Ferroelectric/GaN Heterostructures And Related Field-Effect Transistors

As a binary III/V semiconductor, gallium nitride has excellent properties, such as wide direct band gap, high electron drift velocity, strong breakdown electric field, and high temperature thermostablitization, which promote its applications in electronic, optoelectronic, high-power and high-frequence devices, especially for military and space applications in radiation environments. Integration of ferroelectric thin films which are of benefit of its nonlinear nature of ferroelectric, piezoelectric, electrolight, acousto-optic and photoelectric effects, ferroelectric/GaN heterostruc-tures combine the best features of each to perform their multiple-use functions in novel devices. The target of the dissertation is to clearly understand physical mechanisms of ferroelectric/GaN heterostructures and field-effect transistors, and establish device models of their electrical characteristics.The dissertation focuses on the basic device physical issues and theoretically research device models to describe the mechanism and properties of ferroelectric/GaN heterostructures, with the combination of approaches of theoretical analysis, numerical simulation, and experimental verification. This dissertation obtains the following achievements.Firstly, on the fact that the thermal generation rate of minority carrier is extremely low, analytic expressions of GaN surface charge density and capacitance are theoretically derived, and a model of flat-band voltage of metal/ferroelectric/GaN structure is presented. The model contains the coercive field of ferroelectric thin film, the spontaneous polarization of GaN, the metal-semiconductor work function, and surface charges and interface traps. Model results agree well with the experimental results and the model is useful to analyze the properties of ferroelectric/GaN heterostructures and field effect transistors.Secondly, consistent with the experimental results, a numerical model of hysteresis loops of metal/ferroelectric/GaN structure is proposed arfer explaining the polarization coupling effects at ferroelectric/GaN interface. The model reflects the shielding in accumulated state and depolarization effect in depletion and deep depletion states. To compare with interface polarization coupling effects, a numerical model of hysteresis loops of metal/ferroelectric/insulator structure is also proposed. The results show that the ferroelectric-layer voltage curve is clockwise and insulator component voltage effect is the reason why the hysteresis loop is unsaturated.Thirdly, using the voltage distribution relationship in capacitor network, flat-band voltage and GaN capacitance expression, a numerical capacitance model of metal/ferroelectric/GaN structures is developed and validated. The model reflects the accumulation, depletion and deep depletion states of capacitance curves. Combined with the caculated results of the energey band, the device mechanism by polarization-modulated field effect is clarified. The influences of GaN doping concentration, the coercive field of ferroelectric thin films, and its the relative dielectric constant and thickness, and the relative dielectric constant and thickness of dead layers on the capacitance curve of the metal/ferroelectric/GaN structures are discussed.Finally, output characteristic curves of different metal/ferroelectric/GaN field-effect transistors are given numerical simulation and indicate that the reason for the current of source/drain type transistor is large than that of Schottky-barrier type transistor is former lower barrier, by comparing the channel potential, electric field and carrier distribution and energy band diagram with each orther. A capacitance model of metal/ferroelectric/insulator/GaN structure is got to show that the capacitance curve is clockwise causing by the charge injection effect.These achievements illustrate more profound understanding of mutual constraint relationship between characteristics and parameters including materials, structures, and processing. The dissertation provides novel valuable approaches for the design, fabrication and optimization of ferroelectric/GaN heterostructures and related field-effect transistors.