Ba Sr 1 TiO 3 (BST) thin films have large dielectric constants that can be varied by as much as a factor of 3 with an applied field, making them attractive for radio frequency (RF) circuits as small-area ac bypass/dc blocking capacitors, or high-power varactors. However, BST must be deposited at relatively high temperatures in an oxidizing environment, presenting significant integration challenges for MMIC applications. This letter describes the successful integration of BST films on AlGaN/GaN

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... igh electron-mobility transistor (HEMT) monolithic microwave integrated circuits on sapphire substrates. A sacrificial SiO 2 buffer layer is used to protect the underlying AlGaN during the RF magnetron sputtering of the BST film at an elevated temperature, with a carefully controlled heater ramp rate to avoid degradation of the ohmic contacts on the HEMT. Index Terms-BaSrTiO 3 (BST), ferroelectric varactors, GaN, high electron-mobility transistor (HEMT), monolithic microwave integrated circuit (MMIC).