Abstract

The paper presents results from a study carried out on two FET processes, to demonstrate the enhanced prediction capabilities of the general-purpose FET model COBRA [I], to predict the intermodulation distortion (IMD) characteristics of micro-wave and mm-wave FETs, in comparison with other traditional large-signal models. The first, second and third order derivatives of the COBRA I/V model function are analysed and compared with the corresponding functions for the Statz-Raytheon and a modified Materka model. For the COBRA model case, these derivatives are shown to be continuous over the entire bias plane and in agreement with experimental results previously presented in literature for similar devices and extracted from harmonic measurements. In the case of the other two models, the high-order derivatives are described less accurately, especially around critical bias regions such as knee, pinch-off or soft breakdown. A sequence of two-tone (IMD) large-signal test results are presented for the case of a 0.2um PHEMT process designed for mm-wave applications, and operating in four different bias conditions. The results prove the advantages of the COBRA model in comparison with the other two in predicting high order IM products.