Study of time resolution of low-gain avalanche detectors

Abstract

A Low Gain Avalanche Diode (LGAD) is a semiconductor device that amplifies signals inside a sensor, using an avalanche mechanism. As the charge multiplication occurs within a thin p–n junction and the sensor can be made thin, it can achieve a superior time resolution. For example, a resolution of 30 ps was obtained in our previous beam-test study. In combination with the excellent 3-D spatial information achievable by semiconductor devices, we are aiming to develop a 4-D detector. Although high-energy beams are extremely useful for evaluating the time resolution of LGADs, desktop measurement systems are also helpful for the swift evaluation of LGAD samples under development. Therefore, we are developing a time measurement system using β-rays from a 90Sr source. We have obtained a slightly degraded time resolution than the beam-test results, but the system is found to be useful for our development. In the development of 4-D detectors, the readout needs to be finely segmented (to sub-100μm) while maintaining a uniform gain over the wider area, and a high fill factor. One such device with the required segmentation is the AC-LGAD, in which the gain layer is implanted over the entire sensor area, and the signal is read out via segmented AC pads interleaved with an insulating layer. Several AC-LGAD structures were created in a TCAD simulation, and the optimal sensor design parameters were extracted.