Voltage-controlled emission wavelength switching (VCEWS) is demonstrated in a light-emitting diode (LED) containing a strained Si1-xGex/Si double quantum well (DQW) in the active region. Impact ionization in a reverse-biased Si and SiGe allows the otherwise evasive bipolar VCEWS in LEDs containing type-II anti-electron QWs. A clear spectral dominance switch was observed upon changing the direction of the driving current where electroluminescence (EL) from the QW on the positive electrode side dominated. In transient EL, a 500 kHz bandwidth was obtained, indicating that the impact ionization controls the generation of the carriers. (c) 2005 Published by Elsevier B.V.