Probing Live PN Junctions with Terahertz Waves

It is known that the reflectivity of semiconductor materials at terahertz frequencies is sensitive to changes in free carriers. This phenomenon has been rigorously studied with semiconductor wafers and offline semiconductor devices. Despite this capability for sensing free carriers, terahertz probing of semiconductor devices in operation is nearly non-existent. Here we observe a PN-junction diode in operation, using terahertz waves in reflection. The Shockley diode equation and Drude model are leveraged to explain the observation. We anticipate that this work will be fundamental in establishing the capability to monitor semiconductor devices in operation, potentially unlocking a new wave of terahertz applications in the semiconductor industry and beyond