An InGaAs/GaAs High Electron Mobility Transistor (HEMT) comprises a buffer layer, a main conducting channel, an InGaAs/GaAs thickness-graded superlattice structure, a mono atom -doped carrier supply layer, a Schottky cap layer of gate electrode and an Ohmic cap layer of drain electrode and source electrode which are formed successively on a substrate. The superlattice structure comprises spacer and sub-channel. By using thickness-graded superlattice spacer structure is able to ameliorate lattice-mismatch-induced scattering within heterostucture interfacial, increase range of gate voltage swing in gate electrode, and through real-space transfer generated by bias voltage in high electric field, drain-to-source saturation current proceed step-up phenomenon to forming a HEMT having scalable voltage multi-extrinsic transconductance enhanced portions.