That's a field effect transistor. A diode is made from a P/N junction in normal silicon (i won't say i know it's the same in other materials). A Bipolar Junction Transistor is made from an N/P/N junction, or a P/N/P junction. On the surface it looks like you could make a transistor out of two diodes because of the junctions, but the problem is that you have to bond them on the silicon to get close to the effect of a transistor, the metal leads and bonding wires inside a normal diode prevent that from working. What this does mean however is that you can use half of an NPN or PNP transistor as a diode, and it's a somewhat common thing to do to minimize the types of parts you need on a circuit board if you're already using the transistors for something else.