We report on the fabrication of organic phototransistors based on the spiro-conjugated molecule $\text{2,7-}\mathrm{bis}\mathrm{-(}{\mathrm{N,N}}^{\prime }-\mathrm{diphenylamino}{\text{)-2}}^{\prime }{\text{,7}}^{\prime }-\mathrm{bis}(\mathrm{biphenyl}\text{-4-}\mathrm{yl}{\text{)-9,9}}^{\prime }-\mathrm{spirobifluorene}.$ Intramolecular charge transfer leads to an increase in charge carrier density, providing the amplification effect. The sensitivity is better than 1 A/W for ultraviolet light at 370 nm, making the device interesting for sensor applications.