base-emitter current (current control), or by the base-emitter voltage (voltage control). These views are related by the current-voltage relation of the base-emitter junction, which is the usual exponential current-voltage curve of a p-n junction (diode).

The explanation for collector current is the concentration gradient of minority carriers in the base region. Due to low-level injection (in which there are many fewer excess carriers than normal majority carriers) the ambipolar transport