JPH0828527B2 - Light receiving element with built-in circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in an integrated circuit having a large noise resistance, for example, a differential amplifier using a photodiode.

(Prior Art) Generally, a differential amplifier is used to reduce the influence of external noise. In that case, an amplifier with a pair of similar circuits is used.

FIG. 4 shows an example in which a differential amplifier is used in order to improve the resistance to power line noise of a circuit using a light receiving element with a built-in circuit. The detected photocurrent photodiode D1 is amplified by the first amplifier AMP1, it is amplified by the third amplifier AMP3 through capacitor C1, a resistor R _1, the output of which is input to the comparator COMP. On the other hand, symmetrically, a dummy photodiode D2, which is a light-shielded junction capacitive element having the same capacitance value as the photodiode D1, is connected to the second amplifier AMP2,
Further capacity C2, through a resistor R ₂ connected to a fourth amplifier AMP4, and inputs the output to the comparator COMP. The first amplifier AMP1 and the second amplifier AMP2 are connected to the constant voltage line V _S. When noise enters the power supply line, noise also enters the constant voltage line V _S , and noise also enters the first amplifier AMP1 and the second amplifier AMP2. At this time, if the differential circuit formed by both amplifier systems is completely symmetrical, the noise is canceled in the comparator COMP and does not affect the output. If the symmetry of the differential circuit breaks down,
The noises that have entered the first amplifier AMP1 and the second amplifier AMP2 cannot be canceled by each other, and noise is mixed in the output of the comparator COMP, so that malfunction easily occurs.

The dummy photodiode D2 is the photodiode D1
If it has the same structure as the above, it should be a completely symmetrical differential circuit, so the resistance to power line noise will be better, but in that case, the dummy photodiode D2 has the same size as the photodiode D1, so the chip size Is almost doubled and the cost is high. FIG. 5 is a cross-sectional view of an example of a typical photodiode. For example, an N-type epitaxial layer 2 is formed on the surface of a P-type semiconductor substrate 1 and one of the N ⁺ -type diffusion layers 4 is provided on one side of the N-type epitaxial layer 2. Take out the electrode of
Further, P-type isolation diffusion layers 3 and 3 for separating the N-type epitaxial layer 2 from other portions are provided, and the other electrode is taken out from one of them.

In order to reduce the chip size, a junction capacitance element is usually used for the dummy photodiode D2. FIG. 6A is a plan view of an example of the junction capacitance element, and FIG. 6B is a cross-sectional view of the main part near the center thereof. As shown in the figure, an N ⁺ type buried diffusion layer 5 is provided on the surface of a P type semiconductor substrate 1, and an N type epitaxial layer 2 is grown thereon. Next, a part thereof is surrounded by the P-type separation diffusion layer 3, the P ⁺ -type diffusion layer 7 is formed inside the P-type separation diffusion layer 3, and the N ⁺ -type diffusion layer 4 is formed so as to overlap therewith.

(Problems to be Solved by the Invention) When the photodiode shown in FIG. 5 and the dummy photodiode having the same capacitance as that shown in FIG. 6 are used, the resistance to the noise of the power supply line is Did not reach a sufficient level. When the cause of this is investigated, this is because the frequency characteristics of the photodiode and the dummy photodiode are different due to the difference in the resistance value of the series resistance, and the symmetry of the differential circuit is broken. This state is shown in FIG. solid line
d1 is the characteristic curve of the photodiode D1, whose series resistance is about 2 KΩ. The solid line d2 is a dummy photodiode
It is a characteristic curve of D2, and its series resistance is about 200Ω. The solid lines d1 and d2 greatly differ in capacitance value as the frequency increases. On the other hand, they are almost the same at low frequencies.

An object of the present invention is to improve the symmetry of the differential circuit by matching the frequency characteristics of the photodiode and the dummy photodiode and to improve the noise resistance to a level sufficient for the noise of the power supply line. There is.

(Means for Solving the Problems) In order to achieve the above object, the present invention compares the outputs of a photodiode, a dummy photodiode having the same capacitance value as that of the photodiode and shielded from light, and both the photodiodes. In a light receiving element with a built-in circuit including a comparator, a series resistance adjusting means for matching the high frequency characteristics of the two photodiodes is provided in one of the two photodiodes. Specifically, it is possible to match the high-frequency characteristics of the dummy photodiode by lowering the high-frequency characteristics of the dummy photodiode, but it is not stable in view of process variations. Therefore, by reducing the series resistance of the photodiode, by taking measures to improve the high frequency characteristics at high frequencies, and by matching the high frequency characteristics of the dummy photodiodes, it is possible to match the overall frequency characteristics from low frequencies to high frequencies. However, the high frequency characteristics of the dummy photodiode may be reduced depending on the application.

(Operation) Since the frequency characteristics of the photodiode and the dummy photodiode match, the symmetry of the differential circuit is not impaired even in the high frequency band.

(Example) In order to reduce the series resistance of the photodiode, the resistance value of the material forming the photodiode should be decreased, or the electrodes should be taken out from a plurality of points and the series resistance should be set to the parallel resistance, or they should be used in combination. Can be reduced by

FIG. 1 (a) is a plan view of an embodiment of the present invention, and FIG. 1 (b) is a schematic sectional view thereof. First conductivity type, for example,
On one surface of the P-type semiconductor substrate 1, an N ⁺ -type buried diffusion layer 5 having a second conductivity type of a lattice type, for example, N-type and having a high impurity concentration is provided, and the epitaxial layer 2 is grown thereon to The mold separation diffusion layer 3 surrounds the photodiode. This will be one electrode. In addition, the N ⁺ type buried diffusion layer 5
An N ⁺ type diffusion layer 8 reaching the surface from is formed and used as the other electrode. FIG. 11A shows only the portion surrounded by the P-type separation diffusion layer 3 of FIG. With this configuration,
The series resistance of the photodiode is reduced to about 200Ω, and the frequency characteristic of the capacitance of the photodiode is improved up to the high frequency part, like the dummy photodiode.

