射頻應用之寬汲極橫向擴散金氧半場效電晶體特性與模型參數分析

Abstract

近年來，隨著無線通訊的市場蓬勃發展，用於高功率的射頻電晶體的重要性也與日俱增。橫向擴散金氧半場效電晶體(LDMOS)由於其較佳的射頻表現和製程上的容易度，應用的比率越來越高。在本篇論文中，除了比較傳統的多指狀布局設計的電晶體和八角環狀電晶體結構之外，同時更改了多指狀布局元件的汲極寬度和指根的寬度，探討各種設計的直流特性，射頻表現的差異。本次實驗中，我們採用了安捷倫8510網路分析儀來量測100MHz-20GHz頻率間的S參數，和ATN LP1機台來實行在900MHz與2.4GHz頻率下的功率特性量測。我們發現了加寬汲極的寬度，減半指根的寬度或是改用八角環狀設計，會得到較大的等效漂移區寬度，降低了準飽和效應的影響，而使得元件的特性變好。 接著我們萃取LDMOS小訊號模型參數來描述元件的特性，探討在固定汲極電壓下，不同佈局元件間的模型參數變化，並研究各個模型參數對截止頻率(ft)與最大震盪頻率(fmax)的影響，我們發現閘極-源極電容(Cgs)對截止頻率的影響較大，相對的閘極-汲極電容(Cgd)則是影響fmax較多，當電容上升時ft與fmax將會下降。由於寬汲極多指狀元件擁有較低的汲極電阻和較小的閘極-源極電容，在適當的汲極寬度增加下，若能避免過高的閘極-汲極電容增加量，可以讓我們得到較高的ft和fmax。此外汲極電阻(Rd)的下降為寬汲極多指狀元件特性比傳統多指狀元件還要來的好的主因。

In recent years, with the rapid development of wireless communication markets, the importance of high power RF transistors has been increased. Because the Laterally Diffused Metal-Oxide-Semiconductor (LDMOS) transistors have better RF performances and are easy to manufacture, they have become a mainstream technology for RF power amplifying applications. In this thesis, we compared the device performances between LDMOS with conventional multi-finger and octagonal ring structures. Furthermore we changed the drift width and finger width of multi-finger devices to investigate the DC and RF characteristics for different layout geometries. The S-parameter measurements were performed using Agilent 8510 network analyzer in the frequency range of 100 MHz-20 GHz, and the power characteristics were measured using ATN LP1 load-pull measurement system at 900 MHz and 2.4 GHz. We find that wider drain width, narrower finger width and octagonal ring will have larger effective drift region width, which suppresses the effect of quasi-saturation, so the device performances become much better. Then we extracted the model parameters of LDMOS small-signal equivalent circuit to explain the device high frequency behaviors. By using this model, we discuss the effects of model parameters on cutoff frequency (ft) and maximum oscillation frequency (fmax) under constant drain-source voltage. We find that the gate-source capacitances (Cgs) have large effects on ft, while gate-drain capacitances (Cgd) affect fmax more. With capacitances increasing, ft and fmax will decrease. Owing to smaller Cgs and Rd in wide-drain LDMOS, when the increase of Cgd is diminished with appropriate drain width design, we can get higher ft and fmax. Furthermore, the drain resistance decrease is the main reason for wide-drain finger LDMOS having better performances than conventional multi-finger devices.