Design of an ultra-fast low-noise charge-sensitive microprobe for semiconductor detectors

A low-noise wide-bandwidth charge-sensitive micro-probe is being developed, able to capture the charge signals of semiconductor detectors. The micro-probe works on a single terminated coaxial cable of whatever length, whose quality is crucial to achieving a good dynamic performance. The cable carries both the power supply (DC signal component) and the event pulses (AC signal component). No power-supply filtering capacitor is required. The micro-probe is particularly compact, consisting of a few devices, including a GO surface-mount resistor. Both a discrete-component and an ASIC version are under development. The ASIC version is designed in a 0.35 um 5V CMOS technology and is expected to be fully functional also at cryogenic temperatures. The area occupancy is 0.14 mm2 bonding pads included. Thanks to such a large degree of integration the micro-probe can be placed very close to the detector electrode and is particularly light, compact and hence suited for hostile environments and for applications where a high radio-purity of the front-end is required, like in rare-decay research in underground laboratories. The rise time of the integrated version, as obtained from experimental results, is ≤2ns with a detector capacitance of 16pF. The energy range is beyond 20 MeV, and the power consumption is ~35 mW per channel. The gain stage includes an innovative low-frequency filtering device to provide a bias point to the system even if there are no constant voltage references such as conventional power rails. To achieve this result, CMOS transistors in underthreshold condition are used in combination with adequate capacitors in order to stabilize the source-to-gate voltage of the current generators. The first chip with various micro-probe versions is being tested.