UV Photodetector Based on FG-nMOS in 0.5 μm Standard CMOS Technology

In this work, a photodetector fabricated in standard CMOS 0.5 μm technology useful to detect UV radiation in the 250 nm to 296 nm range is presented. The photodetector consists of an NMOS floating-gate transistor (poly1 to poly2) with a 12 μm/0.6 μm aspect ratio and an input capacitor of 186 fF. Such floating-gate is made of n+ polysilicon surrounded by silicon oxide whose interface presents a barrier of 4.19 eV. Therefore, photoelectrons produced by the incidence of UV radiation (λ ≤296 nm) can cross the interface bandgap and be stored at the floating-gate producing a detectable potential change. The characterization of the transistor I-V curves allows the charge quantification at the floating-gate due to UV exposure; such characterization was done using monochromatic and non-monochromatic sources. The developed mathematical models agree with experimental data. An average responsivity of 4.5 μA/W was obtained, which can be easily improved by using extra circuitry as current amplifiers, op-amps or other analog stages also available in standard CMOS technology.