JPS6148307B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of controlling photoelectric conversion characteristics of an imaging device using a charge transfer device.

Generally, in an imaging device, a method of changing the gamma characteristic of photoelectric conversion in order to increase the size of the dynamic lens is carried out using signal processing, optical elements, or the like.

Conventional charge transfer devices, especially charge-coupled devices (CCDs)
Regarding the method of controlling the photoelectric conversion characteristics of an imaging device using
International Solid-State Circuits
Conference Digest of Technical Papers No. 38
Pages 39 to 39 (Method for Varying Gamma in
Charge—Coupled Imagers)
A method has been proposed in which the dynamic range of a charge transfer imaging device is widened by increasing the bias voltage of a charge storage electrode that stores optical information in a CCD imaging device so that the electrode potential at the end of the charge storage period is higher than that at the beginning of the charge storage period. However, in this conventional method, the maximum amount of accumulated charge during the accumulation period is determined by the storage electrode potential, and if a charge greater than the maximum amount of accumulated charge is generated, that charge is swept out into the substrate. However, this swept-out excess charge diffuses within the substrate and is absorbed into potential wells near the saturated picture elements. This is a blooming phenomenon known to those skilled in the art and is not desirable for an imaging device. When the potential of the storage electrode is increased near the end of the storage period, the already saturated potential well accumulates charge again. Therefore, when observing an imaging screen using a conventional imaging method, an incident image with high light intensity shows a blooming phenomenon (not completely saturated) and an image of a strong light area appears, resulting in an unsightly imaging screen. Ta.

An object of the present invention is to provide a novel method for controlling photoelectric conversion characteristics of a charge transfer imaging device that eliminates such drawbacks.

According to the present invention, in a charge transfer imaging device, an overflow drain is provided in a channel stop region that separates each channel of an imaging region to absorb excess charge exceeding the maximum charge that can be accumulated in a potential well constituting each picture element. Provided is a method for controlling photoelectric conversion characteristics of a charge transfer imaging device, characterized in that the height of the potential barrier in a channel stop region at the beginning of a charge accumulation period is controlled to be lower than at the end of the charge accumulation period in the charge transfer imaging device provided. It will be done.
More specifically, the photoelectric conversion characteristics are controlled by applying a voltage to the overflow drain that is lower at the end of the accumulation period than at the beginning. The photoelectric conversion characteristics of the charge transfer imaging device are controlled by sequentially changing the potential twice or more and lowering the potential stepwise.

Next, a method for controlling the photoelectric conversion characteristics of the charge transfer imaging device of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a plan view of an image pickup device using a charge transfer device known to those skilled in the art, which is called a frame transfer method. In Figure 1, as an example, the N-channel three-phase drive system is used.
Although this is an example of a frame transfer type imaging device with ×3 picture elements, the basic operation is actually the same in any imaging device with m×n picture elements.

The basic operation of the imaging device shown in FIG. 1 will be explained.

Through an insulating layer on the main surface of a semiconductor single crystal substrate,
A series of electrode groups are arranged. The electrode group is divided into a photosensitive array, a storage array, and a readout array, and the electrodes 12a to 12c in the photosensitive array are wire lines.
Electrodes 13a-13c are connected to A ₁ , and electrodes 14a-1 are connected to line A ₂ .
4c is connected to the line _A3 and also to the electrode 15 of the storage array section.
a-15c is connected to the line _B1 , electrodes 16a-16c are connected to the line _B2 , electrodes 17a-17c are connected to the line _B3 , and electrodes 18a-18c of the readout array section are connected to the line B2.
_C1 , electrodes 19a-19c are connected to line _C2 , electrode 2
0a-20c are connected to the line C ₃ respectively, and the lines A ₁ , A ₂ , A ₃ and the lines B ₁ , B ₂ , B ₃ and the lines C ₁ , C ₂ ,
Three-phase pulses are applied to each _C3 to form a three-phase drive charge transfer device. Furthermore, a region 121 indicated by a broken line in FIG. 1 is a channel stop region that defines the active region of the device, for example, by increasing the impurity concentration of the substrate semiconductor or by increasing the thickness of the insulating layer to separate the vertical channels 21, 22, and 23. It is.

When a charge exceeding the maximum charge that can be accumulated in the potential well constituting each picture element in the channel stop region of the photosensitive array section is generated, a reverse biased PN junction (overflow drain) 24 absorbs the excess charge. is provided.

