  We report on transport characteristics of field effect two-dimensional
electron gases (2DEG) in surface indium antimonide quantum wells. The topmost 5
nm of the 30 nm wide quantum well is doped and shown to promote the formation
of reliable, low resistance Ohmic contacts to surface InSb 2DEGs. High quality
single-subband magnetotransport with clear quantized integer quantum Hall
plateaus are observed to filling factor $\nu=1$ in magnetic fields of up to
$B=18$ T. We show that the electron density is gate-tunable, reproducible, and
stable from pinch-off to 4$\times 10^{11}$ cm$^{-2}$, and peak mobilities
exceed 24,000 cm$^2$/Vs. Large Rashba spin-orbit coefficients up to 110
meV$\cdot$\r{A} are obtained through weak anti-localization measurements. An
effective mass of 0.019$m_e$ is determined from temperature-dependent
magnetoresistance measurements, and a g-factor of 41 at a density of 3.6$\times
10^{11}$ cm$^{-2}$ is obtained from coincidence measurements in tilted magnetic
fields. By comparing two heterostructures with and without a delta-doped layer
beneath the quantum well, we find that the carrier density is stable with time
when doping in the ternary Al$_{0.1}$In$_{0.9}$Sb barrier is not present.
Finally, the effect of modulation doping on structural asymmetry between the
two heterostructures is characterized.
