The crystal detector was the earliest type of semiconductor diode. A reversed-biased semiconductor diode exhibits capacitance. Some sets used a crystal detector (semiconductor diode) instead. It also had read-only semiconductor diode memory for programs. Semiconductor diode lasers are typically used in plastic welding. Hence, the net current in a semiconductor diode at zero potential is always zero. The point of contact between the wire and the crystal acted as a semiconductor diode. The JIS semiconductor designation system has all semiconductor diode designations starting with "1S". Its output is rectified by semiconductor diode D, which was a carborundum point contact type. Before the inclusion of integrated circuits, other firmware devices included a discrete semiconductor diode matrix.