Electromagnetic Investigation of Substrate Coupling in Power Integrated Circuits in GaN-on-Si Technology

We present electromagnetic (EM) investigation of substrate coupling in power integrated circuits (ICs) in GaN-on-Si technology with $\left|S_{21}\right|$ maintaining at a low level (less than $\mathbf{- 6 0 ~ d B}$) for multi-MHz signals over a physical separation of $700 \mu \mathrm{m}$. At higher frequencies for shorter separation distance, the signal isolation is inferior with $\left|S_{21}\right| \approx-37 \mathrm{~dB}$ at 100 MHz, that can result in an alarming level of crosstalk voltage (more than 100 mV for a $12-\mathrm{V}$ interference source). The substrate coupling dictates the maximum operation frequencies of power ICs and certain minimum separation distance with circuit components integrated on the same GaN-on-Si chip. The electric field distribution obtained from EM simulation by finite element method (FEM) reveals the coupling mainly through the GaN buffer layer rather than the resistive Si substrate. A thicker GaN buffer layer would cause less coupling because of weaker electric field at the interference source. Conventional guard rings of grounded contacts to the GaN buffer layer has negligible effect on suppressing the coupling. These EM investigation results provide significant implications for the fabrication and monolithic integration of GaN-based power devices and circuits to avoid serious crosstalk and noise coupling on the same chip.