Investigation of capacitance voltage characteristics of strained Si/SiGe n-channel MODFET varactor

• MBE growth of strained silicon quantum well (QW).
• Fabrication of Modulation Doped (MOD) varactor structure.
• Discovery of non-monotonic dependence of capacitance versus voltage curve.
• Explanation of the effect by physical modeling and simulation.

This work is concerned with the investigation of Capacitance-Voltage (CV) behavior of n-channel Si/SiGe MODFET varactors. This investigation provides a valuable insight into the high frequency response of the device under test and its dependence on design parameters; especially regarding the modulation layer doping concentration. The heterostructure under consideration is much more complicated than conventional MOS varactor with respect to non-uniform doping, energy band offsets and the pn-junction in series. Subsequently, CV characterization has never been applied to such MODFET varactor structure. Experimental CV measurements have shown a non-monotonic behavior with a transition point minimum and higher saturation levels on both sides, in contradiction to the conventional high frequency MOS characteristics. This behavior was confirmed qualitatively using simulations. Moreover, we explain some fundamental capacitance properties of the structure, which provide already very interesting perceptions of the MODFET varactor operation, modeling and possible applications using the obtained stimulating results.

Figure optionsDownload as PowerPoint slide