CN110112153A - A kind of TOF imaging sensor demodulation pixel structure of charge fast transfer - Google Patents

Abstract

The invention discloses a TOF image sensor demodulation pixel structure with rapid charge transfer, including a PPD-based double floating diffusion node 4T pixel unit, and the PPD-based double floating diffusion node 4T pixel unit is responsible for transferring photocharges from the PPD The two transfer transistors transferred to the double FD node are characterized in that the gate of each transfer transistor adopts a plurality of adjacent polysilicon gates with a minimum width, and the plurality of polysilicon gates are connected to the resistor array, Power is supplied to the polysilicon gate through the resistor array, so that there is an increasing gate voltage on the polysilicon gate, and the gate voltage is capacitively coupled into the semiconductor to form a lateral drift electric field. The present invention replaces the gate of the transfer tube with a plurality of adjacent polysilicon gates with minimum width, and adds an increasing voltage to the gate to form a lateral drift field of photocharges under the gate of the transfer tube to accelerate the photocharge. transfer.

Description

Demodulation pixel structure of a TOF image sensor with fast charge transfer

technical field

The invention relates to the technical field of image sensors, in particular to a demodulation pixel structure of a TOF image sensor with fast charge transfer.

Background technique

In the TOF image sensor demodulation pixel structure based on clamped photodiode (pinned photodiode, PPD), the photocharge transmission from PPD to FD node is controlled by the diffusion effect in the semiconductor, and the photocharge generated by PPD can only pass through the semiconductor. Diffusion is transferred, which seriously affects the transfer speed of photoelectric charges, which limits the use of pixels at high modulation frequencies. Due to the slow diffusion process, it will affect the performance of PPD. Some solutions obtain a larger photosensitive area by changing the shape of the device to increase the generation of photocharges; there are also solutions that increase the injection of impurities in the photosensitive area to optimize PDD performance; there are also buried trenches under the gates of the transmission tubes TX1 and TX2 to speed up photocharge transmission. of speed.

Contents of the invention

The object of the present invention is to aim at the technical defects existing in the prior art, and to provide a TOF image sensor demodulation pixel structure based on clamped photodiode (pinned photodiode, PPD) to speed up photocharge transfer, by changing the gate of the transfer tube structure to accelerate photocharge transfer.

The technical scheme adopted for realizing the purpose of the present invention is:

A TOF image sensor demodulation pixel structure with fast charge transfer, comprising: a PPD-based double floating diffusion node 4T pixel unit, the PPD-based double floating diffusion node 4T pixel unit is responsible for transferring photocharges from the PPD The two transmission tubes to the double FD node are characterized in that the gate of each of the transmission tubes adopts a plurality of adjacent polysilicon gates with a minimum width, and the plurality of polysilicon gates are connected to the resistor array, through The resistor array supplies power to the polysilicon gate, so that there is an incremental gate voltage on the polysilicon gate, and the gate voltage is capacitively coupled into the semiconductor to form a lateral drift electric field to speed up the process from the generation region The transfer speed of the photo charge transferred to the FD node.

Each resistor in the resistor array has the same resistance value.

Each of the polysilicon gates is connected to a corresponding resistor in the resistor array.

Compared with prior art, the beneficial effect of the present invention is:

The present invention proposes a TOF image sensor demodulation pixel structure with fast charge transfer, by replacing the gates of the transmission tubes TX1 and TX2 with a plurality of adjacent polysilicon gates with the minimum width, and adding an increasing voltage to the gates To form a lateral drift field of photocharges under the gate of the transfer tube to speed up the transfer of photocharges.

Description of drawings

Figure 1 shows the existing PPD-based dual floating diffusion node 4T pixel structure;

FIG. 2 shows the demodulation pixel structure of the TOF image sensor for accelerating charge transfer according to the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

As shown in Figure 1, the existing PPD-based dual floating diffusion node 4T pixel structure includes clamping photodiodes PPD, transmission transistors (TX1, TX2), reset transistors RST, floating diffusion nodes (FD1, FD2), Row gating tubes (RS1, RS2) and source followers (SF1, SF2), floating diffusion nodes (FD1, FD2) are connected to clamp photodiodes PPD through transmission tubes (TX1, TX2), and follow the source Devices (SF1, SF2), and the reset transistor RST are connected, the reset transistor RST and the source follower (SF1, SF2) are connected to VDD, the source follower (SF1, SF2) is connected to the row gate transistor (RS1, RS2), Row strobes (RS1, RS2) are connected to the bus.

