Basically, a bipolar transistor amplifies a small current entering the base to produce a large collector current. It is a current-driven device since the collector current is controlled via the base current. The current gain varies with the collector-emitter voltage (VCE). In the active region shown in the right-hand figure, a bipolar transistor provides a gain called a DC current gain (hFE) . In this region, the collector current remains almost constant regardless of the collector-emitter voltage (VCE). On the other hand, in the saturation region, a bipolar transistor exhibits a DC current gain of only 10 to 20, where the collector current considerably varies with VCE.
A field-effect transistor (FET) controls the width of a current path called the channel that is created by the voltage applied across the gate and source terminals. Changes in the channel width causes the drain-source resistance to change. Therefore, an FET is a voltage-driven device.
An IGBT is a device that combines a MOSFET, a type of FET, in the front stage and a bipolar transistor in the rear stage. An IGBT operates in the same manner as an FET when it turns on. Therefore, an IGBT is also a voltage-driven device.