A related art image sensor is provided with a pixel structure including a photodiode and a transistor. A pixel having such a structure causes electricity to flow by exciting electrons of the photodiode with incident light, and the amount of electricity varies according to light intensity. Thus, a shutter switch is turned on after a predetermined period of time, and a signal is then transmitted to a sense node, thus obtaining a video signal.
However, a signal level is determined according to the capacity of a condenser of the sense node, but a related art sense node has fixed condenser capacity. Therefore, an image sensor has a reduced dynamic range and cannot perform auto-exposure. Thus, an image signal processing block performs the auto-exposure by determining to adjust a sensor exposure time or by applying a gain of a video signal.
To solve the above limitation, there has been proposed a technique in which a varactor using a MOS capacitor is employed so that the dynamic range of the image sensor is expanded by controlling the condenser capacitor of the varactor.
However, this related art has the limitation that the capacitance is formed only when external biasing is applied to the gate of the varactor, which makes it difficult to perform self-biasing control.
Furthermore, a row gate driver is required to control the gate of the varactor according to the related art, and metal lines for connecting them are additionally required, thus lowering an effective fill factor.