A capacitive micro-machined ultrasound transducer (CMUT) device configured to transmit and/or receive ultrasound waves, comprising: - at least one CMUT cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, at least one dielectric layer (21, 23) between the first electrode and the second electrode, and a cavity formed between the substrate and the membrane, the CMUT device further comprising: - an operating bias voltage source configured to supply an operating bias voltage (V B) of a first polarity between the first and second electrode (20, 22) during transmitting and/or receiving ultrasound waves, - a charging voltage source configured to supply an additional charging voltage (Vc) of a second polarity between the first and second electrode (20, 22), the second polarity being the reverse polarity of the first polarity, - a control unit configured to control the charging voltage source to control a charge in the at least one dielectric layer (21, 23) and to supply the charging voltage at a time during no transmitting and/or receiving of ultrasound waves and for a long enough time period to trap charge carriers in the at least one dielectric layer (21, 23), and - a monitoring unit configured to monitor the charge in the at least one dielectric layer (21, 23), the CMUT device being characterized in that: the monitoring unit is adapted to detect when the charge in the at least one dielectric layer (21, 23) is below a predefined value, and the control unit is adapted to control the charging voltage source to reapply the charging voltage (Vc) when the monitoring unit detects that the charge in the at least one dielectric layer (21, 23) is below the predefined value. - at least one CMUT cell comprising a substrate comprising a first electrode, a membrane comprising a second electrode, at least one dielectric layer (21, 23) between the first electrode and the second electrode, and a cavity formed between the substrate and the membrane, - an operating bias voltage source configured to supply an operating bias voltage (V B) of a first polarity between the first and second electrode (20, 22) during transmitting and/or receiving ultrasound waves, - a charging voltage source configured to supply an additional charging voltage (Vc) of a second polarity between the first and second electrode (20, 22), the second polarity being the reverse polarity of the first polarity, - a control unit configured to control the charging voltage source to control a charge in the at least one dielectric layer (21, 23) and to supply the charging voltage at a time during no transmitting and/or receiving of ultrasound waves and for a long enough time period to trap charge carriers in the at least one dielectric layer (21, 23), and - a monitoring unit configured to monitor the charge in the at least one dielectric layer (21, 23), the CMUT device being characterized in that: the monitoring unit is adapted to detect when the charge in the at least one dielectric layer (21, 23) is below a predefined value, and the control unit is adapted to control the charging voltage source to reapply the charging voltage (Vc) when the monitoring unit detects that the charge in the at least one dielectric layer (21, 23) is below the predefined value. the monitoring unit is adapted to detect when the charge in the at least one dielectric layer (21, 23) is below a predefined value, and the control unit is adapted to control the charging voltage source to reapply the charging voltage (Vc) when the monitoring unit detects that the charge in the at least one dielectric layer (21, 23) is below the predefined value.