In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20 kV gun was constructed with a transmission photocathode including an active layer of a GaAs-GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3 mu m for 760 - 810 nm laser wavelength. The brightness of the polarized electron beam was similar to 2.0 X 10(7) A cm(-2) sr(-1), which corresponds to a reduced brightness of similar to 1.0 X 10(7) A m(-2) sr(-1) V-1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8 X 10(8) C cm(-2) was observed for an extracted current of 3 mu A. (C) 2008 American Institute of Physics.