| 摘要: | 本論文一開始針對多種不同基極(base)尺寸的磷化銦鎵/砷化鎵 (InGaP/GaAs)異質接面雙載子電晶體（Heterojunction Bipolar Transistor；簡稱HBT），就直流(dc)、交流(ac)和高頻雜訊的特性，做一完整的測量分析。根據直流和交流測量結果的討論分析，發覺不同基極尺寸的電晶體間的高頻雜訊的差異，與交流電流聚集效應(ac current crowding effect)有關。 直流測量結果顯示基極長度的改變會嚴重影響電晶體直流特性表現，但基極寬度的改變對電晶體直流特性表現則不明顯。在交流方面，電流增益截止頻率(fT)不受基極長度和寬度改變的影響，因為減少的電容和增加的電阻對fT的影響，彼此相互抵銷。但是最大震盪頻率(fmax)則嚴重受到基極長度和寬度改變的影響，因為基極電阻和基極集極間電容的乘積(RB×CBC)大為增加。在雜訊方面，發現量到的最小雜訊指數(NFmin)的數值隨著基極長度和寬度的減少，其在高頻區域的增量也跟著遞減，這是因為交流電流聚集效應的關係。越高的基極阻抗，隨著操作頻率的增加遞減的越多，因此使得NFmin在高頻時的增量遞減。 接著我們建立一個新的高頻雜訊模型，這新模型把基極接觸電容和交流電流聚集效應對電晶體雜訊特性的影響考慮在內，接著推導出新的雜訊參數表示式。根據新的雜訊參數表示式計算出來的結果，和前面測量到的不同基極長度和寬度的電晶體雜訊特性相比較，兩者相當的吻合，可以清楚描述出交流電流聚集效應的影響。除此之外，藉著這個新雜訊模型，我們也觀察到只有基極電組很大的時候，基極接觸電容對高頻雜訊的影響才會較明顯。 In this dissertation, the dc, ac and noise characteristics of InGaP/GaAs HBTs with various base contact size are investigated. According to the discussions of dc and ac measurement results, the difference of noise performance between the HBTs with various base contact size can be attributed to the ac current crowding effect. Besides, we proposed new analytical expressions for four noise parameters of InGaP/GaAs HBT based on the complete small-signal equivalent circuit, which takes base contact capacitance and ac current crowding effect into account. Consequently, this new noise model shows excellent experimental agreement. It can describe the noise behavior of InGaP/GaAs HBT at high frequencies accurately including ac current crowding effect. In chapter 1, the overview of the HBTs has been introduced briefly. The origin of dc and ac emitter current crowding of the HBTs and the introduction of base contact impedance are also described. Besides, the overview of noise characteristics of the HBTs is described as well. In final, the noisy circuit analysis procedure is introduced for the noise parameters expression derivation of HBTs in Chapter 3. The dependence of dc, ac and noise characteristics of InGaP/GaAs HBTs on the base contact size is presented in Chapter 2. The measured results show that the dc performances are not significantly dependent on WB but on LB. In the ac performances, the fT is not apparently dependent on WB and LB but the fmax is significantly degraded with reduced WB and LB due to the increased RB×CBC product. In the noise performance, the measured NFmin values increase slowly with reduced WB and LB at high frequency ranges due to ac current crowding effect. In the studied devices with fT of ~35 GHz, the measured minimum noise figure (NFmin) increases slowly with reduced base contact width (WB) while the operating frequency is over than 10 GHz. The corresponding decrease in noise resistance (Rn) of HBTs while increasing operating frequency indicates a significant decrease in the base resistance (RB) from the ac current crowding effect. The NFmin of device with WB of 2.0 μm is lowest at medium frequencies but higher than that of devices with WB < 2.0 μm at high frequencies. Based on the experimental results, the ac current crowding effect decelerates the increase in the NFmin at high frequencies. In Chapter 3, a complete HBT high frequency noise model including the influences of the base contact capacitance and ac current crowding effect. Based on the proposed noise model, new expressions for the noise parameters of InGaP/GaAs HBTs are derived to describe the high frequency noise behavior in the presence of ac current crowding effect. The validity of the new noise model is presented by analyzing the four noise parameters of the HBTs with various base contact size. Good agreement is obtained between measured and calculated data for HBTs with various base contact size. The effect of ac current crowding on high frequency noise is well described. For device with high equivalent noise resistance (Rn), the proposed noise model with frequency dependent intrinsic base thermal noise describes the drop of Rn at high frequencies accurately. In addition, the base contact capacitance (CB) shows its significant influence on the noise parameters only when the base resistance is high. However, current crowding effect is distributive. Therefore, the method in the noise model that we use Cbi//Rbi to describe the frequency dependent ac current crowding effect may not appropriate. Therefore, it still needs better method to describe the distributive base resistance to meet physical meaning. |
