Why (and How) LGADs Work: Ionization, Space Charge, and Gain Saturation

TL;DR

This paper presents a comprehensive physical model for LGADs that accurately reproduces their timing resolution by including ionization, space charge effects, and gain saturation, validated against multiple experimental data sets.

Contribution

The authors develop and validate a detailed simulation model of LGADs incorporating space charge effects and gain saturation, improving understanding of their timing performance.

Findings

Ionization alone overestimates Landau noise without additional effects.

Space charge effects smooth initial charge distribution during drift.

Gain saturation suppresses large-amplitude fluctuations, matching experimental data.

Abstract

The temporal resolution of Low-Gain Avalanche Detectors (LGADs), also known as Ultra-Fast Silicon Detectors (UFSDs), is governed by two contributions: jitter, arising from electronic noise and signal slew rate, and the Landau noise term, arising from the non-uniform energy deposition of minimum ionizing particles (MIPs). We show that a correct simulation of the initial ionization alone significantly overestimates the measured Landau noise. Two additional physical mechanisms are necessary to reproduce the data: space charge effects during electron/hole drift, which smooth the granularity of the initial charge distribution, and gain saturation during multiplication, which preferentially suppresses large-amplitude fluctuations. All steps of the model have been implemented in the fast simulation program Weightfield2 (WF2). The model is validated against several independent experimental observations: the evolution of the measured charge distribution with gain, the temporal resolution of events in the Landau tail, and the thickness dependence of timing performance. We also discuss a data-driven gain measurement method based on gain saturation, and implications for gain layer design.