Segregation of Si in molecular beam epitaxy grown GaAs quantum wells is investigated using current–voltage characteristics (I–V) of quantum well infrared photodetectors. Theoretical modeling is used to derive the extent of segregation from the measured ratio of dark currents through the device in forward and reverse directions. The segregation length, expressed in Å per decade of concentration decay, increases from 12 to 52 Å on increasing the growth temperature from 550 to 605 °C. The character of this increase indicates that Si segregation is kinetically limited in this temperature range, but approaches thermal equilibrium above 600 °C. The effect of arsenic overpressure on the segregation length at a growth temperature of 595 °C is also studied. An increased overpressure of arsenic suppresses Si segregation for both dimeric and tetrameric forms of As. The effect of As2is very small, with the segregation length decreasing from 52 to 48 Å on increasing the arsenic flux by a factor of eight from its nominal value. The same increase in flux gives a much stronger suppression of Si segregation if As4is used: the segregation length decreases from 51 to 40 Å.