JPH05145109A - Photo conversion device and drive method thereof - Google Patents

Abstract

PURPOSE:To make it possible to carry out accumulation operation having a knee characteristic and drive low voltage operation at a wide dynamic range and a favorable sensibility in an optical conversion device where there is loaded APD having a repeat structure of an inclined band gap. CONSTITUTION:This device is provided with a minimum forbidden band width Eg2 and a maximum forbidden band width Eg3 on both ends as a carrier multiplication layer 612 which multiplies holes and laminates a plurality of inclined band gap layers 612a to 612c whose forbidden band width changes continuously between both ends. There is provided a hetero junction in the minimum forbidden band width Eg2 of the inclined band gap layer on one side and the maximum forbidden band width Eg3 portion of the inclined band gap layer on the other side. A hetero-junction inclined band gap layer at least on one side is designed for an optical conversion device which is a high concentration n type semiconductor layer 612b and its low voltage drive method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device and a driving method thereof, and more particularly, to a photoelectric conversion device and a driving method thereof for reducing a driving voltage of a photoelectric conversion device having a knee characteristic during charge accumulation operation. Regarding the method.

[0002]

2. Description of the Related Art In recent years, facsimiles, digital copying machines,
With the spread of image information processing devices such as image readers and video cameras, long line sensors having one-dimensionally arranged photosensors and area sensors having two-dimensionally arranged photosensors are widely used.

As photoelectric conversion elements used in these, CCD and BASIS (BAse Stored Image) are used.
Ge sensor), MOS type, and other various types of solid-state image pickup devices are generally used. In recent years, as the image quality has been improved, the device density and integration have been further increased, resulting in a decrease in the aperture ratio of pixels. The S / N ratio cannot be sufficiently secured due to an increase in noise of the signal charge transfer system.

On the other hand, a spherical lens is used, or a photoelectric conversion element using a material such as amorphous silicon is used as a CC.
Although a form of stacking on D or the like (Japanese Patent Laid-Open No. 49-91116 et al.) Has been developed and effective use of incident light has been attempted, noise in the transfer system still remains a big problem.

Therefore, the use of an avalanche photo diode (APD) has been devised to amplify the optical signal inside the photoelectric conversion element and relatively reduce the noise in the transfer system (see Technical Report). Report Vol.86 No.208
'86). APD is generally widely used in the field of optical communication, and can amplify output by several tens to several hundreds. If application of APD to an image pickup device can be realized, the S / N ratio can be increased. We can expect a sufficient improvement.

However, Schottky, PN, PI
The driving method of the APD is significantly different from that of a photoelectric conversion element generally used in an image pickup apparatus such as N, and therefore, full-scale application to the image pickup apparatus has been hindered.

Specifically, the APD usually used in the field of optical communication is such that a high voltage is applied between the two terminals of the device and the avalanche multiplication is induced by the electric field acceleration of carriers. It requires a very large voltage of 100 volts, which is an abnormal drive power source for a solid-state image sensor.

Further, since the avalanche multiplication strongly depends on the electric field intensity, a linear output with respect to the input light intensity can be obtained in consideration of use in the charge storage mode which is a general driving method of solid-state image pickup devices. The area does not exist.

Therefore, an additional capacitance is provided in the charge storage portion,
The linearity of the input / output is ensured by suppressing the change of the electric field strength due to the charge accumulation (ITEJ Technical Rep.
ort Vol. 11, No. 28, pp67-72)
The improvement means has been devised, but strictly speaking, the linearity of input and output is impaired according to the voltage change of the output signal,
The problem of still requiring a high voltage power source remains.

Therefore, a structure in which a graded bandgap is combined as a region for causing avalanche multiplication to promote carrier multiplication at the heterojunction portion (Japanese Patent Laid-Open No. 58-58).
157179) have been devised.

