Investigations On Reliability Of RF-LDMOS Under UIS Stress

Radio frequency double diffused metal-oxide semiconductor device(RF-LDMOS)is a very competitive power device by virtue of its good linearity,high gain,high breakdown voltage and good performance of broadband matching,so it has been widely used in the power amplifier in Mobile Communication Base Station,etc.In the application of RF-LDMOS,the repetitive Unclamped Inductive Switching(UIS)stress has become a case of extreme electrical stress that it is often subjected to.The long-term UIS stress will degenerate the electrical parameters of RF-LDMOS,and make it face serious problems of reliability.However,the current research objects are mainly focused on conventional power MOS devices,but there are few reports about the degradation of RF-LDMOS under UIS stress.Therefore,it is of great significance to study the degradation mechanism of RF-LDMOS under UIS stress.With the aid of direct current current–voltage(DCIV)technique,the degradation mechanism of RFLDMOS under UIS stress is investigated in this thesis,based on the UIS degradation test platform and computer aided simulation platform.The research shows that under the UIS stress,the hot-holes are injected into the oxide layer below the end of the gate field plate and a large number of the interface states are generated.The generated interface states affect the transport of carriers in the drift region by scattering and decrease the carrier's surface mobility and the conductivity,which lead to the increase of on-state resistance and the decrease of the transconductance.The injected hot-holes induce an electron mirror layer below it and increase the number of the effective electrons in the drift region,which reduces the width of the barrier area and makes the gate drain capacitance increase.The combination of the two causes decrease the power gain of the device.In addition,the influence of different structure and process parameters on the reliability of RF-LDMOS under UIS stress is further studied.With the aid of computer aided simulation,three high UIS stress reliability RF-LDMOS devices with double LDD(Lightly Doped Drain)structure,Pdown structure and channel P-well structure are proposed respectively.Among them,the RF-LDMOS with double LDD structure has the best reliability,which can reduce the peak current under UIS stress by 32% and the breakdown time of avalanche by 16% compared to the original device,and the hot carrier injection is effectively suppressed,thus the reliability of the RF-LDMOS under the UIS stress is improved.