Direct optical triggering of an avalanche transistor with a short laser pulse has been demonstrated. In some applications this system provides a compact low‐jitter replacement for a laser‐triggered spark gap. The technique has been applied to generating the gating and sweep voltages in a picosecond streak camera for laser pulse diagnostics where it eliminates the need for multiple beam splitters and long delays. The ``trigger'' avalanche transistor was placed as one of a series string of avalanche transistors. A portion of the switched‐out laser pulse to be diagnosed was focused onto the trigger transistor chip. Nanosecond‐rise kilovolt waveforms are thus generated with time jitter of the entire system being less than 100 psec.