An RF-front-end system comparison of SiGe HBT and GaAs pHEMT: limitations in non-linearity and balance

For a successful system design of a wireless transmitter or receiver, it is important to investigate the overall linearity of a particular manufacturing process, where the limit of linearity can be viewed both as the onset of non-linearity above some threshold as well as loss of phase and amplitude balance. This dissertation presents a balanced frequency-doubler circuit as an archetype of a non-linear circuit that would allow the study of harmonics. The source of non-idealities in a frequency doubler is studied that would limit the overall linearity of a system design. In this context, a balun is used as an archetype of the passive structures that complements a frequency doubler when the linearity of a system is considered. For microwave and millimetre-wave circuit de-sign, GaAs processing is well established but it involves high cost. SiGe processing is emerging in the microwave and millimetre-wave arena, promising reasonable performance at a lower cost and equipped with integrated digital logic capability. To compare the two process technologies, GaAs and SiGe, similar frequency-doubler circuits and passive baluns are implemented in both the processes using GaAs pHEMTs and SiGe HBTs. The design challenges, issues with layout and design flows developed for each of the processes are discussed. Analysis of harmonics in frequency doublers using GaAs pHEMTs and SiGe HBTs shows that pHEMTs are inherently more linear than HBTs. Large input power is needed to drive the frequency doubler using pHEMTs compared to SiGe HBTs. However, the area requirement of the GaAs frequency doubler is more than the SiGe counterpart. Measurement results demonstrate that high linearity (> 35dB odd-harmonic suppression) and balance (< 0.15 dB amplitude imbalance and < 2° phase imbalance) is possible from both technologies over comparable bandwidths.