Active Region
The active region, also called the forward-active mode, is the operating condition of a bipolar junction transistor in which the base-emitter junction is forward biased and the base-collector junction is reverse biased, so that carriers injected from the emitter cross the thin base and are collected, producing a collector current that is controlled by the base. It is the region in which a transistor behaves as an amplifier rather than as a switch: small changes at the base produce proportionally larger changes in the collector current, and the device behaves roughly like a current source whose value is steered by its input.
In the active region the collector current is set by the base side of the device, not by the load. The simplest description is IC = β × IB, where β is the current gain, typically somewhere from tens to a few hundred for small-signal silicon devices. The more fundamental description is that the collector current depends exponentially on the base-emitter voltage, IC = IS × exp(VBE / VT), where IS is a small saturation current and the thermal voltage VT is about 26 millivolts at room temperature. The second form explains why a silicon transistor in the active region sits at about 0.6 to 0.7 volt of base-emitter voltage over a wide range of currents, and why a change of only about 60 millivolts in VBE changes the collector current roughly tenfold. The load resistor merely converts the collector current into an output voltage, which is how voltage gain appears.
The reverse-biased collector junction is what makes the collector behave like a current source. Because the collector collects almost every carrier that arrives at the edge of the base regardless of how large the collector voltage is, the collector current changes only slightly as the collector-emitter voltage varies. On a set of output curves, the active region is the nearly flat part of each curve to the right of the steep knee. Small-signal amplifiers are biased so that the quiescent collector voltage sits well inside this flat part, with enough headroom above the saturation knee that the largest expected output swing does not push the device into the saturation region or toward cutoff, where the waveform would clip.
The Early effect explains why the flat part is not perfectly flat. Named after James M. Early of Bell Labs, who described it in the early 1950s, it arises because raising the collector-base reverse voltage widens the depletion region and narrows the effective base, which shortens the carriers' path and increases the collector current slightly. Extrapolating the output curves backward meets the voltage axis at a point called the Early voltage VA, often tens to a few hundred volts, and the transistor's output resistance is approximately VA divided by the collector current. A finite Early voltage limits the voltage gain a single transistor stage can achieve, which is why precision analog designs use current mirrors, cascode stages and long-base devices to raise output resistance.
Active-region operation is temperature-sensitive, which makes biasing the central practical problem. Both β and the base-emitter voltage drift with temperature: VBE falls by roughly 2 millivolts per degree Celsius at constant current, and β rises with temperature, so a poorly biased stage can creep upward in current, heat itself further and drift out of the active region, an extreme case being thermal runaway. Designers counter this with emitter degeneration resistors, voltage-divider bias with a stiff divider, negative feedback and, in integrated circuits, matched devices and current mirrors, all aimed at making the operating point depend on resistor ratios rather than on β. A common misconception is that β is a fixed constant of a transistor type; datasheets quote wide ranges, and β varies with collector current, temperature and individual part.
The naming caveat is that the MOSFET has an amplifying region that is called saturation, not active. A field-effect transistor operated as an amplifier is biased where the channel is pinched off and the drain current is nearly independent of drain voltage, a region named saturation by MOSFET convention, whereas a bipolar transistor in saturation is the closed-switch state. Some texts therefore use "active" or "forward active" for bipolar amplification and reserve "saturation" for the opposite bipolar condition, and cross-reading these vocabularies is a frequent source of confusion. The underlying analogy is sound, however: the bipolar active region and the MOSFET saturation region are both the regions in which the output acts as a controlled current source, which is why the same amplifier stages such as the common-emitter and common-source configurations have close parallels.
Read active region as the amplifier's home territory: the narrow range of bias where one junction injects, the other collects, and the base steers a nearly load-independent current, bounded by saturation on one side and cutoff on the other, and always at the mercy of temperature and the Early effect.