Novel GaAs/AlGaAs heterojunction phototransistors with a delta -doped base have been fabricated. Very high gain and low output noise have been measured. The measured noise is composed ofshol noise associated with collector quiescent bias current and amplifed shot noise due to collector leaf; current for non-passivated devices. For well-passivated devices, the measured noise is, as predicted previously [Y. Wang, E.S. Yang, J. Appl. Phys. 74 (1993) 6978], just composed of the shot noise associated with collector quiescent bias current. The high gain and low intrinsic noise characteristics of these transistors make them very promising in weak light dt tection.