ULTRARAM™ is a III-V compound semiconductor memory concept which exploits quantum resonant tunneling to achieve non-volatility at extremely low switching energy per unit area. Prototype devices are fabricated in a 2x2 memory array formation on GaAs substrates. The devices show 0/1 state contrast from program/erase (P/E) cycles with 2.5 V pulses of 500-µs duration, a remarkable switching speed for a 20 µm gate length. Memory retention is tested for 8x10⁴ s, whereby the 0/1 states show adequate contrast throughout, whilst performing 8x10⁴ readout operations. Further reliability is demonstrated via program-read-erase-read endurance cycling for 10⁶ cycles with 0/1 contrast. A half-voltage array architecture proposed in our previous work is experimentally realized, with an outstandingly small disturb rate over 10⁵ half-voltage cycles.