CMOS image sensor and method for driving a CMOS image sensor with increased dynamic range

摘要

The imager comprises a set of pixels each comprising a photodiode (PD) as a photodetecting element generating charge carriers in proportion to illumination, a capacitor (C1) as a storage element which can be coupled or decoupled from the photodiode in order to store at a memory node (B) a signal measuring the charge carriers generated by the photodiode, and five MOS transistors (M1 to M5). At least one transistor (M1,M3) is connected in series with the photodiode, directly or via a coupling transistor (M2), and is configured so to function at least partially with weak inversion so that in exposition phase the pixel has a logarithmic response for illumination levels higher than a predetermined level. The transistor (M2) couples the photodiode (PD) in reverse bias to the capacitor (C1), that connects the nodes (A) and (B). The transistor (M3) initializes the memory node (B) to a determined voltage. The drains of transistors (M1,M3) are connected to different supply voltages, that is the voltages (VBIAS) and (VDD), respectively, where the voltage (VBIAS) is higher than the voltage (VDD). The transistor (M4) is a source-follower transistor, and the transistor (M5) is a row-selection transistor which ensures the voltage transfer to an output bus common to all pixels in a column. The imager also comprises means for switching the gate voltage of at least one transistor (M1,M3) between the voltages (V1) and (V2), where the voltage (V1) is higher than the supply voltage (VDD) plus a threshold voltage (VTH), and the voltage (V2) is lower than the supply voltage (VDD) plus a threshold voltage (VSS) plus the threshold voltage (VTH). The photodiode (PD) is formed in an n-type well, and the transistors (M1 to M5) are n-MOS transistors. The storage element (C1) is formed as a capacitor protected from light by a metallic layer. A method is claimed for operating the imager and comprises the step of exposition consisting in connecting the transistor (M1) as a resistance, the step of storage consisting in decoupling the photodiode from the capacitor, and the step of reading the stored signal measure; or the steps of initialization, exposition, storage and reading. At the reading step a voltage is applied to the gate of the transistor (M1) so that the charge carriers generated by the photodiode (PD) are drained via that transistor.