Kynnysjännite

Kynnysjännite, often translated as threshold voltage, is a fundamental concept in solid-state physics and electronics, particularly concerning semiconductor devices like transistors and diodes. It represents the minimum voltage that must be applied across a junction or a device to initiate a significant flow of current or to induce a desired state of operation.

In the context of a p-n junction diode, the kynnysjännite, also known as the built-in voltage or barrier potential, is the voltage that opposes the diffusion of charge carriers across the junction when no external voltage is applied. This internal voltage arises from the formation of a depletion region at the interface between the p-type and n-type semiconductor materials. Once an external forward bias voltage exceeds this kynnysjännite, the depletion region narrows, allowing a substantial current to flow.

For a bipolar junction transistor (BJT), the kynnysjännite, often referred to as the base-emitter voltage ($V_{BE(on)}$), is the voltage required across the base-emitter junction to turn the transistor on and allow current to flow from collector to emitter. Similarly, in a field-effect transistor (FET), the kynnysjännite, or threshold voltage ($V_{th}$), is the minimum gate-source voltage ($V_{GS}$) needed to create a conductive channel between the source and drain terminals, enabling current flow. The precise value of the kynnysjännite depends on the semiconductor material, doping concentrations, and device geometry.