HTS scanning SQUID microscopy of active circuits

We have used a high-T-c scanning SQUID microscope to image semiconductor ci rcuits operating in air at room temperature. Our microscope uses a commerci ally available 77 K refrigerator to cool a YBa2Cu3O7-delta de SQUID. The sy stem maintains vacuum isolation of the SQUID even when it is separated from a room-temperature sample by about 30 mu m. When operated in this manner, the SQUID has a magnetic field sensitivity of 20 pT/root Hz above 500 Hz. B y inverting the magnetic field images to generate two-dimensional current d ensity distributions, we localize current paths to within +/-36 mu m at SQU ID-sample separations of 150 mu m, We present images and discuss the spatia l resolution obtained with this technique.