Ning Wang

A novel enhancement mode AlGaN/GaN high electron mobility transistor with split floating gates*

A novel enhancement-mode AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. Specifically, several split floating gates (FGs) with negative charges are inserted to the conventional MIS structure. The simulation results revealed that the decreases with the increase of polarization sheet charge density and the tunnel dielectric (between FGs and AlGaN) thickness, while it increases with the increase of FGs sheet charge density and blocking dielectric (between FGs and control gate) thickness. In the case of the same gate length, the will left shift with decreasing FG length. More interestingly, the split FGs could significantly reduce the device failure probability in comparison with the single large area FG structure.

Trap behaviours characterization of AlGaN/GaN high electron mobility transistors by room-temperature transient capacitance measurement

In this paper, the trap behaviours in AlGaN/GaN high electron mobility transistors (HEMTs) are investigated using transient capacitance measurement. By measuring the transient gate capacitance variance (ΔC) with different pulse height, the gate pulse induced trap behaviours in SiNX gate dielectric layer or at the SiNX/AlGaN interface is revealed. Based on the results, a model on electron traps in AlGaN/GaN HEMTs is proposed. The threshold voltage (Vth) instability in AlGaN/GaN HEMTs is believed to be correlated with the presence of these traps in SiNX gate dielectric layer or at the SiNX/AlGaN interface. Furthermore, trap density before and after step-stress applied on drain electrode is quantitatively analyzed based on ΔC measurement.

A Magnetic Flower State-Based Memory Cell

In an elliptical magnetic dot two vortices and a flower state configuration can be accommodated at remanence. We show that a two-fold degeneracy in the position of the flower state can be geometrically introduced and exploited for solid state data storage. Switching between two, energetically-equivalent, magnetic configurations is obtained by injecting a perpendicular electric current. Switching currents of the order of ~105 A/cm2 and high thermal stability are demonstrated by micromagnetic simulations. This system can be used as a bit-cell for magnetic solid state data storage. The low value of the switching current and the high thermal stability allow direct integration with the current complementary metal oxide semiconductor technology.

A Charge Storage Based Enhancement Mode AlGaN/GaN High Electron Mobility Transistor

In this work, a charge storage based enhancement mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. A stacked gate dielectrics, consisting of a tunnel oxide, a charge trap layer and a blocking oxide are applied in the HEMT structure. The E-mode can be realized by negative charge storage within the charge trap layer during the programming process. The impact of the programming condition and the thickness of the dielectrics on the threshold voltage (Vth) are simulated systematically. It is found that the Vth increases with the increasing programming voltage and time due to the increase of the storage charge. Under proper programming condition, the Vth can be increased to more than 2 V. Moreover, It is also found that the Vth increases with the decrease of the thickness of the dielectrics. In addition, it is found that the breakdown voltage of such HEMT can be adjusted by varying the gate dielectric stacks.

Investigation on Surface Polarization of Al2O3-capped GaN/AlGaN/GaN Heterostructure by Angle-Resolved X-ray Photoelectron Spectroscopy

The surface polarization of Ga-face gallium nitride (GaN) (2xa0nm)/AlGaN (22xa0nm)/GaN channel (150xa0nm)/buffer/Si with Al2O3 capping layer is investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS). It is found that the energy band varies from upward bending to downward bending in the interface region, which is believed to be corresponding to the polarization variation. An interfacial layer is formed between top GaN and Al2O3 due to the occurrence of Ga–N bond break and Ga–O bond forming during Al2O3 deposition via the atomic layer deposition (ALD). This interfacial layer is believed to eliminate the GaN polarization, thus reducing the polarization-induced negative charges. Furthermore, this interfacial layer plays a key role for the introduction of the positive charges which lead the energy band downward. Finally, a N2 annealing at 400xa0°C is observed to enhance the interfacial layer growth thus increasing the density of positive charges.

Three-Mode Behavior of Spin-Transfer Vortex Oscillators With Dynamic Polarizer

Magnetic vortex-based oscillators represent a unique opportunity to study the mutual spin-transfer torque coupling between two ferromagnetic layers in a spin-valve configuration. This will eventually lead to a complete understanding of this phenomenon and, potentially, to electronic devices of practical application. Mutual spin-transfer torque can lead to multiple-mode high-frequency dynamics in a spin-valve system. We here report our study on the emission response of an ohmic nanocontact to a spin-valve multilayer subject to perpendicularly applied magnetic field. In this configuration, three modes are accessible, two of which correspond to the precessional motion of a vortex in one of the two ferromagnetic layers with the other working as a static polarizer. At high currents, the third mode can be observed that is ascribed to the simultaneous precession of two vortices, one in each layer, with the other layer working as a dynamic polarizer. The simultaneous application of a field in the sample plane tends to suppress the mode that corresponds to the precessional motion of the vortex in the thinner ferromagnetic layer.