Relations for the optimum well width, barrier width, and width of the spacer layer which correspond to the highest PVCR based on effective mass and barrier height in RTDs are proposed. The optimum spacer layer is found to be half of the de-Broglie wavelength associated with the bound state of the corresponding finite quantum well. The proposed relations for the optimum parameters can be used to design RTD based on any two appropriate materials to attain the highest PVCR. The effect of doping concentrations on PVCR and peak current was studied. As a case study, we have considered the GaAs/Ga0.7Al0·3As, GaN/Ga0.7Al0·3N, and In0.53Ga0.47 As/AlAs RTDs. The transfer matrix formalism is extended to study the current-voltage relation in polar GaN/AlN RTDs.