Pseudomorphic (In,Ga)As layers are used in GaAs‐based semiconductor‐insulator‐ semiconductor (SIS) structures to shift the threshold voltage from the natural, near‐zero value. The threshold voltage is shifted positively for (In,Ga)As gate layers, and negatively for (In,Ga)As channel layers, by the (In,Ga)As/GaAs conduction‐band offset. The thermionic and field emission barrier heights agree with shifts obtained in capacitance‐voltage characteristics. The structures withstand implant activation anneals, establishing a simple technology to create either enhancement or depletion‐mode devices.