To briefly explain the operation of the imaging device shown in Fig. 1, a positive voltage is applied to, for example, the _A2 line of the photosensitive array.
When A ₁ and A ₃ are set to the ground potential, a potential well is formed on the semiconductor surface (shaded area) directly under the electrode group 13a to 13c to which A ₂ is connected. When the semiconductor surface is irradiated with an optical information pattern in this state, charges (electrons, which are minority charges in this case) corresponding to the brightness of the light are accumulated in each potential well. When the charge accumulation period ends, three-phase clock pulses are applied to the lines A ₁ , A ₂ , A ₃ , B ₁ , B ₂ , and B ₃ to sequentially transfer the signal charges accumulated in the photosensitive array to the storage array. At this time, the electrode 13
A pulse is applied until the signal charge immediately below c is transferred to immediately below 17c of the storage array. After the signal charge of the photosensitive array is transferred to the storage array, a positive pulse voltage is again applied to the line _A2 of the photosensitive array to accumulate the charge due to the optical information.

On the other hand, the signal charges transferred to the storage section are synchronized with the clock of the readout array, and a pulse equivalent to one clock (three electrode transfer) is applied during one horizontal period in which all the signal charges of the readout array are taken out to the output section. That is, the signal charges of the vertical transfer channel are transferred from the bottom of the storage array to the readout array in parallel, and the signal charges of the readout array are transferred to the lines C ₁ ,
By applying high-speed three-phase pulses to C ₂ and C ₃ , the charges in the readout array are read out to the output section 25 as time-series signals.

This is a known method in which this method is sequentially repeated to extract the charge image of the storage array as a video signal in accordance with a normal television system. During such frame transfer operation, the overflow drain 24
The PN junction of is maintained at a constant reverse bias.

FIG. 2 shows the pulse voltage waveform and overflow drain of each line in the photosensitive array area, which are most relevant to the present invention and explain the above-mentioned operation. At this time, the maximum amount of charge that can be stored under the storage electrode during the storage period of the photosensitive array section is determined by the height _V1 of the pulse voltage during the storage period.

Next, a method for controlling the photoelectric conversion characteristics of an imaging device using the above-described charge transfer device of the present invention will be described using an example.

Figures 3a and 3b are diagrams for explaining the present invention, each showing the storage electrode section (A ₂
This is a diagram schematically showing a schematic cross section in the direction perpendicular to the charge transfer direction of the electrodes 13a-13c) connected to the line and the surface potential distribution directly under the electrode.
The same symbols are used for the same parts as in the figures. For example, P
Electrode 13 is arranged on the main surface of type semiconductor substrate 27 with insulating layer 28 interposed therebetween. A channel portion 26 for vertically transferring the signal charge of a picture element is separated from an adjacent vertical channel by a channel stop region 21 having a high substrate P-type impurity concentration, for example. Within each channel stop diffusion region 21 is an overflow drain 24 of an N-type region that absorbs excess charge.
is provided.

FIG. 3b shows a charge accumulation state in FIG. 3a, that is, a positive pulse bias is applied to the electrode 13, and the electrodes connected to the adjacent electrodes (lines A ₁ and A ₃ in FIG. 1) are grounded, and the overflow drain is closed. It schematically shows the surface potential distribution 29 when reverse bias is applied. Curve 30 shows the surface potential distribution directly under the adjacent electrode for comparison.

Photoelectric conversion in the photosensitive section will be explained using FIG. 3. The maximum amount of charge that can be stored in the potential well below the storage electrode during the storage period is determined by the pulsed voltage of the storage electrode, as described above. However, in an imaging device with an overflow drain as shown in Fig. 3, each picture element has a surface potential φ _p directly under the adjacent electrode.
The impurity concentration distribution in the channel stop region is controlled so that the surface potential of each of the potential wells surrounded by the surface potential φ _c of the channel stop region immediately below the storage electrode is in the relationship φ _c > φ _p , and all excess charges are removed from the channel stop region. Blooming as described above is suppressed by absorbing it into the overflow drain across the barrier φ _c of the region. That is,
The maximum amount of charge Q _T that can be stored in each picture element of a charge transfer imaging device with an overflow drain is expressed by the following equation, where φ _s is the surface potential of the storage electrode when a pulse voltage is applied.

