We present a simple silicon circuit for modeling voltage-dependent ion channels found within neural cells, capturing both the gating particle's sigmoidal activation (or inactivation) and the bell-shaped time constant. In its simplest form, our ion-channel analog consists of two MOS transistors and a unity-gain inverter. We present equations describing its nonlinear dynamics and measurements from a chip fabricated in a 0.25 /spl µ/m CMOS process. The channel analog's simplicity allows tens of thousands to be built on a single chip, facilitating the implementation of biologically realistic models of neural computation.