JPH06104477A - Phototransistor - Google Patents

Abstract

PURPOSE:To considerably reduce junction capacitance (Cbc) between a base and a collector by using an amorphous Si film at a light-receiving portion thereby completely depleting the amorphous Si film in an operative bias state. CONSTITUTION:An n-type Si crystal substrate 1, a p-type base diffused layer 2 formed on said Si substrate 1 and an n-type emitter diffused layer 3 formed on said layer 2 constitute an npn-type transistor having the substrate 1 as a common collector. Also, a p-type amorphous Si film 4 deposited on the substrate 1 creates a photodiode between said film 4 and the substrate 1 and generates a photocurrent and acts as a light-receiving portion. Also, the amorphous Si film 4 is electrically connected to the base diffused layer 2 with an electrode pattern 5 formed to a net pattern. Also, an electrode pattern 6 connected to an emitter electrode of transistor is provided. By doing this, the amorphous Si film 4 is completely depleted in operation bias state so that Cbc becomes extremely small.

Description

Detailed Description of the Invention

[0001]

[Industrial application] With the progress of image processing and image communication equipment, the need for high-performance image sensors is increasing. The present invention relates to a phototransistor for forming a high performance image sensor.

[0002]

2. Description of the Related Art An image sensor using an integrated circuit uses a photodiode or a phototransistor as a light detecting element. Photocurrent is about 1 in photodiode
nA / lx / mm ² , about 100n for phototransistor
A / lx / mm ² . The phototransistor has a large photocurrent because the sensing element itself has an amplification function. Further, in the image sensor, the light receiving area becomes smaller and the photocurrent becomes smaller as the resolution becomes higher. Further, in order to improve the sensitivity, various image sensors employ a drive system in a charge storage mode in which photocurrent is stored for a certain period of time and concentratedly output at a signal output timing. For example, the CCD image sensor guides the optical signal charges accumulated in the photodiodes to the potential well of the CCD all at once at the reading timing, and then transfers them to the output amplifier side along the potential well to obtain an image signal. M
The OS image sensor charges a photodiode or a phototransistor with a certain amount of electric charge, then discharges it by a photocurrent until the next reading timing, and obtains an accumulated image signal by the next recharging. A bipolar image sensor charges a phototransistor with a certain amount of electric charge, then discharges it with a photocurrent until the next reading timing, and amplifies by a transistor function at the next recharging timing, and then obtains an accumulated image signal. .

The sensitivity of the light detecting section is highest in the storage system using a phototransistor. Planar and cross-sectional structures of a phototransistor in a conventional example are shown in FIGS. This is a structure in which a base diffusion layer 2 having a transistor amplifying function and a photoelectric conversion function is formed on an n-type Si substrate 1, and an n-type emitter diffusion layer 3 is formed on the surface thereof. The diffusion layer is widened to form a photoelectric conversion unit, and the collector-base capacitance is large. A diffusion layer 8 is formed at the same time as the diffusion of the emitter and serves as a collector electrode. FIG. 5 shows an equivalent circuit of the image sensor including the phototransistor 30, the access field effect transistor (FET) 34, the reset FET 36, and the scanning shift register 37. For easy understanding, the phototransistor 30 is represented by a photocurrent source 31, a base-collector capacitance 32, and a transistor 33. The photocurrent generated at the base-collector junction discharges the electric charge stored in the base-collector capacitance, and as a result, the collector-base voltage decreases. This voltage change is conducted from the emitter electrode of the phototransistor 30 to the access FET 34 at a constant time interval (accumulation time) so that the image output line 3
5 to output an image signal, and subsequently to make the reset FET 36 conductive, the emitter potential of the phototransistor 30 is reset to zero potential. This system has a simple circuit and is quite sensitive, but this phototransistor has a problem that high-speed reading is difficult because of a large time constant for recharging. Therefore, G3
It can be used as a fax machine for classes, but the accumulation time is short,
For G4 class fax with a high scanning frequency, sufficient sensitivity cannot be obtained due to insufficient sensitivity, and afterimage performance deteriorates due to incomplete recharging.

In addition to resetting the emitter electrode,
A method of reducing the afterimage by resetting the base electrode has been proposed (Japanese Patent Laid-Open No. 2-83976), but this method complicates the sensor circuit and the drive circuit.