When working, the transmission tube TX1 and the transmission tube TX2 are turned on alternately. If the transmission tube TX1 is turned on first, the transmission tube TX2 is turned off, and the accumulated photoelectric charge is transferred to the floating diffusion node FD1, and passes through the row gating tube and column. stage readout circuit to complete the integration of a certain phase; then the transfer tube TX2 is turned on, the transfer tube TX1 is turned off, and the readout of the other phase is completed through the same process. Such a dual FD pixel structure can be used in one frame The information of the two phases is read out, and the information of the four phases required by the TOF image sensor can be obtained in two frames.

As shown in Fig. 2, the demodulation pixel structure of the TOF image sensor with fast charge transfer of the present invention is to use 7 gates of the transmission tube TX1 and the transmission tube TX2 in the original double floating diffusion point structure (according to the demand, It can be more) adjacent polysilicon with the smallest width instead, and these polysilicon gates are powered by corresponding resistor arrays outside the pixel, so that there is an incremental voltage on the polysilicon gate, and the gate voltage is capacitively coupled into the semiconductor, forming A lateral drift electric field speeds up the transport of photocharges from the generation region to the FD node.

In the present invention, the gate structures of the transmission transistors TX1 and TX2 in the original structure are replaced by seven adjacent polysilicon gates with the minimum width to form a polysilicon gate 10, and these adjacent polysilicon gates need to be very close to each other through capacitive coupling A stable lateral drift field is formed in the semiconductor, but due to the conductivity of polysilicon, the polysilicon gate cannot contact each other, and the resistor array 20 used outside the pixel (formed by a plurality of resistors 21 connected by wires 22, the input terminal of each resistor 21 It is connected with a polysilicon gate, and the output terminal is connected with the adjacent polysilicon gate 10 and connected with the input end of the adjacent resistor 21), each resistor resistance value in the resistor array 20 is the same, V1 and VD of each resistor array A stable voltage of 7V is added between them. Since the resistance value is the same, a voltage drop of 1V is generated at both ends of each resistor, so that the polysilicon gate is supplied with an increasing voltage, and the gate voltage is capacitively coupled to the semiconductor to form a The lateral drift electric field speeds up the transport speed of the photo-charge transported from the generation area to the FD node.

The present invention proposes a TOF image sensor demodulation pixel structure with fast charge transfer, which is suitable for TOF image sensors with high requirements for distance accuracy and fast response speed. This type of pixel requires high modulation light frequency to collect phase charges representing depth information. This structure The lateral electric field generated by the gate of the medium-transfer tube in the semiconductor enables the rapid transport of photocharges, and the rapid transfer of charges under highly modulated light improves the accuracy.

The demodulation pixel structure for accelerating charge transfer proposed by the present invention can form a lateral drift electric field in the semiconductor under the gate of the transfer tube for rapid transport of photocharges. Compared with traditional demodulation pixels, this structure improves the output of photocharges. This ability significantly improves the working characteristics of the device and enables the TOF image sensor to detect light with high modulation frequency.

The above is only a preferred embodiment of the present invention, it should be pointed out that, for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, these improvements and Retouching should also be regarded as the protection scope of the present invention.

Claims (3)

1. the TOF imaging sensor demodulation pixel structure of a kind of charge fast transfer, including double floating diffusion nodes based on PPD 4T pixel unit, double floating diffusion node 4T pixel units based on PPD are double with being responsible for for optical charge being transferred to from PPD Two transfer tubes of FD node, will be it is characterized in that, the grid of each transfer tube uses multiple phases with minimum widith Adjacent polysilicon gate, multiple polysilicon gates are connect with electric resistance array, by the electric resistance array to the polysilicon gate Pole power supply, so that there is incremental grid voltage on the polysilicon gate, the grid voltage is capacitively coupled to semiconductor In and form the drift field of a transverse direction, to accelerate to be transferred to from generating region the transmission speed of the optical charge of FD node.

2. the TOF imaging sensor demodulation pixel structure of charge fast transfer as described in claim 1, which is characterized in that described Each resistance in electric resistance array is identical.

3. the TOF imaging sensor demodulation pixel structure of charge fast transfer as described in claim 1, which is characterized in that each The polysilicon gate connects a resistance in a corresponding electric resistance array.