Impact of Si substrate resistivity on the non-linear behaviour of RF CPW transmission lines

Non-linear behaviour of RF coplanar transmission lines is analyzed for various values of Si substrate resistivitiy. Based on small-signal measurements performed under different DC bias conditions, voltage dependent capacitance and conductance per unit length of the transmission line are extracted and compared for several silicon substrates. Harmonic distortion of large RF signal at 900 MHz along CPW lines is measured using a spectrum analyzer based setup as well as with a LSNA which gives us access to the phase of the harmonic components. For an input power of +25 dBm, the highest harmonic component (2^nd) is as high as -15, -57, -37 and -63 dBm for resistivity substrates of 20, 500, 5 k and 2 kΩ-cm, respectively. A reduction of 45 and 15 dB for all harmonic components was obtained for the 5 and 2 kΩ-cm HR-Si substrates, respectively, when a trap-rich passivation layer was used at the Si/SiO₂ interface, and for both characterization setups. The impact of the resistivity value on signal distortion with its relation to the bias-dependence substrate characteristic and the efficiency of the trap mechanism of the passivation layer are for the first time introduced from experimental result considerations.