| This thesis presents the realization of high-mobility two-dimensional (2D) electrons in Si/SiGe heterostructures and the transport properties of such systems at low temperatures.;Conventionally, Si 2D electron devices are either Si metal-oxide-semiconductor field-effect transistors or modulation-doped Si/SiGe heterostructures. Combining the device architecture of these two types of device, we demonstrate the operation of enhancement-mode Si/SiGe heterostructure field-effect transistors (HFETs). The observed electron mobility 1.6 x 106 cm 2/Vs is the record value for 2D electrons in a Si quantum well at the time of writing. The electron density limit in the quantum well and the feasibility of complementary circuits are investigated.;Using the high-mobility Si/SiGe HFETs and conventional modualtion-doped Si/SiGe quantum wells, we study the 2D metal-insulator transition (MIT) problem by measuring the dependence of the resistivity on temperature and in-plane magnetic field. Experimental observations and comparisons with prior work are presented, as well as the applicability of some theoretical proposals. The results underscore the importance of the nature of disorder in the 2D MIT phenomena.;At high magnetic fields, the fractional quantum Hall effect (FQHE) is observed in the high-mobility Si/SiGe HFETs. The observed prominent FQHE states are consistent with the composite fermion model with a valley degree of freedom. Features of weak FQHE states, previously obscure or unseen in Si 2D electron systems, are reported. |
