How early could the transistor appear?

How early could transistors have been invented?

Potentially in the late 1920s, but only if metallurgy advanced far enough to implement the already proposed field-effect concept.

The field-effect concept and the materials science needed to manufacture a useful device followed different timelines. Although the idea dates to 1926, practical transistors arrived much later. An alternate 1920s device is therefore conceivable only with a major advance in metallurgy and fabrication.