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Study of Cutoff Frequency Calculation in the Subthreshold Regime of Operation of the SOI - MESFETs

K. Tarik, D. Vasileska and T.J. Thornton
Arizona state University, US

SOI MESFET, subthreshold

Micropower circuits based on sub-threshold MOSFETs are used in a variety of applications ranging from digital watches to medical implants. The principal advantage of a transistor operating in the sub-threshold regime is the minimum power consumption, but the main drawback is its speed. Micropower circuits are limited to operating frequencies below ~ 1MHz due to low cut-off frequency fT=µVT/2 Lg2, where µ is the carrier mobility, VT=kT/e the thermal voltage and Lg is the gate length. In the sub-threshold regime, it is impractical to increase fT by reducing the gate length because of difficulties with transistor matching. The only remaining option to increase fT is to increase the carrier mobility. In a prototypical MOSFET device in the on state, inversion electron mobility is typically 600-700 cm2/Vs but this falls to only 100-200 cm2/Vs in weak inversion, and one expects a cut-off frequency in the range between 40 to 80 MHz for a sub-threshold MOSFET with Lg=1 µm. The above discussion suggests that alternate device structures, like the Schottky Junction Transistor1 (SJT) (or the SOI MESFET architecture), are needed that will satisfy both the low-power and the r.f. requirements and will allow much better operation of, for example, pacemakers. Since the mobility is the key factor in determining the device cut-off frequency, it is the purpose of this study to calculate the cutoff frequency by investigating the electron mobility improvement of SOI MESFET when compared to SOI MOSFET device. To accomplish this goal, we have utilzed our in-house Ensemble Monte Carlo device simulator and performed extensive simulations of similar geometry SOI MOSFETs and Si MESFET channels (see Fig. 1). From the simulation mobility results shown in Fig. 2, one can deduce that in the sub-threshold regime, the MESFET device exhibits higher mobility (5 ~ 10 _ larger in weaker inversion regime) with respect to the bulk or the SOI MOSFETs. For the purpose of verification of the validity of our code, the simulated MOSFET mobility data were compared with experimental values reported in the literature2, 3. So, the mobility based cutoff frequency is 114~126GHz. Another way of extraction of fT is by using fT=gm/2 Cg, where gm is the transconductance and Cg is the gate capacitance of the device. The extracted value is 105GHz, as shown in Fig. 3. This value is quite high with respect to the cutoff frequency of a MOSFET. Due to this enhanced cutoff frequency, we can conclude that the SOI MESFET device is a suitable candidate for application in r.f. micropower circuit design. [1] T. J. Thornton “Physics and Applications of the Schottky Junction Transistor”, IEEE Trans. Electron Devices, 48, 2421 (2002). [2] D. Esseni, M. Mastrapasqua, G. K. Celler, C. Fiegna, L. Selmi, and E. Sangiorgi, “Low Field Electron and Hole Mobility of SOI Transistors Fabricated on Ultrathin Silicon Films for Deep Submicrometer Technology Application”, IEEE Trans. Electron Devices, 48, 2842-2850(2001). [3] J. T. C. Chen and R. S. Muller, “Carrier mobilities at weakly inverted silicon surfaces, ” J. App. Phys., 45, pp. 828-834 (1974).

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