A single-sided modulation-doped 90 AA quantum well with Al/sub 0.45/Ga/sub 0.55/As barriers has been realised using metal-organic vapour-phase epitaxy with selenium doping. Hall effect measurements give a sheet carrier density n/sub s/=2.27*10/sup 12/ cm/sup -2/ and a mobility mu =6050 cm/sup 2/ V/sup -1/ s/sup -1/ at 300 K. At 4.2 K, Shubnikov-de Haas and quantum Hall effect measurements give n/sub s/=1.52*10/sup 12/ cm/sup -2/ and mu =1.28*10/sup 5/ cm/sup 2/ V/sup -1/ s/sup -1/. A persistent increase in n/sub s/ to 2.00*10/sup 12/ cm/sup -2/ is observed at 4.2 K by passing a large current through the sample for a short period of time.<<ETX>>