Solid State Area Imaging Apparatus

This invention relates to a solid state area imaging apparatus which converts a two dimensional optical image into an electrical signal.

An ordinary area imaging apparatus employing a photodiode array is shown, for instance, in the WESCON Tech. papers, session 13-2 August 1967. Such an apparatus consists of photodiodes, switching elements such as MOS transistors and scanning shift registers. In such an apparatus, an electric charge, which is proportional to the intensity of the light incident upon each diode, is stored in each diode and is read out when the switching element which is connected to the diode is turned on by being addressed by both a vertical scanning shift register and a horizontal scanning shift register. A sequential video signal representing a black and white image of an object is obtained as an output of this apparatus.

The advantages of a X-Y addressed area imaging apparatus, such as that described above, are less blooming, a high sensitivity, a high quantum efficiency for light of short wave lengths (450 nm) and less dark current, such advantages being obtained by utilizing a p-n junction as a photosensor.

On the other hand, the disadvantages of a X-Y addressed area imaging apparatus include the existance of fixed pattern noise (FPN) caused by the non-uniformity of the horizontal multiplex switches, and a high random noise level which is caused by the large output capacitor and which increases in proportion to increases in the number of picture elements.

With respect to an area imaging apparatus which operates by means of charge transfer techniques, two methods, namely, the frame transfer method and the interline transfer method, have been proposed. An area imaging apparatus employing the frame transfer method is shown, for instance, in IEEE Trans. on Electron Device, ED-20, No. 3, p244, 1973. Such a frame transfer imaging apparatus, for example, consists of a photosensitive CCD array, a temporary storage CCD array and an output CCD register. A pattern of stored charges in the CCD array which represents an input optical image and which is formed by the entirety of the charges stored during one field interval is transferred into the temporary storage array during a vertical blanking period. Then, the pattern is shifted one horizontal line at a time into the output CCD register during a horizontal blanking period, and is read out by a high speed horizontal clock to produce a sequential video signal.

An imaging apparatus employing the interline transfer method, which is shown, for instance, in 1974 IEEE Intercon Tech. papers, session 2-2, consists of alternate columns of photosensitive elements, vertical transfer CCD registers and an output CCD register. During a vertical blanking period, signal charges detected which are by the photosensitive elements are transferred into the vertical transfer registers. These signal charges are then shifted one horizontal line at a time into the output CCD register during a horizontal blanking period, and transferred out by a horizontal clock.

Advantages of such charge transfer area imaging apparatus are a low FPN arising from the dark current which increases at a high temperature and less random noise due to less output capacitance.

On the other hand, disadvantages thereof are decreases in the output signal, which is caused by transfer losses, a small maximum transferable charge value and poor blooming and smearing characteristics.

An object of the invention is to provide a solid state area imaging apparatus having the advantages of both an X-Y addressed area imaging apparatus and a charge transfer area imaging apparatus.

FIG. 1 is a schematic diagram showing an embodiment of the invention;

FIG. 2 is a schematic diagram showing an another embodiment of the invention;

FIG. 3 is a schematic diagram showing a further embodiment of the invention;

FIGS. 4A, 4B, 4C, 4D and 4E are schematic potential diagrams for describing the operation of the embodiment of FIG. 3.

FIG. 1 shows the construction of one embodiment of a solid state area imaging apparatus according to this invention.

A sensing area A in FIG. 1 has the same construction as that of the conventional X-Y addressed area imaging apparatus and a transfer area B has the same construction as the output CCD register of a CCD area imaging apparatus.

When an addressing pulse from a vertical scanning shift register 101 is applied to a row line 104 during a horizontal blanking period, the MOS FETs 103 gate electrodes which are connected to the row line 104 turn on, and then the signal charges which are stored in the p-n junction of photodiodes 102 and which are in proportion to the intensity of the incident light from an object are transferred into column lines 105. The abovementioned operation of the imaging area A is the same as that of conventional X-Y addressed area imaging apparatus.

The charge signals which are transfered into the column lines 105 one line at a time, flow through respective resistors 107 to a d-c voltage source 106A. The signal voltages which appear at the resistors 107 are applied to input gates 108. Under the control of the voltages of the input gates 108, the charges from an input diode 109, which is inversely biased by a d-c voltage source 106B, are injected into predetermined positions of corresponding bits of an output CCD register 113. The direction of the charge injection is shown by the arrows 115 in FIG. 1. Each bit is formed by a set of MIS (metal-insulator-silicon) elements 119, 120 and 121. In FIG. 1, the charges are injected into MIS structures 120. As the result thereof, one line of signal charges whose values are proportional to those of the stored signal charges are injected into the corresponding bits of the output CCD register 113. The d-c voltage sources 106A, 106B, registers 107, input diodes 109 and input gates 109 constitute a parallel-serial conversion (p-s conversion) area.

