High-field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mi>f</mml:mi></mml:mrow></mml:math> noise in hBN-encapsulated graphene transistors

$1/f$ electronic noise is a conductance fluctuation, expressed in terms of mobility ``$\ensuremath{\alpha}$-noise'' by Hooge and Kleinpenning. Understanding this graphene key for high-performance electronics. Early investigations pointed out deviation from the standard formula, with free-carrier density substituted constant ${n}_{\mathrm{\ensuremath{\Delta}}}\ensuremath{\sim}{10}^{12}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Here we investigate hBN-encapsulated transistors where high gives access to velocity-saturation regime. We show that $\ensuremath{\alpha}$-noise still accounted formula on substituting differential $G$, resulting bell-shaped dependence flicker bias voltage. The same analysis holds Zener regime at even larger bias, two main differences. first one strong enhancement parameter reflecting hundred-times coupling interband excitations hyperbolic phonon-polariton (HPhP) modes midinfrared Reststrahlen (RS) bands hBN, which supported microwave thermometry measurements. second an exponential suppression large fields, attribute decoherence effects. phenomenology supports quantum-coherent bremsstrahlung interpretation $\ensuremath{\alpha}$-noise.