Published online by Cambridge University Press: 05 July 2011
This chapter discusses memory effects exhibited by transistors and how they affect their large-signal RF performance. Memory effects include self-heating, traps, and parasitic bipolar effects in SOI-MOSFETs. Distributed and transient thermal models will be discussed first. Large-signal measurement techniques for characterizing memory effects in transistors using pulsed biased and pulsed-RF large-signal measurements will be presented. Results from combined deep-level optical spectroscopy and large-signal measurement will then be introduced. Finally, the correlation between trapping and noise will be discussed. The chapter will conclude then with a discussion of the cyclostationary effect, according to which the average trapping population and device noise characteristics can be altered by the fast RF signals under large-signal RF operation.
Importance of memory effects in RF devices
GaN HEMTs provide a good example of devices strongly affected by memory effects. High-electron-mobility transistors (HEMTs) are among the most successful heterostructure three-terminal devices to have emerged over the last couple of decades [1]. GaN-based HEMTs show high transconductance, high cutoff frequencies, and good thermal management such that they are suitable for high-power and high-speed applications with minimal cooling [2] [3].
One of the obstacles for GaN HEMTs is current collapse or knee walk-out, which is known to result from the effects of surface trap states [4] [5] [6]. Many studies concerning the reduction of current collapse using various device processing techniques such as surface passivation, modified buffer layer designs, and field plates have been reported [3] [7].
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