Critical Charge and Threshold LET

In many cases, a device's SEU threshold is expressed in terms of an effective linear energy transfer (LET) value instead of a critical charge. These two quantities are related by the formula:

Qc = (L_th T d e) / X

where:

• Qc is the critical charge
• L_th is the threshold effective LET
• T is the device thickness
• d is the material density (2.32 g/cm³ for Si; 5.32 g/cm³ for GaAs)
• e is the electronic charge = 1.602 x 10^-7 pC
• X is the energy needed to create one electron-hole pair (3.6 eV in Si; 4.8 eV in GaAs)

Putting in the numbers, this formula yields:

• Qc = 1.03 x 10^-2 (L_th T ) pC for Si
• Qc = 1.78 x 10^-2 (L_th T ) pC for GaAs
• where
  • L_th is expressed in MeV-cm²/milligram (not per gram!); and
  • T is expressed in microns.