JPS62166561A - Image sensor - Google Patents

Abstract

PURPOSE:To obtain a photodetector or an image sensor which is controlled constantly in its optical sensitivity by combining a lateral bipolar transistor with photodiodes to amplify an optical signal current obtained from the photodiode newly as an optical signal current from each picture element unit. CONSTITUTION:An optical signal current amplified due to a variation in a base potential is obtained from an optically excited carrier generated by a light incident to the vicinity of a boundary between a collector region 2 and a base forming epitaxial layer 3 and an electric field present region, and the degree of this amplification is represented by hFE. The hFE is determined by the impurity density profile of a base layer and near the boundary of the base layer and the thickness of the base layer. In case of a lateral bipolar transistor, the regions 1 and 2 are formed in one masking step. Thus, since the controllability of the thickness of a base layer and an impurity density profile near a base layer boundary, the irregularity in the hFE is small and uniform hFE is provided not only in one wafer but in one lot.

Description

[Detailed description of the invention] Industrial application field The present invention is directed to a lateral transistor (lateral transistor).
This invention relates to an image sensor in which a plurality of light-receiving elements are arranged.

In conventional image sensors, photodiodes are exclusively used as light receiving elements. Here, in order to increase the sensitivity, it is not usual to use a phototransistor or to add a bipolar transistor to each photodiode as a light receiving element to amplify the optical signal current. Therefore, the optical signal current in each pixel is small.

Problems to be Solved by the Invention Although it is desirable that the optical signal for each pixel be large, the reason why photodiodes equipped with phototransistors or amplifying nobolar transistors are not used as light-receiving elements is because of the lack of amplification. The reason is that the emitter-grounded small signal current amplification factor (hereinafter referred to as hFC) of each polar transistor having the function is relatively rarely uniform over the entire image sensor.

Means for Solving the Problems In order to solve the above problems, the present invention uses lateral bipolar transistors for optical signal current amplification.
In combination with a diode, the optical signal current obtained from the photodiode is amplified and converted into an optical signal current from each pixel unit, or it is involved only in the Pn junction used by applying reverse bias of a lateral bipolar transistor. The amplified optical signal current is obtained by using a lateral phototransistor, whose terminal side (collector side) is a light receiving element, as a light receiving element.

Function The present invention uses a lateral structure bipolar transistor for optical signal current amplification with the above-described configuration.
In a lateral structure bipolar transistor, the collector and emitter regions are simultaneously formed in the base region in one process using one IC mask, so the uniformity of the hFP is good. Therefore, a line image sensor with the light receiving element array shown in Figure 4 (al) and an area image sensor with the light receiving element array shown in Figure 4 (bl) (both 30 and 31 are individual light receiving elements) are created using the light receiving element having the above configuration. When this is done, it is possible to realize an image sensor that has both high and uniform photosensitivity over the entire sensor area.In addition, there is no complication of the manufacturing process and no change in the spectral sensitivity spectrum of the light receiving section.

Embodiment FIG. 1 shows a lateral phototransistor to be used in an image sensor according to an embodiment of the present invention, which has a basic configuration of a light-receiving element arrangement as shown in FIGS. 4 (a) and (bl).
(Al is a cross-sectional view, (bl is a plan view. 1 is a P-type (or n-type) emitter region, 2 is a P-type (or n-type) collector region, 3 is an n-type (or P-type) forming the base. ) It is an epitaxial layer, and the region 14 sandwiched between the emitter region 1 and the collector region 2 becomes the base region. 4, 5.6 consists of the same type of semiconductor as the base region forming epitaxial layer 3,
4 is a low resistance buried layer, 5 is a low abrasive resistance area under the base terminal, and 6 is a low resistance area for contacting the base electrode, all of which are mainly used to take out the base electrode 10, and in terms of wood quality. , including the base electrode 10. Reference numeral 7 denotes an element isolation layer, which is made of a semiconductor of a different type from that of the base forming epitaxial layer 3, and in this figure is made of the same type of semiconductor as the substrate semiconductor.

