Abstract
Terahertz signals emitted from three photoconductive antennae based on semi-insulating GaAs and with different gap sizes are tested. These signals represent that the distribution of electrical field between electrode gaps and electrical field enhancement on the anodes is testified. Two main causes of this phenomenon are the different movabilities of electrons and holes and the induced current which is brought by the electrons on arriving at the anodes. The electrical field distributes in a large region, which extends from tens to hundreds of micrometers and it is decided by the gap size.