The low-frequency noise behaviour of graded-channel SOI nMOSFETs

It is shown that the low-frequency noise in graded-channel (GC) SOI nMOSFETs is generally of the flicker or 1/f noise type. The corresponding input-referred noise spectral density is markedly higher than for the conventional uniformly doped or the intrinsic undoped fully depleted n-channel SOI transistors. However, this increase can only be partially explained by the effective channel length reduction provided by the lightly doped region of the GC structure. It is furthermore demonstrated that the underlying noise mechanism for the GC structures is rather related to carrier number fluctuations compared with mobility fluctuations for the intrinsic or the uniformly doped fully depleted device. It is concluded that for optimal analog performance of GC SOI nMOSFETs, high gain has to be traded off for higher 1/f noise.