A semiconductor device includes a substrate, an active layer, a transistor, and a capacitor. The active layer is disposed on the substrate, and the active layer is divided into a first portion and a second portion. The transistor and the capacitor are disposed on the substrate. The transistor includes the second portion, a source electrode, a drain electrode, and a gate electrode. The source electrode and the drain electrode are respectively and electrically connected to the second portion. The gate electrode is disposed on the second portion. The capacitor includes the first portion, a first electrode, a first insulating layer, and a second electrode. The first electrode is electrically connected to the first portion and the source electrode. The first insulating layer is disposed on the first portion. The second electrode is disposed on the first insulating layer and is electrically connected to the gate electrode.