides two separate read-out paths from each pixel with different amplification. The first path contains the regular, highly amplified signal. The second path contains a signal with lower amplification, to capture the information that is clipped in the first path. Both paths feed into the camera's A/D converters, delivering a 14 bit image for each path. These images are then combined into a single 16 bit high dynamic range image."