[0005]

The phototransistor requires a wider base region than an ordinary transistor, and therefore the junction capacitance Cbc between the base and collector becomes large. Therefore, the sensitivity of the signal voltage represented by photocurrent × accumulation time / Cbc does not increase in proportion to the light receiving area. Further, in the operation in the charge storage mode, as shown in FIG. 5, Cbc is charged with the base electrode in a floating state, but a base-emitter junction is interposed in series in the charging circuit, which becomes a non-linear resistance. Therefore, the charging time to the junction capacitance increases extremely with Cbc. Therefore, during high-speed scanning in which the charging time cannot be long, recharging is incomplete and the afterimage performance deteriorates. Attempts have been made to reduce Cbc by forming a phototransistor on a high-resistance Si substrate, but there is a limit to further increase the resistance of the substrate in terms of manufacturing technology.

[0006]

A phototransistor according to a first embodiment of the present invention has a transistor having a collector layer, a base layer, and an emitter layer formed on a Si crystal substrate and the same conductivity type as the base of the transistor. It is composed of an amorphous Si film which is ohmic-connected to the base layer and forms a p / n junction with the collector layer of the transistor.
The amorphous Si film forms a photodiode junction with the collector layer and supplies photocurrent to the base layer. In the case of a substrate common type in which the transistor uses the substrate as a common collector, an amorphous Si film is deposited on the base film and the substrate, and the base layer of the transistor and the amorphous Si layer are ohmic-connected. In this structure, the photodiode is directly formed between the amorphous Si film and the substrate.

The phototransistor according to the second embodiment of the present invention is provided on a substrate and a transistor having a Si crystal substrate as a collector, a base layer formed on the substrate by a diffusion method, and an emitter layer formed on the base layer by a diffusion method. And a photodiode formed of a pin 3 layer film of amorphous Si. The p-type amorphous Si film is ohmic-connected to the base layer by the back electrode, and the n-type amorphous Si film is ohmic-connected to the collector that is the substrate. It is also possible to form a photodiode of an amorphous Si film on the transistor in order to improve the aperture ratio.

[0008]

The phototransistor of the present invention has a composite structure of a normal transistor and a photodiode including an amorphous Si film and a collector layer. The base-collector junction capacitance Cbc of the transistor is small because the base-collector junction area is small, and the amorphous Si film and crystalline S
Although the photocurrent of the photodiode formed of the collector layer made of the i substrate is almost the same as that of crystalline Si, its junction capacitance is extremely small in the operating bias state because the amorphous Si film is completely depleted. . Therefore, Cbc can be reduced while maintaining the photocurrent, the signal voltage sensitivity of the phototransistor is increased and the afterimage performance is improved in the operation in the charge storage mode. Further, in the case of a photodiode having an amorphous Si film having a three-layer structure, its junction capacitance is further reduced and the signal voltage sensitivity is improved.

[0009]

