An extensive numerical analysis is performed to study and evaluate the impact of a dielectric sidewall spacer layer on the various device parameters associated with analog circuit performance of In0.75Ga0.25As channel asymmetric nMOSFETs with InP drain at two different channel lengths (L-g) of 20 and 30 nm. The numerical simulation deck is calibrated with asymmetric InGaAs MOSFET experimental characteristics reported in the literature. Our investigations reveal that device parameters such as transconductance gm, transconductance generation factor, and voltage gain Av exhibit significant improvement when a spacer of high dielectric constant k, such as 25, and small length L sp, such as 5 nm, are used for both Lg= 20 and 30 nm. On the contrary, the output conductance and unity gain cutoff frequency are found to reduce and increase, respectively, with lower k and larger L sp of the spacer. Our studies suggest that improved analog performance of In-rich asymmetric InGaAs MOSFETs can be achieved by spacer layer engineering at advanced technology nodes. © 1963-2012 IEEE.