Image sensor having an array with integrated control circuitry which includes constantly-illuminated photodiodes

An image sensor having an imaging area comprising an array of image sensing pixels, each of which is responsive in operation of the sensor to light input dependent on the image being sensed, and peripheral control circuitry comprising a plurality of elemental devices arranged outside the imaging area and which are connected to the array and are operable to drive the image sensing pixels of the array, the image sensing pixels of the array and the elemental devices of the peripheral control circuitry being provided on a common substrate and being defined by layers provided on the substrate, each image sensing pixel and each elemental device of the control circuitry comprising a diode stack comprising a photodiode and a switching diode connected in series with opposite polarity and disposed one above the other with respect to the substrate, the image sensor further comprising illumination means arranged to illuminate constantly the photodiodes of the elemental devices of the control circuitry during operation of the sensor, the illumination being such that said photodiodes of the elemental devices function as constant current sources connected in series with said switching diodes of the elemental devices during operation of the sensor.