## Purpose of Guard-Rings and Edge Termination
In CZT hemispherical detectors, the large-area cathode is susceptible to surface leakage currents and edge-related electric field distortions. Guard-ring and edge-termination structures are implemented to confine the active electric field, prevent charge leakage along the detector perimeter, and stabilize high-voltage operation. These designs enhance energy resolution, suppress dark current, and protect the detector from breakdown.
## Single or Multiple Guard Rings
A common approach involves one or more concentric guard rings around the hemispherical cathode periphery. These rings are biased slightly below the cathode potential to intercept surface leakage currents before they reach the small anode. Multiple rings can be used in a stepped potential configuration to gradually reduce surface fields, minimizing localized high-field regions that could trigger leakage or breakdown.
## Edge Passivation and Termination
The detector perimeter is typically passivated with dielectric materials such as SiO₂, Si₃N₄, or polymer coatings. Edge termination may involve beveling or chamfering of the crystal surface to reduce field crowding at corners and edges. Combined with guard rings, these treatments prevent high-field concentrations that contribute to surface leakage and improve long-term stability under continuous bias.
## Floating vs. Biased Rings
Guard rings can be either electrically floating or biased relative to the cathode. Floating rings collect leakage currents passively but may be less effective in shaping surface fields. Biased rings, connected via resistive networks to a controlled voltage slightly below cathode potential, actively shape the electric field and guide surface currents away from the active volume. Biased designs generally provide better suppression of dark current and improved spectral performance.
## Integration with Hemispherical Geometry
For hemispherical detectors, the curvature of the cathode complicates uniform guard-ring implementation. Rings are often fabricated as concentric circular electrodes on the curved surface or as planar annular structures along the crystal edge. Electrical modeling ensures that the guard rings do not distort the radial field critical for single-carrier electron collection while still intercepting edge leakage.
## Summary
Guard-ring and edge-termination designs in CZT hemispherical detectors involve one or multiple concentric rings around the cathode, combined with dielectric passivation and beveled edges. These structures can be floating or biased to intercept surface leakage and shape the electric field, preserving high-resolution spectroscopic performance and preventing edge-related breakdown while maintaining the integrity of the hemispherical charge collection geometry.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/what-guard-ring-or-edge-termination-designs-are-implemented-around-the-hemispherical-cathode-of-a-czt-hemispherical-detector.html
