Physical Interaction
Mechanical strain alters the electrical conduction properties of semiconductor devices by modifying their band structure. This phenomenon, called the piezo-FET effect, occurs when external stress shifts the energy bands and changes the carrier mobility within the channel of a field-effect transistor. This coupling provides a direct link between mechanical deformation and electrical signal modulation.
Transducer Mechanism
Gate voltage control is augmented or countered by the polarization charges induced by applied force. The stress-induced piezopolarization charges act as an additional internal gate bias, which modulates the drain current of the transistor. This behavior allows the transistor to function simultaneously as a force sensor and an amplifier.
Sensitivity Modulation
Device orientation and channel geometry determine the magnitude of the current response to stress. Micro-electromechanical systems leverage this orientation dependence to maximize the transducer response in designated directions. By fabricating the channel along specific crystallographic axes of silicon, the current modulation per unit force can be maximized.
This directional sensitivity is exploited in highly sensitive force transducers and strain gauges.
Design Challenge
Parasitic strain in packaging can cause unwanted drift in the transistor characteristics. This thermal or assembly stress degrades the calibration of precision sensors. Specially designed isolation structures are required to shield the transistor from ambient mechanical noise.
