In this paper, results from a numerical study on characteristics of plasmon-polar phonon coupled modes in InAs p-i-n diode structure are presented. A superposition phenomenon of two new modes in frequency spectra of built-in electric field was observed when plasmon-polar phonon interaction was taken into account. Besides, a dependence of modes frequencies on carrier density is found to be similar to dispersion relations of bulk plasmon-polar phonon coupled modes. Consequently, these two new modes can be acknowledged as plasmon-polar phonon coupled modes in diode structure. Furthermore, replicated small amplitude modes due to clipping by diode structure are also found.