Feng Shiwei

Channel Temperature Measurement of AlGaN/GaN HEMTs by Forward Schottky Characteristics

Channel temperature measurements of multi-finger AlGaN/GaN HEMTs by forward Schottky characteristics are presented. The temperature dependence of the forward gate-source Schottky junction voltage is investigated and it is used as the temperature sensitive parameter (TSP) by pulsed switching technique. The channel-to-mounting thermal resistance of the tested AlGaN/GaN HEMT sample is 19.6°C/W. Compared with both the measured results by micro-Raman method and simulated results of a three-dimensional heat conduction model, the physical meaning of the channel temperature for AlGaN/GaN HEMT tested by pulsed switching electrical TSP method is investigated quantitatively for the first time.

Thermal analysis in high power GaAs-based laser diodes

The thermal characteristics of 808 nm AlGaAs/GaAs laser diodes (LDs) are analyzed via electrical transient measurements and infrared thermography. The temperature rise and thermal resistance are measured at various input currents and powers. From the electrical transient measurements, it is found that there is a significant reduction in thermal resistance with increasing power because of the device power conversion efficiency. The component thermal resistance that was obtained from the structure function showed that the total thermal resistance is mainly composed of the thermal resistance of the sub-mount rather than that of the LD chip, and the thermal resistance of the sub-mount decreases with increasing current. The temperature rise values are also measured by infrared thermography and are calibrated based on a reference image, with results that are lower than those determined by electrical transient measurements. The difference in the results is caused by the limited spatial resolution of the measurements and by the signal being captured from the facet rather than from the junction of the laser diode.

Determination of channel temperature for AlGaN/GaN HEMTs by high spectral resolution micro-Raman spectroscopy

Channel temperature determinations of AlGaN/GaN high electron mobility transistors (HEMTs) by high spectral resolution micro-Raman spectroscopy are proposed. The temperature dependence of the E2 phonon frequency of GaN material is calibrated by using a JYT-64000 micro-Raman system. By using the Lorentz fitting method, the measurement uncertainty for the Raman phonon frequency of ±0.035 cm−1 is achieved, corresponding to a temperature accuracy of ±3.2 °C for GaN material, which is the highest temperature resolution in the published works. The thermal resistance of the tested AlGaN/GaN HEMT sample is 22.8 °C/W, which is in reasonably good agreement with a three dimensional heat conduction simulation. The difference among the channel temperatures obtained by micro-Raman spectroscopy, the pulsed electrical method and the infrared image method are also investigated quantificationally.

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au(15 nm/220 nm/40 nm/50nm) multiplayer contacts to n-type GaN (Nd = 3.7 × 1017 cm−3, Nd = 3.0 × 1018 cmT−3). The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the Al layer even the Ti layer.

Evaluation of the drain—source voltage effect on AlGaAs/InGaAs PHEMTs thermal resistance by the structure function method

The effect of drain—source voltage on AlGaAs/InGaAs PHEMTs thermal resistance is studied by experimental measuring and simulation. The result shows that AlGaAs/InGaAs PHEMTs thermal resistance presents a downward trend under the same power dissipation when the drain—source voltage (VDS) is decreased. Moreover, the relatively low VDS and large drain—source current (IDS) result in a lower thermal resistance. The chip-level and package-level thermal resistance have been extracted by the structure function method. The simulation result indicated that the high electric field occurs at the gate contact where the temperature rise occurs. A relatively low VDS leads to a relatively low electric field, which leads to the decline of the thermal resistance.

Thermal time-constant spectrum extraction method in AlGaN/GaN HEMTs

The transient temperature rise in the active region in AlGaN/GaN high electron mobility transistors (HEMTs) is measured using an electrical method. The original data are smoothed and denoised by a nonparametric fitting algorithm, called locally weighted scatterplot smoothing (LOWESS). The thermal time-constant spectrum is extracted to analyze the physical structure of the heat-conduction path in AlGaN/GaN HEMTs. The thermal time-constant spectra extracted using the LOWESS algorithm are richer and the RC network obtained is greater compared with those with the traditional denoising method (multi-exponential fitting). Thus, the analysis of the heat-flow path is more precise. The results show that the LOWESS nonparametric fitting algorithm can remove noise from measured data better than other methods and can retain the subtle variation tendency of the original discrete data. The thermal time-constant spectra extracted using this method can describe the subtle temperature variations in the AlGaN/GaN HEMT active region. This will help researchers to precisely analyze the layer composition of the heat-flow path.

Identifying the failure mechanism in accelerated life tests by two-parameter lognormal distributions

The failure mechanism stimulated by accelerated stress in the degradation may be different from that under normal conditions, which would lead to invalid accelerated life tests. To solve the problem, we study the relation between the Arrhenius equation and the lognormal distribution in the degradation process. Two relationships of the lognormal distribution parameters must be satisfied in the conclusion of the unaltered failure mechanism, the first is that the logarithmic standard deviations must be equivalent at different temperature levels, and the second is that the ratio of the differences between logarithmic means must be equal to the ratio of the differences between reciprocals of temperature. The logarithm of distribution lines must simultaneously have the same slope and regular interval lines. We studied the degradation of thick-film resistors in MCM by accelerated stress at four temperature levels (390, 400, 410 and 420 K), and the result agreed well with our method.

Self-heating and traps effects on the drain transient response of AlGaN/GaN HEMTs

The effects of self-heating and traps on the drain current transient responses of AlGaN/GaN HEMTs are studied by 2D numerical simulation. The variation of the drain current simulated by the drain turn-on pulses has been analyzed. Our results show that temperature is the main factor for the drain current lag. The time that the drain current takes to reach a steady state depends on the thermal time constant, which is 80 μs in this case. The dynamics of the trapping of electron and channel electron density under drain turn-on pulse voltage are discussed in detail, which indicates that the accepter traps in the buffer are the major reason for the current collapse when the electric field significantly changes. The channel electron density has been shown to increase as the channel temperature rises.

The degradation and recovery properties of AlGaN/GaN high-electron mobility transistors under direct current reverse step voltage stress

Direct current (DC) reverse step voltage stress is applied on the gate of an AlGaN/GaN high-electron mobility transistor (HEMT). Experiments show that parameters degenerate under stress. Large-signal parasitic source/drain resistance (RS/RD) and gate-source forward I—V characteristics are recoverable after breakdown of the device under test (DUT). Electrons trapped by both the AlGaN barrier trap and the surface state under stress lead to this phenomenon, and surface state recovery is the major reason for the recovery of device parameters.

The analysis of thermal characteristics of the laser diode by transient thermal response method

Using the terminal voltage method, we measured and analyzed the thermal characteristics of the laser diodes (LD). From heating response curves, it is possible to determine the thermal resistance, R/sub th/ of different layers of the packaged LDs. The dependence of the R/sub th/ on the working current was measured. It shows much difference between below and above threshold current. This gives much information about the coupling between thermal and optical properties.