Solid State Imaging Apparatus

This invention relates to a solid state imaging apparatus utilizing a photodiode array, a MOS transistor array, a charge transfer device such as a CCD (Charge Coupled Device) and a BBD (Bucket Brigade Device) or a combination thereof, and more particularly to such apparatus which stores charges in proportion to the intensity of incident light and produces a sequential video signal.

As is well known, it is preferable to use a solid state area imaging device for a television camera in place of a pick up tube for converting an optical image of an object to an electrical video signal, in the case where a compact size, long life, high stability and a lower power consumption are required. However, there remain problems in utilizing such device as a practical imaging apparatus.

One of the typical problems is the blooming phenomenon. Blooming is especially objectionable in an imaging devices with high sensitivity, since even a usual light source causes overload at high light portions in a low-light-level environment. Blooming is caused by excess charges generated in a photosensor such as a photodiode. To make such a solid state imaging apparatus capable of operating without blooming in the presence of excessive high light portions, several methods of suppressing blooming have been proposed.

One of the practical methods employs an OFD (Over Flow Drain) within the image sensor to absorb excess charges, is shown in Bell Syst. Tech. Journal, Vol. 51, PP. 1923-1926, 1972. But this method has several problems which are as follows:

The first problem is the limitation of the blooming suppression due to the limitation of the OFD's space in the imaging area. The reason thereof is that quantity of excess charge absorption is mainly proportional to the area of the OFD. The second problem is the decrement of the sensitivity of the imaging apparatus, because of the decrement of the effective area of photosensor due to the existence of the OFD.

Another practical method for suppressing the blooming phonomenon is providing such an n-p-n structure in the imaging area so as to absorb excess charges into the n type substrate of the n-p-n structure, such as shown in ISSCC (International Solid-State Circuits Conference) Dig. of Tech. Papers, pp. 192-193, Feb. 1972. This technique also has problems in that the effect of the blooming suppression is not so high because of the spreading of generated excess charges through the n type substrate of the n-p-n structure into adjacent regions.

An object of the present invention, therefore, is to provide a solid state imaging apparatus in which the blooming is satisfactorily suppressed without decreasing the sensitivity.

Another object of the present invention is to provide a solid state imaging apparatus, in which it is possible to read video signal from its imaging area with a low power consumption and with a large dynamic range.

According to the present invention, there is provided an electrode which is capacitively coupled to a node at which a photosensitive element and a switching element are connected in series, and where a bias voltage including a blooming suppression pulse is supplied to the electrode. The blooming suppression pulse is in synchronism with a read pulse which is supplied to the switching element to read accumulated charge signals from the photosensitive element and functions so as to expand the voltage supplied to the photosensitive element.

According to an embodiment of the present invention, there is provided a plurality of pairs of combinations comprising a first diode and a switching transistor formed into a matrix, a plurality of second diodes respectively coupled to the first diodes, one of the first and second diodes being a photodiode, wherein a bias voltage including a blooming suppression pulse is supplied to each of the first and second diodes. Each connection point of the first and second diodes is clamped at the bias voltage when an accumulated charge signal is about to exceed a predetermined amount due to a large intensity light.

FIG. 1 is a sectional view of a solid state imaging apparatus according to the present invention;

FIG. 2 is a schematic diagram illustrating an equivalent circuit of the apparatus shown in FIG. 1;

FIGS. 3a, 3b, 3c and 3d are diagrams showing waveforms for describing the operation of the embodiment of FIG. 1.

FIG. 4 is a graph illustrating an effect of suppressing blooming according to the embodiment of FIG. 1;

FIGS. 5a and 5b are respectively a sectional view showing an another embodiment of the invention and an equivalent circuit diagram thereof;

FIGS. 6a and 6b are respectively a sectional view showing a further embodiment of the invention and an equivalent circuit diagram thereof; and

FIG. 7 is a diagram showing a waveform for describing the operation of a further embodiment of the invention.

FIG. 1 and FIG. 2 show one cell of the imaging area of one embodiment of a solid state imaging apparatus according to this invention. The cell includes a p-n heterojunction photoconductor D.sub.N, a diode D.sub.S, a read-gate FET T.sub.R and a BBD gate FET T.sub.B. The diode D.sub.S is constructed by forming a n.sup.+ type diffused region 1 in a P type substrate 2, which diffused region 1 also constitutes a drain region of the read-gate FET T.sub.R. A P.sup.+ region 3 operates as a potential barrier for preventing an injection of electrons from the n.sup.+ region 1 to a n.sup.+ region 4 which constitutes a source region of the read-gate FET T.sub.R. The p.sup.+ region 3 is provided only in the case where vertical scanning shift registers for transfering signal charges from imaging cells to an output horizontal register consist of CCDs. In case the vertical scanning shift registers are constituted by BBDs, the n.sup.+ type region 4 is constructed in the substrate as shown in the said figure. Since the transfer function of CCDs and the BBDs is substantially same, the operation of the embodiment of this invention is described as follows in the case of utilizing BBDs.

