JPS61228667A - Solid-state image pick-up device - Google Patents

Abstract

PURPOSE:To obtain a solid-state image pick-up device, in which a numerical aperture can be improved, the amount of a saturation signal is large and an accumulated signal can be amplified and read, by providing a specified MOS transistor photoelectric conversion element and a specified reading means. CONSTITUTION:Photoelectric conversion is performed by a P-N junction diode 14, which is formed by a source part 3, a drain part 4 and a back gate part 2 on a semiconductor substrate 1. A photoelectric conversion signal is accumulated in an MOS transistor photoelectric conversion element 30 by the potential change caused by incident light into the back gate part 2 in the floating state. The back gate part 2 in each photoelectric conversion element 30 is sequentially made to be in the floating state. When the threshold voltage of the MOS transistor 30 is changed by the potential change of the back gate part 2, a current flows in correspondence with the gate potential as a signal current. A reading means 40, which reads the signal current, is provided. Thus, the amount of the signal can be amplified and taken out, and the sensitivity is improved to a large extent. The solid-state image pick-up sensor having a large numerical aperture and the large dynamic range can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a circuit system for a solid-state imaging device.

[Conventional technology]

Conventionally, there was a device of this type as shown in Figure 50.
In the figure, 51 is a p-type semiconductor substrate, 52 is the source of the MOS transistor 4, and 53, 54, and 57 are the MO3
These are the drain, channel, and gate of I-transistor 4. 55 is a field oxide film, 56 is a vertical signal line, 58
is a transparent insulating film. Further, 59 is a flattening film, 60 is a color filter, 61 is a protective film, and 62 is a light shielding film.

In this device, the source 52 of the MOS transistor 4 is a photoelectric conversion section, and the junction surface 63 between the source 52 and the substrate 51 is a signal charge storage section, and the entire transistor 64 having a switching function and the vertical signal line 56 are The pixel portion is the scanning circuit section of the pixel.

FIG. 5 shows a cross-sectional structure within one pixel of a solid-state imaging device, and FIG. 6 shows a circuit diagram of the entire device in which these are arranged. In FIG. 73 is a vertical scanning circuit, 73 is a vertical signal line, and 74 is a horizontal signal line. A portion 75 surrounded by a broken line and including the transistor 64 corresponds to one pixel unit shown in cross section in FIG.

Next, the operation will be explained. In FIG. 5, the light that is separated by the color filter 60 and reaches the source part 52 of the MOS transistor 4 is absorbed there, and generates an electronic tag pair. In the source section 52, the vertical signal fi56. A constant voltage is supplied from the drain 53, so that carriers are accumulated in the junction 63 between the source section 52 and the substrate 51, and the potential of the source section 52 is in a floating state. When electron-hole pairs are supplied to the source section 52 by photoexcitation in such a state, the electrons and holes are separated by the drift electric field of the junction section 63, and the separated electrons and the correct tag were accumulated in advance. It recombines with the carriers, and the potential of the source part 52 changes depending on the light intensity of the substrate 51.
approaches the potential of The amount of carriers flowing into the source part 52 from the drain 53 during signal readout, that is, the magnitude of the current flowing through the vertical signal line 56 when a voltage higher than a predetermined threshold is applied to the gate 57, is determined within the pixel. It corresponds to the intensity of the incident light.

[Problem that the invention seeks to solve]

Conventional solid-state imaging devices are configured as described above, so in order to prevent carriers from flowing into the drain during periods other than readout, it is necessary to limit the aperture area exposed to light to only the source portion using a light-shielding film. As a result, the aperture ratio, which indicates the proportion of the amount of light that is effectively photoelectrically converted, was very low at 20 to 30%.Also, since the only carrier storage area is the pn junction capacitance below the source part, However, the disadvantage was that the saturation signal amount was low.

This invention was made to solve the above-mentioned problems, and its purpose is to provide a solid-state imaging device that can improve the aperture ratio, have a large amount of saturated signal, and can amplify and read out accumulated signals. shall be.

[Means for solving problems]

The solid-state imaging device according to the present invention performs photoelectric conversion using a PN junction diode disposed on a semiconductor substrate and formed by each of a source section, a drain section, and a back gate section, and a potential of a bank gate section in a floating state due to incident light. As a result of the change, the MOS transistor photoelectric conversion element that accumulates the photoelectric conversion signal and the bank gate portion of each photoelectric conversion element are sequentially brought into a floating state, and the threshold voltage of the MOS transistor is changed due to the potential change of the bank gate portion. A readout means for reading out a current flowing in response to a predetermined gate potential as a signal current is provided.

