We demonstrate cryogenic switching devices based on HEMT technology for scalable quantum computer (QC) control and readout signal routing. The switches are implemented in a quantum well network design without metal contacts between gates, and exhibit extremely steep cryogenic subthreshold swing of similar to 5 mV/decade, fast switching, along with on/off ratio of 10(7) and isolation leakage of 10 pA/mu m. The use of a Pt gate metal layer that diffuses into the gate barrier is shown to significantly enhance subthreshold properties. The results indicate that HEMT-based switching circuits could offer advantages over Si CMOS at cryogenic temperatures for future high-density ultra-low power QC signal routing.