The surface photovoltage (SPV) technique is often used to determine the diffusion length in hydrogenated amorphous silicon (a‐Si:H). The use of this technique fora‐Si:H is disputed because it has been shown by computer simulations that the drift component is not negligible compared to the diffusion current in SPV measurements ona‐Si:H. For crystalline semiconductors the SPV technique gives the minority‐carrier diffusion length while the drift current is negligible. We found from the solution of the transport equations that the SPV technique can be used in the case ofa‐Si:H, but gives the ambipolar diffusion length which consists of the mobility and lifetime of both holes and electrons. However, the termdiffusionlength is misleading since the hole drift current and the hole diffusion current are of the same order of magnitude fora‐Si:H. This result is in accordance with the results of the computer simulations mentioned earlier. The drift current assists the diffusion current and increases the measured diffusion length by a factor (2)1/2compared to the situation with diffusion current alone.