High-speed bipolar phototransistors in a 180 nm CMOS process

Several high-speed pnp phototransistors built in a standard 180 nm CMOS process are presented. The phototransistors were implemented in sizes of 40×40 μm² and 100×100 μm². Different base and emitter areas lead to different characteristics of the phototransistors. As starting material a p⁺ wafer with a p^- epitaxial layer on top was used. The phototransistors were optically characterized at wavelengths of 410, 675 and 850 nm. Bandwidths up to 92 MHz and dynamic responsivities up to 2.95 A/W were achieved. Evaluating the results, we can say that the presented phototransistors are well suited for high speed photosensitive optical applications where inherent amplification is needed. Further on, the standard silicon CMOS implementation opens the possibility for cheap integration of integrated optoelectronic circuits. Possible applications for the presented phototransistors are low cost high speed image sensors, opto-couplers, etc.