High gain bipolar junction phototransistors with finger-shaped emitter for improved optical gas sensing in the blue spectral region

In thiswork, we report on the design, fabrication, electrical and optical characterization of innovative silicon npn bipolar junction phototransistors, with a finger-shaped emitter implant. Different finger width and interfinger distances, emitter geometries, metal contacts and doping profiles have been implemented to study the impact on the current gain and the spectral responsivity. Such a finger emitter/base junction proved to significantly enhance both the current gain and the responsivity in the blue spectral region with respect to the standard fully implanted detector (maximum current gain of 158 and 72, and responsivity = 0.25 and 0.20 A/W, respectively). BJTs featuring an additional N-channel implant below the emitter n+-fingers were also successively fabricated and exhibit an improved current gain of 650. The comparison of experimental results for all the implemented geometries allow the identification of the best performing BJT, in terms of current gain and responsivity, and of a figure of merit which takes into account the two fundamental layout parameters affecting the optical and electrical performance: number of emitter fingers and their total perimeter. The new finger-BJTs will be employed as gas sensors after being coated with a metalloporphyrin sensing film, that changes its light absorption properties in the blue when interacting with the gas molecules. A functional measurement campaign has been scheduled with ethanol vapors at different concentrations.