I. Introduction
With the rapid development of InP HBT technology, the operation frequency of the millimeter-wave (mmWave) and sub-mmWave integrated circuits manufactured in InP HBT have surpassed well above 200 GHz [1]. GSG pads are commonly used in most of the integrated circuit design for on-wafer testing [2]. Thus, accurate compact models are especially valuable for efficient simulation in mmWave and sub-mmWave circuit design.