We report on combined RF-mu SR and electric fields (EF) mu SR measurements in semi-insulating Gap. At low temperature the capture of excess carriers by a neutral muonium (Mu = mu(+) + e(-)) center is suggested to be a dominant process in muonium charge dynamics. The charged muonium fraction (Mu(-) or Mu(+)) formed on the microsecond time-scale is increased with lowering the temperature. Contrary to the usually observed scenario, at T<40 K an EF prevents formation of the final charged state.
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