CN103928485B - A kind of radioscopic image sensor of high output amplitude - Google Patents

Abstract

The present invention provides an X-ray image sensor with high output amplitude, which includes a timing control circuit, a pixel unit circuit, a sampling and holding circuit, and an output buffer circuit. one. The reset switch tube adopts a PMOS transistor structure, and the PMOS transistor fabricated on the P-type substrate is fabricated in an N-well, and the charge collector is a diode formed between the N-well and the P-type substrate (N-well/P substrate diode). In the improved pixel circuit of the present invention, the reset transistor in the pixel structure is changed from a common NMOS transistor to a PMOS transistor, and the PMOS reset transistor is built in an N-well structure forming a charge collection diode. In this way, the PMOS transistor plays both the functions of the collector well and the reset transistor. On the one hand, it avoids the effect of introducing redundant N wells on the collection of radiation charges. On the other hand, because the reset transistor is changed from an NMOS transistor to a PMOS transistor, the cathode of the charge collector The voltage can be better reset to the power supply voltage, thereby increasing the signal voltage output amplitude.

Description

A High Output Amplitude X-ray Image Sensor

Technical field:

The invention relates to the technical fields of radiation imaging and high-energy particle detection, in particular to an X-ray image sensor with high output amplitude.

Background technique:

In the prior art, a CMOS image sensor mainly includes a timing control circuit, a pixel unit circuit, a sample-and-hold circuit and an output buffer circuit, as shown in FIG. 1 . Among them, the pixel unit circuit is the most basic unit for detecting X-rays, and the quality of its output signal directly determines the final performance of the image sensor.

In the existing X-ray image sensors, the pixel circuit of the CMOS image sensor that directly detects X-rays generally uses an N-well on a P-type silicon substrate to form an N-well/P-substrate diode for collecting radiation excitation. Because the production of PMOS transistors also requires N wells, in order to avoid the influence of the extra N wells introduced by PMOS transistors on the collection of charges excited by radiation, generally in the pixel of X-ray radiation imaging CMOS active pixel image sensors NMOS transistors are used in the circuit, that is, all MOS transistors in the pixel are NMOS transistors, and the structure of the collector element is N well/P substrate diode, as shown in Figure 2. Since only NMOS transistors can be used in the pixel to build structures such as reset, source follower, and strobe, the existence of the threshold voltage of the transistor will make the amplitude of the output signal continuously decrease, and the dynamic range of the final signal obtained is small, and the signal-to-noise ratio is small, which is not conducive to signal Post-processing imaging. Obviously, using NMOS transistors to construct the pixel circuit avoids the introduction of other N wells outside the radiation charge collection well, but it greatly limits the amplitude of the pixel output signal.

Invention content:

In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide an X-ray image sensor with a wide dynamic range of the output signal and a high output amplitude which is beneficial to the later imaging processing.

To achieve the above object, the present invention adopts the following technical solution: an X-ray image sensor with high output amplitude, including a timing control circuit, an M×N-bit pixel circuit, an N-bit sampling and holding circuit, and an output buffer circuit, characterized in that, M ×N-bit pixel circuit, N-bit sample-and-hold circuit and output buffer circuit together form a CMOS active pixel image sensor under the control of a sequential circuit; the collection element of the pixel circuit and the reset transistor are combined into one, and a PMOS transistor structure.

