1. Field of the Invention
The present invention relates to a solid state image pickup device and its driving method, and more particularly to a solid state image pickup device having a horizontal charge transfer element capable of parallel receiving charges from a plurality of vertical charge transfer elements and serially outputting the charges, and to a method of driving such a solid state image pickup device.
2. Description of the Related Art
After establishment of mass production techniques of CCD's (charge coupled devices), image pickup apparatuses such as electronic still cameras and digital video cameras are rapidly prevailing, which apparatuses utilize CCD-type solid state image pickup devices as area image sensors.
In a CCD-type solid image pickup device, a number of photoelectric conversion elements are formed in a matrix shape in columns and rows in a photosensitive area defined in one surface of a semiconductor substrate. A photodiode is generally used as the photoelectric conversion element. The total number of photoelectric conversion elements in one solid state image pickup device is, for example, several hundred thousands to several millions.
A vertical charge transfer element (VCCD) is disposed near each photosensitive conversion element column. Charges accumulated in each photoelectric conversion element are read into the associated vertical charge transfer element which transfers the charges to a horizontal charge transfer element (HCCD) formed outside the photosensitive area. In this case, charges are read from each photoelectric conversion element constituting each photoelectric conversion element row into a corresponding vertical charge transfer element. The vertical charge transfer elements transfer the charges read from one photoelectric conversion element row to the horizontal charge transfer element at the same phase.
The horizontal charge transfer element receives in parallel charges from the vertical charge transfer elements, and transfers the charges serially to an output unit.
The output unit converts charges received from the horizontal charge transfer element into image signals (voltage signals). These image signals are supplied to a video signal proceeding circuit formed, for example, in a semiconductor substrate different from that of the solid state image pickup device.
Recent image pickup apparatuses utilizing CCD type solid image pickup devices are provided with various functions such as an automatic exposure function (hereinafter abbreviated to “AE”), an automatic focus function (hereinafter abbreviated to “AF”) and a digital zoom function, in order to make the apparatuses easy to use.
When the function such as AE, AF and digital zoom is used, image signals which correspond to charges read from some photoelectric conversion elements disposed in a part region, e.g. in a central area region of the photosensitive area are used for performing the function.
However, in this case, charges are read from all photoelectric conversion elements constituting one frame or field. All charges read into the vertical charge transfer elements are transferred down to the output unit. Charges unnecessary for performing the function such as AE, AF and digital zoom are drained at the output unit.
As well known, the horizontal charge transfer element is driven at high speed. A consumption power of the element is therefore large. For example, a maximum percentage of the consumption power of the horizontal charge transfer element to the consumption power of the image pickup apparatus is about 30% when the function such as AE, AF and digital zoom is performed.
Furthermore, it takes a relatively long time to complete an operation of AE or AF for an electronic still camera having, for example, one million to several million pixels. A release time lag from a shutter release to an actual image pickup becomes relatively long in some cases. An actual image pickup may lose a decisive photographing chance.
If unnecessary charges can be drained at high speed without driving the horizontal charge transfer element, the consumption power of the apparatus during performing the function of AE, AF and digital zoom can be reduced. The release time lap can also be shortened.
JP-A-5-183820 discloses a CCD type solid state image pickup device capable of draining unnecessary charges from a horizontal charge transfer element (HCCD) at high speed. This solid state image pickup device has a signal-discharge drain disposed in stripe-shape along the horizontal charge transfer element, and a gate is formed between the signal-discharge drain and horizontal charge transfer element. The horizontal charge transfer element (charge transfer channel), gate and signal-discharge drain constitute the structure similar to a MOS transistor.
When a predetermined control voltage is applied to a gate electrode constituting the gate, charges in the horizontal charge transfer element are drained to the signal-discharge drain at a time. Unnecessary charges are drained from the horizontal charge transfer element to the signal-discharge drain in the unit of a photoelectric conversion element row.
In the solid state image pickup element described in the above-cited unexamined patent gazette, if the gate electrode and each horizontal transfer electrode constituting the horizontal charge transfer element are to be made of polysilicon, it becomes necessary to use first to third three polysilicon layers.
The manufacture yield of solid state image pickup devices is therefore likely to lower and the manufacture time is likely to prolong. Productivity is likely to lower.