PICOSECOND PULSE-PROPAGATION ON COPLANAR STRIPLINES FABRICATED ON LOSSY SEMICONDUCTOR SUBSTRATES - MODELING AND EXPERIMENTS

A simple model for the propagation of high-frequency signals on coplan ar striplines with lossy semiconductor substrates is proposed and demo nstrated. This model incorporates the effect of a conductive substrate through the loss tangent in a distributed-circuit analysis extended t o high frequencies. Very strong attenuation and dispersion due to the substrate are observed even when the GaAs conductance is only 1 mho/cm , corresponding to a doping density of around 10(15) cm(-3). The accur acy of this model is tested with a direct comparison to experimental d ata of picosecond pulse propagation on a doped-GaAs coplanar stripline (CPS) measured in the time domain using the electro-optic (EO) sampli ng technique. Good agreement is found in terms of the attenuation and phase velocity of the distorted pulses at four propagation distances u p to 300 mu m. The pulse propagation on a multiple modulation-doped la yer is also studied experimentally as a prototype of high-frequency si gnal propagation on the gate of a modulation-doped field-effect transi stor (MODFET). The attenuation shows linear frequency dependence up to 1.0 THz, contrary to the cubic or quadratic dependence of coplanar tr ansmission lines on low-loss substrates.