Investigatioh On Electrical Characteristics Of GaN-based MFSFETs

GaN-based metel-ferroelectric-semiconductor field-effect transistors (MFEFETs) have recently attracted much attention due to their excellent potential for memory applications, such as non-volatile storage, nondestructive readout, simple structure, high temperature and power. In order to design well performance GaN-based MFSFETs, the relationship between device electric characteristics and design parameters is need to deeply comprehend. So far, howerver, few literatures have systematically reported the device models about GaN-based MFSFET performance. Therefore, this dissertation is to research and discuss the carrier transport and current-voltage characteristics of GaN-based MFEFETs, based on the fundamental semiconductor equations and boundary conditions where both ferroelectric polarization and semiconductor spontaneous polarization are included and consideration of the influence of various scattering mechanisms on carrier mobility.Firstly, the electrical properties of GaN-based MFS capacitors under ideal conditions and non-ideal conditions have been investigated meanwhile. On account of the dead layer at the interface between ferroelectric and GaN substrate, the capacitor characteristic curves, including P-V, C-V, and I-V curves, present obviously shift and shape changing duo to its non-switchable ferroelectric and lower dielectric constant. Moreover, the reliability of the numerical results has been verified by the experimental results. It is found that the thicker dead layer would make the device performance worse and even failure of devices.Secondly, the influence of dead layer thickness ratio, ferroelectric layer thickness, and channel length on electrical characteristics of GaN-based MFSFETs is synthetically considered and carrier densities and mobilities as well as transfer and output characteristics are present.Analytical expressions of threshold voltage and memory window have been obtained with experimental verification, which demonstrate that the dead layer has much more effects than ferroelectric layer and even determines the electrical conduct mechanisms and application purposes of devices.Thirdly, the influence of the ferroelectric dimensional and material parameters on GaN MFSFET electrical properties has been analyzed in details and some useful rules are obtained:(1) Decreasing the ferroelectric spontaneous polarization Ps and ferroelectric dielectric constant value εF, increasing ratio of Pr/Ps and decreasing the thickness of ferroelectric film tF would make the ferroelectric film polarization become saturation under lower applied bias. Amplifying the coercive electric field Ec and tF, the memory window would be widened, however, if the ferroelectric polarization need to maintain at the saturation state, the GaN MFSFET work bias must be enhanced.(2) Reducing Ps and εF and increasing tF, the drain current and the ratio Ion/Ioff of GaN MFSFET would be decreased.(3) In order to keep GaN MFSFET having excellent retention characteristics, the ferroelectric polarization must be saturation. Therefore the minimum work bias for this device is the bias which was applied to keep the ferroelectric polarization saturated.(4) The transfer characteristics of GaN MFSFET is investigated with the length of ferroelectric layer changing from4μm to0.2μm. The results demonstrate that the variation tendency of transfer characteristics is quite different and even in the opposite on each side of1μm. When the channel length shortens from4μm to1μm, drain current as well as transconductance decrease with insignificant differences of threshold voltage and sub-threshold characteristics. When the channel length further shortens to0.2μm, drain current and transconductance turn to increase while the threshold voltage reduces. When it is longer than0.4μm, the sub-threshold characteristics of GaN MFSFET approach to the ideal state, while when it is shorter than0.4μm, the sub-threshold slope becomes to increase and makes the sub-threshold characteristics go to the bad. Especially below0.2μm, the sub-threshold slope becomes infinity, resulting in abnormal sub-threshold characteristics, and the minimum value of drain current is as high as0.25mA, which makes it unable to store information effectively as a memory because of its too large power consumption.