A light sensor including a photo transistor is provided. A gate of the photo transistor receives a gate driving signal. The photo transistor senses a light source based on the gate driving signal to generate a light current signal. The photo transistor includes a metal-oxide active layer. The gate driving signal has a first voltage level during a trap period and has a second voltage level during a read period. The first voltage level is higher than the second voltage level. The gate driving signal of the photo transistor introduces a mechanism to rapidly eliminate excess carriers. Accordingly, the photo transistor has a rapid response while maintaining good light responsibility. Furthermore, a method for driving the foregoing photo transistor is also provided.