Analytic modeling of tunnel Field-Effect-Transistors and experimental investigation of GaN High-Electron-Mobility-Transistors
High density and lower power drive the aggressive scaling down of CMOS transistors. Yet, the scaling of Si bulk MOSFETs are approaching physical limits, suffering from poor electrostatic control due to short channel effects, gate leakage current caused by gate oxide tunneling, and most importantly the non-scaled supply voltage imposed by thermionic emission limitation. Tunnel FETs (TFETs) based on band-to-band tunneling current injection mechanism, have emerged as promising candidates to deliver steep turn-off slopes, thus enables a sharp reduction of supply voltage to below 0.5 V.
This dissertation is primarily devoted to develop an accurate analytic model for TFETs with a double-gate structure, providing physical insights to the design principles. At the core of the model is a gate-controlled channel potential that satisfies the source and drain boundary conditions. The potential is of an exponential profile with a characteristic scale length given by the device thickness. Both the source-to-channel tunneling and source-to-drain tunneling are developed and included in the model. It has been verified by numerical simulations for a wide range of bandgaps and channel lengths. Also incorporated in the model are the short-channel effect, source doping effect, ambipolar effect, and de-bias of gate voltage by channel charge. Based on these, the guidelines for scaling TFETs to sub-10-nm channel lengths are brought forth. The model is continuous, physical and predictive in the sense that there is no need for ad hoc fitting parameters.
For high-power and high-frequency applications, GaN high-electron-mobility-transistors (HEMTs) stand out as promising candidate devices for achieving high breakdown voltage, high output current and high transconductance characteristics. Yet, the performance of GaN HEMTs suffers from mobility degradation due to poor thermal dissipation of conventional epitaxial substrates. This dissertation also experimentally demonstrates the GaN HEMTs fabricated on diamond substrate with extraordinary thermal management capability. The self-heating induced current droop is effectively absent in the saturated Ids-Vds characteristics of the resulting devices, thus paving the way for enhancing the energy conversion efficiency.