Thin film transistor modeling: Frequency dispersion

Using new materials (including organic materials), scaling down to shorter device sizes, using printed TFT technology, and operating Thin Film Transistors (TFTs) at higher frequencies all bring upfront the issues of frequency dispersion of the TFT current-voltage and capacitance-voltage characteristics. This dispersion depends on the interplay of the contact phenomena and electron transport in the device channel. The standard way to account for the frequency dispersion is using the Elmore model (see Fig. 1 a) [1,2], which introduces the RC delays to reflect the propagation time of the carriers along the channel. However, this model is not sufficient for the quantitative modeling or parameter extraction for three reasons. First, it does not account for varying carrier and potential distributions. This could be remedied by using an RC transmission line model with varying parameters along the channel and further improved by making accounting for nonlinearities of the transmission line segments, similar to what has been done in the THz SPICE [3] (see Fig. 1b). Second, in contrast to crystalline field effect transistors, traps in the TFT channel also play a crucial role in determining the TFT frequency dispersion. This has been accounted for by introducing frequency dependent temperature model Variable Dispersion Model (VDM). [4] Third, the TFT contacts might determine the frequency dispersion and in short channel TFTs, and the non-uniform current distribution along the contact needs to be accounted for. In this paper, we review the emerging modeling approaches for reproducing the frequency dispersion in different types of TFTs with emphasis on the compact models that could be used for the device characterization, device and circuit design (when implemented in SPICE) and for the parameter extraction. Please click Additional Files below to see the full abstract

THz detection and amplification using plasmonic Field Effect Transistors driven by DC drain currents

We report on the numerical and theoretical results of sub-THz and THz detection by a current-driven InGaAs/GaAs plasmonic Field-Effect Transistor (TeraFET). New equations are developed to account for the channel length dependence of the drain voltage and saturation current. Numerical simulation results demonstrate that the effect of drain bias current on the source-to-drain response voltage (dU) varies with the device channel length. In a long-channel TeraFET where plasmonic oscillations cannot reach the drain, dU is always positive and rises rapidly with increasing drain current. For a short device in which plasmonic oscillations reach the drain, the current-induced nonuniform electric field leads to a negative response, agreeing with previous observations. At negative dU, the amplitude of the small-signal voltage at the drain side becomes larger than that at the source side. Thus, the device effectively serves as a THz amplifier in this condition. Under the resonant mode, the negative response can be further amplified near the resonant peaks. A new expression of dU is proposed to account for this resonant effect. Based on those expressions, a current-driven TeraFET spectrometer is proposed. The ease of implementation and simplified calibration procedures make it competitive or superior compared with other TeraFET-based spectrometers.Comment: 23 pages, 11 figures, 1 tabl