Si is often used as an n type dopant in MBE grown GaAs epi - layers. In order to determine the dop ing level, we have grown several GaAs layers on a semi insulated GaAs substrate. In each layer, the carrier concentration is different. We use electrochemistry to determine the carrier concentration in each layer, then we can get the relation between the Si dop ing level and the temperature of Si cell. How to get abrup t interface with imp roved growth conditions is also introduced.