A Fabrication Technique for Significantly Reducing the Capacitance of Large-Volume Ge LID Detectors

Practical drift depths for Ge LID detectors are limited to less than 15 mm so that large-volume detectors have a relatively high capacitance because of their increased junction area. A fabrication technique has been developed and evaluated which significantly reduces this capacitance by increasing the effective thickness of the intrinsic material between the junctions and decreasing the junction area. In making a large-volume, low-capacity detector, a conventional Ge diode is first drifted to a depth greater than 1 cm. The original junctions used for the drifting operation are then removed and new junctions are applied to the compensated material in a more favorable geometry. A number of these detectors have been fabricated with a contact structure consisting of a gold surface-barrier P-type contact and a diffused-lithium N-type contact. The diodes that were made had an active volume of 4.5 cm3 and a final capacity of less than 2 pF as compared to their original capacity of 9 pF. The detector performance with a preamplifier and a source confirmed the improvement in resolution that would be expected for this decrease in the diode capacitance without a significant reduction in the active detector volume. This technique may be extended to larger detectors and would make possible the fabrication of a 30-cm3 Ge LID detector with a capacitance less than 5 pF.