JPH02216876A - Optical sensor - Google Patents

Abstract

PURPOSE:To suppress a reduction in a photocurrent, to reduce a junction capac ity and to obtain an optical sensor having high sensitivity by forming first conductivity type region only at a part of a low impurity concentration second conductivity type region having a long life time to be exposed on a surface. CONSTITUTION:In a sensor array, a high impurity concentration N<+> type layer 3 having rectangular spaces at an equal interval by impurity diffusion from the surface of a low impurity concentration N-type silicon substrate 2 of approx. 10<5>/cm<2>, and P-type layers 1 disposed at parts of the spaces are formed. In this case, since a P-N junction is formed between the layer 1 and the layer 2, a junction area is reduced as compared with the P-N junction generated between the regions 1 and 2, and its capacity is reduced that much. A photocur rent conversion efficiency is also reduced, but since the life time of carrier is normally 100musec or more in a low impurity concentration region, a photocur rent flows by diffusion in case of approx. 100mum of sensor size. Accordingly, the photocurrent is not so reduced, and its sensitivity is totally improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a charge storage type optical sensor comprising a photodiode having a PN junction.

[Conventional technology]

Conventionally, an optical sensor consisting of a photodiode having a PN junction has a second conductivity type region 2 surrounding a first conductivity type region 1 constituting each sensor, as shown in Fig. 2+II) and crest).
3 surrounds it, and a photodiode is formed using the junction of regions 1 and 2.3.

Figure 2 (a) is a plan view, and the second open mountain) is (c of al.
FIG. 3 is a sectional view taken along line c. Here, both regions 2 and 3 are of the second conductivity type, but region 3 is usually set to have a high impurity concentration for separation between sensors, and region 2 is set to have a low impurity concentration. A depletion layer region 4 indicated by a dotted line is formed around the pn junction surface between these regions.

The light incident on the substrate is in regions 1, 2.3 and 4,
Generates electron-hole (carrier) pairs and generates a photocurrent depending on the amount of light.

The charge accumulation type detection circuits of these sensors are roughly divided into the following two types.

One is a circuit shown in FIG. 3, which accumulates charge for a certain period of time and detects the amount of charge as a voltage.

This is used in MDS type solid-state image sensors, etc., and operates according to the timing chart shown in FIG.

First, time 1. The transistor 6 previously connected in series with the photodiode 5 is made conductive. A reverse bias is applied to the junction capacitance 8 of the photodiode 5, making the transistor 6 non-conductive at time t when charge is accumulated, and turning off the transistor 6 at time t! When the photodiode 5 is turned on again, the charge accumulated in the junction capacitance 8 of the photodiode 5 during the time T-1, -1, is discharged in an amount corresponding to the amount of light. Further, when the transistor 6 is made conductive at time tyo, a current flows through the load resistor 7 so as to charge the junction capacitance 8 of the photodiode 5. At this time, the output voltage V is the maximum a [V a as shown in FIG.

The series-connected transistors 6 are kept conductive.

In this case, as in the former case, a reverse bias is applied to the junction capacitance 8 of the photodiode 5, and when the transistor 6 is made non-conductive at time t1 during which charge is accumulated, the junction capacitance of the photodiode 5 increases. 8 performs discharge according to the amount of light, and the potential 1 increases as shown in the timing chart. When this potential V becomes equal to the comparison voltage V,a of the comparator 10, the output of the comparator V,
is reversed.

By measuring this time t, T! I understand tl-t. At this time, T becomes the following formula.

ct + ct Here, 5 - photocurrent according to the amount of light, CI is the value of the junction capacitance 8 of the photodiode 5, and C8 is the value of the circuit parasitic capacitance 9.

The other type has a circuit configuration shown in FIG. 5, and measures the time until a constant voltage is reached. This type of operation is shown in the timing chart of FIG.
Previously, the photodiode 5 and [Problems to be Solved by the Invention] In recent years, optical sensors are required to have higher sensitivity.

Increasing the sensitivity means increasing V in equation (1), and lowering T in equation (2).

In this case, there are two possible methods: increasing the current or lowering the CI + CI.
cX has a lower limit value determined by the detection circuit, and cannot be reduced below that value.Also, if the light-receiving area of the optical connection is increased, l
However, increasing the light-receiving area is undesirable because it lowers the resolution of the sensor.In addition, in conventional optical sensors, increasing the light-receiving area also increases the junction capacitance C1, so it is not desirable to increase the sensitivity. cannot be expected.

