An investigation on RF CMOS stability related to bias and scaling

Abstract

In recent times, CMOS devices have played an increasing important role in the area of RF-ICs to implement part of the high-frequency circuits. As a result, the high-frequency characteristics of CMOS devices, including stability, should be investigated in-depth. In the design of RF amplifier, the consideration to avoid oscillation in the circuits is a must. Basically, the instability involved in designing RF circuits can be reduced if the transistor is stable in the interesting frequency region. In this study, the stability factor of CMOS device is discussed. First, the stability factor (i.e. k-factor) is derived based on the microwave small-signal model of MOSFET. Therefore, small-signal parameters that affect stability can be carefully examined. In addition, the effects of biasing and dimensions on k-factor are also discussed. Since various operating point (or biasing) can cause device to have different effects on high frequency, the correlation between biasing and stability is discussed in this study. Due to the fact that CMOS device has continuously shrunk in size, a detailed discussion of the effect of MOSFETs dimension on stability is included. It is found from our experimental results that the smaller the gate length, the greater is the frequency range for CMOS device to become potentially unstable.