2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

Single Election Trapping in Nanoscale Transistors; RTS(Random Telegraph Signals) and l/f Noise

L. Forbes, D.A. Miller and M.Y. Louie
Oregon State University, US

Keywords:
single electron, traps, noise, l/f noise, RTS

Abstract:
Random telegraph signal, RTS, noise[1] is most obvious in submicron devices with traps, for example in nanoscale MOS transistors, MOSFET’s, with a low number of carriers. RTS is sometimes an indicator of poor device quality in larger devices and indicative of giant traps capturing and emitting many electrons at one time. RTS is however present in all devices and in particular MOSFET’s, RTS is easily observed in small submicron MOSFET’s as individual steps in the drain current associated with the capture and emission of single electrons. For a single type of trap the power spectral density is a Lorentzian spectrum and depends on the number of traps, N, and the root mean square occupancy of the traps, . For strong asymmetric RTS noise approaches zero. and there is no noise. Symmetric traps can become asymmetric in a MOSFET, by applying switching bias, but asymmetric ones without bias switching can become symmetric after bias switching.[1] McWorter has shown how a combination of these individual trapping events over many different traps with different time constants can result in l/f noise.[2] RTS noise is easily observed submicron or nanoscale size MOSFET’s. RTS is however present in all devices and in particular MOSFET’s and can result in transients in the measured noise and long term transient effects in the noise of large MOSFET’s. We have also observed and investigated these effects on very large micron size MOSFET’s. Following the application of switched bias the noise at 1kHz is reduced due to accumulation of the surface and holes collecting at the surface of the transistor channel. This results in many surface states and oxide traps capturing holes and changing charge states and remaining in a fixed charge state over an extended period of time. As such the charge state can not fluctuate and contribute to noise. What is new and different here in the time domain is that there are long term transients in the l/f noise after the application of switched bias. A variety of different types of transient behaviors have been observed on devices from two different manufacturers obtained over different time periods years apart. In some case the l/f noise will decrease after the application of fixed bias and stay at a lower level for long periods of time, however, the next time switched bias is applied it will recover from this lower level. In other cases the l/f noise will initially decrease with switched bias but recover in shorter periods of time. Annealing or wear out effects are observed in the long term transients. All of these phenomena can be explained by RTS. [1] S. Machlup, ''Noise in Semiconductors: Spectrum of a Two-Parameter Random Signal,'' Journal of Applied Physics, vol. 25, no. 3, pp. 341-343, 1954. [2] A. L. McWhorter, “l/f noise and germanium surface properties,” in Semiconductor Surface Physics (U. Penn., 1957) pp. 207-228. [3] C.W. Zhang, M.Y. Louie and L. Forbes, ''MOSFET l/f noise measurement under switched bias conditions, ''Workshop on Microelectronics and Electron Devices, Boise, Idaho, 16 April 2004, pp. 79-81. [4] M.Y. Louie and L. Forbes, ''Long term transients in MOSFET 1/f noise under switched bias conditions, '' Workshop on Microelectronics and Electron Devices, Boise, Idaho, 15 April, 2005, pp. 55-58. [5] M.Y. Louie and L. Forbes, ''MOSFET 1/f noise under switched bias conditions,'' Proc. 3rd Int. Symp. On Fluctuations and Noise, Austin, TX, 23-25 May, 2005, pp. 215-225. [6] M.Y. Louie, D.A. Miller, M.E. Jacob and L. Forbes, ''Long term transients in MOSFET 1/f noise with switched bias,” IEEE Device Research Conference, Santa Barbara, CA, 20-22 June 2005, pp. 79-80.

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