Design Criteria in Sizing Phase-Change RF Switches

This paper presents design criteria for four-terminal phase-change (PC) RF switches as a function of their dimensions, (heater width, RF gap, and barrier thickness), materials, and connection topology. Reducing heater width is shown to reduce switch actuation power, with this reduction ultimately being limited by the maximum allowed heater current density, as set by projected reliability. Narrower RF gaps are shown to increase switch cut-off frequency F
_CO
, because they decrease on-state resistance more than they increase off-state capacitance. The balance of this increased F
_CO
versus the estimated reduction in power handling as the RF gap shrinks is quantified. Barrier layers of AlN thicker than 200 nm are shown to be sufficient to electrically decouple the switch from the heater trace. Coupling to heater pads can also be significant unless pad sizes are reduced as in a monolithically integrated switch. Finally, it is demonstrated that division of the switch into multiple parallel segments is a viable approach for lowering the heater actuation voltage for CMOS integration. A 12.5% increase in COFF (~1.5 fF) was observed and 35% increase in actuation power at temperature was needed in a two-segment parallel switch, as compared to a single segment switch.