Study On The Influence Of Polarization Field On The Performance Of PIN Diodes

Nowadays,power electronic devices have been rapidly developed and widely used.However,there is still tremendous space for futher improving the device performance,such as increasing the breakdown voltage and reducing the forward voltage.Targeted on these two important cretirions,this work tries to investigate the impact of the polarization inuduced electric field on the breakdown voltage for GaN based PIN diodes.In addition,this thesis has also reviewed the background,the current status,the applications for GaN based PIN power electronic devices before presenting our results regarding the optimization process for GaN based PIN diodes.The purpose for this works is mainly to increase the breakdown voltage by proposing GaN-based PIN diode with a thin AlGaN layer in the drift region,and then the AlGaN/GaN interface can produce the polarization effect to modulate the electric field distribution.We use the polarization induced electric field to reduce the electric field intensity in the drift layer,thus increasing the breakdown voltage.We have also studied the impact of such AlGaN insertion layer on the forward voltage when the PIN diodes are in the on-state.The reseach stragegy for this thesis arises from the fact that the polarization effect for GaN-based material system can affect the breakdown voltage for GaN based PIN diodes.The entire work for this thesis is based on theoretical designs and calculations.This thesis includes the following innovations:(1)using the polarization induced electric field at the GaN/AlGaN interface to control the electric field magnitude in the drift region;(2)we design two types of GaN-based PIN diodes,i.e.,a p-side down PIN diode along the[0001]direction and an n-side down PIN diode along the[000-1]direction to increase the breakdown voltage,such that the polarization induced electric filed for the mentioned devices can compensate the external-bias-generated electric field;(3)from the aspect of actual epitaxial growh,it is difficult to growth thick AlGaN insertion layer with a high Al composition,and therefore we further propose the stair-cased-AlGaN-PIN structure that uses AlGaN insertion layer with staire-cased Al compoistion,and this design can still maintain a large breakdown voltage.Then,this thesis has conducted theoretical analysis and data processing on different GaN-based PIN diodes,and an in-depth physical invesitigtion is also made to thoroughly investigates the influence of the polarization effect on the breakdown voltage.Moreover,the on-state characteristics for the proposed GaN-based PIN diodes are also investigated.We firstly study GaN-based PIN diodes that are along the[0001]orientation,such that p-side-down PIN diodes and n-side-down PIN diodes.We investigate the impact of the drift layer thickness,the position for the AlGaN insertin layer,the Al composition for the AlGaN insertion layer,the doping concentration for the drift layer on the breakdown voltage.We find that the p-side-down GaN based diodes can have a larger breakdown voltage.We also study the on-state characteristics for the mentioned devices and we find that the AlGaN insertion layer can slightly increase the forward voltage because of the additional energy barrier.Secondly,considering the difficuly in annealing the p-side-down GaN PIN diodes,we further propose n-side-down GaN PIN diodes that are along the[000-1]orientation.However,from the point of epitxial growth view,it is difficult to grow Al-rich AlGaN insertion layer,and thus we suggest using stair-cased AlGaN insertion layer,i.e.,the Al composition is varied in a stair-cased way.We find that such design is also able to increase the breakdown voltage.Lastly,we summarize:(1)the drift layer cannot be thinner than 4?m to avoid reach-throgh breakdown;(2)we have to manipulate the polarization induced electric field so that it is opposite to the external-bias-generated electric field,and by doing so,the breakdown voltage gets increased;(3)the AlGaN insertion layer shall be placed at the n~--GaN/p~+-GaN junction where the strongest electric field occurs;(4)it is possible to further increase the breakdown voltage by increasing the Al composition in the AlGaN insertion layer;(5)the stair-cased-AlGaN PIN diodes are more realistic in actual epitaxial growth and fabrication processes.This thesis also reports the tradeoff between the increased breakdown voltage and the larger forward voltage,and hence the futher research shall be focused on the device optimization so that the breakdown voltage can be enhanced while the forward bias can be simutanesouly decreased.We believe the findings in this work can provde more understanding for power electronic deivces.