Early effect

In a bipolar junction transistor, the Early effect is the unwanted change in collector current that occurs when the base-to-collector voltage changes, even though the base current stays constant. This happens because the depletion region at the base-collector junction expands as reverse bias increases, effectively squeezing the neutral base region narrower. A thinner base means fewer recombination paths for charge carriers, so more current flows from emitter to collector. This parasitic variation degrades the transistor's output impedance and introduces nonlinearity in amplifier circuits.

The effect is named after James M. Early, who characterized it in silicon transistors during the 1950s. It appears in nearly all real bipolar devices, though its magnitude varies with design. In germanium transistors and early silicon planar types, Early voltages might be 50 to 150 volts; in modern epitaxial designs with careful doping profiles, they can exceed 1000 volts. The Early voltage is defined as the voltage at which a graph of output collector current, extrapolated backward, would cross the horizontal axis, typically ranging from 50 to 300 volts for general-purpose transistors.

Impact on circuit behavior

For a given base current, the collector current is not truly constant but increases slightly with collector-to-emitter voltage. In a 2N3904 small-signal transistor, this effect might cause collector current to vary by 10 to 20 percent across a 10 volt swing at the collector. In precision analog circuits, logarithmic converters, and current mirrors, the Early effect introduces gain errors and temperature drift that designers must actively compensate. Current mirrors relying on matched transistor pairs are particularly sensitive because even tiny differences in collector voltage between the reference and mirror legs can cause significant current mismatches.

The Early effect becomes more pronounced at low collector-emitter voltages because the base width modulation is a larger fraction of an already narrow base. Conversely, saturation operation, where the collector-base junction becomes forward biased, disrupts the entire effect model because the depletion region shrinks rather than expands. Power transistors designed for switching applications tolerate larger Early effect because speed and blocking voltage take priority over linearity, whereas precision analog transistors and integrated circuit designs incorporate layout techniques like base implantation profiles and emitter geometry to minimize it.

Designers quantify the Early effect through the transistor's output resistance, which equals the Early voltage divided by the collector current. Higher Early voltage means higher output resistance and better current source behavior. In integrated circuits, designers often use cascode configurations, which stack transistors to boost effective output impedance without requiring impractically high Early voltages. Understanding and accounting for the Early effect remains essential for anyone designing transimpedance amplifiers, precision gain stages, or current-steering analog circuits, where seemingly small voltage variations can accumulate into circuit-level performance loss.