In power amplifiers, signal levels are no longer small, so that issues such as power-conversion efficiency and signal distortion become important. Power transistors are designed and mounted so as to remove heat efficiently from collector or drain, which have a large area and are in direct contact with a metal side or case of the transistor. Relatively high power outputs imply large-signal operation that departs from linearity and introduces nonlinear distortion. In many applications, the power transistor is operated in a switching mode, in which it is repeatedly turned on and off. The main advantage is that power efficiency is high because the transistor is either fully conducting or cut off. Another structural feature of power bipolar junction transistors (BJT) is the interleaving of emitter and base regions. Insulated Gate Bipolar Transistors combine the advantages of metal–oxide–semiconductor field-effect transistors and BJTs while having intermediate power handling capability and speed of response.