Gas sensors based on high blue spectral responsivity photodiodes

Silicon (Si) photodiodes with a finger p–n junction have been modeled, designed, fabricated and tested. The surface depletion region (down to 200 nm from top of the Si substrate) between neighboring n+-finger implants provides for a very effective collection of photons in the blue spectral range, thus enhancing the responsivity, especially for devices with larger interfinger distance. At 420 nm wavelength and 5 V reverse bias the measured responsivity is 0.24 A/W with respect to 0.14 A/W standard reference photodiodes made on the same substrate, with a maximum value of 0.44 A/W observed at 20 V reverse bias. The finger photodiodes have been evaporated by metalloporphyrin films, able to change their absorption spectrum in the Soret band, around 440 nm, on exposure to volatile organic compounds (VOCs). A blue light source has been employed to test the sensors response to ethanol at room temperature and concentrations down to 300 ppm were measured with the Zn-T(heptyloxy)PP.