According to this method, the energy required for ionizing the carriers is given by the energy step of the heterojunction, so that the multiplication factor does not change due to the change in the power supply voltage due to the charge accumulation. Further, since the voltage applied to the multiplication layer may be such that each graded bandgap layer is depleted, specifically, a voltage of 1 to 2 V corresponding to the number of graded layers may be applied to each graded layer. For example, there are 5 graded layers and each layer has 2 layers.
If V is applied, a voltage of 10 V in total is sufficient.

However, in the photoelectric conversion device as described above, the linearity of input and output, that is, the carrier multiplication factor does not change even when the effective applied bias to the element changes in the charge accumulation operation, can be stably obtained. Since the highest priority is given, the reduction of the dynamic range is not compensated for depending on the driving conditions such as the applied bias.

Therefore, a high-concentration impurity layer is provided on one side or both sides of the heterojunction portion between the maximum bandgap layer and the minimum bandgap layer to provide a knee characteristic in the charge storage operation. An attempt is made to realize a photoelectric conversion device having high sensitivity and a wide dynamic range, and a driving method thereof.

However, in general, when an impurity-doped layer is included in the depletion layer, the voltage required for depletion increases, so the idea of low-voltage driving is required unless measures are taken to suppress the applied voltage as much as possible. There was a problem of getting out of. The following is a brief description.

FIG. 5 is an energy band diagram showing the structure of a conventional photoelectric conversion device, and FIG. 5 (a) is an energy band diagram of an APD having a carrier multiplication section of three stages as an element structure at zero bias. Is. In the figure, Eg ₁
Indicates the required forbidden band width, Eg ₂ is the minimum forbidden band width, Eg
₃ indicates the maximum forbidden band width, and V ₀ is Eg
It is the energy of the difference in electron affinity chi ₃ of _second material of the electron affinity chi ₂ and Eg ₃ material (χ ₂ -χ _3).

Further, FIG. 5 (b) is a diagram showing an energy band when a bias necessary for causing carrier multiplication is applied, in which E _ion is carrier ionization energy, and V ₀ is , Eg ₂ material electron affinity χ ₂ and Eg ₃ material electron affinity χ ₃ (χ ₂ −
χ ₃ ), V _DA represents energy obtained by adding an energy step difference corresponding to the electric field acceleration to the above V ₀ , and has a relationship of V _DA ≧ E _ION ≧ V ₀ .

FIG. 6 is a diagram showing current-voltage characteristics of the photoelectric conversion device shown in FIG.

In the example of FIG. 5, the band offset energy of the conduction band at the heterojunction between the graded bandgap layers for carrier multiplication is less than the ionization energy of the minimum bandgap at the time of zero bias. A high concentration of p-type doping is applied to the band width portion, and when bias application to the device is advanced, electric field concentration is caused to cause ionization of electrons.

[0019]

However, in the above-described conventional APD based on the multiplication of holes, a high concentration p-type semiconductor is obtained by performing high concentration p-type doping on the minimum forbidden band width portion. In the APD described above, after a current corresponding to a quantum efficiency of 1 starts to flow at 14 to 15 V, a voltage of about 20, 40, 60 V must be added thereafter until multiplication of 2, 4, 8 times occurs. I won't. This is the conventional electric field acceleration A
This is a high value comparable to the voltage used to be a problem in PD, which is not a practical condition.

That is, in the APD having the repeating structure of the graded bandgap layer on the premise of multiplication of the holes, the maximum forbidden band width layer and the minimum forbidden band width layer are set so that the dynamic range is not deteriorated by driving conditions such as applied bias. The conventional photoelectric conversion device in which the high-concentration p-type semiconductor is arranged at the heterojunction between the width layer and the conventional photoelectric conversion device has a problem that the driving voltage becomes a high voltage.

(Object of the Invention) The present invention is to solve such a conventional problem, and on the premise of multiplication of holes,
In a photoelectric conversion device equipped with an APD having a repeating structure of a tilted band gap, a storage operation having a knee characteristic is possible, and a low voltage operation as much as possible is realized.
It is an object to propose a structure and driving conditions of a photoelectric conversion device.

