JPH0448862A - Image sensor and its drive method - Google Patents

Abstract

PURPOSE:To improve an after-image rate considerably by connecting a reverse junction diode by diffusion of a 2nd emitter and a reset switch between power supplies for a reset. CONSTITUTION:A base region of reset switches 7(7a-7g) is reset via a reverse junction diode. A base electrode of a photo transistor is not completely floated, but a very small current is injected to the base via the reverse junction diode of a 2nd emitter. Thus, since not only a collector voltage but also a base voltage is increased at the latter half of charging, the sensor gets active in a shorter time than that of a conventional sensor. As a result, an after-image rate is considerably improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an image sensor that makes it possible to read document information at high speed and with high quality. The need for high-performance image sensors is increasing Even though image sensors using integrated circuits use photodiodes or phototransistors as light detection elements, the sensitivity of photodiodes when directly outputting photocurrent using the direct method is approximately 5nA/<Iux-mrn”), and approximately 100 nA/(lu x-mm2) for a phototransistor.The phototransistor has a high sensitivity because the sensing element itself has an amplification function.In addition, the image sensor has a high resolution. As the height increases, the light-receiving area becomes smaller, and the sensitivity becomes insufficient in the direct method.In order to increase the sensitivity from the drive method, photocurrent is accumulated for a time corresponding to the output interval of the image signal.
Although various image sensors use a drive method based on the accumulation method that outputs signals intensively at the timing of signal output, for example, in a CCD image sensor, the optical signal charges accumulated in the photodiode at the reading timing are transferred to the CCD image sensor.
The signal is guided to the potential well D, and then transferred along the potential well to the output amplifier side to obtain an image signal.
MOS image sensors charge a photodiode or phototransistor with a certain amount of charge, then discharge it with a blocking current until the timing of the next reading, and obtain an accumulated image signal by recharging.Bipolar image sensors use a phototransistor. Even if the accumulated image signal is obtained after discharging it with a photocurrent and amplifying it by the transistor function at the next recharge timing, the sensitivity of the photodetector section is The storage method used is the one with the highest Ls.The time constant for recharging is long, and the problem is that there is a large afterimage, making high-speed reading difficult. A circuit diagram of a conventional image sensor shown in Fig. 1 (1a-I
g) is a phototransistor, 2 (2a to 2g) is the output terminal of the scanning circuit, 3 (3a to 3g) is the charging switch a, 4 is the input terminal of the bias power supply for setting the charging voltage, 5 is the image signal output Terminal 20 is a scanning circuit.
According to the scanning signal, the switch 3 is turned on and off, and a charging current flows to the phototransistor 1, and an image signal is obtained from the terminal 5.More detailed explanation is as follows. When the switch 3 is turned on, the photocurrent generation function by the collector junction, its junction capacitance Cbo, and a transistor without photoresponse is charged with a constant charge determined by the charging voltage Vk6 and Cbo. Switch 3 is pressed for a certain period of time (
When the phototransistor 1 is turned off (accumulation time) and light is shining on the light receiving window of the phototransistor 1, a charge proportional to the amount of light is discharged. The quantity is obtained from the image signal terminal 5.

Problems to be Solved by the Invention Image sensors consisting of a photodetector section consisting of a phototransistor array and a scanning circuit have a large time constant for recharging, and especially when performing high-speed reading, there is a large afterimage, making clear reading difficult. be. Conventionally, when a phototransistor operates in charge storage mode, its base electrode is floating, and it is necessary to charge the collector-base junction capacitance.In this case, a base-emitter junction is inserted in series with the charging circuit. Because this becomes a nonlinear resistance, there is little discharge charge in the low exposure area, and sufficient base-emitter voltage is not provided, making charging time short.
As a result, the afterimage rate increases rapidly, especially in low-exposure areas.In high-speed scanning, this problem becomes even more severe because charging time cannot be long enough.The present invention takes into account the above problems and significantly reduces afterimages. Sensitivity It is an object of the present invention to provide an image sensor capable of high-speed reading and a method for driving the image sensor. A charging switch is provided between the collector of each of the phototransistors and the charging power source, and a reset switch is provided between the second emitter of each of the phototransistors and the reset power source. The first emitters are commonly connected to obtain an image signal.

In addition, the image sensor driving method of the present invention (1) outputs the charging current of the phototransistor as an image signal at the timing when only the charging switch is conductive, and then outputs the charging current of the phototransistor as an image signal at the timing when only the charging switch is conductive, and when the reset switch and the charging switch are activated following the timing when only the charging switch is conductive. The base region of the phototransistor is reset at the timing of both conduction.

