Memory device including composite metal oxide semiconductor channels and methods for forming the same

A three-dimensional memory device, comprising:

• an alternating stack of insulating layers and electrically conductive layers;
• a memory opening vertically extending through the alternating stack;
• and
• a memory opening fill structure located in the memory opening and comprising a memory film and a vertical composite metal oxide semiconductor channel comprising an outer semiconducting metal oxide channel layer having a first band gap and an inner semiconducting metal oxide channel layer having a second band gap that is different from the first band gap,
• wherein:
• a conduction band offset of a conduction band energy level of the outer semiconducting metal oxide channel layer relative to a conduction band energy level of the inner semiconducting metal oxide channel layer is positive, and a valence band offset of a valence band energy level of the outer semiconducting metal oxide layer relative to a valence band energy level of the inner semiconducting metal oxide layer is negative;
• the vertical composite metal oxide semiconductor channel further comprises an intermediate semiconducting channel layer having a graded band gap that is interposed between the outer semiconducting metal oxide channel layer and the inner semiconducting metal oxide channel layer;
• and
• the intermediate semiconducting channel layer comprises indium gallium zinc oxide or indium gallium zinc oxynitride in which an atomic percentage of gallium and indium increases with the distance from the outer semiconducting metal oxide channel layer.