Intracellular voltage transients of cultured cells are recorded by transistors and other planar electrodes as local extracellular voltages. The theoretical relationship between extra- and intracellular voltage is investigated with a two-compartment circuit using the approximation of a fast, weak and small cell-silicon junction. It is shown that extracellular recording relies on the difference of specific ionic conductances in the attached and free regions of the cell membrane. The result rationalizes various observations with neuron transistors. It guides the optimization of extracellular recording and the development of cell-based chemical sensors.