Capacitance–voltage (${C}$ – ${V}$) characteristics and carrier transport properties of 2-D electron gases (2DEGs) in an undoped Si/SiGe heterostructure at ${T}= {4} - {35}$ K are presented. Two capacitance plateaus due to density saturation of the 2DEG in the buried Si quantum well (QW) are observed and explained by a model of surface tunneling. The peak mobility at 4 K is 4.1 $\times 10^{{5}}$ cm2/ $\text{V}\cdot \text{s}$ and enhanced by a factor of 1.97 at an even lower carrier density compared to the saturated carrier density, which is attributed to the effect of remote carrier screening. At ${T}\,\,=35$ K, the mobility enhancement with a factor of 1.35 is still observed, which suggests the surface tunneling is still dominant. [ABSTRACT FROM AUTHOR]