A semiconductor device that can measure a minute current. The semiconductor device includes a first transistor, a second transistor, a node, and a capacitor. The first transistor includes an oxide semiconductor in a channel formation region. The node is electrically connected to a gate of the second transistor and a first terminal of the capacitor. The node is brought into an electrically floating state by turning off the first transistor after a potential V0 is supplied. Change in a potential VFN of the node over time is expressed by Formula (1). In Formula (1), t is elapsed time after the node is brought into the electrically floating state, τ is a constant with a unit of time, and β is a constant greater than or equal to 0.4 and less than or equal to 0.6. V FN ⁡ ( t ) = V 0 × ⅇ - ( t τ ) β ( 1 )