Complex sentence: In a semiconductor, the work function is sensitive to the electric field in the vacuum, due to the field effect: essentially, the doping of the surface can be controlled by electric field.
Split into simpler sentences: In a semiconductor, the work function is sensitive to the doping level at the surface of the semiconductor. In turn, the work function is also sensitive to the electric field in the vacuum, due to the field effect: the doping near the surface can be controlled by electric field.