EXAMPLE A plan view and a sectional structure of a phototransistor in Example 1 of the present invention are shown in FIGS.
The n-type Si crystal substrate 1, the p-type base diffusion layer 2 formed on the Si substrate 1, and the n-type emitter diffusion layer 3 formed on the base diffusion layer 2 constitute an npn-type transistor using the substrate as a common collector. To do. Reference numeral 4 denotes a p-type amorphous Si film deposited on the substrate 1, which forms a photodiode with the substrate 1 and generates a photocurrent to act as a light receiving portion. Further, the amorphous Si film 4 is electrically connected to the base diffusion layer 2 of the transistor by the electrode pattern 5 formed in a stitch shape. Reference numeral 6 is an electrode pattern connected to the emitter electrode of the transistor. The electrode patterns 5 and 6 are preferably Al-Si-Cu films capable of good ohmic contact with crystalline Si and amorphous Si films. Reference numeral 7 is a SiO ₂ film formed on the substrate 1. Generally amorphous Si
The carrier concentration of the film is smaller than that of crystalline Si, but in this embodiment, it is at least smaller than the carrier concentration of the n-type Si substrate 1. When forming a junction by the diffusion method, the carrier concentration of the diffusion layer must be larger than the carrier concentration of the substrate, and the depletion layer does not spread to the side of the diffusion layer with a large carrier concentration, thus increasing the junction capacitance. However, with the deposition method, it is possible to form a thin film having a carrier concentration smaller than that of the substrate on the substrate, and it is possible to suppress an increase in capacity. FIG. 2 is an equivalent circuit when the phototransistor according to the present invention is operated in the charge storage mode. The phototransistor 10 according to the present invention is equivalent to an amplifying transistor 11, a base-collector capacitance 12 (capacitance value, Cbc) of the transistor, and a photocurrent source 13 generated by a junction between an amorphous Si film and a substrate.
It is represented by the junction capacitance 14 (capacitance value, Ca). The access FET 15 acts to guide the optical signal charge generated in the phototransistor 10 to the output line 16 according to the scanning signal from the scanning circuit, and the resetting FET 17 sets the potential of the emitter electrode of the phototransistor after reading to zero or a desired potential. To reset to. 1
8 is a positive power supply line, 19 is an access pulse input terminal,
Reference numeral 20 is a reset pulse input terminal. By applying a reset pulse to the gate electrode of the reset FET 17 at an accumulation time interval, the emitter voltage of the phototransistor is reset to zero or a desired voltage, and the access pulse is input to the access pulse input terminal 1 immediately before each reset.
Application to 9 leads the signal voltage appearing at the emitter of the phototransistor to the output line 16. Figure 1
When the phototransistor of Example 1 shown in FIG. 2 is operated by the circuit shown in FIG. 2, a voltage of several V is applied between the collector and the base, and the amorphous Si film is completely depleted.
Since the amplifying transistor 11 does not need a base region as a light receiving portion, the base area is small and the capacitance value of the capacitor 12 is small. Also, the junction capacitance between the amorphous Si film and the substrate 1
In No. 4, the carrier concentration of the amorphous Si film is small, almost completely depleted in the operating bias state, and the capacitance value Ca becomes extremely small.

Therefore, although the collector-base capacitance Cbc is about 1.2 pF in the conventional phototransistor having an area of the base layer including the light receiving portion of 7000 μm ² , it is Cbc in the phototransistor of the same size according to the present invention. It became about 0.15 pF. Even when the amorphous Si film is completely depleted, the photocurrent generated by photoexcitation is almost the same as when it is not depleted. The output signal voltage is theoretically I
It is represented by L × Tint / Cbc. So IL is the photocurrent,
Tint is the accumulation time. As a result of the experiment, the output signal sensitivity of the conventional phototransistor is about 1
Although it was 1 V / Lx.s, the output signal sensitivity of the phototransistor of this example was about 80 V / Lx.s, which was extremely high. The afterimage performance was about 7% for the conventional phototransistor, but about 2% for the phototransistor of the present invention. Therefore, according to the phototransistor of the present embodiment, the sensitivity and the afterimage performance are significantly improved, the illuminance on the sensor surface can be reduced, and the cost of the light source in the sensor unit can be reduced. Furthermore, since the band gap of the amorphous Si film is larger than that of crystalline Si, the spectral sensitivity is closer to the luminosity curve, which is advantageous.

Although FIG. 1 shows the case of the n-type substrate, the same action and effect can be exhibited even if the conductivity type is completely reversed. That is, after forming the base diffusion layer 2 composed of an n-type diffusion layer on the p-type substrate 1 and the p-type emitter diffusion layer 3 on the base diffusion layer 2, an n-type amorphous Si film is deposited on the substrate 1. By doing so, a photodiode including a pnp transistor and an n-type amorphous Si film and a crystal substrate is formed. The amorphous Si film and the base layer are ohmic-connected with an electrode pattern. The second base diffusion layer can be completely depleted in the operation bias state. As a result, the effective collector-base capacitance can be significantly reduced as in the phototransistor of FIG.