(Function) In this invention, charge conversion is performed by the PN junction diode formed by each of the source part, the drain part, and the back gate part of the MOS transistor photoelectric conversion element, so the area where photoelectric conversion is possible is increased, and the aperture In addition, since signal charges are accumulated in the back gate portion of the MOS transistor, the signal accumulation area is increased compared to the conventional case, and the amount of saturated signal can be increased.Furthermore, the amount of saturation signal can be increased by changing the back gate potential. Since the threshold voltage of the MOS transistor changes and the ease with which the source-drain current flows is controlled, the accumulated signal can be amplified and read out to the outside.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows a solid-state imaging device according to an embodiment of the present invention, and is a cross-sectional view of a portion corresponding to one pixel. In figure 0, 1 is an n-type semiconductor substrate, 2 is a p-type semiconductor region, and 3 is an n
4 is an n-type semiconductor region, which is the source part of the MOS transistor 30 formed in the p-type semiconductor region 2 (forming the drain part of the MOS transistor 30). is a gate oxide film, 6 is a gate electrode, 7 is a silicon oxide film, 8,10°ztLl:
Arrangement 9 is a puffy basin membrane.

A circuit diagram for explaining the operation of this MOS transistor 30 is shown in FIG. 2. In FIG. 2, 11 to 13 are DC power supplies, 14 is a photodiode, 15 is a back gate potential, 16 is a gate, and 17 is a load. 18 is an output terminal, and 19.20 is a switch. Reference numeral 40 denotes a reading means, which comprises both the switches 19 and 20 and a control circuit (not shown) for opening and closing them.

Also, in Figure 3, in Figure 0 showing an example of a pattern corresponding to Figures 1 and 2, the same numbers as in Figure 1 indicate the same numbers, and 31, 32, and 33 are contact holes. .

In addition, a time chart for explaining the operation of the device of this embodiment is shown in FIG. 4. In FIG.
In the figure, the timing of opening and closing of the switch 19 is shown, (0) in the same figure shows the timing of opening and closing of the switch 20, and (e) of the figure shows changes in the intensity of light irradiated to the solid-state imaging device. Figures (b) and (d) show the back gate potential 15 which changes depending on the intensity of light.
and changes in the potential of the output terminal 18 are shown.

Next, the operation will be explained.

In FIG. 2, when the switch 19 is turned on, the back gate potential 15 becomes a negative DC potential.

At this time, a reverse bias is applied to the photodiode 14. In this state, when the switch 19 is turned off, the back gate potential 15 becomes a floating state, and electron-hole pairs are generated in the p-type semiconductor layer 2 due to the presence of incident light, and the reverse bias charge of the photodiode 14 is regenerated. Decreased by combination. That is, the back gate potential 15 rises as shown in the 4th section), and the electron-hole pairs are actually formed between the p-type semiconductor layer 2 and the n-type semiconductor region 1, 3°4.
This occurs in each depletion layer between the two.

By the way, the current-voltage characteristics of a MOS transistor are VDS
In the saturated region of >Vsat, it is expressed by the following equation.

105- (1/2)-β (VGS-VTR)”
However, IDS is the drain current, VCS is the potential difference between the gate and source, VTH is the threshold voltage of the MOS transistor, and β is a constant.

The threshold voltage VTR mentioned above depends on the back gate potential VSB as shown in the following equation.

VTII-VTHO-5it J9〒+BK vsa
+21F+VTHCON -VDS However, V THO is 0 bias threshold voltage BK is the back gate effect coefficient VSB is the back gate potential ΦF is the Fermi potential ('-0, 3 V) V TlIC0N is the drain dependence coefficient of VTH.

Therefore, when the bank gate potential changes depending on the amount of incident light, the drain current of the MOS transistor changes as shown in the following equation. When calculated in the saturation region, aVSB 2) VTHaVsB -β (VGS-VTR) -BK -2VSB+
24) It becomes F.

Let's do some numerical calculations here.

Now, μm600cm depth /V −5ec +
Cox = 4 X 1 0-'pF/t'm"
+ W-10um + L-5am,
β-4,8X l O-' (＾Ha town, VGS-5V, VTR-IV, BK -1゜v
sa-ov, ΦF -0,3V, -4,
8Xl0-' (A/V).