[0022]

As a means for solving the above problems, the present invention provides a light absorption layer having a required forbidden band width Eg ₁ disposed between charge injection blocking layers, and _one or a plurality of slopes. The photoelectric conversion unit includes a carrier multiplication unit including a band gap layer, and the tilted band gap layer has a minimum forbidden band width Eg ₂ and a maximum forbidden band width Eg ₃ therein. , And are arranged so that they are in contact with each other to form a heterojunction, and further Eg ₂ <Eg ₄ is formed at both ends.
<Equipped with band gap of Eg ₃ becomes Eg _4, from the area of both the forbidden band width Eg _2, Eg _3, between the Eg _4, the forbidden band width is continuously changed, and the In a photoelectric conversion device having a structure in which the energy step of the valence band at the heterojunction is larger than the energy step of the conduction band, at least the part of the minimum forbidden band width Eg ₂ among the parts forming the heterojunction is high. The present invention provides a photoelectric conversion device characterized by being an n-type semiconductor having a high concentration.

Further, the bias V _V applied to both ends of the light absorption layer and the carrier multiplication portion of the photoelectric conversion device is set to a high potential on the side of the light absorption layer, and the value is set to a maximum forbidden band width. Energy step of the valence band at the heterojunction part that changes discontinuously from the value to the minimum value: E _{V OFF} , film thickness of the light absorption layer: d _p , film thickness of the carrier multiplication part: d _A , graded band gap Thickness of one layer: d _{GRD int} , thickness of one layer with maximum forbidden band width doped with high concentration: d _{EG2 dop} ,
And the number of heterojunctions between the layer having the maximum forbidden band width and the layer having the minimum forbidden band width: n, By giving in, a practical driving condition is realized.

[0024]

According to the present invention, in the photoelectric conversion device equipped with the APD having the repeating structure of the slanted bandgap on the premise of multiplication of the holes, the minimum forbidden band width Eg ₂ portion of the slanted bandgap layer and the maximum forbidden bandgap are formed. By forming a high concentration n-type semiconductor layer in at least the minimum forbidden band width portion at the heterojunction portion with the forbidden band width Eg ₃ portion, it is possible to have a knee characteristic in the charge storage operation and to expand the dynamic range. Can be planned.

Further, the bias V _V applied to the device is higher on the side of the light absorption layer than on the side of the carrier multiplying part, and is set to V _V satisfying the above-mentioned mathematical expression, so that the device is driven at a lower voltage than before. be able to. This will be described in detail below.

In order to prevent the applied voltage V _V from increasing, it must first be considered that a large electric field is not applied to the light absorption layer having a large film thickness.

Here, on the premise of multiplication of holes, in order to flow holes from the light absorption layer to the carrier multiplication portion, the light absorption layer side must have a higher potential than the carrier multiplication portion side. .. That is, it is necessary to apply a positive voltage to the terminal on the light absorption layer side. In this case, when the voltage is increased to deplete the impurity layer, when the impurity is an acceptor, an electric field is generated on the light absorption layer side. In the case of a donor, the electric field becomes stronger, and the electric field becomes stronger on the carrier multiplication portion side.

That is, as shown in the present invention, when the impurity is the donor and the semiconductor type is the n-type, the electric field applied to the light absorption layer is small, and the applied voltage to the element can be small.

Next, the voltage applied to each layer is shown below in order to estimate the magnitude of the video bias V _V applied to the present photoelectric conversion element.

However, in the following equation, each symbol is
E _{V OFF:} energy step of the valence band at the heterojunction that forbidden band changes discontinuously from the maximum value to the minimum value, d
_p : film thickness of light absorption layer, d _A : film thickness of carrier multiplication part, d
_{GRD int} : thickness of one graded bandgap layer, d _EG2
dop : thickness of one layer with maximum forbidden band width doped with high concentration, n: number of heterojunctions between layers with maximum forbidden band width and minimum forbidden band width (number of impurity layers), k: 1 to
n, the position of the impurity layer counted from the light absorption layer side, N _I : impurity concentration, Eg: required forbidden band width, Eg ₂ : minimum forbidden band width, Eg ₃ : maximum forbidden band width, ε: dielectric constant, q : Electricity,
V _V : represents the voltage applied to the element.

