Regional Signal-Delay Analysis Applied to High-Frequency Carbon Nanotube FETs

Abstract

Abstract-A regional signal-delay analysis is presented for fieldeffect transistors intended for operation at very high frequencies. For the example used here of a doped-contact carbon nanotube field-effect transistor, the analysis reveals that tunneling into the channel region of the device, and modulation of the space-charge regions in the source and drain adjacent to the channel, are the principal contributors to the overall delay. A recently proposed lower limit to the signal delay time in the channel is critically examined via the introduction of a local signal velocity.Index Terms-Carbon nanotube, field-effect transistors, high frequency, signal delay.

Comparison of p-i-n and n-i-n carbon nanotube FETs regarding high-frequency performance

“…As V DS is increased, electrons will attain and exceed this energy on entering the drain. Thus, use of the constant-effective-mass model will overestimate the velocity in this region, leading to an underestimate of the signal delay time in the drain [8], and, consequently, to an over-optimistic value of f T . This fact is demonstrated in Fig.…”

“…where D b is the degeneracy of sub-band b, G C;b is the local density of states arising from coupling to contact C [8], and f C is the Fermi function at contact C. The parameter u is used to differentiate between electrons and holes:…”

“…An energy-dependent effective-mass (EEM) model, rather than a constant-effective-mass (CEM) model, is applied to our Schrödinger-Poisson solver [7] to achieve more accurate simulation results for devices in which high electric fields are expected to be present. This situation is likely to arise in the drain region of the device at high drain-source bias and, if not correctly treated, could lead to an underestimate of the signal delay time in this region [8], and to a corresponding overestimate of f T [9]. We also explore the effect of chirality, thereby extending the work on n-i-n CNFETs that has been presented recently [10].…”

Critique of high-frequency performance of carbon nanotube FETs

“…The results shown in Fig. 7 indicate the improvement for a particular CNFET on increasing V GS from 0.4 to 0.7 V, with V DS held at 0.7 V [25]. This improvement may be mitigated at other combinations of V GS and V DS , due to the bias dependence of C Gi [12].…”

“…7. Effect on f T of: gate bias [25]; non-quasi-static response [26]; phonon scattering [27]. Arrows indicate increasing parameter.…”