Imaging devices comprising photovoltaic detector elements

Abstract

In an imaging device, photocurrent generated by photovoltaic detector elements (1), e.g. cadmium mercury telluride photodiodes, is integrated in resettable capacitors (2), and an output signal (S) is derived by reading the potential of the capacitor (2) at the end of its integration period, e.g. using a source-follower MOST (3). In accordance with the invention, blooming-protection means (11,48,12) is coupled to each capacitor (2) to inhibit forward-biasing of the detector elements (1) and inversion of the capacitor potential when the capacitor (2) becomes fully discharged by excessive photocurrent. The blooming-protection means comprises a further gate (11) which has substantially the same threshold voltage as the injection gate (10) via which the photocurrent is injected into the capacitor (2). The further gate (11) which is connected at substantially the same control potential as the injection gate (10), is most conveniently formed together with the injection gate (10) as alternate integral parts of a common gate stripe (37) extending at one side of the capacitors (2). When a capacitor (2) becomes fully discharged, the further gate (11) couples it to a source (12) which supplies current to the capacitor (2) to stabilize the potential of both the capacitor (2) and its detector element (1) and so to protect against signal blooming.