First, when the number of impurity layers is n, the voltage applied when the impurity layer closest to the light absorption layer is just depleted is V _{Hk in} each layer, In addition, the electric field strength E _Dk at the end of each impurity layer is Given in. Since this electric field is also applied to the k-th graded band gap layer, the voltage V _AI applied to all the graded band gap layers is Here, the impurity concentration N _I is Is set by.

Further, considering that the voltage necessary for depleting the graded bandgap layer is superposed before the depletion of the impurity layer, the superposed voltage is multiplied by the multiplication layer component V _{AI DEP} . V _{AI DEP} = n · E _{V OFF The} amount V _{P DEP} distributed to the light absorption layer is V _{P DEP} = V _{AI DEP} · (d _P / d _A ), and the total V _DEP is V _DEP = n · E. _{V OFF} · ((d _P + d _A ) / d _A ) ... (3) As a result, the applied voltage V _{V of the} element is the sum of (1), (2), and (3), but it can be considered that the ratio of (1) is small. By applying the value given by, the minimum drive voltage can be determined.

Therefore, by using the means of the present invention, it is possible to provide a photoelectric conversion device having a knee characteristic in the charge storage operation and capable of being driven at a low voltage.

(Examples of Embodiments) Examples of embodiments of the present invention will be described below.

FIG. 1A is a sectional structural view of an embodiment of the photoelectric conversion device of the present invention. In FIG.
Is an MO formed as a switch element on the substrate 600.
S is the source or drain of S, and 604 is its drain. Further, 611 is an electron blocking layer (charge injection blocking layer), 612 is a carrier multiplication part, 613 is a light absorbing layer, 614 is a hole blocking layer (charge injection blocking layer),
Reference numeral 616 is an upper electrode, and 615 is a passivation layer. The carrier multiplication section 612 is composed of three layers 612a and 61a.
2b and 612c, which form a light absorption layer 613 and a photoelectric conversion unit. Further, in this example, the 612b layer forming the three-layer carrier multiplying part 612 is a high-concentration n-type layer.

The substrate of such an embodiment of the present invention may be an insulating substrate, a semiconductor substrate, a metal substrate, or the like, and the deposition temperature of the film formed on the substrate and the acid, alkali, or solvent for the treatment. Anything that is resistant to For example, as the insulating substrate, glass, quartz, ceramics, and those on which a switch element, a scanning element, and the like are formed are often used, and a heat-resistant resin film is also used. Also, a semiconductor substrate having a signal processing circuit such as a switch element, an amplifier, and a scanning element is used. Typical signal processing circuits are CCD and MO.
S type, BBD, BASIS (JP-A-60-12759,
JP-A-60-12760) and the like, and any of them may be used.

Next, as the electrode film, a translucent electrode is used for light incidence on one side, and as its type, a metal film having a film thickness of several tens of Å to several hundred Å, SnO ₂ , IT.
A conductive metal oxide film of O, ZnO ₂ , IrO _x or the like is used. A metal film is mainly used as the electrode film on the other side, but it may be selected according to required conditions such as resistivity and chemical properties.

For the photoconductive film and the carrier multiplying part, semiconductors capable of controlling valence electrons are targeted for the present invention, and are mainly tetrahedral (group IV) semiconductors or III-V group semiconductors. A compound semiconductor is used. Group IV semiconductors include Ge, Si, C, and their composites SiGe, SiC, SiGeC, and SiN, Si.
O, SiSn, etc.

The structure of the semiconductor may be crystalline, polycrystalline, microcrystalline or amorphous for the i layer portion (undoped or slightly doped layer). Here, the term “microcrystal” refers to a structure in which a crystal grain of several tens to several hundreds of liters is contained in an amorphous material and the crystallization rate is up to several tens to 100%.

