This paper describes the design, construction, and characterization of a variable‐temperature wafer apparatus for use in the detection of electrically active defects which produce deep levels in the band gap of silicon. In its present form, the wafer chuck can heat and cool wafers as large as 51 mm in diameter over the temperature range from −196° to 350°C. Heating rates as high as 7°C/s have been achieved. Sensitivity for electrical measurements is sufficient to allow current measurements as low as 0.2 pA or capacitance changes (1 MHz) as small as 5 fF. The use of this apparatus is illustrated by wafer mapping the gold defect density in diodes fabricated across a silicon wafer.