Su, Y.-H.Y.-H.SuChou, K.-Y.K.-Y.ChouChuang, Y.Y.ChuangLu, T.-M.T.-M.LuJIUN-YUN LI2020-06-112020-06-11201900218979https://scholars.lib.ntu.edu.tw/handle/123456789/498459https://www.scopus.com/inward/record.uri?eid=2-s2.0-85067550681&doi=10.1063%2f1.5094848&partnerID=40&md5=2f72c919832e7d1ca5d1d38f9cce645eWe investigate the effects of surface tunneling on electrostatics and transport properties of two-dimensional electron gases (2DEGs) in undoped Si/SiGe heterostructures with different 2DEG depths. By varying the gate voltage, four stages of density-mobility dependence are identified with two density saturation regimes observed, which confirms that the system transitions between equilibrium and nonequilibrium. Mobility is enhanced with an increasing density at low biases and, counterintuitively, with a decreasing density at high biases as well. The density saturation and mobility enhancement can be semiquantitatively explained by a surface tunneling model in combination with a bilayer screening theory. © 2019 Author(s).Electron gas; Carrier screenings; Gate voltages; Mobility enhancement; Non equilibrium; Saturation regime; Si/SiGe heterostructures; System transitions; Tunneling models; Two dimensional electron gasjournal article10.1063/1.50948482-s2.0-85067550681