The injected signal charges in the CCD register 113 are transfered in the direction of the arrow 116 by 3.phi. horizontal transfer clocks 110, 111 and 112 during the horizontal scanning period, and then read out from the output terminal 117 through an output gate 114. After all the signal charges are read out from the output register 113, the next row line is accessed by the vertical scanning shift register 101 and the signal charges stored in the next line of the photodiodes 102 are transfered into the corresponding column line 105 during the horizontal blanking period. A next line of signal charges are injected from the input diode 109 into the same position as that of the previous horizontal scan, and then read out from the output CCD register 113 during the horizontal scanning period to produce a sequential video signal.

According to this embodiment, low blooming, a high sensitivity, a high quantum efficiency for light of short wave lengths (450 nm) and low dark current characteristics due to the X-Y addressed photodiode imaging array 102, and a low FPN and low random noise characteristics due to the CCD output register 113 are obtained.

FIG. 2 shows an another embodiment of the present invention. The difference of this embodiment with respect to previous embodiment is the construction of the p-s conversion area which consists of a floating gate amplifier.

A line of signal charges, which are transferred into the column lines 105, flow through a polysilicon floating gates 206 into a d-c voltage source 201B. In response to the flow of the signal charges, MOS FETs 203, having the floating gates 206, conduct and then the charges are injected into diffusion regions 204 from a d-c voltage source 201A. The value of the injected charges are in proportion to the signal charges. The charges injected into the diffusion regions 204 are transferred into the corresponding positions of the output CCD register 113 through switching gates 205 under the control of a gate pulse applied to a terminal 207 during the horizontal blanking period. MOS FETs 202 are switching transistors for resetting the floating gates 206 so that the voltage of the gates is equal to that of a d-c voltage source 210C.

The transfered signal charges in the corresponding positions of the CCD 113 are read out therefrom during the horizontal scanning period to produce the sequential video signal in the same manner as the embodiment of FIG. 1.

FIG. 3 shows a further embodiment of the present invention utilizing a temporary storage capacitor in the p-s conversion area.

In this embodiment, before the row line 104 is addressed, a gate pulse .phi..sub.G1 is applied to the gate electrodes of MOS FETs 301 during the horizontal blanking period. Therefore, MOS FETs 301 turn on, so that temporary storage capacitors 303 are charged to a potential which is substantially equal to the voltage V.sub.B of a d-c voltage source 304, such as shown in FIG. 4A. Then, a line of signal charges are read out into the column line 105 by the application of an address pulse from the vertical scanning shift register 101. FIGS. 4A, 4B, 4C, 4D and 4E are schematic potential diagrams for describing the operation of this embodiment, wherein Qs is the read out signal charge and Q.sub.b is the bias charge stored in the temporary storage capacitor 303. Subsequently, a gate pulse .phi..sub.G2 is applied to the gate electrodes of MOS FETs 302 to turn them on. Therefore, the bias charges Q.sub.b of capacitors 303 are injected into the corresponding column line 105 so as to be combined with the read out signal charges Q.sub.s, such as shown in FIG. 4B. After this combination of charges, a pulse .phi..sub.c which has the same polarity as that of the gate pulse .phi..sub.G1 or .phi..sub.G2 is applied to other terminals of the temporary storage capacitors 303. As a result, both charges Q.sub.s and Q.sub.b are transfered back into capacitors 303 in a Bucket Brigade mode during the application of the pulse .phi..sub.c. After the MOS FETs 302 are cut off by removing the gate pulse .phi..sub.G2, the pulse .phi..sub.c is turned off. Accordingly, combined charges Q.sub.s and Q.sub.b are temporarily stored in the capacitors 303, such as shown in FIG. 4D. When a gate pulse .phi..sub.G3 is applied to switching gates 305, the combined charges Q.sub.s and Q.sub.b are read out into the corresponding positions of the output CCD register 113, such as shown in FIG. 4E. These charges are then transfered out during the horizontal scanning period from the CCD register 113 with the transfer clocks 110, 111 and 112 so as to produce the video signal.

According to this embodiment, the bias charges are combined with the signal charges, and a high transfer efficiency is obtained even if the signal charges are very large.

Although this invention has been particularly shown and described, it is contemplated that various changes and modifications may be made without departing from the scope of the invention as set forth in the following claims. The embodiments of the present invention in which an exclusive property or privilege is claimed are defined as follows.