Phenomena And Mechanism Of Surface Flashover Across Semiconducting Materials Under Pulsed Voltage

With the rapid development of pulsed power technology, solid switch devices are being gradually applied. In these devices, the photoconductive semiconductor switch (PCSS) with good performances arouses broad attention. However, the flashover phenomena across a semiconductor device always occur at a much lower electric field stress than its intrinsic breakdown strength, which have limited its development in the field of high voltage and power. Much attention has been ever paid on the flashover phenomena across semiconductor all over the world, but because the phenomena are so complicated and uncertain, up to now the latent mechanism is not very clear and some key problems are still in debate. So it is greatly instructive to study the phenomena and its mechanism for promoting the lifetime and reliability of high-field semiconductor devices. Furthermore, the research is also an important branch of the flashover across solid dielectric.Firstly, a high-voltage vacuum (atmosphere) discharge experimental platform is designed and implemented. The main part of the platform include: the vacuum chamber and vacuum pumps, a one-stage Marx generator of short pulse, two kinds of planar electrode configurations, the light intensity measurement system based on photomultiplier tube (PMT), and the image recording system, consisting of high speed image converter camera, CCD system with infrared filter lens and ICCD camera. It is proved that the platform can satisfy the demand of the experiments in this thesis.For investigating the mechanism of the flashover across semiconductor, the silicon wafers with different types are employed. The influences of dielectric ambient, electrode system and surface status on flashover phenomena are discussed. Based on the experimental results, some key procedures during flashover development are analyzed by numerical method. The main works are as following: 1. The flashover experiments in atmosphere, vacuum and deionized water are performed. The characteristics and process of flashover channel in air and vacuum are compared. It is found that, the starting position of the flashover channel depends on the type of the semiconductor, and the perforative light channel has been formed in the period of the voltage climbing up. Considering the observed gas desorption phenonmena during the flashover in vacuum, two different models of flashover in vacuum and air are presented, respectively. It is recognized that the flashover is originated from the injected current. The model about air emphasizes that injected current in surface layer could cause the disturbance of electric filed, which may arouse the ionization of gas ouside. The model in vacuum regards that the gas desorption process induced by the injected current is more important. 2. The effects of metalized layer contact and its annealing craft on flashover phenomena are compared and analyzed. Based on the results, two cognitions are given: the good and uniform contact may decrease the electric field stress near the electrode and increase the flashover voltage; the interface states at the contact area are able to restrain the injected current and elevate the flashover voltage. The experimental sample with a novel structure of insert contact is also tested, and its flashover performance is related with the inserting depth. The higher flashover voltage was acquired for the sample with larger depth. It is found that the flashover current behaves some relations with applied voltage polarity, and it is considered that minority carriers play main role on the preflashover current. Considering the relaxation condition of majority carriers and the infrared image of filament current, a theory that minority carriers play major role on inducing current filament is presented.3. Based on the experimental results about the effects of etching and passivation surface on the flashover, the relationship between surface states and flashover characteristics is analyzed. A theoretical model that removing surface states could elevate flashover voltage is presented. The model can not only interpret the effects of surface etching and passivation but also provide a support for the abonormal flashover phenomena of the sample with coarse surface in air and vacuum.4. In the study on the process of the flashover channel, the electric field at the edge of electrodes is calculated with the numerical method, and the electric field disturbance aroused by contact barrier is also calculated. Based on the filament current channel model, the related thermal process is simulated, which proves that the temperature is high enough to reach the melting point of silicon. The simulating results can explain the damage phenomena on silicon surface and also support the presented theoretical model of gas desorption aroused by thermal process.