The switch-on time of a GaAs n-type triangular barrier switch (TBS) has been measured from the 10 to 90 percent points to be 300 ps. The real part of the impedance of the 75-µm-diameter TBS changed from 6.7 kΩ in the OFF state to 9.75 Ω (including device contact resistance) in the ON state. After the switching threshold level was exceeded, the current density through the switch increased exponentially with a time constant of 112 ps. The minority-carrier effective base transit time of 86 ps accounted for most of the TBS switch-on time constant.