A gas sensor including: a gas sensing element including first and second ceramic layers. The transistor includes a first gate, a second gate disposed on one side of the first gate, a first semiconductor layer, a second semiconductor layer, an oxide layer, a first insulation layer, a second insulation layer, a source, and a drain. The PDA unit determines whether the cellular phone unit is in the off state when the PDA unit transitions to the enabled state, and transitions to the disabled state after activating the timer when disabled.