An object of the present invention is to provide a higher-performance optical sensor with improved sensitivity without lowering resolution.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a photodiode having a PN junction in which a region of a first conductivity type is surrounded by a region of a second conductivity type is uniformly distributed on the surface of one semiconductor substrate. In the optical sensor, a region of the first conductivity type exists on the surface of the substrate, and one of each of a plurality of regions of the second conductivity type with a low impurity concentration is separated into regions of the second conductivity type with a high impurity concentration. It shall be established only in the department.

[Effect]

Since the region of the first conductivity type is provided only in a part of the region of the second conductivity type, the junction capacitance of the photodiode, that is, (
11, (The CI in Equation 21 decreases. In the depletion layer region, almost 100% of the carriers generated by the light incident on the light receiving section become a photocurrent due to the electric field, but in areas other than the depletion layer region, they become a photocurrent due to diffusion. In the above photosensor structure, the area of the depletion layer decreases, so the photocurrent conversion efficiency also decreases, but the carrier lifetime is limited to the low impurity concentration region (10”
/j), the time is usually 100 μsec or more, so if the photosensor size is about 100 μs, the photocurrent will flow due to diffusion, so the photocurrent will not decrease that much, so the overall sensitivity will improve.

[Example] Figure 1 (a) to [CI] Figure 1 shows the structure of a sensor array using an optical sensor according to an embodiment of the present invention (Ml is a plan view, (b) is an AA' of Tal) Cross-sectional view along the line, (C
) is a sectional view taken along line BB' of lal, and parts common to those in FIG. 2 are given the same reference numerals. This sensor array is constructed by diffusing impurities from the surface of an N-type silicon substrate 2 with a low impurity concentration of about 101/-, and includes a highly doped N° layer 3 with rectangular cavities at equal intervals and a part of the voids. It was formed by PJII, which is located at In this case, since a PN junction is formed between PJII and N layer 2, the junction area is smaller than that of the PN junction formed between region 1.2 in FIG. 2, and the capacitance is reduced accordingly.

Carriers caused by light incident on the N0 layer 3 have a short lifetime, so they hardly become a photocurrent, but carriers incident on the N layer 2 usually have a lifetime of 100 μs or more, and therefore have a diffusion length of 100 n or more. , a part of the carriers diffuses to the junction between the two layers 1 and becomes a photocurrent. Therefore, the effect of the low range of capacitance C6 is greater than the effect of the reduction in photocurrent i due to the reduction of the junction area, and (11
l in the equation becomes larger, and T in the equation becomes smaller, improving sensitivity in both cases.

〔Effect of the invention〕

According to the present invention, by forming a region of the first conductivity type only in a part of the region of the second conductivity type with a low impurity concentration and a long lifetime exposed on the surface, the effective light receiving area of the optical sensor can be increased. By reducing only the area of the PN junction without changing it, it was possible to suppress the reduction in photocurrent and reduce the junction capacitance, and it was possible to obtain a highly sensitive optical sensor.

[Brief explanation of the drawing]

Figures 1(a) to (e) show a sensor array consisting of optical sensors according to an embodiment of the present invention, (II) is a plan view, (b
) is a cross-sectional view along the line A-A' of ta+, (C) is a cross-sectional view along the line B-8' of (a), and Figures 2 (A) and (b) are a sensor array consisting of a conventional optical sensor. (a) is a plan view, (b) is a cross-sectional view along the line C-c' of (ml), FIG. 3 is a circuit diagram showing an example of the detection circuit of the optical sensor, and FIG. 4 is its timing. Fig. 5 is a circuit diagram showing another example of the detection circuit of the optical sensor, and Fig. 6 is its timing chart. 1: 2 layer (first conductivity type region) 2: N layer (low impurity concentration region) 2-conductivity type region) 3iN'' layer (high impurity! (b) ~2 2nd f!1 Figure 1 Figure 5 Figure 6

Claims (1)

[Claims] 1) In a photodiode having a PN junction in which a region of the first conductivity type is surrounded by a region of the second conductivity type, the photodiode is uniformly distributed on the surface of one semiconductor substrate, and The region is provided only in a part of each of a plurality of regions of the second conductivity type with a low impurity concentration that are separated into regions of the second conductivity type with a high impurity concentration existing on the surface of the substrate. light sensor.