Measurement of High-Frequency Characteristics of CNTFETs and Equivalent Circuit Model Analysis

Abstract

Carbon nanotube field effect transistors (CNTFETs) are high-mobility devices that operate at very high-speeds. Theoretical analyses suggest that the cut-off frequency ( fT) of an ideal CNTFET is between 800 GHz and 1.3 THz when its gate length is 0.1 μm (1; 2). Since this frequency is much higher than that of state-of-the-art Si, GaAs, and InP transistors, CNTFETs are promising candidates for future nanoelectronic devices. Singh et al.(3) measured frequency responses of top-gated CNTFETs up to 100 MHz. Li et al.(4) observed 2.6-GHz operation of CNTFETs with an LC impedance-matching circuit. However, as Li et al. pointed out (4), measuring high-frequency performance of high-impedance devices, such as CNTFETs, is quite difficult. This is because their output impedances are much higher (∼105 Ω) than the impedance of the measurement system (50 Ω) using a network analyzer. To perform accurate high-frequency measurements, especially those to determine fT values of such devices, we must measure S-parameters with a network analyzer even though large impedance mismatches hinder us from obtaining accurate measurement data. Kim et al.(5) measured S-parameters of multifinger CNTFETs by using a network analyzer and obtained an fT value of 2.5 GHz. They also concluded a maximum oscillation frequency ( fmax) of more than 5 GHz was obtained using the maximum stable gain (Gmsg). Le Louarn et al.(6) obtained intrinsic fT value of 30 GHz by measuring a CNTFET the channel of which was fabricated using dielectrophoresis to increase the CNT density. They also obtained Gmsg value of more than 10 dB at 20 GHz. This chapter will describe a method for accurately measuring and modeling the highfrequency characteristics of CNTFETs, with reference to our experiment and analysis (7). In the experiment, we first decreased the device impedance to be able to measure the S-parameter using network analyzer. This was achieved by developing a high-density multiple-channel CNTFET structure the output impedance of which is much lower than that of the conventional single-channel CNTFETs. Then we used a de-embedding procedure to remove existing errors in measured S-parameters of small-signal devices in order to obtain the current gain and unilateral power gain (U) that can determine accurate fT and fmax values. For accurate RF modeling of CNTFETs, we developed an equivalent circuit RF model that includes parasitic resistances and capacitances of the CNTFET. Then the expression of the fT ( fmax) was derived as a function of them. Not ignoring the higher order parasitic resistances and capacitances neglected in the cases of current RF transistors, an accurate model was obtained that can fully explain the experimental results. 13