(Invited) Review of Identification Methods of 1/f Noise Mechanisms in Polymer-Based OTFTs

We study the low frequency noise, also known as flicker noise (1/f), in polymer-based Organic Thin Film Transistors, and we review techniques to identify the mechanisms of the 1/f noise based on a specific model of the device. We found that the quality of the gate dielectric and its interface may determine the dominant noise mechamism. Three theories have been formulated to explain the origin of 1/f noise in FET devices: the carrier number fluctuation (∆N) theory[1], the mobility fluctuation (∆µ) theory [2], and the ∆N - ∆µ theory [3], which assumes that effects of insulator charge fluctuations affect the carrier mobility at the interface. The ∆N - ∆µ theory reduces to the ∆N model when the surface mobility is not affected by carrier number fluctuation. According to the ∆N - ∆µ theory [3]the plot of SID(f)/ID 2 and [gm/ID]2 against ID (gm being the transconductance and ID the drain current) allows us to identify the noise mechanism. When both curves are parallel, 1/f noise must be due to carrier number fluctuation. If there is a slight deviation from this parallel behavior, it must be due to the correlated surface mobility fludtuation (∆N - ∆µ model). A number of analysis of the noise in OTFTs have been published. In most of them, it is found to be a bulk phenomenon[4,5], and as a result the main mechanism is assumed to be mobility fluctuation. However, very often these analysis are not very complete and are based on crystalline MOSFET models instead of specific OTFT models. On the other hand, it was demonstrated that the analysis must be made over a range wide of bias in order to accurately determine the origin of noise [6,7]. We studied the origin of 1/f noise two polymeric p-type OTFT technologies developed by CEA´Liten (France). We found that in the older technology the slopes, in log-log scale, of SID(f)/ID 2 and [gm/ID]2, versus ID had quite different values. This means that the main mechanism of noise in OTFTs is not the fluctuation of the number of carriers. Plotting SID(f)/I2 Dvs (VGS-Vth) in log-log scale (Vth being the threshold voltage) for devices with different channel lengths, channel width W=2000 µm and at VDS = 1 V, we observed that the extracted slope is close to -1 (Table 1). This indicates that noise is due to the fluctuation of carrier mobility, due to the lower quality of the materials and fabrication defects. The threshold voltage was extracted assuming a physically-based OTFT model with a power-law based model of the field effect mobility [8]. In the second technology, mobility values are higher, 2-3 cm2/(V s). The normalized current noise PSD follows the squared-transconductance ratio variation (Fig. 1), not the inverse of the drain current. We concluded that the low frequency in the targeted devices of the second technology is originated from the carrier number fluctuation due to trapping/release of charge carriers by trap states uniformly distributed within the gate oxide. The extracted values of gate dielectric trap-states density are comparable to values obtained in conventional Si-based MOSFETs [7], indicating the higher quality of the gate dielectric and its interface with the polymeric material compared to the first studied technology and other previously reported OTFT devices. [1] McWhorter A. L.. "1/f noise and germanium surface prosperities," Semiconductor Surface Physics. R. H. Kingdton (Ed), University of Pennsylvania Press, Philadelphia, PA, 1957, pp. 207-228. [2] Hooge F. N., "1/f noise is no surface effect", Physics Letters, 29A (3), 1969, 139–140 [3] Ghibaudo, G., et al..,‘Improved analysis of low frequency noise in field-effect MOS transistors’, Phys. Status Solidi. A , 1991, 124, pp. 571-581 [4] L. Ke, S., et al., “Low frequency noise analysis on organic thin film transistors,” Journal of Applied Physics 104, 124502 (2008). [5] H. Kang and V. Subramanian, “Measurement and analysis of 1/f noise under switched bias in organic thin film transistors,” Applied Physics Letters 104, 023301 (2014). [6] Y. Xu, et al., “Origin of low-frequency noise in pentacene field-effect transistors,” Solid-State Electronics 61, 106–110 (2011). [7] W. E. Muhea, et al.. “1/f noise analysis in high mobility polymer-based OTFTs with non-fluorinated dielectric,” Appl. Phys. Lett. 114, 243301 (2019). [8] C.H. Kim; et al., “A Compact Model for Organic Field-Effect Transistors With Improved Output Asymptotic Behaviors,” IEEE Trans. on Electron Devices, vol. 60, no. 3, pp. 1136-1141, March 2013. Figure 1