TheI‐Vcharacteristic of dc and rf magnetron sputtering systems has been shown to fit an expression of the formI=&bgr;(V−V0)2, whereV0is the minimum voltage necessary to maintain a discharge. New experimental data is presented for a dc planar magnetron with Al, Cd2Sn, Cr, and CdSe targets in argon discharges and for a dc Research S‐gun with an Al target in argon and argon/oxygen discharges. Literature data for planar, S‐gun, and cylindrical magnetrons has also been shown to fit the above expression; for rf magnetrons,Iis the rms current andVis the target self‐bias voltage.V0decreases from about 400 V at 0.1 Pa to 250 V at 1 Pa and then decreases at higher pressures. &bgr; increases from about 50 to ≤300 A/kV2as the pressure increases from 0.1 to 10 Pa. The actual values ofV0and &bgr; depend on the system, target, and sputtering gas. It is shown that theI∝V2dependence is due to a space‐charge‐limited electron current in the magnetron geometry which reduces the electron mobility by several orders of magnitude from the zero magnetic field value; the mobility ratio depends on the electron‐atom collision frequency.