Piezoelectronic Transistor - Piezoresistors
Piezoresistors are materials that change resistance when under pressure. Piezoresistance has found use as a strain sensor for solid-state accelerometers. Silicon and germanium exhibit piezoresistance, and have been used in many MEMS applications.
For a piezoelectronic transistor, it is important to use a piezoresistor that has a large on/off ratio. This will enable a logic device to consume low power when turned off, while transmitting large current when turned on. Rare-earth chalcogenides are known to have very strong pressure-dependent conductivity, with bulk single crystals showing up to 10,000,000x change in conductivity (Jayamaran et al., Phys. Rev. Lett. 25, 1430 (1970)).
The large piezoresistance observed for SmSe and other rare-earth chalcogenides such as SmTe, SmS, TmTe and SmEuS is due to their band structure. These compounds have a fully occupied 4f band, and an unoccupied 5d band. When compressed, the 4f band energy is raised, approaching the 5d conduction band. The density of carriers in the conduction band increases, leading to an increase in conductivity. The transition from insulator to metal is continuous, and reversible. Furthermore, the change does not involve a rearrangement of atomic bonds, but is an isostructructural transition. The PET merely needs to compress bonds, without moving atoms. For this reason, the piezoresistor can be cycled multiple times without the fatigue mechanisms associated with atomic motion.