Current collapse and device degradation in AlGaN/GaN heterostructure field effect transistors

TK Electrical engineering. Electronics Nuclear engineering

Balaz, Daniel ; Asenov, Asen

University:University of Glasgow

Department:School of Engineering

关键词: AlGaN, GaN, HFET, HEMT, converse piezoelectric effect, stress, strain, current collaspe, device degradation, Poole-Frenkel;   

Others  :  http://theses.gla.ac.uk/2676/1/2011BalazPhD.pdf

来源: University of Glasgow

【 摘 要 】

A spectrum of phenomena related to the reliability of AlGaN/GaN HEMTs are investigatedin this thesis using numerical simulations. The focus is on trap related phenomena that leadto decrease in the power output and failure of devices, i.e. the current collapse and thedevice degradation. The current collapse phenomenon has been largely suppressed usingSiN passivation, but there are gaps in the understanding of the process leading to thiseffect. Device degradation, on the other side, is a pending problem of current devices andan obstacle to wide penetration of the market.Calibration of I-V measurements of two devices is performed with high accuracy toprovide a trustworthy starting point for modelling the phenomena of interest. Traditionally,in simulations of nitride based HEMTs, only direct piezoelectric effect is taken intoaccount and the resulting interface charge is thence independent of the electric field. In thiswork, the impact of the electric field via the converse piezoelectric effect is taken intoaccount and its impact on the bound charge and the drain current is studied, as a refinementof the simulation methodology.It is widely believed that the current collapse is caused by a virtual gate, i.e. electronsleaked to the surface of the device. We have found a charge distribution that reproducedthe I-V measurement that shows current collapse, hence validating the concept of thevirtual gate. While it was previously shown that the virtual gate has a similar impact on theI-V curve as is observed during the current collapse, we believe that this is for the first timethat a wide range of gate and drain voltages was calibrated.High gate/drain voltage leading to permanent degradation was also investigated. Thehypothesis that stress induced defects and dislocations might be responsible for thedegradation was tested but not fully confirmed.Finally, the leakage of electrons thought to be responsible for formation of the virtual gateand the current collapse due to the Poole-Frenkel emission, is simulated in order to explainthe surface charge distribution responsible for the current collapse and deduced inChapter 5.