Heterogeneous integration of low-temperature metal-oxide TFTs

The breadth of circuit fabrication opportunities enabled by metal-oxide thin-film transistors (MO-TFTs) is unprecedented. Large-area deposition techniques and high electron mobility are behind their adoption in the display industry, and substrate agnosticism and low process temperatures enabled the present wave of flexible electronics research. Reports of circuits involving complementary MO-TFTs, oxide-organic hybrid combinations, and even MO-TFTs integrated onto Si LSI back end of line interconnects demonstrate this technology's utility in 2D and 3D monolithic heterogeneous integration (HI). In addition to a brief literature review focused on functional HI between MO-TFTs and a variety of dissimilar active devices, we share progress toward integrating MO-TFTs with compound semiconductor devices, namely GaN HEMTs. A monolithically integrated cascode topology was used to couple a HEMT's > 200 V breakdown characteristic with the gate driving characteristic of an IGZO TFT, effectively shifting the HEMT threshold voltage from -3 V to +1 V.