Photoexcited carriers generated by light incident near the interface between the collector region 2 and the base-forming epitaxial layer 3 and in the electric field region change the base potential to obtain an amplified optical signal current, and the degree of amplification is expressed by hFE. . h
FE is determined by the base layer, the impurity concentration profile near the base layer boundary, and the thickness of the base layer. In the case of a lateral bipolar transistor, since the emitter region 1 and the collector region 2 can be formed in one mask process, it is possible to control the base layer thickness and the impurity concentration profile near the base layer boundary with good controllability, which reduces the variation in hFF:. Small and uniform hFE not only within one wafer but also within 10.7 wafers
has value. On the other hand, a typical vertical bipolar phototransistor is shown in FIG.

As in the case of FIG. 1, the fluctuation of the base potential due to the excited carriers generated by the incident light near the interface 29 between the collector region 16 and the base region 17 and into the electric field existing region amplifies the optical signal current. The base layer thickness and the concentration profile near the base layer boundary, which determine the hFE value, greatly depend on the two steps of forming the base region 17 and forming the emitter region 15. Therefore, it is difficult to control the hFE value. Bad sex. As for the light receiving section, the interface 29 should be as shallow as possible from the surface in order to increase sensitivity on the blue side, and it is also desirable to avoid complicating the bipolar IC process. These things make it even more difficult to control the hF11 value to a constant value.

Therefore, the amplification of the optical signal current by a lateral bipolar transistor, which can be easily realized in a bipolar IC process, does not cause a decrease in blue sensitivity or complicate the bipolar IC process, and is optimal in an image sensor.

This is the same whether the lateral bipolar transistor is used as a phototransistor or the photodiode signal is amplified by the lateral bipolar transistor, but the former is preferable in terms of improving device integration.

FIG. 3 shows an example of a pond. FIG. 3 (al indicates a photodiode 26 and a lateral npn transistor) is an example in which a function equivalent to that of an npn phototransistor is obtained by connecting a transistor 27. Figure 3 tC) is similar to Figure 3 (a+), but the cathode of the photodiode 26 and the lateral npn
) This is an example where the collector of the transistor 27 is not used at the same potential. Figure 3 (bl is photodiode 26 and lateral!n+)) By connecting to transistor 28, p
This is an example of obtaining the same function as an np phototransistor.

FIG. 3 (dl is also the same as bl, but the anode of the photodiode 26 and the collector of the transistor 28 are not used at the same potential).

These all use lateral type bipolar transistors and have good controllability of the hFE value, so bipolar ICs
To provide a light-receiving element and eventually an image sensor that maintains a shallow pn junction without complicating the process and changing the photosensitivity spectral characteristics and does not cause a decrease in blue sensitivity.

Effects of the Invention As described above, according to the present invention, bipolar IC
It is possible to produce a light-receiving element or image sensor in which the light sensitivity is controlled to be constant without being affected by delicate individual manufacturing conditions, without complicating the process, without reducing the blue sensitivity of the spectral sensitivity characteristic.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a phototransistor used in an image sensor according to an embodiment of the present invention, FIG. 2 is a cross-sectional view of a conventional vertical bipolar transistor, and FIG. 3 is a cross-sectional view of a photodiode according to another embodiment of the present invention. FIG. 4 is a cross-sectional view of a combination of lateral bipolar transistors, and a plan view showing the basic arrangement of light-receiving elements of a line image sensor and an area image sensor, respectively. 1...Emitter region, 2...Collector region, 14...Base region, 26...
Photodiode, 27...lateral npn
) transistor, 28...lateral pnp transistor. 30.31... Light receiving element. Name of agent Patent attorney Toshio Nakao 1 person Figure 3 (α) treno (Cn (dλ) ((1)

Claims (2)

[Claims] (1) An image sensor having a phototransistor as an optical signal reading pixel, wherein the phototransistor has a basic structure of a lateral transistor and is sensitive to light. (2) An image sensor in which a photodiode is used as an optical signal reading pixel and a transistor for optical signal amplification is added to each photodiode in order to amplify the optical signal obtained from each photodiode, and the optical signal amplification transistor is added to each photodiode. An image sensor characterized in that a transistor for use in the image sensor is a lateral transistor.