射頻橫向擴散金氧半場效電晶體之小信號特性分析與模型

Abstract

摘 要 近年來由於與日俱增的無線通訊應用市場需求，用於高功率放大的高功率射頻電晶體也變得非常重要。而隨著半導體製程的發展，橫向擴散金氧半場效電晶體 (LDMOS)由於有著比雙載子電晶體更優異的射頻表現以及成本的優勢，已被廣泛的運用在這方面的應用。在本篇論文中，我們研究LDMOS的小訊號特性並提出了一個新的小訊號模型。在此模型中，我們以基板寄生網路來描述矽基板的損耗，為了改進模型在高頻的準確度，我們將一個汲極電容(Cd)與汲極電阻(Rd)並聯。量測與模擬的資料在20GHz之內都得到相當地吻合。 接著我們研究Rd與Cd在不同偏壓下的變化。在閘極電壓與汲極電壓增加時，由於漂移區的有效導通面積的下降造成Rd的上升;同樣地也使得Cd成反比的變化。我們也研究了閘極電容的行為，元件在線性操作時，閘極-汲極電容(CGD)在臨界電壓時呈現了一個峰值；而當元件操作在飽和區時，閘極電容在元件進入準飽和狀態時也有此種現象。最後，我們研究LDMOS在高頻時的溫度效應，我們發現截止頻率(fT)與最大震盪頻率(fmax)隨著溫度上升有下降的趨勢。而當溫度上升時fmax劣化得比fT嚴重。我們萃取了不同溫度下的小訊號參數來解釋這些行為。我們發現gm隨溫度的變化是最主要的因素，而fmax劣化更嚴重的原因是由於Rd隨溫度的上升。

Abstract For the increasing market demand of wireless application in recent years, high power RF transistors for power amplification have become very important. And with the progress of semiconductor process, Lateral Diffused Metal-oxide-semiconductor (LDMOS) transistors have been proven to be very popular for these applications owning to superior RF performance and highly cost-effective compared to bipolar transistors counterparts. In this thesis, we investigate the small-signal characteristics of LDMOS and also propose a new LDMOS small-signal model. In our model, a substrate parasitic network is included to describe the silicon substrate loss. To improve the model accuracy at high frequency, one additional drain capacitance (Cd) is added and is parallel with the extrinsic drain resistance (Rd). A good agreement between modeled and measured data has been achieved up to 20 GHz. Then we investigate the variation of Rd and Cd with different biases. Rd increases with increasing gate and drain voltages, but Cd shows an inverse trend, owing to the reduction of effective conduction area of the drift region. The behaviors of the gate capacitances are also studied. In linear region, the gate-to-drain capacitance (CGD) exhibits a peak at the threshold voltage. In saturation region, owing to the existence of the drift region, the gate capacitances show a peak at the onset of quasi-saturation. Finally, we study the temperature effects on the high-frequency characteristics of the LDMOS. We find that both cutoff frequency (fT) and maximum oscillation frequency (fmax) decrease with increasing temperature. And fmax suffer from more degradation compared to fT as the temperature goes higher. We extracted the small-signal model parameters with different temperatures to explain the temperature behaviors of fT and fmax. We find that the change of gm with temperature is the main factor to affect the temperature dependences of fT and fmax. In addition, the reduction of fmax with temperature is enhanced by the drain resistance.