The requirements of resonant tunneling diodes (RTDs) in general and molecular diodes in particular for use as local refresh in low-power DRAM memory cells are discussed. Simulations show that none of the so far published molecules showing negative differential resistance have adequate electrical properties. Further, simulations show that present RTDs in III-V materials or SiGe are not compatible with the demands of future DRAM generations. A detailed list of requirements on the electrical properties of molecular RTDs or RTDs made of nanocrystals is presented, For instance, the valley current of the RTDs should be in the 10(-16) A range. The issues of acceptable differences in the number of active molecules constituting the two RTDs, and the maximum acceptable contact resistance between the molecules and the silicon substrate are addressed.