A novel kind of phototransistor with high gain and sub‐&mgr;m spatial resolution is fabricated by focused laser beam‐induced Zn doping of ann‐modulation doped GaAs/Al0.4Ga0.6As quantum well structure. To produce this lateralnpn‐structure,p‐doped lines are directly written over a mesa without degrading the quality of the sample. The local Zn doping causes an effective potential barrier for electrons. Photogenerated holes reduce this barrier and amplify the thermionic electron current. Spatially resolved photocurrent measurements show typical responsivities above 103A/W and linewidths as small as 605 nm. ©1995 American Institute of Physics.