Lü Changzhi

Accelerating the life of transistors

Choosing small and medium power switching transistors of the NPN type in a 3DK set as the study object, the test of accelerating life is conducted in constant temperature and humidity, and then the data are statistically analyzed with software developed by ourselves. According to degradations of such sensitive parameters as the reverse leakage current of transistors, the lifetime order of transistors is about more than 104 at 100 °C and 100% relative humidity (RH) conditions. By corrosion fracture of transistor outer leads and other failure modes, with the failure truncated testing, the average lifetime rank of transistors in different distributions is extrapolated about 103. Failure mechanism analyses of degradation of electrical parameters, outer lead fracture and other reasons that affect transistor lifetime are conducted. The findings show that the impact of external stress of outer leads on transistor reliability is more serious than that of parameter degradation.

High-temperature current conduction through three kinds of Schottky diodes

Fundamentals of the Schottky contacts and the high-temperature current conduction through three kinds of Schottky diodes are studied. N-Si Schottky diodes, GaN Schottky diodes and AlGaN/GaN Schottky diodes are investigated by I–V–T measurements ranging from 300 to 523 K. For these Schottky diodes, a rise in temperature is accompanied with an increase in barrier height and a reduction in ideality factor. Mechanisms are suggested, including thermionic emission, field emission, trap-assisted tunnelling and so on. The most remarkable finding in the present paper is that these three kinds of Schottky diodes are revealed to have different behaviours of high-temperature reverse currents. For the n-Si Schottky diode, a rise in temperature is accompanied by an increase in reverse current. The reverse current of the GaN Schottky diode decreases first and then increases with rising temperature. The AlGaN/GaN Schottky diode has a trend opposite to that of the GaN Schottky diode, and the dominant mechanisms are the effects of the piezoelectric polarization field and variation of two-dimensional electron gas charge density.

High-temperature characteristics of AlxGa1−xN/GaN Schottky diodes

High-temperature characteristics of the metal/AlxGa1−xN/GaN M/S/S (M/S/S) diodes have been studied with current–voltage (I–V) and capacitance–voltage (C–V) measurements at high temperatures. Due to the presence of the piezoelectric polarization field and a quantum well at the AlxGa1−xN/GaN interface, the AlxGa1−xN/GaN diodes show properties distinctly different from those of the AlxGa1−xN diodes. For the AlxGa1−xN/GaN diodes, an increase in temperature accompanies an increase in barrier height and a decrease in ideality factor, while the AlxGa1−xN diodes are opposite. Furthermore, at room temperature, both reverse leakage current and reverse breakdown voltage are superior for the AlxGa1−xN/GaN diodes to those for the AlxGa1−xN diodes.

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.

Storage life of power switching transistors based on performance degradation data

NPN-type small and medium power switching transistors in 3DK series are used to conduct analyses and studies of accelerating degradation. Through three group studies of accelerating degradation in different temperature?humidity constant stresses, the failure sensitive parameters of transistors are identified and the lifetime of samples is extrapolated from the performance degradation data. Average lifetimes in three common distributions are given, when, combined with the Hallberg?Peck temperature?humidity model, the storage lifetime of transistor samples in the natural storage condition is extrapolated between 105?107 h. According to its definition, the accelerating factor is 1462 in 100?C/100% relative humidity (RH) stress condition, and 25?C/25% RH stress condition. Finally, the degradation causes of performance parameters of the test samples are analyzed. The findings can provide certain references for the storage reliability of domestic transistors.

The expression correction of transistor current gain and its application in reliability assessment

Considering the impacts of ideal factor n, VBE and band gap changes with the temperature on current gain, the current gain expression has been corrected to make the results closer to the actual test. Besides, the accelerating lifetime study method in the constant temperature—humidity stress is used to estimate the reliability of the same batch transistors. Applying the revised findings from the expression, the current gains before and after the test are compared and analyzed, and, according to the degradation data of the current gain, the transistor lifetimes in the test stress are respectively extrapolated in the different failure criteria.

The storage lifetime model based on multi-performance degradation parameters

According to the multi-performance degradation of the bipolar transistor in the accelerating storage process, an extrapolation model of the storage lifetime is proposed. In this model, using the Wiener process simulates the mono-degradation process of each feature degradation; using the copula function describes the correlation among these feature degradations. The Wiener process and parameters in the copula function are considered to associate with the temperature, and their relationships can be represented by the converted equations. Through the maximum likelihood estimation, the parameters in the Wiener process can be found; introducing Kendalls tau, those in the copula function can be estimated. By conducting the regression analyses of the estimated values of the parameters in each stress, their corresponding converted equations can be shown. Based on the storage test data of bipolar transistors, with the estimation method, the storage lifetime is found. The findings show that the model is reasonable for the prediction of storage lifetime.

Study on the reliability of Ni/Au/AlGaN/GaN HEMTs at high temperature

Fundamentals of current transports of the Ni/Au/AlGaN/GaN HEMTs have been studied under the temperature between 27°C and 250°C. Its found that the maximum drain-source current (IDS) decreases with rising temperature, and the temperature dependent of IDS has a negative coefficient of −0.314mA/mm.°C. The decrease in mobility is considered to be the main cause of this deterioration. Its also found that a rise in temperature accompanies an increase in barrier height and a decrease in ideality factor. The remarkable finding is that reverse leakage current increases first and then decreases with rising temperature.