  We review recent precision measurements on semiconductor tunable-barrier
electron pumps operating in a ratchet mode. Seven studies on five different
designs of pumps have reported measurements of the pump current with relative
total uncertainties around $10^{-6}$ or less. Combined with theoretical models
of electron capture by the pumps, these experimental data exhibits encouraging
evidence that the pumps operate according to a universal mechanism, independent
of the details of device design. Evidence for robustness of the pump current
against changes in the control parameters is at a more preliminary stage, but
also encouraging, with two studies reporting robustness of the pump current
against three or more parameters in the range of $\sim\!5 \times 10^{-7}$ to
$\sim\!2 \times 10^{-6}$. This review highlights the need for an agreed
protocol for tuning the electron pump for optimal operation, as well as more
rigorous evaluations of the robustness in a wide range of pump designs.