A first gate electrode 5 has a overlapping area with the n.sup.+ type region 4. A gate oxide layer 6 insulates the first gate electrode 5 from the substrate 2. A insulator 7 is formed on the first gate electrode 5 and insulates a first matal electrode 8 from the first gate electrode 5 and the substrate 2. The electrode 8 is in contact with n.sup.+ type region 1, and is utilized as the cathode electrodes of both diode D.sub.S and the p-n heterojunction photoconductor D.sub.N constructed therewith, for example, ZnSe 9 and Zn.sub.1-x Cd.sub.x Te 10. A transparent electrode 11 is deposited on the photoconductor D.sub.N. A symbol T represents the direction of the signal charge transfer through the vertical scanning shift register.

FIG. 2 is an equivalent circuit of the cell shown in FIG. 1. A numeral 12 represents a terminal for supplying a read pulse .0..sub.R and transfer pulse .0..sub.T such as shown in FIG. 3a to the first gate electrode 5 of the transistors T.sub.R and T.sub.B. A symbol C.sub.B represents a capacitance of a bucket for counstructing BBD and Cj represents a junction capacitance. The cathode electrode of the photoconductor D.sub.N and the diode D.sub.S are connected to each other at a node M. Symbols C.sub.N and C.sub.S represent the equivalent capacitance of the photoconductor D.sub.N and the diode D.sub.S, respectively. A numeral 13 is an input terminal which corresponds to the transparent electrode 11 in FIG. 1, and to which a bias voltage .0..sub.B such as shown in FIG. 3b is supplied.

In this embodiment, the bias voltage .0..sub.B includes a blooming suppression pulse .0..sub.BS which is in synchronism with the read pulse .0..sub.R supplied to the input terminal 12. The operation of the above mentioned embodiment is described hereinafter.

FIG. 3a is a diagram showing the waveform of a clock pulse .0. composed of the read pulse .0..sub.R for reading signal charges from picture elements, i.e. photoconductors D.sub.N, and the transfer pulse .0..sub.T for transfering the signal charges. The voltages of the read pulse .0..sub.R and the transfer pulse .0..sub.T are V.sub.CH and V.sub..0., respectively. FIG. 3b is a diagram showing the waveform of the bias voltage .0..sub.B which includes the blooming suppression pulse .0..sub.BS. The bias level of the bias voltage .0..sub.B is V.sub.C with a positive polarity and the voltage of the blooming suppression pulse .0..sub.BS is V.sub.L with a negative polarity.

With the application of the clock pulse .0. and the bias voltage .0..sub.B to the terminals 12 and 13, respectively, the potential of the node M in FIG. 2 changes such as shown by wave form of FIG. 3c. The operation of the apparatus of this embodiment is described in detail at respective periods T.sub.1 -T.sub.4 of FIG. 3c as follows.

(i) Period T.sub.1

When the voltage of the terminal 13 changes from V.sub.C to V.sub.L in response to the blooming suppression pulse .0..sub.BS at the beginning of the period T.sub.1, the potential of the node M changes to V.sub.1 such as determined by the formula: ##EQU1## where V represents the voltage of the node M at the end of the preceeding period T.sub.0. Therefore, v.sub.1 is expressed by: ##EQU2##

Assuming that the node M has been clamped at the voltage level V.sub.C due to large intensity light in the preceeding period T.sub.0, such as shown in FIG. 3c, V is equal to V.sub.C. Then v.sub.1 becomes V.sub.1 and is expressed by: ##EQU3##

(ii) Period T.sub.2

At the beginning of the period T.sub.2, the read pulse .0..sub.R is supplied to the terminal 12. The voltage V.sub.CH of the read pulse .0..sub.R is divided by capacitances C.sub.B and Cj, and the divided voltage is applied to the node A of the transfer stage T.sub.B. At this instant, the potential of the node A changes from V.sub.R to V.sub.h, and is expressed as follows: ##EQU4## where V.sub.R is the preceeding potential of the node A, which is set up by the transfer pulse .0..sub.T and expressed as V.sub.R =V.sub..0. -V.sub.TB, V.sub.TB : the threshold voltage of the FET T.sub.B. With the read pulse .0..sub.R, the gate FET T.sub.R is on, and charges which are in proportion to the intensity of the light are transfered into the node M from the node A; in other words, signal charges are read cut through the FET T.sub.R into the capacitor C.sub.B. As a result, the potential of node M rises to the cut-off voltage of the FET T.sub.R. The cutoff voltage V.sub.2 is expressed as: ##EQU5## where V.sub.TC is the threshold voltage of the read gate FET T.sub.R, and K.sub.SB is an increased component of the threshold voltage due to a substrate bias effect.

