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In this letter, a novel silicon-embedded coreless transformer is proposed and demonstrated. The transformer is fabricated in the thick bottom layer of a silicon substrate and connected to the frontside through vias opened in the thin top layer where all other components of the system can be fabricated. A 5-turn coreless transformer fabricated using this monolithic transformer technology achieves a small area of 2 mm2 and a good voltage gain of larger than -0.8 dB (load = 50 Ω, best reported so far) from 12 to 100 MHz. This technology shows great potential for on-chip isolated signal transfer.