According to the present invention, the base of each phototransistor is not floating, but the base of each phototransistor is connected to the reset power supply. The reset switch is turned on after accessing the phototransistor and obtaining the accumulated image signal.
Due to the action of the reverse junction diode, when the reset switch is on, the voltage between the base and the first emitter is increased at high speed, and the phototransistor becomes active in a shorter time than conventional ones, resulting in charging. The speed is increased and the afterimage rate is greatly improved.
FIG. 1 is a circuit diagram of an image sensor according to an embodiment of the present invention.a I Na-1g) is a phototransistor;
~2g) is the output terminal of the first scanning circuit 12, 3 (3a
~3g) is a switch for charging the phototransistor 1. These switches can be formed with a PNP transistor or a field effect transistor (FET). 4 is an input terminal of a bias power supply for setting the charging voltage;
5 is an image signal output terminal +, 6 (6a to 6g) is an output terminal of the second scanning circuit 13, and 7 (7a to 7g) is a reset switch that resets the base region via a reverse junction diode. An input terminal of a reset power supply for setting a reset voltage, 9 an input terminal (STI) of a start pulse manually applied to the first scanning circuit 12, and 10 an input terminal of the second scanning circuit 1.
Input terminal (Sr1) of the start pulse input to 3,
11 is a clock pulse input terminal (CK), 12 is a first scanning circuit 13 is a second scanning circuit 4 In the present invention, the base electrode of the phototransistor is completely floating. Even if a small current is injected through the reverse junction diode with two emitters, not only the collector voltage but also the base voltage is increased during the latter half of charging. The afterimage rate has been significantly improved. Figure 2 is a timing chart of the input and output of the scanning circuit in this embodiment. When a clock pulse (Figure 2 a) is input to the clock terminal, the same pulses as the start pulse (Figure 2 d) and (Figure 2 f) are sequentially output to the output terminal as shown in the figure. The image sensor according to the present invention outputs the charging current that flows between when the charging switch is turned on and when the reset switch is turned on as an accumulated image signal, and ignores the current that flows while the reset switch is turned on. By doing this, it is possible to obtain an output with a low afterimage rate without sensitivity variations due to variations in the characteristics of the reverse junction diode of each phototransistor and without switching noise for reset.

FIGS. 3(a) and 3(b) are a plan view and a cross-sectional view showing the device structure of a phototransistor, respectively. 14 is a base diffusion plate; 15 is a diffusion plate for protruding a collector bow; FIG.
I6 is the first emitter diffusion region 17 is the second emitter diffusion region K 18 is the P-type semiconductor base with 19tL bipolar I
Effects of the Invention According to the present invention, even if the second emitter is open to the base diffusion layer to form a reverse junction. It has become possible to significantly reduce afterimages in image sensors that use phototransistors as photodetecting elements, and achieve high sensitivity and high-speed reading, and the industrial effects are tremendous.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an image sensor according to an embodiment of the present invention Fig. 2 is a timing chart of input and output of two scanning circuits Fig. 3 is a device structure of a phototransistor used in the image sensor of the present invention @ Fig. 4 is an equivalent circuit diagram. FIG. 5 is a circuit diagram of a conventional image sensor. DESCRIPTION OF SYMBOLS 1... Phototransistor 2... Output terminal of the first scanning circuit, 3... Switch for charging, 4... Power supply for charging 5... Image signal output terminal 'Yu'6... Output terminal of second scanning circuit, 7... reset switch,
8...Reset electric I/1% 9...STI, 1
0...ST2, 11...CK, 12...First scanning circuit 13...Second scanning circuit a 14...Base diffusion region 15...Diffusion region #L for collector extraction
16...First emitter diffusion region @ 17...Second
Emitter diffusion region slope 18...Silicon substrate 19...
・Separate tilt plate 20... Name of the agent in the same area of scanning Patent attorney Shigetaka Awano Haka 1 person 14-
Draw S on the base 15-1, cover S1, scattering trowel for soiling 16-, emitter f, scatter lI area

Claims (2)

[Claims] (1) In an image sensor consisting of a phototransistor array, a scanning circuit, and a switch element, each phototransistor belonging to the phototransistor array has a first
and a second emitter, a charging switch inserted between the collector of each of the phototransistors and the charging power source, and a reset switch inserted between the second emitter of each of the phototransistors and the reset power source. An image sensor comprising: the first emitters being connected in common to obtain an image signal. (2) The charging current of the phototransistor is output as an image signal at the timing when only the charging switch is conductive, and the phototransistor is output at the timing when both the reset switch and the charging switch are conductive, following the timing when only the charging switch is conductive. 2. The method of driving an image sensor according to claim 1, further comprising resetting a base region of the transistor.