The goals of inductor design for MMIC applications are large inductance with small size, high Q-factor and high self-resonant frequency (SRF). But the spiral inductors based on SiC substrate have never been individually reported before. So SiC-based GaN MMIC has a good prospect in power and microwave applications. It is an ideal substrate for AlGaN/GaN heterojunction epitaxy, because SiC has excellent thermal conductivity and superior lattice matching with GaN material. With these advantages, SiC can be a splendid candidate for high frequency and high power applications. With smaller relative dielectric constant and higher thermal conductivity, the parasitic capacitance of the SiC-based devices and circuits is smaller and the thermal reliability of them is improved. Abundant spiral inductors based on GaAs and low-resistivity silicon substrates with different structures have been investigated intensively and outstanding performances have been reported.

The 1.4-nH inductors have Q-factors of 30 at 5.2 GHz and 40 at 5.8 GHz for the HRS and the sapphire substrates, respectively. , spiral inductors have been fabricated by using copper-damascene interconnects on high-resistivity silicon (HRS) and sapphire substrates. With lower parasitic capacitance and smaller substrate loss, the Q-factor has been improved to 10.6, with a self-resonant frequency of 10 GHz. , an air-bridge based on-chip suspended inductor on sapphire has been proposed. Large amounts of experimental and theoretical work have been done to investigate the characteristics of microwave spiral inductors on different substrate. Microstrip spiral inductors are important passive components for radio frequency (RF) chocking, tuning, matching and filtering and are extensively used in monolithic microwave integrated circuits (MMIC) design. Through the electromagnetic field distribution simulation, the details of the electric field around the air-bridge have been presented which demonstrate the formation and the variation principles of the coupling effect. By means of establishing the lumped model, the parasitic coupling capacitance of the air-bridge has been extracted and presents the exponential decay with the air-bridge heights decreasing which indicates that this capacitor is directly related to the coupling effect of the air-bridge. All the coefficients in the formula have specific meaning. From the S-parameters measurements, the exponential decay phenomenon is observed for L, Q-factor, and SRF with the air-bridge height decreasing, and an analytic expression is concluded to exactly fit the measured data which can be used to predict the performance of the spiral inductor. The fabricated 1-nH spiral inductor on SiC substrate demonstrates a self-resonant frequency of 51.6 GHz, with a peak Q-fact of 12.14 at 22.1 GHz.

The coupling effect of air-bridges on broadband spiral inductors in SiC-based MMIC technology has been investigated deeply.

Airbridge broadband