IcVbe

IcVbe, or the Ic–Vbe characteristic, describes the collector current of a Bipolar Junction Transistor (BJT) as a function of the base-emitter voltage (Vbe) at a given collector-emitter voltage (Vce). It is a fundamental way to visualize how the base-emitter diode controls the current flowing in the collector channel, reflecting the transistor’s exponential diode behavior in forward-active operation.

In basic transistor models, the forward-active Ic–Vbe relationship is often approximated by Ic ≈ Is exp(Vbe / Vt), where Is is the saturation current and Vt is the thermal voltage (approximately 25 mV at room temperature). This exponential dependence means small increases in Vbe produce large increases in Ic. The actual collector current also depends on temperature, device geometry, and Early effect, which introduces a dependence on Vce. In practice, Ic is related to the base current by the current gain β, with Ic ≈ β·Ib in forward-active operation, while Vbe largely sets Ib through the base-emitter diode.

IcVbe curves are used to extract transistor parameters for device models and SPICE simulations, to design biasing schemes, and to study temperature sensitivity. They illustrate transconductance, where the small-signal transconductance g_m ≈ Ic / Vt is directly tied to the slope of the Ic–Vbe curve. Device-to-device variation and measurement conditions (especially Vce and temperature) can significantly influence the curve.

Limitations include the breakdown of the simple exponential model outside forward-active operation, saturation at low Vce, and substantial variability among devices.