Enhancement-mode quantum transistors for single electron spin

Using an InAs/GaSb composite quantum well, we demonstrate an enhancement mode single electron transistor. With a Hall bar geometry, we show that the device undergoes a transition from accumulation of two-dimensional (2D) holes in GaSb to a complete depletion and finally to an inversion layer of 2D electrons in InAs. When the top-gate area is reduced to nanometer scale, the inversion electrons are confined to a quantum dot, and the transistor displays single electron characteristics: a series of conductance peaks resulting from electrons tunnelling through the quantum dot. The occurrence of the first peak is the signature of one electron occupying InAs quantum dot. The unique configuration of enhancement quantum dots makes it possible to upscale to 2D arrays for manipulation and transporting of single spins.