Yiqiang Chen

Effects of surface tension and axis stress on piezoelectric behaviors of ferroelectric nanowires

The effects of surface tension and axis stress on piezoelectric behaviors of ferroelectric nanowires with radius polarization were investigated by the time-dependent Ginzburg-Landau theory. When surface tension increases, both of coercive field and remnant strain decrease. The larger the surface tension is, the more they decrease. The axis compressive stress enhances the coercive field and remnant strain, while the axis tensile stress has contrary effect. The reason for the stress-modulated piezoelectricity is that radius polarization is forced by axis compressive stress but restrained by surface tension and axis tensile stress. The research is useful for ferroelectric nanostructures in strain engineering.

Reliability Investigations of AlGaN/GaN HEMTs Based on On-State Electroluminescence Characterization

In this paper, we investigated ON-state electroluminescence (EL) characteristics in fresh GaN-based HEMTs under different ON-state bias conditions. It demonstrated that: 1) the intensity of the ON-state EL emitted by the GaN-based HEMTs with relatively high VDS (VDS > 10 V) monotonically increased as the gate voltage increased; and 2) the distribution of the intensity of the ON-state EL was uniform along the gate and across the device before stress tests. The degradation process and mechanisms of the devices in the high temperature operation (HTO) stress were also analyzed based on the ON-state EL characterization, the electrical measurement, and failure analysis. It demonstrated the following. First, the ON-state EL distribution of the device after step HTO (S-HTO) stress tests was no longer uniform along one of the fingers, which indicated that the degradation was related to this finger. This hypothesis was confirmed by the SEM observation after the deprocess of the gate metal and the SiN passivation layer. Second, a new degradation mechanism of AlGaN/GaN HEMTs that caused the abrupt degradation in the S-HTO stress was identified and related to the emergent, localized, and jagged crack under the gate metal. Third, there was no abrupt degradation in the long-term HTO (LT-HTO) stress test. However, the ON-state EL intensity of the inner fingers in the device after the LT-HTO stress test decreased more than that of the outer fingers, which meant that the degradation of the inner fingers was faster than that of the outer fingers as a result of the higher temperature caused by self-heating in the inner region of the device. Finally, the gradual degradation in the LT-HTO stress test was ascribed to the gradual formation of structural damage along the drain side of the whole gate edge, which was confirmed by the SEM observation.

Using Termination Effect to Characterize Electric and Magnetic Field Coupling Between TEM Cell and Microstrip Line

In this paper, we change terminal loads of a microstrip line to separate electric field coupling and magnetic field coupling between a microstrip line and a transverse electromagnetic (TEM) cell during radiated emission measurement. Such separation can also be validated by another terminal load. After validation, the effect of termination on both couplings is investigated, and the requirements of termination impedance to suppress the two couplings are revealed. In addition, we find that the linear physical models deriving both lumped coupling parameters only hold below a limited frequency.

Effect of Hydrogen on Electrical Properties of Metal-Ferroelectric (SrBi 2 Ta 2 O 9 )–Insulator (HfTaO)–Silicon Capacitor

Metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with SrBi2Ta2O9 as a ferroelectric layer and HfTaO as an insulator layer were fabricated, and the electrical properties were investigated before and after the hydrogen treatment. The size of memory window for the MFIS capacitors seriously decreases and even vanishes. This could be attributed to the diffusion of hydrogen ions into the ferroelectric layer, which leads to the degradation of coercive field for ferroelectric layer. Moreover, it was found that the leakage current density of the MFIS increases as much as one order of magnitude after the hydrogen treatment, which could be attributed to the electrons produced by the ionized of oxygen vacancy and H atoms. The results could provide useful guidelines for the design and application of ferroelectric memory.

Effect of temperature on the electrical properties of a metal-ferroelectric (SrBi2Ta2O9)-insulator (HfTaO)-silicon capacitor

A metal-ferroelectric (SrBi2Ta2O9)-insulator (HfTaO)-semiconductor capacitor was fabricated, and the temperature dependence of its electrical properties was investigated. Within the temperature range of 300–220 K, the maximum memory window is up to 1.26 V, and it could be attributed to a higher coercive field of the ferroelectric film at a lower temperature, which is induced by the deeper and more box-shaped potential well based on the defect-domain interaction model. The memory window decreases with increasing temperature from 300 to 400 K, and the larger sweep voltage leads to a smaller memory window at a higher temperature, which could be attributed to temperature-dependent polarization of the ferroelectric film and charge injection from an Si substrate of the capacitor. With the temperature increasing from 220 to 400 K, the leakage current density increases with temperature by about one order, and the corresponding conduction mechanism is discussed. The results could provide useful guidelines for design and application of ferroelectric memory.

