New Solid LBCAST

Introduction
Before the end of 2003, solid image sensors were also classified as CCD and CMOS, but Nikon, Japan, overridden this history in late 2003 by using a new solid LBCAST JFET image sensor (Lateral Buried Charge Accumulator and Sensing Transistor array Junction Field Transistor) in its C2H lens-converted anti-single digital camera, which was released in July 2003. It can be said that the combination of CCD and CMOS technology fully reflects the advantages of CMOS low power consumption and CCD high speed data reading, with a size of 23.3mm × 15.5mm with a diagonal length of 28.4mm and a total number of pixels of 4.260 million (2560) × 1664), 4.1 million effective pixels, 9.4 pixel interval μ M.

1 JFET and MOSFET
Field Effect TubeThere are mainly junction field effect transistors (JFETs) and insulated gate field effect transistors (IGFETs), which are often called MOSFETs because of their aluminum grid. They have the advantages of high input impedance, low noise, low power consumption, high thermal stability and strong radiation resistance. The difference is that the conductive mechanism and current control principle are fundamentally different. JFET uses the width change of the depletion zone to change the width of the conductive channel to control the drain current. MOSFET uses the electric field effect of a semiconductor surface.inductanceThe amount of charge charged changes the conduction channel to control the current. The difference in their properties is that JFET is often used in the input stage (front stage) of the amplifier, while MOSFET is used in the last stage (output stage) of the amplifier. But under some working conditions, the input of MOSFETresistanceIt is not high enough to meet the requirements, and when working at high temperature, the resistance value of PN junction decreases significantly as the reverse current of PN junction increases, and the leakage current is also large.
Features of 2 LBCAST JFET
In the competition for image sensor with high bandwidth and low power consumption, CMOS image sensor has shown more advantages in design than CCD: small size, low system cost and low power consumption on the premise of definite product quality. However, noise becomes a barrier to the success of CMOS, which will lead to the degradation of image quality. This is also one of the reasons why the noise problem must be solved. LBCAST JFET has a number of advantages, in addition toamplifierIn addition to using JFET, it is also related to its internal structure and working characteristics.
2.1 LBCAST JFET Reading Method
The current common reading methods for CCD and CMOS are sequential charge transfer and X-Y address and transmission. Figure 1 (a) The sequential charge transfer method commonly used for traditional Interline CCD image sensors is that the electrical signal converted from an optical signal is first transmitted to a column transfer register and then to an image processing unit, thereby limiting the speed. In addition, in theory, because sequential charge transfer requires continuous, high-speed drive conversion registers, this requires more power. Figure 1 (b) is an X-Y addressing and transmission method commonly used by CMOS image sensors in which each pixel has its own amplifier that transmits signals through column and row scans and outputs them to the image processing unit. It has a separate data transmission line and therefore achieves a high speed, but if you look closely at its output image, you can see that it is prone to image distortion on separate lines.

The JFET image sensor also uses X-Y addressing and transmission. The data is read out by two signal lines in different colors, which makes it faster to read the image and has the advantage of freely extracting high-density pixel data. The data distribution line of the JFET image sensor replaces the area method with the color method (green, blue and red). This improves the operation speed and image quality, and solves the problem that the output image is easy to reproduce distortion on separate lines.
The JFET image sensor separates the source according to the color. All green signals are output by one line, while all blue and red signals are output by another line, which makes the image unaffected by output amplifier fluctuations and ensures image quality. Because the human eye is particularly sensitive to green, the green signal line only processes the green signal, and it is especially important in image sharpening and setting the contrast of the image. When reading row data, apply the data line to the left of the column (G signal above, B signal below) and then read the second row. Then apply the data line to the right of the column (G signal above, R signal below) and proceed in turn (Figure 2), you can see that the R:G:B ratio is 1:2:1, usually colored.Wave filterRates are also designed based on this principle.
Functions of 2.2 JFET
Extracting Pixel Data from LBCAST JFETcrystalThe tube is a JFET, and each pixel contains a pair of charge accumulation components (i.e., photosensitive elements) and a JFET for detecting amplification.transistorIt can realize photoelectric conversion, storage and amplification. The amplifier in CMOS image sensor is MOSFET amplifier. When the camera shutter closes, the light is received at the end of a flash, the MOSFET grid used for transfer is opened, and all stored charge is transferred to the JFET grid. In addition, the JFET grid is equivalent to a measuring cup, through which you can read how much light is transferred to this "cup". JFET grid voltage follows from photoelectricdiodeThe transferred charge rises. In this case, the JFET causes the signal voltage to rise correspondingly and output it as the data read by the column signal line. After the image signal is read out, the JFET grid sends a charge to the MOSFET to reset it, so that the JFET grid can be switched on and off. In other words, JFET functions like a pixel switch that can be closed when a signal needs to be read. Compared with CMOS image sensors, the path of JFET is much simplified, which makes the speed greatly improved, the reliability enhanced, and the defective rate reduced.
2.3 Analysis of Internal Structure Features
In LBCAST JFET, because the charge accumulation part is transversely embedded, JFET becomes a channel structure clamped in the Gate (switch), an ideal amplifier element, and has higher sensitivity and lower noise than CMOS.
First, for a given signal, the LBCAST cup, although small, provides a larger voltage increment and higher resolution. Secondly, in the CMOS image sensor, the signal is channeled to the surface of the silicon; In LBCAST, the signal is transmitted through an internal channel, so the big noise is reduced to almost one-third of the previous level, and the dark current is very small, which can effectively suppress the dark noise. On the other hand, dual-channel extraction of pixel signals can achieve high-speed processing. In terms of structure, LBCAST camera pixels have one fewer metal layer than CMOS in wiring construction, and lower wiring density and fewer inter-layer connection holes. Thus, the goal of simple structure, few manufacturing failures and high yield is achieved.

The JFET sensor pixel selection switch consists of three transistors: transfer, JFET and reset. The CMOS sensor consists of four transistors, and the fourth transistor is used for pixel selection. Therefore, LBCAST has a simpler structure and higher efficiency than CMOS, and because photodiodes per unit area can be increased, it also improves its functionality. Internal connections (including opaque layers) are also simple and suitable for one polysilicon layer and two material layers. The CMOS is composed of four layers. The smaller the number of layers required, the shorter the distance between the photodiode and the microlens. LBCCAST sensor can effectively reduce the image noise taken in dark light by BPD (Burie D PhotoDiode), internal FPN (Fixed Pattern Noise) and other technologies. Similar to the CMOS two-channel reading method, it improves the data reading rate. Microlenses that are very close to the photosensitive unit improve the light efficiency while effectively improving the consistency of the center and corners of the picture. The LBCAST JFET pixel structure is shown in Figure 3 and the LBCAST JFET pixel profile is shown in Figure 4.
3 Concluding remarks
The main goal of the newly developed LBCAST JFET sensor is to focus on speed. Its total pixels are not very high, only 4 million. Nikon therefore positioned D2H cameras for LBCAST JFET image sensors in news reporting and sports photography.
LBCAST has higher sensitivity and lower noise than CMOS, and has simple structure, fewer manufacturing failures and higher product yield. Because of its simple structure, it can adopt the same manufacturing process as CMOS. It is expected that the manufacturing cost can be greatly reduced in the future, and the application prospects are very promising.

Source:Xiang Xueqin