Low-bias-noise spectroscopy of field-effect transistor channels: Depletion-region trap models and spectra

Abstract

The use of noise spectroscopy as a tool to characterize the materials and processing of FETs is demonstrated by using a simple model which accounts for resistance fluctuations in the FET channel by charge fluctuations in the FET Space Charge Region (SCR). The charge fluctuations are modeled as random electron capture by traps in the SCR from simultaneous and distinct concentrations of traps in the bulk region of the SCR, and in surface region immediately below the gate. The general form of the model relates the SCR width fluctuations to these charge fluctuations via Gauss' Law, and couples the width changes to the channel resistance variations. The dependence of the spectrum of the width fluctuations, S w ( f), upon the SCR width W, is evaluated at a low drain bias where a one-dimensional analysis of the uniform channel is adequate. The model is simplified by assuming independent, single electron traps with uniform distributions, from which the concentrations are found as the regression coefficients from a plot of W 2 S w ( f) vs W 4 at a fixed frequency. Gallium arsenide MESFETs are used as a specific example, with the uniform and independent trap assumption being adequate to fit both ion implanted, and MBE grown channel layers. The trap density lineshape (2 πf · S( f)) obtained, is best fit by a broadened Lorentzian lineshape of [ 2ωτ (1+ω 2τ 2)] 1 2 , which is characteristic of a one-dimensional diffusion process.