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The ring-shaped phototransistor with a floating bulk enclosed by a ring-shaped photodiode is proposed to enhance the responsivity for the ultraviolet/blue spectral range. The P-channel metal-oxide-semiconductor field-effect transistor and N-channel metal-oxide-semiconductor field-effect transistor phototransistors were manufactured using a standard 0.35-mum complimentary metal-oxide-semiconductor (CMOS) technology. When the phototransistors were illuminated with 400-nm light, the measurement results for 3-V bias demonstrated a responsivity higher than 1500 A/W, which is also superior to that of other reported photodetectors manufactured using a standard CMOS technology. Even for very small bias voltages such as 0.1 V, the phototransistor can exhibit a responsivity of 17.9 A/W.