As impurities added for controlling valence electrons, V group substances are used for N-type control, and Group III substances are used for P-type control. Any of P, As, Sb, Bi may be used in the group V, and B, Al, Ga, In, Ta may be used in the group III.

The high-concentration impurity layer having a polycrystalline or microcrystalline structure is formed mainly by the plasma CVD method, the photo-CVD method, the thermal CVD method, the uW-CVD method, the LP-CVD method, and the atmospheric pressure CV.
Various CVD methods such as the D method and the sputtering method are used. The crystal grain size depends on power, substrate temperature, light amount, pressure,
Controlled by selecting conditions such as gas flow rate (Evaluation of the crystal grain size when determining the conditions is not limited to the X-ray diffraction method and TE
It is performed by the direct observation method using M or the like. ).

AlGaAsS is a III-V group semiconductor.
b, InAsSb, InGaAsSb, InGaAl
As, InAsPSb, InGaAsSb, AlG
aP and the like. Group VI substances are used for N-type control for valence electron control, and Group II substances are used for P-type control. VI group S,
Se, Te, Po, Be, Mg, Ca, Sr,
Any of Ba, Ra, Zn, Cd, and Hg may be used.

In the heterojunction portion, in order to compensate for the lack of offset energy, the film thickness and the impurity addition amount of the impurity added layer for causing electric field concentration may be determined as follows.

The film thickness may be determined according to the mean free path of the carrier to be multiplied for the material to be used.
Further, the film thickness of the single layer of the graded bandgap layer is also determined in accordance therewith, and may be set to 3 times or more, and preferably 10 to 20 times, the film thickness of the impurity added layer in the heterojunction portion.

The amount of added impurities is determined so that the voltage applied when the impurity layer is completely depleted is larger than the insufficient offset energy. That is, it is determined as follows.

[0046] (However, ε: dielectric constant of impurity layer, N: impurity concentration, χ
₃ : electron affinity of the layer having the maximum forbidden band width, χ ₂ : electron affinity of the layer having the minimum forbidden band width, E _ion : ionization energy of carriers, d: film thickness of the impurity layer). In the above equation, the subscripts 3 and 2 correspond to the wide gap and the narrow gap, respectively.

E _g3 and E _g2 are band gaps of the impurity layer.

[0048]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional structure (a) of a photoelectric conversion device according to an embodiment of the present invention, an energy band diagram (b) of a photoelectric conversion element section at a zero bias, and a diagram showing necessary for causing carrier multiplication. The energy band diagram (c) when a bias is applied is shown.

FIG. 4 is a sectional process drawing showing a manufacturing process of the photoelectric conversion device of the present invention.

First, the photoelectric conversion device of this embodiment will be described along the manufacturing steps (a) to (f) of FIG.

First, according to the ordinary MOS process technology,
A scanning IC substrate composed of MOS transistors is manufactured (a).

Then, in order to laminate the photoelectric conversion elements, a substrate surface flattening step is performed. First, the extraction electrode 608 is formed in the step (a), and then SiO 2 is formed.
_Two films 609 are deposited on the surface of 8000 Å, and ordinary photoresist is spin-coated to make the surface flat. After that, etching back is performed by RIE to expose the top of the electrode 608 while maintaining the flatness (b).

After that, Cr is formed by an ordinary sputtering method.
2000 Å are deposited, and then, a pixel electrode 610 is formed into a desired shape by a normal photolithography process (c). Then, the sample was set in a capacitively coupled plasma CVD apparatus, SiH ₄ gas was introduced into the apparatus at 60 SCCM, B ₂ H ₆ gas diluted to 10% with H ₂ gas was introduced into the apparatus at 20 SCCM, and the total gas pressure was 0. About 5.5 at 2 Torr
High frequency discharge is performed for about 1000 Å to form an electron blocking layer 611 made of a p-type high concentration impurity layer of a-Si: H.
Deposited to a thickness of. At this time, the substrate temperature was 300 ° C. and the discharge power density was about 0.2 W / cm ² .

