the fermi level of the gate metal is matched to the pinning point, which is 1.2 ev below the conduction band. with the reduced algaas layer thickness, the electrons supplied by donors in the algaas layer are insufficient to pin the layer. as a result, band bending is moving upward and the two-dimensional electrons gas does not appear. when a positive voltage greater than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.