How does the flicker noise corner frequency of a transistor affect close-in phase noise performance?

The flicker noise (1/f noise) corner frequency of a transistor directly determines the close-in phase noise performance of oscillators built with that transistor. Flicker noise is low-frequency noise whose power spectral density increases inversely with frequency (S_n proportional to 1/f^a where a is approximately 1). In an oscillator, this low-frequency flicker noise is upconverted to phase noise around the carrier through the nonlinear mixing action of the oscillator's active device; specifically, the flicker noise modulates the oscillator's frequency, creating phase noise that falls off as 1/f^3 close to the carrier (within the 1/f noise corner region) rather than the 1/f^2 slope predicted by Leeson's model for white noise alone. The 1/f noise corner frequency (f_c) of common RF transistors ranges from: SiGe HBT (1-10 kHz, best for low phase noise oscillators), InP HBT (1-10 kHz), GaAs pHEMT (10-100 kHz), Si MOSFET (100 kHz to 10 MHz, worst), and GaN HEMT (10-100 kHz). In the oscillator's phase noise spectrum, the transition from 1/f^3 slope (flicker-dominated) to 1/f^2 slope (white-noise-dominated) occurs at approximately f_c divided by the oscillator's loaded Q factor squared (for a simple van der Pol analysis) or at approximately f_c (Leeson's model, simplified). Choosing a transistor with a low 1/f corner frequency is critical for applications requiring low close-in phase noise: radar (Doppler resolution), communication systems (reciprocal mixing), and precision instrumentation.