Research On Modeling And Realization Of UV Enhanced Photodetector Used For Single Chip Integration

Nowadays, UV detectors are mainly made of SiC, GaN, AlGaN and other wideband gap semiconductor materials. They are expensive and have great bulk. Secondly,because they can’t be fabricated with signal readout and signal processing circuits inan integrated chip, more than three chips are required to realize a UV detection system,making the UV detection systems huge, complex and high power consumption.Although traditional silicon based UV photodiodes can be achieved in single chipwith CMOS readout circuit, their response and low quantum efficiency are generallylow. For the sake of solving the low responsivity and low quantum efficiency ofsilicon based UV photodiodes, this thesis combines the advantages of MOSFET(Metal-Oxide-Semiconductor Field Effect Transistor) devices (high dynamic rangeand low noise etc.) with the exponential type big current of photodiode, using thecarrier injection characteristics of PN junction and overcoming the shortcoming ofsingle carrier conduction of MOSFET devices, which makes the current density ofMOSFET small. A silicon-based combined UV enhanced photodetector used forsingle-chip integration is presented in this thesis.The UV enhanced photodetector proposed in this work consists of a MOSFETand a ring-shaped photodiode, the body of the MOSFET is left floating and thephotodiode is realized by lateral PN junctions. Conductive channel of MOSFET isformed when it is under certain biases. To detect a drain current of the MOSFET whenthere is no light. We name it the first drain current. Once light is illuminated on theMOSFET, photogenerated carriers are driven into the body of MOSFET with the helpof built-in electric field of the lateral PN junctions, changing the substrate potentialand causing the threshold voltage of the MOSFET smaller. In a condition of constantbias voltages, threshold voltage of the MOSFET is decreased while the output draincurrent of which is increased. We detect a drain current again and name it the seconddrain current. A photo responsive current of the combined photodetector is obtainedby subtracting the two drain currents.Firstly, longer discussions of gate width, stripe-shaped gate structure of thenovel device are made using Silvaco TCAD process and device simulation tool. Thepurpose is to study the effects caused by gate width on carrier collection ability of thedepletion potential well that under the conductive channel, and to simulate itsabsorption characteristic to short wavelength affected by stripe-shaped gate structure. Structures for tape out are confirmed by judging the effects that the gate width andstripe-shaped gate structure play on the device performances. Secondly, for thestructure to be fabricated, further simulations and analyses of electrical and opticalcharacteristics, including body potential change, transfer characteristics, outputcharacteristics, spectral response and DC characteristics, are carried out.The simulated results show that the designed device has higher response currentto ultraviolet (short wavelength) than to near infrared waves (long wavelength). Itshows amazing DC responsibility for weak optical signal. The UV enhancedphotodetector has very high potential value for application in ultraviolet detection andweak light detection. Finally, layout design and tape out fabrication of the proposeddevice have been realized in a standard0.5μm CMOS process.