A BJT can be used as a switch by driving it between cutoff and saturation rather than keeping it in the forward-active region used for amplification.

In an npn low-side switch, insufficient base-emitter drive leaves the transistor in cutoff and the collector path carries little current. Sufficient base current drives the device on; if the external circuit limits the collector current below the value implied by $\beta I_B$, the transistor enters saturation.

In saturation, a simple model uses a small collector-emitter voltage $V_{CE(sat)}$ rather than the forward-active gain relation.

A base resistor is usually required to limit drive current. The design must provide enough base current to guarantee saturation under worst-case device gain while respecting the source's current capability.

Stored charge in saturation can slow turn-off, so switching speed and drive conditions matter in addition to the static on/off states.