Two different types of planar nano-scale (4–5 nm) field emitters, bowtie and diode devices, were characterized for use as electron sources for vacuum nano-transistors. I-V measurements were performed, and long-term stability tests were conducted in a dedicated life-time test chamber under high vacuum environment (~5×10−8 Torr). Experiments showed that the maximum field emission current was ≈ 3 nA from bowtie devices for a bias voltage of 6 V and ≈ 15 nA from diode devices at a bias voltage of 8 V. A long-term stability study showed that although one diode device failed at ≈ 300 hr, three other devices emitted a stable current (< 5 % Drop) for > 1000 hrs. of continuous operation at a DC bias of 6 V.