1314 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 4, APRIL 2005 The Influence of Transistor Nonlinearities on Noise Properties Sungjae Lee, Member, IEEE, and Kevin J. Webb, Fellow, IEEE Abstract—A nonlinear field-effect transistor equivalent-circuit model is examined to identify the fundamental mechanisms that up-convert baseband noise to near-carrier sideband noise when the device is operated in the large-signal regime. This model captures all physical noise sources and nonlinearities in the tran- sistor, and thereby allows a general cause-and-effect treatment. The noise sources in the equivalent-circuit model are deter- mined using low-frequency spectrum analyzer and microwave noise-figure meter data. Using the example of an AlGaN/GaN high electron-mobility transistor, the developed model correctly describes both the measured near-carrier sideband amplitude and phase noise simultaneously. Index Terms—Amplitude noise, gallium nitride, noise measure- ment, nonlinearities, phase noise, semiconductor device noise. I. INTRODUCTION T RANSMITTER or oscillator noise is constituted by noise sidebands modulating a carrier. In the case of transistors, this sideband noise is primarily attributed to baseband noise, which is mixed up to the microwave carrier frequency due to transistor nonlinearities. This near-carrier sideband noise limits system performance because it degrades the signal-to-noise ratio for the near-carrier signals. A transistor operated under large-signal conditions up-con- verts the intrinsic noise into amplitude- and phase-modu- lated sidebands (amplitude and phase noise) around the carrier, and these can be discriminated in a measurement. For example, in GaAs transistors, the measured amplitude and phase-noise levels have been found comparable [1]. Therefore, a complete large-signal noise equivalent circuit should correctly describe both amplitude and phase noise. Oscillator amplitude noise is typically much smaller than the oscillator phase noise due to the self-amplitude-limiting mechanism [2]. As a consequence, there have been major efforts over a long time period to under- stand the dominant phase noise in transistor-based oscillators (see, e.g., [3]–[7]). A measurement-based nonlinear noise equivalent-circuit model allows study of fundamental mechanisms which will Manuscript received May 27, 2004; revised September 16, 2004. This work was supported by the Air Force Office of Scientific Research under Contract F49620-03-1-0405 and by the Office of Naval Research under Contract N00014-99-C-0172. S. Lee was with the School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 USA. He is now with IBM Microelec- tronics, Essex Junction, VT 05452 USA. K. J. Webb is with the School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 USA (e-mail: webb@purdue.edu). Digital Object Identifier 10.1109/TMTT.2005.845763 up-convert the baseband noise to the near-carrier side- band noise. To produce this model, the linear and nonlinear equivalent-circuit elements need to be determined, as well as the noise sources, i.e., the baseband noise needs to be measured. Driving the transistor in an amplifier configuration, rather than as an oscillator, allows for precise control over the operating conditions, facilitating parameterization of a nonlinear device noise model [8]–[11]. The resulting nonlinear transistor model, configured as an amplifier, has been treated using harmonic-balance simulation, where the coefficients in a finite Fourier expansion of voltage or current are determined numerically. This type of analysis has been used to relate a nonlinear model to amplitude noise [10] and to phase noise [8], [9]. The impact of transistor nonlinearities on the baseband noise up-conversion have been investigated through an analytic study of field-effect transistor (FET) oscillators [12], [13]. The general understanding that has evolved is that, in FETs, the nonlinear gate–source capacitance gives rise to phase noise and nonlinear transconductance and, to a lesser degree, the output conductance , contribute to the ampli- tude noise [13]. This view has led to simplified nonlinear noise equivalent circuits, and has also led to the use of only a gate equivalent noise source [13]. In reality, and as we show, this delineation of the noise source contributions is not, in general, possible. Furthermore, it is necessary to consider both noise sources at the input and output. We present an example measurement-based large-signal noise equivalent-circuit transistor model, which includes all physical noise sources, and apply this model to an AlGaN/GaN high electron-mobility transistor (HEMT). This model correctly describes both amplitude and phase noise in the transistor. The major noise sources and nonlinearities are captured in a simpli- fied analytic model, thereby allowing a conceptual theory to be developed. These models allow the fundamental understanding of how the near-carrier amplitude and phase-noise sidebands are generated when the device is operated in the large-signal regime. The AlGaN/GaN HEMT used for this study was grown on an SiC substrate and processed as described elsewhere [14]. The HEMT has significant noise that will form the domi- nant sideband noise in a nonlinear amplifier or an oscillator. Section II describes an underlying noise theory using a sim- plified model that is subsequently related to the more general transistor nonlinear model of Section III. Noise sideband mea- surements and model predictions for the GaN HEMT are pre- sented in Section IV. 0018-9480/$20.00 © 2005 IEEE