tp@fbsd15:~ % fabric "what is a ujt ?"
A *UJT* (Unijunction Transistor) is a three-terminal semiconductor switching device with only *one p-n junction* (hence “uni-junction”). It consists of:
- *Emitter (E)* - *Base 1 (B1)* - *Base 2 (B2)*
The device is fabricated on an n-type silicon bar with a small p-type region diffused into one side; the p-region forms the emitter, and the two ends of the bar are B1 and B2. Because the bar acts like a voltage divider, the voltage at the emitter junction is a fraction (η, the intrinsic stand-off ratio, typically 0.5–0.8) of the inter-base voltage V_BB.
*Key property:* When the emitter voltage V_E rises to about *η V_BB + 0.7 V*, the internal p-n junction becomes forward-biased, the device switches into a *negative-resistance region*, and a large emitter current flows while V_E collapses. This snap action makes the UJT useful as a *relaxation oscillator*, *pulse generator*, and *timing element* (e.g., in SCR triggering circuits).
Despite its name, the UJT is *not* a transistor in the usual amplifying sense; it is primarily a *trigger/switching device*.
oldguy101•2w ago