Subsequently, in the CVD apparatus, SiH
₄ gas from 60SCCM to 6 SCCM, to 54SCCM the GeH ₄ gas from 0 SCCM, continuously varied,
High-frequency discharge is performed for 2.8 minutes under the conditions of a substrate temperature of 300 ° C., a total gas pressure of 0.2 Torr, and a high-frequency power density of about 0.5 W / cm ² , and an a-SiGe: H graded bandgap layer 612a of about 500 Å is formed. It was deposited to a thickness (deposition process 1).

Next, in the CVD apparatus, SiH
₄ gas 6SCCM, GeH ₄ gas 54SCCM, H
Introduce 6 SCCM of PH ₃ gas diluted to 10% with ₂ gases, substrate temperature 300 ° C., total gas pressure 0.2 Torr,
The n-type layer 612 of a-SiGe: H is formed by performing high frequency discharge for 34 seconds under the condition of the high frequency power density of about 0.5 W / cm ^2.
b was deposited to a thickness of about 100Å (deposition process 2).

Subsequently, in the CVD apparatus, SiH
₄ gas flow rate from 24 SCCM to 60 SCCM, NH ₃
The gas flow rate was gradually changed from 36 SCCM to 0 SCCM, discharge was performed for about 4.7 minutes under the conditions of a substrate temperature of 300 ° C., a total gas pressure of 0.3 Torr, and a high frequency power density of about 0.2 W / cm ² , a A -SiN: H graded bandgap layer 612c was deposited to a thickness of about 500Å (deposition process 3).

Thereafter, the deposition processes 1 to 3 are repeated twice to form a total of three graded bandgap layers.

Subsequently, the flow rate of SiH ₄ gas was set to 30 SCCM and the flow rate of H ₂ gas was set to 30 SCCM in the CVD apparatus, the substrate temperature was 300 ° C., the total gas pressure was 0.3 Torr, and the high frequency power density was about 0.2 W / cm ² . Under the conditions, discharge was performed for about 75 minutes to form a light absorption layer 613 having a thickness of about 8000Å. Subsequently, the SiH ₄ gas flow rate was changed to 24 SCCM, H
₂₀ SCC of PH ₃ gas diluted to 10% with ₂ gases
M and NH ₃ gas flow rates of 36 SCCM were introduced, and other deposition conditions were the same as in the deposition process 3 and discharge was performed for approximately 4.7 minutes to form a hole blocking layer 614 having a thickness of approximately 500Å (d).

Then, the semiconductor layers 611 to 614 were patterned into a desired shape to isolate the device (e).

Then, the sample is subjected to another capacitively coupled plasma C
S set in VD equipment and diluted to 10% with H ₂ gas
Introducing 10 SCCM of iH ₄ gas and 100 SCCM of 99.99% NH ₃ gas, substrate temperature of 300 ° C., total gas pressure of 0.4 Torr, high frequency power density of about 0.01 W / c
Discharged for about 60 minutes under m ² condition, thickness about 3000
A protective layer 615 made of the SiH _x film of Å was formed. After that, a through hole was opened in the protective layer 615, and an ITO film 616 having a thickness of 700 Å was formed by a sputtering method to obtain a photoelectric conversion device (f).

FIG. 1 is a diagram showing the photoelectric conversion device of this example manufactured in this way. In this example, the energy step of the valence band at the heterojunction is more than the energy step of the conduction band. Has a large structure, and has a carrier multiplication portion 612 which is premised on multiplication of holes.

FIG. 1B is an energy band diagram of the photoelectric conversion device of this example manufactured in this way,
Similar to FIG. 6 of the conventional example, Eg ₁ indicates the required forbidden band width, Eg ₂ indicates the minimum forbidden band width, Eg ₃ indicates the maximum forbidden band width, and V ₀ is (electron affinity χ of the material of Eg _{2 2} and Eg ₃ material electron affinity chi ₃ of the energy difference (χ ₂ -χ ₃₎
Is shown.

