橫向擴散的射頻金氧半場效電晶體之特性分析與模型建立

Abstract

近年來，射頻橫向擴散金氧半場效電晶體（RF LDMOS）在手機基地台中，作為功率放大氣得重要元件。為了能符合新一代通訊標準的需求，必須不斷地改善LDMOS的特性。在本論文中，我們將探討四種結構：fishbone、square、octagon 和 circle的直流、高頻和射頻功率特性。為了得到較低的汲極電阻，我們採用了不同於傳統結構的三種「環狀」結構。除此之外，為了抑制square結構所附帶角落的影響，我們將四方形的環修正為八角形和圓形的環。另外，還針對square結構，去探討不同通道寬度的影響，得知通道較小的元件具有較佳的直流表現，但是，在高頻部分則呈現較差的結果。為了進一步地了解元件參數對高頻特性的影響，我們建立了小訊號等校電路，將其參數萃取出並分析比較。實驗結果顯示，我們所設計的環狀結構，透過汲極面積的增加，能夠有效地降低汲極端的寄生電阻，因而改善元件的截止頻率（fT）和最大震盪頻率（fmax）。此外輸出功率、功率增益以及附加功率效率均有較佳的表現，而線性度和崩潰電壓則和fishbone結構不相上下。使用circle結構可以得到較大的電流和轉導，因為能更有效地減少汲極端寄生電阻。由於circle結構只更動光罩之設計，至程流程並無改變，因此實為一大優點。 另一方面，我們也完整的分析元件的電容特性。由於LDMOS的通道為非均勻摻雜且具有漂移區，因此電容會有峰值產生。我們發現square結構會出現第二個峰值，而circle結構則和fishbone結構一樣只有單一峰值。這是因為square結構再轉角處的電流密度較小，使得電子速度需要較大的閘極偏壓才能進入類飽和狀態。 我們使用修改過的MM20模型去建立fishbone和circle結構的元件模型，藉由使用T-CAD模擬軟體，得到橫向電場和空乏區的分佈。另外得知具有field plate結構的元件操作在發生quasi-saturation下，在漂移區中，其電場較為均勻並能使電阻降低。利用模擬的結果去修改MM20模型成為一個簡單且沒有太多複雜運算的模型，能容易地去描繪出元件的電性。利用修改過的模型去萃取出fishbone 和circle結構的元件參數並比較分析，結果顯示，這些參數所表達的訊息與第二章小訊號參數的結果相輔相成，具有相似的趨勢。因此，使用這模型模擬不同步局設計的RF LDMOS，所得到的I-V和C-V曲線也與量測的電性曲線相符，確實得到精確的一致性。

RF LDMOS nowadays plays an important role in the RF power amplifier applications in base stations for personal communication systems. In order to meet the demands imposed by new communication standards, the performance of LDMOS is subject to continuous improvements. In this thesis, four types of layout structures, fishbone, square, octagon and circle were studied for DC, high-frequency, and RF power characteristics. To achieve lower drain resistance, we adopted “ring” structures in the layout design. In addition, to reduce corner effect, we modified the square ring structures to octagon and circle rings. For square structures, variation of channel widths was investigated. The device with smaller Wch shows better DC performance but shows worse RF performance. In order to determine the effect of device parameters on high-frequency characteristics more clearly, small-signal equivalent circuit was built to be analyzed. From the simulation results, the smaller drain parasitic resistance in the ring structures could be the key factor for improving fT and fmax contrasting to fishbone structure. As for microwave power characteristics, output power, power gain and power added efficiency (PAE) were improved with a similar linearity with the same breakdown voltage. The extra areas in the drift region would have lowered the drain parasitic resistance and improve the on-resistance. By using the circle structure, higher drain current and transconductance were shown by the reason of larger equivalent W/L and lower drain parasitic resistance comparing to square. Its reveals that the circle structure had a better performance, without altering the process flow. In another part of this thesis, we discussed and analyzed the capacitance characteristics completely. For having a non-uniform doping channel and the existence of the drift region, CGS+ CGB and CGD exhibit a peak in LDMOS. In the square structure, the second peaks in a capacitance-voltage curve have been observed at high drain voltages for the first time. Besides, the circle structure has the same capacitance characteristics as the fishbone structure that indicates only one peak in the capacitance curve. While the corner region of the drift in the square shows lower current density than the edge region, it needs higher gate voltage to enter quasi-saturation. By increasing the gate voltage, the current in the corner region is high enough to make the velocity of electrons in the drift saturated. The device models have been built for fishbone and circle structures by using the modified MM20. We obtain the lateral electric field distribution and the depletion distribution by using T-CAD simulated software. The device with field plate has uniform electric field and lower resistance in the drift region as device enters quasi-saturation. From the result of simulation, we modify the MM20 to a simple model. It is easier to describe the electrical characteristics of device. The extracted model parameters were also investigated for fishbone and circle structures. These parameters present the similar information as chapter 2. Therefore, this model shows an accurate description on I-V and C-V curves, and provides a good agreement between simulated and measured data for the RF LDMOS with different layout designs.