Q _T =C _px (φ _s −φ _c ) (1) where C _px is the oxide film capacitance.

Further, assuming that the accumulation time is constant, the charge generated and accumulated in each potential well is proportional to the amount of incident light. That is, it is also known that gamma is 1.

Also, in Figure 3, increase the overflow drain voltage (increase the reverse bias voltage).
The surface potential φ _c of the channel stop region is modulated by the fringing electric field of the overflow drain, resulting in a surface potential as shown by curve 31 in FIG. 3b, and the surface potential of the channel stop region becomes φ _c ~φ _c ′.
Changes to In other words, since φ _c changes depending on the voltage of the overflow drain, the maximum amount of accumulated charge can be controlled, as can be understood from equation (1).

Next, the photoelectric conversion characteristic control of the present invention will be explained using the pulse waveform shown in FIG. Charge storage electrode 1
2 is kept at a constant potential of V ₁ during the charge accumulation period, and the potential of the overflow drain is kept high at V _D1 at the beginning of the accumulation period. If the surface potential of the channel stop region at this time is φ _c ', the maximum amount of accumulated charge Q _T ' at the initial stage of accumulation is C _px・(φ _s - φ _c '), and any additional charges generated are absorbed by the overflow drain. . Furthermore, when the overflow drain potential is changed to V _D2 from any point t ₁ during the accumulation period t ₀ to t ₁ , the surface potential of the channel stop region decreases from φ _c ′ to φ _c (the barrier wall potential increases), and the maximum accumulation is reached. From equation (1), the amount of charge is C _px (φ _c −φ
_c ′). Therefore, the initial period of accumulation t ₀ to _{t 1}
The picture elements that have already reached the saturation charge amount during the period can again accumulate signal charges generated by optical information. The solid line in FIG. 5 shows the change in the amount of accumulated charge with respect to the amount of incident light and the photoelectric conversion characteristics at this time. For comparison, the conventional photoelectric conversion characteristics are shown with a broken line.

It can be seen from FIG. 5 that the range of incident light that can be imaged is expanded by the above-described method according to the present invention.

FIG. 6 shows a pulse waveform during the accumulation period of the overflow drain to explain another embodiment of the present invention, and when the potential of the overflow drain is changed twice at t ₁ and t ₂ during the accumulation period t ₀ to _{t 3} , a photovoltaic effect occurs. The conversion characteristic is the seventh
As shown in the figure, three bending points are formed, and the range of incident light that can be imaged can be further expanded. Furthermore, by changing the voltage of the overflow drain shown in FIGS. 4 and 6, the contrast of a screen with high brightness can also be adjusted.

As explained above, according to the present invention, in a charge transfer imaging device equipped with an overflow drain that absorbs excess charge, the voltage of the overflow drain during the accumulation period is made smaller at the end of the accumulation period than at the beginning of the accumulation period, thereby causing the blooming phenomenon. It is possible to arbitrarily control the photoelectric conversion characteristics of an imaging device without

In this embodiment, a two-dimensional frame transfer imaging device has been described, but it is obvious that the present invention can also be applied to other types of two-dimensional imaging device or one-dimensional charge transfer imaging device.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of an imaging device to which the present invention is applied, FIG. 2 shows a pulse waveform of a photosensitive section used in the embodiment shown in FIG. 1, and FIG. 3 shows charges in the photosensitive array shown in FIG. 1. 4 and 6 show the pulse waveforms of the overflow drain used in the present invention, and FIGS. 5 and 7 show the horizontal partial cross-sectional view a of the storage region and the surface potential distribution immediately below it. This shows the photoelectric conversion characteristics of the imaging device used.

Claims (1)

[Claims] 1. In an imaging device using a charge transfer element, a region of a conductivity type opposite to that of the substrate semiconductor is provided in a channel separation region that has the same conductivity type as the substrate semiconductor that separates transfer channels in the imaging region and has a higher impurity concentration than the substrate semiconductor. In a charge transfer imaging device in which a P-N junction is formed by the P-N junction and an excess charge greater than the maximum amount of charge that can be accumulated in a potential well constituting a picture element of the imaging device is absorbed into the P-N junction, the P-
A method for controlling photoelectric conversion characteristics of a charge transfer imaging device, characterized in that the reverse bias voltage applied to the N junction is lowered at the end of the charge accumulation period than at the beginning of the charge accumulation period.