A compound semiconductor made from equal quantities of group-III material gallium and group-V material arsenic (see periodic table). This crystalline semiconductor material has a direct energy gap, and a conduction-band structure that permits high electron mobility and saturated velocity; it also exhibits the Gunn effect. In undoped or intrinsic form, GaAs exhibits a high resistivity, which can be used for a semi-insulating substrate in high-frequency transistors and integrated circuits. GaAs is used as the basis for microwave transistors, such as MESFETs, HEMTs, and HBTs, and for monolithic microwave integrated circuits (MMICs), where the material advantages are superior to silicon for applications at these frequencies. The direct energy gap is exploited in optoelectronic applications, such as light-emitting diodes (LEDs) and semiconductor lasers.