Fig. 4. TBJT for signal frequency monitoring.

a, b Schematic illustration of the device structure with contact-separating mode and sliding mode, respectively. c The collector current, when the FEP film vertically contacts and separates from the copper film with a frequency ranging between 1–5 Hz with collector voltage 0.5 V. d The collector current, when the FEP film and the copper film slide against each other with a frequency ranging between 1–5 Hz with collector voltage 0.5 V. e Relationship between the collector current and the vibration frequency; the sensitivity is 1.00 μA Hz⁻¹. f Relationship between the collector current and sliding frequency; the sensitivity is 0.98 μA Hz⁻¹