Effects of Stress and Depolarization on Electrical Behaviors of Ferroelectric Field-Effect Transistor

The effects of stress and depolarization on current-voltage characteristics of ferroelectric field-effect transistor (FeFET) were investigated by Landau-Khalatnikov theory in conjunction with Pao and Sahs model. Output characteristics current increases under compressive stress but decreases under tensile stress. Transfer characteristics curve is enlarged by compressive stress but compressed by tensile stress, and it shifts to the negative voltage axis under compressive stress gradient while to the opposite direction under tensile stress gradient. Because of depolarization, output characteristics current is obviously affected, and transfer characteristics curve is seriously distorted. These results show that stress and depolarization should be considered in device design to prevent failure of FeFET.

Movable Noncontact RF Current Measurement on a PCB Trace

This paper develops a movable noncontact probing method of a radio frequency current on a printed trace based on the electromagnetic induction. The current measurement method is validated by comparing a series of reconstructed periodic and pulse currents with those as known input signals on a microstrip line. According to our measurement, the transfer impedance from the current under measurement to the probe has a linear frequency range from 10 MHz to 2.2 GHz. The reconstruction measurement can be achieved even for a random periodic noise with a duration of 1 ns and a single pulse with a rise time of 2 ns. Owing to the flexible relocation of the probe, the measurement accuracy due to the errors of spatial displacement and the probe placement angle is investigated in detail. It is shown that the dynamic parameters of reconstructed current, such as fall time and rise time, are insensitive to the spatial errors, and instead the amplitudes of both periodic and pulse currents are highly sensitive, especially to a horizontal displacement.

Orientation Effect of Field-to-Line Coupling in a TEM Cell

In this paper, we study the orientation effect of magnetic and electric field-to-line coupling (FtLC) between a microstrip line and a TEM cell by using a square-shaped transmission line pulse as an interference source. With the detections at enough orientations, the electric and magnetic FtLCs can be extracted with high accuracy. It is found that in the TEM cell, the ratio of electric to maximum magnetic FtLC is dominated by the characteristic impedance of the studied microstrip line due to the determined value of width-to-distance. For a certain ratio of electric to maximum magnetic FtLC, there is a singular angle at which the microstrip line gives no response for any electromagnetic interference excitation. Meanwhile, at the supplementary angle, the response of the microstrip line shows no dependence on the termination load.

Hydrogen-dependent low frequency noise and its physical mechanism of HfO2 resistance change random access memory

The effect of hydrogen on low frequency noise characteristics of HfO2 resistance change random access memories (RRAMs) was investigated in this paper. The experimental results show that HfO2 RRAMs after hydrogen treatment take on the better uniformity of switch characteristics and the conduction enhancement behavior. Furthermore, it was found that the low frequency noise characteristics of the HfO2 RRAMs was significantly impacted by the hydrogen treatment, and at three kinds of typical resistance states, the low frequency noises of the HfO2 RRAMs after hydrogen treatment are larger than those of the fresh HfO2 RRAMs. The mechanism could be attributed to H induced oxygen vacancies, which serve as the additional traps for conduction due to the trap-assisted tunneling process. This will result in more random trap/detrap processes in the conducting filament, which gives rise to the larger low frequency noise in the HfO2 RRAMs. The results of this study may be useful in the design and application of HfO2 RRAMs.

Reliability assessment of Algangan Hemts for high voltage applications based on high temperature reverse bias test

We have submitted the AlGaN/GaN High electron mobility transistors (HEMTs) to the high temperature reverse bias(HTRB) stress to assess their reliability for high voltage operations. The effects of HTRB stress as a function of the VDG and stress time on the DC parameters and trapping effects were investigated. The study was based on combined DC and pulsed characterization, transient measurement. It provides the following information: 1) the exposure to HTRB may result in a permanent degradation in on-state current, the transconductance, and a slightly positive shift of threshold voltage; 2) the high temperature (TA=175°C) step reverse bias (VDG stepped from 20V to 80V) stress tests revealed that the gate leakage current IGS increased abruptly after 50V stress voltage VDG; 3) interestingly, the long term high temperature moderate reverse bias(VDG≤40V) stress resulted in some positive effects such as the decrease of gate leakage current, improved gate lag characteristics and the decrease of current collapse in pulsed measurement during the whole test; 4) the reverse-bias voltage is the key factor but for reliability assessment of AlGaN/GaN HEMTs for high voltage application based on HTRB stress; 5) The degradation mechanism of the gate leakage current under relatively high voltage HTRB test has been ascribed to the degradation metal and semiconductor interface; 6) the improved I-V characteristic under moderately voltage HTRB test has been attributed to the reduction of lateral tunneling current.