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The RF performance of two different Si-based resonant interband tunneling diodes (RITD) grown by low-temperature molecular beam epitaxy (LT-MBE) were studied. An RITD with an active region of B δ-doping plane/2 nm i-Si0.5Ge0.5/1 nm i-Si/P δ-doping plane yielded a peak-to-valley current ratio (PVCR) of 1.14, resistive cutoff frequency (fr0) of 5.6 GHz, and a speed index of 23.3 mV/ps after rapid thermal annealing at 650°C for 1 min. To the authors' knowledge, these are the highest reported values for any epitaxially grown Si-based tunnel diode. Another RITD design with an active region of 1 nm p+ Si0.6Ge0.4/B δ-doping plane/4-nm iSi0.6Ge0.4/2 nm i-Si/P δ-doping plane and annealed at 825°C for 1 min had a PVCR of 2.9, an fr0 of 0.4 GHz, and a speed index of 0.2 mV/ps. A small signal model was established to fit the measured S11 data for both device designs. Approaches to increase fr0 are suggested based on the comparison between these two diodes. The two devices exhibit substantially different junction capacitance/bias relationships, which may suggest the confined states in the δ-doped quantum well are preserved after annealing at lower temperatures but are reduced at higher temperature annealing. A comprehensive dc/RF semi-physical model was developed and implemented in Agilent advanced design system (ADS) software. Instabilities in the negative differential resistance (NDR) region during dc measurements were then simulated.