The base current components in semiconductor on insulator symmetric lateral bipolar transistors are examined in detail to yield insight into the underlying device physics. For Si-OI devices, the base current component due to recombination in the quasineutral base is negligible, the component due to injection into the emitter has a weaker than 1 NE dependence because of the effect of heavy doping. The component due to recombination in the emitter-base diode space-charge region is associated with the fabrication process, and could be reduced to a negligible level with process optimization. For SiGe-OI devices, the effect of heavy doping appears to be less than for Si-OI devices. The result is higher maximum current gain for SiGe-OI devices than for comparably doped Si-OI devices.