The low-performance "lateral" bipolar transistors occasionally employed in CMOS processes are sometimes designed symmetrically, i.e., with no distinction between forward and backward operation.
Small modifications in the voltage applied across the base-emitter terminals causes the current that flows among the emitter and the collector to change considerably. This influence can be employed to amplify the input voltage or current. BJTs can be thought of like voltage-controlled current sources, but are much more simply characterized as current-controlled current sources, or current amplifiers, because of the low impedance at the base.
Early transistors were prepared from germanium but most current BJTs are made from silicon. A significant minority is as well now made from gallium arsenide, particularly for very high speed applications.