FIG. 3 shows a plane and sectional structure of a phototransistor according to the second embodiment of the present invention. The n-type Si substrate 1, the p-type base diffusion layer 2 formed on the Si substrate 1, and the n-type emitter diffusion layer 3 formed on the base diffusion layer 2 are
This is an npn transistor having a so-called substrate as a common collector. Reference numeral 21 denotes an AL-Si-Cu film, and the pi formed by the three-layer amorphous Si film 22 deposited on the surface of the AL-Si-Cu film.
It is a substrate-side electrode of the n-photodiode and is connected to the base layer of the transistor. The three-layered amorphous Si film 22 is sequentially deposited from the substrate side in the order of a p-type film, an i-type film, and an n-type film. Reference numeral 23 is an electrode pattern for ohmic-connecting the surface of the photodiode and the substrate, and is formed in a stitch shape on the surface of the photodiode. Since this photodiode has a three-layer structure including an intrinsic layer of an amorphous Si film, the junction capacitance is further reduced. That is,
This device structure is characterized in that Cbc is small even when the operating bias voltage is low.

In order to apply the phototransistor according to the present invention to a one-dimensional image sensor, an image sensor is formed from this phototransistor which becomes a one-dimensional array of pixels and a one-dimensional scanning shift register according to the circuit shown in FIG. Can be configured. Further, it can be applied to a two-dimensional sensor by forming a two-dimensionally arranged phototransistor and a scanning circuit for scanning each phototransistor in an XY matrix.

[0014]

In the phototransistor of the present invention, the amorphous Si film is completely depleted in the operating bias state by using the amorphous Si film in the light receiving portion, and
The capacity between collectors can be significantly reduced. Therefore, the sensitivity of the signal voltage output to the emitter electrode of the phototransistor and the afterimage performance are significantly improved. Therefore, the present invention is useful as a photodetector operating in the charge storage mode and an image sensor for reading a document, and its industrial effect is great.

[Brief description of drawings]

FIG. 1 is a plan view and a sectional view of a phototransistor according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit showing the operation of a phototransistor in a charge storage mode.

FIG. 3 is a plan view and a sectional view of a phototransistor according to a second embodiment of the present invention.

FIG. 4 is a plan view and a sectional view of a phototransistor in a conventional example.

FIG. 5 is an equivalent circuit diagram of the image sensor.

[Explanation of symbols]

1 Si Substrate 2 Base Diffusion Layer 3 Emitter Diffusion Layer 4 Amorphous Si Film 5, 6 Electrode Pattern 10 Phototransistor 11 Transistor 12 Transistor Base-Collector Capacitance 13 Photocurrent Source 14 Amorphous Si Film-Collector Interlayer Capacitance 15 Access FET 16 image signal output line 17 reset FET 21 photodiode-side electrode of amorphous Si film 22 three-layer amorphous Si film 23 electrode pattern connecting photodiode surface and substrate 30 phototransistor 34 access FET 35 image signal Output line 36 Reset FET 37 Scanning shift register

Claims (5)

[Claims] 1. A transistor comprising a collector layer, a base layer, and an emitter layer formed on a Si crystal substrate, and a transistor having the same conductivity type as that of the base layer of the transistor, connected to the base layer of the transistor, and a collector layer of the transistor. Between p
A phototransistor characterized by comprising an amorphous Si film deposited so as to form a / n junction and acting as a light receiving portion, and significantly reducing the base-collector capacitance. 2. Amorphous Si deposited on a substrate, which is connected to a transistor formed of a Si crystal substrate as a collector, a base layer formed on the substrate by the diffusion method, and an emitter layer formed on the base layer by the diffusion method, and the base layer. 2. The phototransistor according to claim 1, wherein the phototransistor is made of a film and extracts an image signal from the emitter electrode. 3. A Si crystal substrate is an n-type and amorphous Si
The phototransistor according to claim 2, wherein the film is p-type. 4. A transistor comprising a Si substrate, a base layer having the substrate as a collector and formed on the substrate by a diffusion method, and an emitter layer formed on the base layer by a diffusion method, and a base layer and an ohmic layer provided on the substrate. A photodiode including a first amorphous Si film having the same conductivity type as the base layer connected thereto, an intrinsic second amorphous Si film, and a third amorphous Si film having the same conductivity type as the collector layer ohmic-connected to the collector layer. And a phototransistor characterized in that the base-collector capacitance is significantly reduced. 5. The Si substrate is n-type and the transistor is npn.
5. A mold, wherein first to third amorphous Si layers are formed on a region including a transistor.
Phototransistor.