Nikon's D2H SLR digital camera uses a new type of solid image sensor LBCAST JFET. The device has greatly improved in the reading method, internal structure, etc., compared with CCD, CMOS image sensor, has transient start, high sensitivity, high resolution, low energy consumption, high yield and low noise.
Prior to the end of 2003, the solid image sensor was also divided into CCD type and CMOS, but Nikon, Nikang, at the end of 2003, has rewritten this history, which is used in the D2H lens conversion-type anti-single digital camera released in July 2003. A new type of solid LBCAST JFET image sensor (Lateral Buried Charge Accumulator and Sensing Transistor Array Junction Field Effect Transistor). It can be said that the product of CCD and CMOS technology advantage, fully reflects the advantage of CMOS low power consumption and CCD high speed data read, the size is 23.3mm × 15.5mm, the diagonal length is 28.4mm, the total pixel is 4.26 million (2560 × 1664), the number of effective pixels is 4.1 million, and the pixel interval is 9.4 μm.
1, JFET and MOSFET
Field effect tube mainly tuned field effect tube (JFET) and insulated gate type field effect tube (IGFET, due to its metal aluminum is often referred to as MOSFET), has high input impedance, low noise, low power consumption, heat Advantages such as high stability and strong anti-radiation resistance. Their difference is that the conductive mechanism and the current control principle are at all. The JFET is a width change of the exhaustion zone to change the width of the conductive channel to control the drain current; the MOSFET is the electric field effect of the semiconductor surface, how much is electrically charged Change the conductive ditch to control the current. The difference in nature is that JFETs are often used in the amplifier input stage (front level), and the MOSFET is used in the active amplifier (output stage). However, under some working conditions, the input resistance of the MOSFET is not high enough, it is difficult to meet the requirements, and when operating at high temperatures, due to the increase in the PN junction, the resistance will be significantly decreased, and the drain current is also large.
2, characteristics of lbcast jfet
In the pursuit of high bandwidth, low-power image sensor competition, the CMOS image sensor exhibits more advantageous characteristics than CCD in the design: small size, low system cost, and low power consumption in its product quality. However, noise becomes the largest obstacle on CMOS success, which will lead to a decrease in image quality. This is also one of the reasons why noise problems must be solved. LBCast JFET has many advantages, in addition to the use of JFETs with the amplifier, it is related to its internal structure and work characteristics
2.1 LBCAST JFET reading method
Current readings commonly used in CCD and CMOS: sequential charge transfer mode and X-YDetry and transmission mode. Fig. 1 (a) is a sequential charge transfer method commonly employed by a conventional interline CCD image sensor first transmitted to the column transfer register, and finally output to the image processing unit, so the speed is limited. In addition, in theory, since the sequential charge transfer mode requires continuous, high-speed drive conversion registers, this requires more electrical power. Figure 1 (b) is a XY addressing and transmission mode commonly used by the CMOS image sensor. In this manner, each pixel has its own amplifier, transmitted by column scanning and row scanning, and outputs the image processing unit . It has a separate data transmission line, so it can achieve a high speed, but if it is carefully observed, it is possible to find that image distortion is prone to separatelines.
The JFET image sensor also uses X-Y addressing and transmission. The data is read out by two signal lines, so that the speed of reading the image is faster, and there is an advantage that the high density pixel data can be randomly extract. The JFET Image Sensor's data allocation line is used for the color method (green, blue and red) instead of the region's way, and improves the image quality while increasing the operating speed, solving the output image is easy to reproduce this. A problem.
The JFET image sensor is separated from the color separation of the signal; all green signals are output through a line, while all blue and red signals are output through another line, which makes the image not affect the influence of output amplifier, ensuring image quality. Since the human eye is particularly sensitive to the green, the green signal line only processes the green signal, and the green of the image sharpening and setting image contrast is particularly important. When reading the first line of data, apply the data line on the left (the G signal above, the B signal is output below), then read the second row data, at which point the data line (the upper output G signal is output, the following output R) Signals), continue (Figure 2), then you can see R: G: B ratio is 1: 2: 1, usually the ratio of color filter is also based on this principle.
2.2 JFET function
The transistor of extracting pixel data in the LBCAST JFET is a JFET, and each pixel includes a pair of charge accumulation portions (i.e., photosensitive elements) and JFET transistors detected, photoelectric conversion, storage, and amplification. The amplifier in the CMOS image sensor is a MOSFET amplifier. At the camera shutter, it is an end of a moment, and the MOSFET gate is opened, while all stored charges are transferred to the JFET gate. In addition, the JFET gate is equivalent to the amount of cups, and how many light loads can be read from the "cup" through the JFET. The JFET gate voltage increases from the charge transfer from the photodiode. At this time, the JFET enables the signal voltage corresponding to the data output as the column signal line. After the image signal is read, the JFET gate will be charged to the MOSFET to reset it so that the opening and closing of the JFET gate can be controlled. In other words, the functionality of the JFET is like a pixel switch, and it is closed when a signal is needed. Compared to the CMOS image sensor, the path has a simplification of the path, which makes it greatly improved, and the reliability is enhanced, and the order rate is lowered.
2.3 Analysis of internal structures
In the LBCAST JFET, since the charge accumulation is used in transverse embedded mode, JFET is a channel constructor sandwiched in GATE (switch), which is an ideal growth amplifying element, and CMOS has higher sensitivity and lower noise. .
First, for a given signal, the LBCAST usage cup is small, but provides a relatively large voltage increment and a high resolution. Next, in the CMOS image sensor, the signal is a surface of the channel to silicon; and in LBCAST, the transmission of the signal is passed through the internal channel, so the large noise is almost 1/3 of the previous level, and the dark current is particularly small. It can effectively suppress dark noise. On the other hand, the pixel signal is simultaneously extracted, and high speed processing can be achieved. In terms of structure, the wiring structure of the LBCAST camera pixel is less than a metal layer in CMOS, and the wiring density is also relatively low, and the interlayer connection holes are also relatively small. This achieves a simple structure, less manufacturing fault, and high yield.
The JFET sensor pixel selection switch consists of 3 transistors: transfer, JFET, and reset. The CMOS sensor consists of four transistors, and the fourth transistor is used as a pixel selection. Therefore, LBCAST has a simple structure than CMOS, high efficiency, and because the photodiode of the unit area can increase, it improves its functionality. The internal connection (including non-transparent layer) is also very simple, suitable for a polysilicon layer and 2 material layers. The design of CMOS is consisting of four layers. The less the number of layers required, the shorter the distance from the photodiode and the microlens, and the LBCCAST sensor can effectively reduce image noise taken under dark light by BPD (Burie D PhotoDioDe), internal FPN (Fixed Pattern Noise). It is similar to the dual channel reading method of CMOS to improve the data readout rate, and the microlens that are very close to the photosensitive unit effectively improve the interaction of the center and the corner angle of the picture at the same time. The LBCAST JFET pixel structure diagram is shown in Figure 3, and the LBCAST JFET pixel section is shown in Figure 4.
3, conclude
The main goal of the newly developed LBCAST JFET sensor is to pay attention to "speed." Its total pixels are not high, only 4 million. Therefore, Nikon will be applied to the D2H camera applied to the LBCAST JFET image sensor in terms of news reports and sports photography.
LBCAST and CMOS have higher sensitivity and low noise effect, and the structure is simple, the manufacturing fault is small, and the finished product is high. Since it is simple, it can be used with the same manufacturing process as CMOS. It is expected that the manufacturing cost can be greatly reduced, and the application prospect is very optimistic.
Editor in charge: GT, read full text
Our other product:
