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A demateralisation circuit was an essential part of a Type 40 TARDIS which enabled it to de-materialise from normal space into the Vortex and re-materialise later back into it. (DW: Terror of the Autons)
During his exile on Earth, the Time Lords removed the "dematerialisation codes" from the Doctor's mind and made the dematerialisation circuit of his TARDIS inactive. He therefore coveted the working dematerialisation circuit of the Master's TARDIS (DW: Terror of the Autons, The Mind of Evil, EDA: The Eight Doctors). Once the Time Lords had rescinded his exile after his defeat of the renegade Time Lord Omega, the Time Lords restored this knowledge to the Doctor memory. (DW: The Three Doctors)