JP2764776B2 - Bipolar phototransistor device. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bipolar phototransistor for a semiconductor image sensor device, which converts incident energy of light such as light and radiation into an electric signal.

[0002]

2. Description of the Related Art An example of light will be described below as a radiation source. FIG. 5 shows a conventional bipolar phototransistor. The element surface of the bipolar transistor is covered with a silicon oxide film 6. When the light 9 is incident on the phototransistor, charges are accumulated between the base 2 and the collector 5, the base potential increases, and a base current flows from the base 2 to the emitter 1. A collector current IC obtained by multiplying the base current by a DC current amplification factor (hereinafter referred to as hFE) flows from the collector terminal 51 toward the emitter 1.

FIG. 2 shows an equivalent circuit of a semiconductor image sensor device using a bipolar phototransistor. In FIG. 2, the bipolar phototransistor of FIG. 5 is incorporated as P1. When the switch S1 is turned on,
The collector current IC1 is output as a current to the capacitor C1. As an external output, the electric charge accumulated in the capacitor C1 is taken out as an output voltage VO1.

[0004]

When the intensity of incident light is read by the image sensor device, reproducibility and linearity are required between the input light quantity and the output voltage. FIG. 6 shows a base voltage-hFE characteristic of a conventional bipolar transistor. h
FE has a base voltage dependency, and in a region where the base voltage is low, as hFE is low and the base voltage is high,
hFE increases. The base voltage-hFE dependence varies from element to element as shown in FIG. 6, and changes with time even for the same element.

[0005] This phenomenon is mainly due to the fact that carriers generated by light are combined with interface levels 10 existing at the interface between the surface of the base 2 and the silicon oxide film 6 in FIG. Therefore, as the interface level increases, the hFE decreases in a region where the base voltage is low. Since most of these interface states are mainly generated by dangling bonds of the silicon substrate, hydrogen is added to suppress dangling bonds. However, since the bonding force between hydrogen and dangling bonds is weak, and hydrogen easily diffuses in the silicon oxide film, the suppression of dangling bonds by hydrogen is unstable. Further, the interface state amount varies every time the device is manufactured, or fluctuates due to a bias, a temperature, and the like in the device during operation of the device. For this reason, the base voltage dependence of hFE occurs, and the dependence itself changes.

FIG. 7 shows the photoelectric conversion characteristics when such a conventional bipolar transistor is used as a phototransistor. In an area where the input light quantity is low, the increase in the output voltage is small with respect to the increase in the input light quantity, and a phenomenon occurs in which the increase in the output voltage increases as the input light increases. As shown in (1), there has been a problem that the elements vary.

SUMMARY OF THE INVENTION An object of the present invention is to provide a bipolar phototransistor in which the output voltage does not vary with respect to the input light amount and the increase in the output voltage with respect to the increase in the input light amount is constant regardless of the input light amount. It is.

[0008]

In order to solve the above-mentioned problems, in the present invention, a dense impurity layer of the same conductivity type as the base is provided on the base surface of the phototransistor. Further, a gate electrode was provided on the base surface at the interface between the emitter and the base.
Further, the silicon oxide film covering the phototransistor surface is covered with a material having a smaller diffusion coefficient of hydrogen than the silicon oxide film, for example, a silicon nitride film, so that hydrogen is confined in the silicon oxide film.

[0009]

The surface of the phototransistor is stabilized, the fluctuation of the hFE of the phototransistor and the dependence on the hFE / base voltage are eliminated, and the reproducibility and linearity of the input light quantity and the output voltage can be stably obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a sectional view of one embodiment showing the structure of a bipolar phototransistor according to the present invention. A gate electrode 20 is provided on the base surface around the emitter 1 of the phototransistor. On the base surface, a high-concentration base 30, which is a dense impurity layer of the same conductivity type as the base, is diffused away from the emitter 1. The distance between the high-concentration base 30 and the emitter 1 is usually equal to or less than the width of the gate electrode 20. Silicon oxide film 6 covering the entire surface of the phototransistor
Are completely covered with the silicon nitride film 7 and are not exposed to the air at all. Hydrogen is confined in the silicon oxide film 6. In FIG. 1, the number of hydrogen-permeable films covering the surface silicon oxide film does not need to be one, and a plurality of combinations are possible.

The base concentration is usually 1E15 / cm ^{3 to} 1E
The concentration is about 17 / cm ³ . Various charges are taken into the silicon oxide film 6, and the surface concentration of the base is 1E1.
If it is less than 6 / cm ^3, it is affected by the charge in the oxide film,
The surface condition of the base fluctuates. Also, N-P-N
In the case of a phototransistor, the base concentration is 1E17 / c
Even if it is m ³ , segregation of boron occurs at the interface between the base surface and the silicon oxide film 6. That is, when the base surface made of silicon containing boron is oxidized, Si is easily dissolved by the silicon oxide film, so that the boron concentration on the Si surface decreases. For this reason, the concentration on the base surface is lowered by one digit or more and is unstable. In the case of a phototransistor, a large base surface area is required to receive light at the base surface and accumulate photocharges at the base-collector junction. Therefore, if the surface state is unstable, the accumulation of photocharges is affected. Therefore, by increasing the concentration of the base surface, the surface of the base is stabilized, and the amount of accumulated photocharge is stabilized. The base surface concentration needs to be 1E17 / cm ³ or more. However, when the high-concentration base 30 comes into contact with the emitter, the emitter injection efficiency is reduced, and the hFE is greatly reduced. Therefore, it is necessary to separate the high concentration base portion from the emitter. The separation distance is such that the depletion layer extending from the emitter to the low concentration base does not reach the high concentration base 30.