That is, when the back gate voltage changes by lv, the source/drain current 105 becomes 4.8 Xl0-'=
This will result in a change of 48μA, and this amount of change can be calculated by applying the load resistance 1
7 as a voltage change.

Here, the area A of the detector part of one pixel is 100, um"
, the junction capacitance CJ is 0.01 pP, and the incident light P is 10” (5
50 nm), the amount of charge Δ generated by the incident light is
QHani-h -c 680X6.6 Xl0-"X3 XIO"-6,5
4X10-'h(C) Therefore, the change in back gate voltage ΔvS8 becomes ΔVSB-ΔQ/CJ -65,4 (mV), and therefore the change in source-drain current Δ105 becomes A I 05
-,--4,8 X1O-sX AVSB --4,8
Xl0-'X 65.4 Xl0-'--31.4X1
0-ma(A)--3μA.

Here, if the load resistance value is 10Ω, the voltage change detected from the output terminal is 3Xl0-6XIO'-3Xt
The voltage becomes O-Ten(V)-30 mV, and a voltage change that can be sufficiently detected even with a very small amount of light appears at the output terminal.

That is, as shown in the above calculations, in this embodiment, even when a small amount of light is incident, the gate voltage remains constant.

By appropriately adjusting the values of the drain voltage and load resistance, the output value can be made sufficiently detectable. This is an advantage that could not be obtained with conventional solid-state imaging devices that do not have an amplification function. Note that in the case of an n-channel transistor, the amount of current taken out can be increased by increasing the gate potential or drain voltage.

In addition, in the conventional photoelectric conversion element of this embodiment, only the source region 52 is the photoelectric conversion region, and the PN junction 6 under the region 52 is
3 capacitance was the signal accumulation region, but the photoelectric conversion region is the depletion layer between the n-type semiconductor region 1.3°4 and the p-type semiconductor region 2, and the signal accumulation region is the p-type semiconductor region 2. Therefore, the aperture ratio is larger than that of the conventional one, and the amount of saturation signal can be increased.

In the above embodiment, the element is formed on an n-type semiconductor substrate, but it may be formed on a p-type semiconductor substrate, and the same effects as in the above embodiment can be obtained. However, in this case, the conductivity type of each semiconductor region formed on the substrate must naturally be reversed.

〔Effect of the invention〕

As described above, according to the present invention, since the amount of accumulated signal is expressed by the change in the back gate potential of the MOS transistor, the source-drain current changes greatly depending on the amount of accumulated signal, and the gate voltage and The amount of signal can be amplified and extracted by the drain voltage, and a solid-state imaging device with significantly improved sensitivity can be obtained. Moreover, since the areas of the photoelectric conversion region and signal accumulation region are larger than those of conventional ones, the aperture ratio is increased and a large amount of saturated signal can be obtained, resulting in a solid-state imaging device with a wide dynamic range. There is.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a solid-state imaging device according to an embodiment of the invention, FIG. 2 is a circuit diagram of a solid-state imaging device according to an embodiment of the invention, and FIG. 3 is a solid-state imaging device according to an embodiment of the invention. FIG. 4 is a timing chart showing the operation of the solid-state imaging device according to the present invention, FIG. 5 is a cross-sectional view of a conventional solid-state imaging device, and FIG. FIG. 2 is a circuit diagram of a conventional example. 30...MOS transistor photoelectric conversion element, 1...
n-type semiconductor substrate (first conductivity type semiconductor substrate), 2...
p-type semiconductor region (second conductivity type semiconductor region), 3.4.
...Role type semiconductor region, 14...PN junction diode,
19.20...Switch, 17...Load resistance, 40
...Reading means.

Claims (2)

[Claims] (1) Photoelectric conversion is performed by a PN junction diode arranged on a semiconductor substrate and formed by a source section, a drain section, and a back gate section, and the photoelectric conversion signal is generated by a potential change due to incident light on the floating back gate section. a MOS transistor photoelectric conversion element that accumulates
The back gate portion of each photoelectric conversion element is sequentially brought into a floating state, and when the threshold voltage of the MOS transistor changes due to a change in the potential of the back gate portion, a current flowing in response to a predetermined gate potential is read as a signal current. What is claimed is: 1. A solid-state imaging device, comprising: reading means for reading out data. (2) The MOS transistor is formed in a second conductivity type region formed for each pixel on the semiconductor substrate of the first conductivity type. The solid-state imaging device according to item 1.