- 學位論文
Negative Differential Conductivity of Two Dimensional Electron Gas System at High Magnetic Field
二維電子系統強磁場下負電導之研究
摘要
A new kind of nonlinear effect is observed a two-dimensional electron gas system at high magnetic field. Understanding of negative differential conductivity (NDC) in bulk semiconductors in the absence of magnetic fields has been established since nearly four decades ago [1]. It is well-known that a semiconductor exhibiting NDC is inherently unstable and could be characterized by a multivalued dependence in the current density-electric filed (J-E) relation. The instability of the system with NDC in the presence of magnetic field, however, is less theoretically studied. Kurosawa et al.[2] suggested that the macroscopic instability due to NDC is much more easily induced to break the homogeneous states under a strong magnetic field. According to Ref [2], the criterion of NDC is given as ΔJ •ΔE <0. As the criterion is fulfilled, the additional current ΔJ moves inwardly and hence the space charge sheet start growing. Since the Hall angle ψ is almost near π/2 in the presence of the strong magnetic field, even a week nonlinear effect could cause ψ > π/2 and the current instability would emerge. Theoretically the same conclusions could be extended to apply in two dimensional electron gas system (2DEG ) case. The nonlinear transport is characterized by the current driven instability at high lattice temperatures. The threshold of current instability is found to be associated with the occurrence of negative differential conductivity. The origin of the nonlinearities is attributed to the suppression of the scattering rate with increasing electric field because of the peculiar shape of the density of states in quantum Hall effect. By comparing the data with the existing theories we discuss the nature of current instability and bode a phase diagram for the future studies.