The N ⁺ type buried diffusion layer 5 and the N ⁺ type diffusion layer 8 described above reduce the series resistance of the epitaxial layer 2, and other arrangements are possible. In that case, the series resistance is
It has been found that the resistance to power line noise can be set to a level at which there is no practical problem if it is approximately 1 KΩ or less. Therefore, the structure of the photodiode can be variously configured within this range.

FIG. 2 shows a comparison of the characteristics between the present embodiment and the conventional example. The solid line A is that of this embodiment and the solid line B is that of the conventional example. The vertical axis represents the noise level causing the output malfunction, and the horizontal axis represents the frequency. As is apparent from the figure, in the conventional example, the capacitance is not balanced, so that the malfunction tends to occur from around 100 KHz, but in the present embodiment, the change in the malfunction noise level is small. Here, the reason why the malfunction noise level rises in the part where both solid lines A and B exceed 1 MHz is that the response of the amplifier cannot follow.

In the present embodiment, the N ⁺ type buried diffusion layer 5 is provided in a lattice shape. However, the larger the area, the larger the photodiode capacitance value, and accordingly the dummy photodiode capacitance value is increased. This is because the necessity arises, that is, the area of the dummy photodiode is increased and the chip size is increased. If the influence on the chip size is small depending on the application, the N ⁺ type buried diffusion layer 5 can be provided on the entire lower surface of the epitaxial layer 2 in the photodiode section.

If the series resistance of the photodiode is sufficiently reduced,
To match this, it is necessary to further reduce the series resistance of the dummy photodiode. FIG. 3 corresponds to that.

FIG. 3 (a) is a plan view of another embodiment of the present invention, and FIG. 3 (b) is a schematic sectional view thereof. The element shown in FIG. 3 is a junction capacitance type element, in which an N ⁺ type buried diffusion layer 5 is provided on the surface of a P type semiconductor substrate 1, and an N type epitaxial layer 2 is grown thereon. Next, a P ⁺ -type diffusion layer 7 is provided in a part of the portion surrounded by the P-type separation diffusion layer 3, and a plurality of P ⁺ -type diffusion layers 7 are formed so as to overlap with the P ⁺ -type diffusion layer 7.
N ⁺ type diffusion layers 4, 4 ... Are provided. The figure (a) shows only the part surrounded by the P-type separation diffusion layer 3 of the figure (b).

In the conventional junction capacitance type element, since the N ⁺ type diffusion layer 4 is continuous as shown in FIGS. 6A and 6B, the P ⁺ type diffusion layer immediately below the N ⁺ type diffusion layer 4 is formed. The resistance value of portion 7 was large, and the capacitance value was low on the high frequency side. In the present embodiment, the N ⁺ type diffusion layer 4 is divided, and a plurality of electrode outlets of the P ⁺ type diffusion layer 7 are provided by utilizing the gaps. According to this configuration, the series resistance is reduced, the deterioration of the frequency characteristic of the capacitance value is improved, and the resistance to the noise of the power supply line can be improved as in the embodiment of FIG.

When increasing the series resistance of the dummy photodiode, it can be easily adjusted by changing the material or connecting a resistance to the material.

(Effect of the Invention) According to the present invention, since the symmetry of the circuit of the differential amplifier at high frequencies can be improved, it is possible to provide a light receiving element with a built-in circuit having high resistance to power line noise.

[Brief description of drawings]

FIG. 1 (a) is a plan view of an embodiment of the present invention, and FIG. 1 (b).
Is a schematic cross-sectional view thereof, FIG. 2 is a graph showing the relationship between output malfunction level and frequency between one embodiment of the present invention and a conventional example, and FIG. 3 (a) is a plan view of another embodiment of the present invention , The figure (b)
Is a schematic sectional view thereof, and FIG. 4 is a circuit diagram of an example of a differential amplifier,
FIG. 5 is a schematic sectional view of a photodiode, FIG. 6 (a) is a plan view of an example of a junction capacitive element, FIG. 6 (b) is a schematic sectional view thereof, and FIG. 7 is a photodiode and a dummy photodiode in a conventional example. 5 is a graph showing a comparison of the characteristics of FIG. 1 ... P-type semiconductor substrate, 2 ... N-type epitaxial layer,
3 ... P-type isolation diffusion layer, 4 ... N ⁺ -type diffusion layer, 5 ... N ⁺ -type buried diffusion layer, 6 ... P-type diffusion layer, 7 ... P ⁺ -type diffusion layer,

Claims (2)

[Claims] 1. A photodiode composed of a semiconductor substrate of a first conductivity type and a semiconductor layer of a second conductivity type formed on the surface thereof, and a photodiode formed on a semiconductor substrate common to the photodiode and having a capacitance value In a light receiving element with a built-in circuit, which has a dummy photodiode shielded by an equal junction capacitance element and a differential amplifier comparing the outputs of the two photodiodes, the photodiode portion is formed on a first conductive type semiconductor substrate and its surface. Forming a second conductivity type buried diffusion layer having an impurity concentration higher than that of the semiconductor layer at a boundary between the second conductivity type semiconductor layers, and matching the series resistances of the two photodiodes to improve high frequency characteristics. A light receiving element with a built-in circuit. 2. The light receiving element with a built-in circuit according to claim 1, wherein the second conductive type buried diffusion layer has a lattice shape along the surface of the semiconductor substrate.