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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 = (Lth T d e) / X

where:

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