Further, the pixel circuit is composed of a PMOS reset transistor, a source-following transistor, a line-selecting transistor and a radiation charge collection element; the PMOS reset transistor is connected to the NMOS source-following transistor, and the voltage output by the source-following transistor is output through the row-selecting transistor;

The radiation charge collection element is an N well structure constructed on a P-type substrate to form an N well/P substrate diode, and the source and drain are formed by P+ implantation in the N well used as the charge collection element. A PMOS transistor is fabricated in an N well structure; the substrate of the PMOS transistor, that is, the N well, is used for charge collection, and the gate of the PMOS transistor is controlled to realize integral reset of the charge collection unit.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention constructs a PMOS reset transistor by means of an N well used as a collector, and combines the reset transistor and the collector structure into one, without affecting the collection of radiated charges, and at the same time can greatly increase the amplitude of the output signal. In the three-tube pixel structure using 5V--0.5um technology, the output voltage of the pixel in this structure is increased from 2.04V in the conventional structure to 3.32V, which is about 65% higher, so that the amplitude of the output signal is greatly improved, and the dynamic The range is extended and the signal-to-noise ratio is improved.

The improvement of the pixel circuit in the invention greatly increases the output voltage amplitude of the pixel signal, so that the performance of the image sensor is greatly improved.

Description of drawings:

Figure 1 is a circuit block diagram of an X-ray active pixel image sensor.

Fig. 2 is a schematic diagram of a pixel circuit in an X-ray active pixel image sensor.

FIG. 3 is a schematic diagram of the pixel circuit of the X-ray active pixel image sensor of the present invention.

detailed description:

As shown in Figure 1, a typical X-ray image sensor includes a timing control circuit, an M×N-bit pixel circuit, an N-bit sampling and holding circuit, and an output buffer circuit. Under the control, they jointly form a CMOS active pixel image sensor. The present invention mainly improves the pixel circuit of the X-ray image sensor. As shown in FIG. 3 , the collecting element and the compound tube are combined into one, and a PMOS transistor structure is adopted.

As shown in Figure 3, the pixel circuit is composed of a PMOS reset transistor, a source follower transistor, a row selection transistor, and a radiation charge collector; the PMOS reset transistor is connected to the NMOS source follower transistor, and the voltage output by the source follower transistor is output through the row selection transistor ; The radiation charge collection unit is an N well/P substrate diode on a P-type substrate, and the source and drain are formed by P+ implantation in the N well used as the charge collection unit, and are made in the N well. PMOS transistor; the substrate of the PMOS transistor, that is, the N well, is used for charge collection, and its gate is controlled to realize the integral reset of the pixel circuit. In the present invention, the radiation charge collecting element adopted by the pixel circuit is an N well/P substrate diode on a P-type substrate, and the making of a PMOS transistor also needs to build an N well on a P-type substrate. The construction of PMOS transistors on the bottom plays the role of charge collector and reset MOS transistor at the same time. The PMOS transistor itself is used as a reset transistor to replace the original NMOS reset transistor to increase the output voltage. On the other hand, the N-well structure of the PMOS transistor is used. As a radiation charge collection trap to achieve charge collection. This allows the PMOS transistor to simultaneously function as a radiation charge collection trap and as a reset transistor. The improvement of the pixel circuit in the invention greatly increases the output voltage amplitude of the pixel signal, which can greatly improve the performance of the image sensor.

Claims (1)

1. An X-ray image sensor with a high output amplitude, comprising a timing control circuit, an M × N bit pixel circuit, an N bit sample-and-hold circuit and an output buffer circuit, characterized in that the M × N bit pixel circuit and the N bit The sample-and-hold circuit and the output buffer circuit jointly form a CMOS active pixel image sensor under the control of the sequential circuit; the collection element and the reset transistor of the pixel circuit are combined into one, and a PMOS transistor structure is adopted; The pixel circuit is composed of a PMOS reset transistor, a source-following transistor, a line-selecting transistor and a radiation charge collection element; the PMOS reset transistor is connected to the NMOS source-following transistor, and the voltage output by the source-following transistor is output through the row-selecting transistor; The radiation charge collection element is an N well structure constructed on a P-type substrate, and a source and a drain are formed by P+ implantation in the N well used as the charge collection element, and a PMOS transistor is fabricated in the N well structure; The substrate of the PMOS transistor, that is, the N well, is used for charge collection, and the gate of the PMOS transistor is controlled to realize integral reset of the charge collection unit.