This paper presents experimental results regarding optical and electrical characteristics of a partially gated p-i-n structure that has an extension in the channel region coated with transparent material. The correlation between band to band tunneling and photo current is discussed. Four main phenomena are observed under illumination: (1) negative transconductance can be obtained under reverse bias conditions (2) the off current is governed by the photocurrent and subthreshold slope is degraded (3) a kink in the saturation current appears in the output characteristics (4) the light sensitivity of the transconductance in P mode operation can be tuned with the back gate bias.