We  describe the operation of single electron transistors with a double-gate 
geometry defined by Ga focused ion beam implanted in-plane gates and line Schottky 
gates. The in-plane gates are used to squeeze the channel and to increase the 
charging energy. The Schottky gates are placed on the channel to form the tunnel 
junctions. Independent control of these gates is useful to define the geometry of 
single electron transistors. We found strong resonances exhibiting negative 
differential resistance in the small devices, which is attributed to tunneling 
through zero-dimensional states when the barrier has a parabolic-shaped potential.