Kink-effect-related noise in InAlAs/InGaAs short-channel HEMTs

Abstract

InAlAs/InGaAs HEMTs have demonstrated exceptional performance for low-noise high-frequency applications. However they have still some drawbacks to be removed, like the kink effect, which limits their applications by leading to a decrease in the gain and an enhancement in the noise level for high-enough values of the drain-to-source voltage. This effect is typically associated with the pile up of holes (generated by impact ionization) in the source-gate portion of the channel. In this work we investigate the noise properties of a 100 nm T-gate recessed In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As HEMT in the presence of kink effect. For the calculations we make use of a 2D ensemble Monte Carlo (MC) simulator that incorporates all the microscopic processes at the basis of this effect. Impact ionization, which leads to the appearance of holes responsible for the kink, is included using the Keldysh approach with parameters adjusted to reproduce the impact ionization coefficients in bulk materials. Hole recombination is also considered, with a characteristic time τ ranging between 0.01 and 1 ns. The accumulation of holes in the source-gate region leads to a decrease of the potential barrier controlling the current through the channel, which is further opened and, as a consequence, the drain current increases. This phenomenon appears accompanied by a significant raise of the noise in the device that spoils its performance. The aim of this work is to analyze this excess noise and explain its physical origin by means of MC simulations. Impact ionization and hole trapping mechanisms lead to fluctuations of the hole concentration in the channel. Since these fluctuations are strongly coupled to the drain-current fluctuations by the high transconductance of the transistor, with the onset of the kink effect an important increase of the noise takes place, with a characteristic cutoff frequency related to the impact ionization rate and the hole recombination time. This is clearly observed in the calculated current correlation functions and corresponding spectral densities, that will be presented at the conference.