From the expressions (3) and (4), V.sub.2 is given as follows: ##EQU6## This voltage V.sub.2 is, therefore, constant.

In this state, charges Q.sub.M stored in the capacitors C.sub.N and C.sub.S is expressed by: ##EQU7##

(iii) Period T.sub.3

At the beginning of the period T.sub.3, the potential of the terminal 13 changes from V.sub.L to V.sub.C, and so the potential of the node M rises from V.sub.2 to V.sub.3 because of the capacitive coupling by the junction capacitance C.sub.N. The voltage V.sub.3 is defined by the following equation: ##EQU8## Therefore, from the equations (6) and (7), the potential V.sub.3 is given by: ##EQU9## If, in a practical embodiment, the capacitances and the voltages are selected, for example, to have the following values: ##EQU10## V.sub.1, V.sub.2 and V.sub.3 are given as follows: ##EQU11##

(iv) Period T.sub.4

In this period, the signal charges read out into the capacitor C.sub.B during the read pulse period and temporarily stored therein are transfered out through the vertical scanning shift register in the direction T in FIG. 1 under the control of the transfer clock pulse .0..sub.t. On the other hand, the potential of the node M deminishes in proportion to the intensity of the incident light upon the photoconductor D.sub.N. Even if the intensity of the incident light is excessively large, such as in this case, the potential of the node M does not decrease to below V.sub.C, because the node M is clamped at the bias level V.sub.C by the heterojunction photoconductor D.sub.N, when the potential thereof is about to exceed the potential V.sub.C. As a result the excess charge is removed through the electrode 11 and the terminal 13, so that the excess signal charges are not diffused into the substrate, and therefore the blooming phenomenon is significantly suppressed.

The voltage V.sub.C is selected to be at a certain level which is higher than the voltage of the transfer pulse .0..sub.T so that the signal charges are not read out by the transfer pulse .0..sub.T through the read gate FET T.sub.R. If the voltage V.sub.C is high, the effect of blooming supression becomes large but the dynamic range of the imaging apparatus becomes small. Therefore, the amplitude of the voltage V.sub.C should be selected preferably in consideration of such a tradeoff.

The dynamic range of the imaging apparatus is decided by the voltage difference .DELTA.V, i.e. the saturation signal voltage, such as given by:

The signal charges representing the total amount of the incident light during one charge accumulation period, that is, almost one field or frame period of the conventional solid state imaging apparatus, are read out by the read pulse .0..sub.R through the read gate FET T.sub.R into the vertical transfer channel.

According to this embodiment, in the period T.sub.3, the voltage [C.sub.N /(C.sub.N +C.sub.S)].multidot.(V.sub.C -V.sub.L) is added to the potential .sup.V 2 of the node M. Therefore, the dynamic range is enlarged with such an added increment in the potential of the node M. Thus, the range operable without blooming is enlarged. In the case when the values of the capacitances and the voltages are given such as mentioned above the saturation signal voltage .DELTA.V is given from the formula (9) as ##EQU12##

On the other hand, when the blooming suppression pulse .0..sub.BS is not supplied to the electrode 11 as illustrated in FIG. 3d, the saturation signal voltage .DELTA.V' is given as ##EQU13##

It is obvious from the expressions (10) and (11), that according to this embodiment the dynamic range is, enlarged by V.sub.3 -V.sub.2, that is 7 volts, with respect to the latter apparatus. The experimental data representing blooming suppression characteristics of the embodiment described above are shown in FIG. 4. In the same figure, the abscissa axis represents the intensity of the incident light which is expressed as a multiple of a saturation exposure, and the ordinate axis represents the ratio of the blooming signal to the saturation signal in percent. In this case, the amount of the blooming signal is defined as the amount of quasi-output signal which is generated in the direction of the vertical transfer in the imaging apparatus when the signal charges are read out from the black background surrounding a white spot which is located at the center of the imaging area. The amount of the saturation signal is defined as the amount of the output signal read out from the white spot. In the same figure, the dotted curve represents a characteristic of the conventional case where the blooming suppression pulse is not supplied, and the solid curved line represents a characteristic of the embodiment of this invention. As is evident from the FIG. 4, if the limit of the allowable blooming signal rates is 5%, the apparatus of this embodiment has the effect of blooming suppression against light having an intensity more than a hundred times as large as that of the saturation exposure. On the other hand, the conventional apparatus has the effect against light having an intensity which is less than twice as large as that of the saturation exposure.

