Sub-5nm ultrashallow junctions in planar and non-planar semiconductors are formed by use of a molecular monolayer doping method and conventional spike annealing. ∼70% of the dopants are found to be electrically active, allowing for a low sheet resistance for a given dopant areal dose, and minimal junction leakage currents (<1 μA/cm2) are observed. This indicates the high-quality of the ultrashallow junctions formed by this monolayer doping method. In addition, temperature-dependent current-voltage (I-V) behavior of individual InAs nanowire field-effect transistors is used to study the field-effect mobility as a function of nanowire radius. The field-effect mobility is observed to decrease with decreasing radius. The low-temperature transport behavior reveals the significant impact of surface roughness scattering on mobility degradation in smaller radius nanowires. The successful demonstration of a monolayer doping technique that does not introduce defects into the substrate, combined with a better understanding of diameter-dependent electron mobility in nanowires, contributes toward the advancement of nanoscale, electronic materials.