Since carriers flow toward the emitter 1, the disappearance of carriers due to surface recombination occurs most around the emitter. The largest cause of surface recombination is the interface state. The interface state is generated by dangling bonds of silicon, and the dangling is caused by discontinuous bonding of a silicon-silicon oxide film. In addition, the silicon-silicon bond may be broken due to damage due to processing using plasma (plasma etching, plasma CVD) during the manufacturing process. By providing a gate electrode on the base surface at the interface between the emitter and the base, the surface can be protected from damage caused by the plasma, whereby the interface state can be kept low. The same effect is obtained whether the gate potential is fixed or floating.

[0013] Silicon dangling bonds cannot be suppressed theoretically. Therefore, hydrogen is added to bond with silicon dangling bonds to make them electrically inactive. In the present invention, since hydrogen is added to the silicon oxide film 6 and confined by the silicon nitride film 7, the hydrogen bond is stable.

Another embodiment is shown in FIG. In FIG.
The provision of the gate electrode 20 and the high-concentration base 30 is not shown in FIG.
However, the silicon oxide film 6 is covered with the aluminum wiring 8 and the silicon nitride film 6. Therefore, the hydrogen confined in the silicon oxide film is stable without escaping. The same effect can be obtained if the wiring material in FIG. 8 is a film such as polysilicon or silicide having a diffusion coefficient of hydrogen smaller than that of an oxide film.

The method of adding hydrogen is as follows.
The addition may be performed by a known method such as high-temperature hydrogen annealing or ion implantation. By using a plasma silicon nitride film as the silicon nitride film, the addition of hydrogen and the confinement can be performed simultaneously. FIG. 3 shows the hFE-base voltage characteristics of the phototransistor according to the present invention. According to FIG.
Has a small base voltage dependence. Also, there is almost no variation due to the elements. FIG. 4 shows the photoelectric conversion characteristics of the phototransistor according to the present invention. The photoelectric conversion has high linearity. Also, there is almost no variation due to the elements.

The covering with the high concentration base 30, the gate electrode 20, and the silicon nitride film 7 in the present invention is effective when used in combination or alone.

[0017]

As described above, according to the present invention, in a bipolar phototransistor, (1) by providing a high-concentration base, the surface of the base is hardly affected by the outside. (2) With the following configuration, the interface state on the base surface was lowered.

By covering the silicon oxide film with the silicon nitride film, hydrogen is confined in the silicon oxide film and dangling bonds on the surface of the silicon substrate are suppressed. By providing the gate electrode on the base surface around the emitter, plasma damage to the base surface around the emitter was suppressed. As a result, the DC current gain can be kept constant irrespective of the base voltage, thus achieving the remarkable effect of improving the linearity and reproducibility of the output electric signal with respect to the amount of received light. I do.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a phototransistor for a semiconductor image sensor device according to the present invention.

FIG. 2 is an equivalent circuit of the semiconductor image sensor device.

FIG. 3 is a diagram showing the hFE-base voltage dependence of the phototransistor of the present invention.

FIG. 4 is a diagram illustrating photoelectric conversion characteristics when a phototransistor of the present invention is used.

FIG. 5 is a cross-sectional view of a conventional phototransistor.

FIG. 6 is a diagram showing hFE-base voltage characteristics of a conventional phototransistor.

FIG. 7 is a diagram showing a photoelectric conversion characteristic when a conventional phototransistor is used.

FIG. 8 is a sectional view of another embodiment of a phototransistor for a semiconductor image sensor device according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 emitter 2 base 3 gate 4 bias voltage 5 collector 6 silicon oxide film 7 silicon nitride film 8 aluminum electrode 20 gate electrode 30 high concentration base layer P1 phototransistor S1 arranged MOS analog switch IC1 output current C1 capacitor VO1 output voltage

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/10

Claims (3)

(57) [Claims] A semiconductor substrate; A collector region and a surface formed on the collector region;
A base region and an emitter formed on the surface of the base region.
Formed in the surface region of the semiconductor substrate.
Bipolar phototransistor, Formed on the surface of the base region apart from the emitter region
And a high concentration of impurities of the same conductivity type as the base.
Concentration-based, A structure formed on the bipolar phototransistor
A silicon oxide film; Separated from the high concentration base at the periphery of the emitter
The silicon oxide film above the base to cover the region
An electrode formed on the surface of Silicon nitride film formed on the surface of the silicon oxide film
Bipolar phototransistor characterized by comprising
Star device. 2. The high-concentration base includes an emitter region.
And is provided so as to surround the emitter region.
The bipolar phototransistor device according to claim 1. 3. A collector region and a surface portion of the collector region.
A base region formed in the base region and a surface portion of the base region.
A surface of the semiconductor substrate comprising an emitter region formed
A bipolar phototransistor formed in the region, Formed on the surface of the base region apart from the emitter region
And a high concentration of impurities of the same conductivity type as the base.
Concentration-based, It has a surface portion and a side portion, and has a silicon nitride film and a first wiring.
Line, completely covered with the bipolar phototransistor.
A silicon oxide film formed on the resistor, Separated from the high concentration base at the periphery of the emitter
The silicon oxide film above the base to cover an area
Characterized by comprising a second electrode formed on the surface of
Bipolar phototransistor device.