FIGS. 5a and 5b show a cell structure of an another embodiment of the invention and the equivalent circuit thereof, respectively. The difference of this embodiment with respect to the previous embodiment of FIGS. 1 and 2 exists in a construction of a photosensitive element D.sub.N ' and a diode D.sub.S '. In this embodiment, the diode D.sub.S ' and the photodiode D.sub.N ' are utilized in place of the heterojunction photoconductor D.sub.N and the diode D.sub.S shown in FIG. 2, respectively. In FIG. 5a, 21 is an n.sup.+ diffused region formed in the substrate 2 of p type semiconductor. The photodiode D.sub.N is constructed by the n.sup.+ region 21 and the substrate 2. A p.sup.+ diffused region 22 is provided in the n.sup.+ region 21, so as to construct the diode D.sub.S ' in combination with the n.sup.+ diffused region 21. The p.sup.+ region 22 is connected electrically to an electrode 23. In the equivalent circuit of FIG. 5b, C.sub.S ' is the junction capacitance of the diode D.sub.S ' and C.sub.N is that of the diode D.sub.N '. In FIGS. 5a and 5b, the constructions of the portions represented by the same symbols as those in FIGS. 1 and 2 are the same as those in FIGS. 1 and 2. In the solid state imaging apparatus of this embodiment, the bias voltage .0..sub.B, including blooming suppression pulse .0..sub.BS shown in FIG. 3b, is supplied to the terminal of the electrode 23. With respect to blooming suppression, the function of this embodiment is similar to that of the preceeding embodiment of FIGS. 1 and 2. It is well understood from FIGS. 3a, 3b, 3c and 3d, and the description thereof. Although the heterojunction photoconductor D.sub.N or the diode D.sub.S ' is utilized as the element through which the bias voltage is supplied in the above mentioned embodiments, it is not essential for the invention. It is also possiblle to utilize a capacitor in place of such element.

FIGS. 6a and 6b illustrate a further embodiment employing such construction. FIG. 6a shows a cell structure of this embodiment, and FIG. 6b the equivalent circuit therof. In these figures, the elements with the same symbols as in FIGS. 1a and 1b have the same construction as those in FIGS. 1a and 1b. A numeral 31 in FIG. 6a is an n.sup.+ diffused region such as the region 1 in FIG. 1. An electrode 32 is formed on the insulating layer 7 so that it has an overlapping portion in part with the n.sup.+ diffused region 31. The n.sup.+ region 31, the oxide layer 6, the insulating layer 7, and the electrode 32 constitute a capacitor C. A waveform of a blooming suppression pulse which is supplied to the electrode 32 is almost the same as that of the bias voltage .0..sub.BS except that it does not include a DC level. Therefore, the operation of this embodiment is almost the same as those of the preceeding embodiments except that the node M is not clamped at a certain voltage level in the accumulation period T.sub.4. Only the dynamic range of the imaging apparatus is enlarged in this case. That is to say, the potential of the node M rised by [C/(C+C.sub.S)].multidot.(V.sub.C -V.sub.L) in this apparatus when the blooming suppression pulse is supplied to the electrode 32. The increment of the node potential corresponds to that in the embodiment of FIGS. 1 and 2, that is [C.sub.N /(C.sub.N +C.sub.S)].multidot.(V.sub.C -V.sub.L). Therefore, the allowable range of the incident light in which the apparatus is operable without blooming is enlarged.

FIG. 7 represents a clock pulse .0..sub.0 of a further embodiment in which lower power consumption is obtained. The construction thereof may be the same as that of FIG. 1 and a bias voltage is also the same as FIG. 3b. The salient feature of this embodiment is the shape of the clock pulse .0..sub.0 in which the amplitude of the read pulse .0..sub.R0 is smaller than that of the embodiment of FIG. 1 so as to be almost equal to the amplitude of the transfer pulse .0..sub.T0. This feature makes it possible to realize the imaging apparatus with a lower power consumption. In spite of the fact that the read pulse .0..sub.R0 has such a small amplitude, a sufficient dynamic range of the apparatus is preferably obtained by selecting the amplitude of the blooming suppression pulse .0..sub.BS.

Although this invention is described in reference to the apparatus employing the inter-line transfer method the use of such a method is not essential for the invention. It is possible to apply this invention to apparatus employing other methods such as the frame transfer method, and also to a linear imaging apparatus.

It is obvious that this invention is also applicable to the imaging apparatus which operates in an interlace mode, though the above-disclosure refers only to noninterlace imaging apparatus for the simplicity.