An Accurate Gate Resistance Spice Model for RF IC Applications
Dr. Steve S. Chung
|Keywords:||射頻電路模組;閘極電阻;鬆弛時間;非類穩態效應;溫度效應;RF Spice Model;Gate resistance;Relaxation time;Non-quasi-static effect;Temperature effect|
除此之外，我們還計算在不同偏壓下，矽晶氧半場效電晶體的通道鬆弛時間（Relaxation Time），並且利用Spice模擬它對元件在高頻的影響。此鬆弛時間所造成的非類穩態效應（Non-Quasi-Static effect）可用一個簡單的RC電路來模擬。另外，我們首次探討高頻參數的溫度效應。閘極電阻與鬆弛時間將隨溫度上升而變大，並且降低電流增益。相反的，閘極電容不隨溫度而改變。換句話說，在高溫下，閘極電阻與鬆弛時間將會嚴重影響到元件的高頻表現。
As the gate-length of a MOSFET reduces, its high-frequency characteristics can be greatly improved. However, its model becomes more complicate. Current Spice models are based on dc measurement data and simple capacitance models which can only approximate the high-frequency device characteristics up to a fraction of the device unity current gain frequency (ft). Thus, it is important to investigate the high-frequency characteristics and then incorporate the small-signal equivalent circuit parameters in Spice. On the other hand, an advanced RF model by considering the distributed effects in both gate and the channel is needed to accurately describe the RF behavior of device over a wide bias range. In this thesis, an analytical expression of the y-parameter is established and calibrated against measurements from a 0.18mm NMOSFET. From the extracted data, we found that the gate resistance depends largely on the bias and temperature. It will greatly impact the device performance at high frequency (e.g., Y11, Y21). For the first time, a simple physical-based gate resistance model is developed in this thesis and can be implemented in Spice. The gate resistance is modeled by a parallel interconnection of the intrinsic gate resistance and a resistance coupled from the channel. The Spice simulation result of this model is more accurate than that of a constant Rg model. A constant Rg model will overestimate the value of Y11, however, the proposed nonlinear gate resistance model with both bias and frequency dependent features can achieve very good accuracy. Furthermore, we calculate the relaxation time of the MOSFET at various bias, and show the Spice simulation result. The non-quasi-static effect due to the relaxation time can be simulated easily with a simple RC series network. Besides, the temperature effect of RF parameters were studied first. The gate resistance and the relaxation time increase with temperature and then reduce the current gain. In contrast, the gate capacitance remains a constant at different temperatures. In other words, the gate resistance and the relaxation time affect the device performance seriously at high frequency and high temperature. In summary, the proposed gate resistance model is important for RF circuit simulation and can predict the RF behavior of device accurately.
|Appears in Collections:||Thesis|