| In the current information society which has rapidly developed, the optical technology has become an important channel to gain light information or other information extracted from light. It enables human beings to expand their visual range better, the same time, it can record number of picosecond and even femtosecond ultra-fast phenomena. In the photoelectric detection technology, optoelectronic devices plays a dominant position, while the semiconductor optoelectronic devices with excellent electrical characteristics, simple process and cheap price have become the dominant optical device categories among which, the silicon photoelectric detection devices are more widely used. At present, silicon optoelectronic devices are mainly PIN photodiode, avalanche photodiode APD and general phototransistor. As the key role of optical detection devices, it determines that the response rate, response time, response wavelength and other features have to achieve high performance, but these devices we talked above have obvious shortcomings.PIN photodiode is the most popular optical detection device, it's most important feature is that between high-doped P type and N-type semiconductor layer there grows an intrinsic zone called I layer, it's main characteristics: response time is very short, about the 10-9s, broadband can be up to 10 GHz. And, as I layer is very thick and very affordable to large reverse bias, so the linear output range is very wide. When the incident light less than 10mW power inject into the device, it will not saturate. The weakness is that the resistant of I layer is so big that leads output current is very small, only a fewμA. APD is a semiconductor device which works at a high reverse bias with avalanche effect. It has huge internal gain, up to 106. This device is characterized by particularly fast response time, it's bandwidth can be as high as 100GHz, and the response speed is fastest among current photodiode. The biggest drawback is the additional noise is too large as the avalanche effect which severely limits the device signal to noise ratio. Phototransistor, which is similar to a current transistor, has ability to amply current. But when the incident light power decrease, the output signal to noise ratio deteriorated very quickly, which severely limits detection of weak light signals. To improve the signal to noise ratio under low light conditions and obtain high sensitivity and wide dynamic range, people have been proposed and developed several improved phototransistor. But so far, there has not emerge a photoelectric detection device which could meet a standard CMOS process completely, simple structure, low dark current, and in very low light conditions, high photoelectric response rate, and also has a wide dynamic range. Therefore, this thesis is very meaningful.This paper presents a high sensitivity and wide dynamic range silicon phototransistor. It first presents the basic structure of the device and describes its working principle. Use the theory of thermal electron emission to discuss the current of the device transport process, and then simulate and analyze with the numeric value. The device consists of two different NPN transistor. The two transistors'emitter and collector is constituted by the active region, and are connected by metal wire which leads to the two electrodes. The difference is the length of different base and different light transmission, in which the illuminated transistor provides light-induced carriers, electrons are immediately scanned into collector and photo-hole under the effect of the concentration gradient spread to the opaque base made it's base region's barrier height reduced. This will lead to more electronic emitter injection, to achieve the objective of the current amplification. Even in the case of weak incident light, the transistor base region will lead to changes in barrier height, then the electron current increased significantly, and this is the reason that the device has a high sensitivity.This thesis analysis the high sensitivity and wide dynamic range phototransistor's current transport process and working conditions in the DC-gain mechanism, derived the basic properties of the device parameters, defined that weaker incident is , sensitivity rate increased. Photoelectric sensitivity under low light should be a fixed value. Therefore, the device has a very good response to low light for the detection of weak light signals. We use thermionic emission theory to simulate two-dimensional numerical value. The characteristics we discussed are voltage-current characteristic, response rate, carrier distribution and potential distribution.Before the device was manufactured, we design some of the basic parameters of its size using Silvaco simulation. And optimize the structure. The basic parameters we simulated are base doping concentration, the length of the two transistor base region and active region junction depth. Simulation results show that the lower base doping concentration, the more likely the device reach the punch-through state; Shorter the base of the opaque transistor is ,easier to reach punch-through state, that is, the smaller the device turn-on voltage is. Longer the base of the illuminated transistor is, the larger the photocurrent could be obtained. Greater junction depth is the more easily the device reaches punch-through. But because we use standard CMOS process, there is little change in junction depth, so it plays the least impact on device performance. We also designed three structures, respectively, they are: Using the N+ active region for the emitter and collector as the structure A; Using N well for the emitter and collector as the structure B; Using N+ active region and N well for emitter and collector respectively as the structure C. After discussion, the best choice to manufacture is structure A. Then we use CSMC 0.5μm 2P2M CMOS Process to fabricate the device, and then a brief introduction of its fabrication process is proposed.Finally, we test the device, the main characters we test are: voltage-current characteristic, photoelectric response rate, transient response, and the spectral response and so on. Test results show that when incident light applied to the device surface, the device will produce the photocurrent, and with the incident light intensity larger, the photocurrent goes larger, and in bright light conditions, the photocurrent will not reach saturation. In the smallest incident is about 0.01μW/cm2, the photo current is 34.4nA. The sensitivity raises when the light intensity increases. When the incident light power density reaches 0.01μW/cm2, the device sensitivity can be as high as 106. The device has a very large dynamic range which can reach six orders of magnitude. Finally, the device's transient and spectral response were also tested and the result was give. |
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