The photoelectric conversion device of the present invention shown in FIG. 1 (b) has a structure in which three graded band gap layers are heterojunction, as in the conventional example shown in FIG. 5 (a). In the energy band diagram of the conventional example of), the Eg ₂ portion is p-type, E
The g ₃ portion is i-type, but the energy band structure of the photoelectric conversion device of the present invention is different in that Eg ₂ is n-type and Eg ₃ is i-type, which is a feature of the present invention. Is.

Further, FIG. 1C is a diagram showing the time when the bias is applied, and the effect that the electric field applied to the portion of the light absorption portion Eg ₁ is relatively weakened and the bias applied to the entire element is reduced. Is obtained.

FIG. 2 is a diagram showing the current-voltage characteristics of the APD having the three-stage carrier multiplication section as shown in FIG. 1 as the element structure. As shown in FIG. 2, in the vicinity exceeding ionization starting voltage 1V _IONL, rapidly starts to flow current corresponding to the quantum efficiency 2, then the current corresponding to the quantum efficiency 4,8 from near V _ion2, V _Ion3 It flows out in steps.

FIG. 3 shows the input / output relationship when a storage operation is performed by applying a bias of V _ion3 or more to the device. As shown in FIG. 3, in the present embodiment, when the output exceeds a certain light amount, the slope with respect to the input changes, and the slope becomes gentle by a number corresponding to the step of the current-voltage characteristic, and reaches a saturation. Show. The output saturation characteristic as described above can be approximately regarded as what is called a so-called Knee characteristic, and the dynamic range can be expanded by utilizing this characteristic.

That is, if the boundary of the target light quantity region is overlapped with the point where the saturation characteristic starts, the dynamic range is about the same as the multiplication factor (8 in this example) as compared with the case without the knee characteristic. It will be expanded.

Next, the method of the present invention will be described below as to the method of driving the photoelectric conversion device of the present embodiment manufactured as described above at a voltage as low as possible.

As described above, the bias voltage V _V applied across the light absorption layer and the carrier multiplication layer of the photoelectric conversion device is (However, in the above equation, E _{V OFF:} energy step of the valence band at the heterojunction that forbidden band changes discontinuously from the maximum value to the minimum value, d _p: thickness of the light absorbing layer, d _{A Is} the thickness of the carrier multiplication layer, d _{GRD int is} the thickness of one layer of the graded band gap layer, d _{EG2 dop is} the thickness of the layer having the maximum forbidden band width with high concentration of impurities, and n is the maximum forbidden band width. The number of heterojunctions between the layer having a gap and the layer having a minimum band gap must be satisfied.

In the case of this embodiment, each of the above parameters is
(Eg ₂ = 1.2, Eg ₃ = 2.5, E _{V OFF} = 0.
2, E _{V OFF} = 1.1), d _p = 8000Å, d
_A = 3000Å, d _{GRD int} = 1000Å, d _{EG2 dop}
= 100Å, n = 3, and applying these parameters to the above equation (4), Therefore, as the bias voltage (video voltage) V _V ,
If 30 V is applied, the above formula can be satisfied.

Therefore, in the photoelectric conversion device formed by the above process, the video absorption voltage V _V is set to be lower on the light absorption layer side than on the carrier multiplication layer side, and
When a voltage of 30 V was applied, when the surface illuminance of the photoelectric conversion device reached about 300 lux, a knee characteristic began to be seen in the output and saturates at about 2000 lux, and
A dynamic range of B was obtained.

Further, the sensitivity is 8 times higher than that when an element such as a normal PN, PIN, Schottky is used, and high sensitivity,
Wide dynamic range operation was confirmed.

