AlGaAs/InGaAs pseudomorphic high-electron-mobility transistor (pHEMT) single-pole-single-throw (SPST) switches with various upper/lower delta-doped ratio designs were fabricated and investigated for the first time. Both off-state capacitance and the specific on-resistance (R-on) of pHEMT are dominated factors and showed characteristics of sensitive to upper/lower delta-doped ratio for RIF switch applications. By adopting the series-shunt architecture, upper/lower ratio of 3:1 switch achieved the lowest insertion loss compared to 4:1 design owing to the device shunt to ground (M2) of 4:1 design exhibited a worse fundamental signal isolation especially at high power level. As to the isolation under same architecture, however, due to the lowest Ron can be obtained, the 4:1 design provided better isolation performance. In addition, the M2 also dominated the second and third harmonics suppression and meanwhile, the lowest R-on of 4:1 design was found to be beneficial to the reduction of the harmonics power transmitted to the output terminal. (C) 2008 Elsevier Ltd. All rights reserved.