The effect of device channel thickness on the transconductance performance of a Ga0.52In0.48P/In0.2Ga0.8As/ Ga0.52In0.48P double heterojunction pseudomorphic high electron mobility transistor (DH-pHEMT) grown on GaAs substrate was investigated using a two-dimensional (2D) device simulator. The electron sheet concentration values for different channel thickness were calculated using an analytical model. Simulation results revealed that devices with narrower channels exhibited transconductances that were comparable to those of devices with wide channels, due to a trade-off between the electron sheet concentration and the gate-to-channel separation. Subsequently, the devices were fabricated and their measured results agreed with the transconductance trend predicted by simulation. A second transconductance peak of lower magnitude was observed for the Ga0.52In0.48P/In0.2Ga0.8As/Ga0.52In0.48P DH-pHEMT at more negative biases. This phenomenon was investigated and described qualitatively by a model, which involves the incorporation of interfacial quaternary layers at the lower In0.2Ga0.8As channel-Ga0.52In0.48P interface.