[0073]

As described above, according to the present invention,
In a photoelectric conversion device equipped with an APD having a repeating structure of a tilted bandgap on the premise of multiplication of holes,
On the other hand, the minimum forbidden band width Eg ₂ of the above inclined band gap layer
In the heterojunction between the portion and the other maximum forbidden band width Eg ₃ of the above-mentioned graded bandgap layer, at least the portion having the minimum forbidden band width Eg ₂ is formed as a high-concentration n-type semiconductor layer, so that the charge It is possible to have a knee characteristic in the accumulation operation, and it is possible to increase the dynamic range and increase the sensitivity.

[0074] Further, the bias V _V to be applied to the device, lower than the carrier multiplication layer side in the light absorption layer side, by the V _V satisfying the condition of the bias V _V of the present invention, conventionally It can be driven with a low voltage.

[Brief description of drawings]

FIG. 1 is a structure and energy band diagram of a photoelectric conversion device according to an embodiment of the present invention.

2 is a current-voltage characteristic of the photoelectric conversion device according to the embodiment of FIG.

FIG. 3 is a diagram showing input / output characteristics when a charge storage operation of the photoelectric conversion device of the embodiment of FIG. 1 is performed.

FIG. 4 shows a manufacturing process of a photoelectric conversion device according to an embodiment of the present invention.

FIG. 5 is an energy band diagram showing a structure of a photoelectric conversion element formed by a conventional method.

6 is a diagram showing current-voltage characteristics of the conventional photoelectric conversion device of FIG.

[Explanation of symbols]

600 substrate 601 pixel isolation region (p ⁺ type impurity implantation part) 602 switch MOS source, drain (n ⁺ type impurity implantation part) (switch element) 603 gate insulating film 604 gate electrode 605 interlayer insulating film 606 signal transfer electrode 607 interlayer insulating Film 608 Read-out electrode 609 Planarization layer 610 Pixel electrode 611 Electron blocking layer (charge injection device layer) 612 Carrier multiplication unit 613 Light absorption layer 614 Hole blocking layer (charge injection device layer) 615 Passivation layer 616 Top electrode (transparent) electrode)

Claims (3)

[Claims] _1. A photoelectric device including a light absorption layer having a required forbidden band width Eg ₁ disposed between charge injection blocking layers, and a carrier multiplication portion including one or more inclined band gap layers. And the graded bandgap layer has a minimum forbidden band width Eg.
₂ and includes a maximum forbidden band width Eg ₃ therein, and includes a bandgap of being arranged so they are in contact to form a _{heterojunction, Eg 2 <Eg 4 <Eg} 3 becomes Eg ₄ in both the end, The forbidden band width continuously changes between both regions of the forbidden band widths Eg ₂ and Eg ₃ and the Eg ₄ , and the energy step of the valence band at the heterojunction portion is a conduction band. In the photoelectric conversion device having a structure larger than the energy step of the photoelectric conversion device, at least a portion having the minimum forbidden band width Eg ₂ of the portion forming the heterojunction is a high-concentration n-type semiconductor. apparatus. 2. The method for driving a photoelectric conversion device according to claim 1, wherein a bias voltage V _V applied to both ends of the light absorption layer and the carrier multiplication portion of the photoelectric conversion device is applied to the light absorption layer side. A driving method of a photoelectric conversion device, characterized in that the photoelectric conversion device is driven at a higher potential than the carrier multiplication layer side. 3. The bias voltage V _V applied to both ends of the light absorption layer and the carrier multiplication portion of the photoelectric conversion device is: (However, in the above equation, E _{V OFF:} energy step of the valence band at the heterojunction that forbidden band changes discontinuously from the maximum value to the minimum value, d _p: thickness of the light absorbing layer, d _A : Film thickness of carrier multiplication part, d _{GRD int} : film thickness of one layer of the graded band gap layer, d _{EG2 dop} : film thickness of one layer having the maximum forbidden band width doped with high concentration impurity, n: maximum forbidden band width 3. The method of driving a photoelectric conversion device according to claim 2, wherein the photoelectric conversion device is driven by applying a voltage given by the number of heterojunctions of the layer having the gap and the layer having the minimum forbidden band width.