Evaluating Negative Capacitance Technology for RF MOS Varactors

We exploit the negative capacitance (NC) property of the ferroelectric for enhancement of the capacitance tuning ratio (C_ratio) of MOS varactors which are key elements of frequency control in an RF voltage controlled oscillator (VCO). We show that by replacing a conventional High-κ layer by a ferroelectric material in a High-κ Metal Gate (HKMG) stack, a larger C_ratio can be achieved at the same voltage. In other words, the same C_ratio can be achieved at a lower voltage. This can further be increased by increasing the thickness of the ferroelectric layer (t_fe), however, at the cost of a lower Quality (Q) factor and a higher variation in the VCO gain/sensitivity. We further investigate the impact of geometrical and material parameters and find that in general, a small interfacial oxide thickness (t_ox), a smaller area of the ferroelectric layer (A_fe) compared to the internal gate area (A_MOS), and the ferroelectric materials with a high coercive field (E_c) and a low remnant polarization (P_r) can all boost the C_ratio.