Physics and technologies of silicon LDMOSFET for radio frequency applications
This thesis is devoted to the investigation of devices and technologies of Lateral Double-Diffused- Metal-Oxide-Semiconductor-Field-Effect-Transistor for Radio Frequency (RP) applications. Theoretical analysis and extensive 2-D process and device simulation results are presented. Theoretical analysis and simulations are carried out on RESURF LDMOS in both bulk and SOI substrate in terms of breakdown characteristics, transconductance, on resistance and CV characteristics. Quasi-saturation is a common phenomenon in DMOS devices. In this work, the dependence of quasi-saturation current on device physical and geometrical parameters is investigated in SOI RP LDMOS. Physical insight is gained into quasi-saturation on SOI RP LDMOS with different top silicon thickness and the same drift dose. It reveals that the difference in thick and thin film SOI lies in the different potential drop in the drift region. The influence of RESURF effect on quasi-saturation is also presented. It is shown that quasi-saturation current level can be affected by RESURF due to its influence on the drift dose. The mechanism of self-heating is presented and the influence of top silicon thickness, buried oxide thickness, voltage bias is studied through simulations. The change of peak temperature and its location with bias is due to the shift of electric field with voltage bias. A back-etch structure and fabrication process have been proposed to achieve a superior thermal performance. The negative differential conductance is not present in the non-isothermal IV curves. The temperature rise in the back-etch structure is less than 114 of that in the bulk structure. An RP LDMOS with a step drift doping profile on SIMOX substrate is evaluated. The fabrication process for the drift formation is proposed. The presented results demonstrate that step drift device has higher breakdown voltage than the conventional uniformly doped (UD) device, which provides the possibility to integrate LDMOS with low voltage CMOS for 28V base station application. This structure also has the advantage of suppressed kink effect due to the reduced electric field within the drift region. The step drift structure also features lower capacitance, improved drain current saturation behaviour and reduced self-heating at class AB bias point. For the first time, a novel sandwich structure for lateral RF MOSFET has been analysed based on silicon-on-nothing (SON) technology. The influence of device parameters on BV, CV and thermal performance has been investigated. Partial SON structure is found preferable in terms of heat conduct ability. Comparison on the electrical and thermal performance is made between SON LDMOSFET and conventional SOI alternative with BV of 40V. It is found that SON structure shows improvement in output capacitance and substrate loss. However, the temperature rise in SON device is higher compared to SOI alternative. The performance of the proposed sandwich SON structure has also been investigated in 28V base station applications, which requires breakdown voltage of 80V.
- PhD