| Summary: | 碩士 === 國立交通大學 === 電子工程系所 === 97 === 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.
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