This is a high-speed field-effect transistor that uses gallium arsenide (GaAs) as the semiconductor material rather than silicon. It is generally used as a very high frequency amplifier. 

A GaAsFET (MESFET) consists of a length of n-type or p-type doped GaAs called the channel. The ends of the channel are called the source and the drain. The terminal with the arrowhead on its symbol represents the gate.