As the forward bias in a semiconductor P-N junction increases, the current through the junction also increases, up to a certain point. When the bias is increased beyond that point (called the saturation point), no further increase in the current occurs. A bipolar transistor becomes saturated when the base-emitter current reaches a maximum under conditions of changing bias. In the characteristic curve (current versus forward bias) of an amplifying or switching device, saturation is indicated by a leveling off of the curve as the forward bias increases.
In a ferromagnetic substance such as laminated or powdered iron, saturation is a condition in which an increase in magnetizing force fails to produce an increase in the magnetic flux density. When the magnetizing force is small, the flux density increases as the magnetizing force increases. But there is a limit to the flux density that can be obtained in a ferromagnetic material. When this limit is reached, the material is said to be at its saturation point. Magnetic saturation is usually undesirable in electronic circuits that use iron-core inductors or transformers.