A5 Electronic Noise

Topic

The thermal agitation of free charge carriers within this semiconductor material—present even when no net current flows through it—produces thermal noise, known as Johnson-Nyquist noise; these free charge carriers, agitated thermally in all directions within the material, generate a voltage fluctuation across its terminals that is independent of the measurement frequency and distributed evenly across the entire frequency spectrum. Consequently, a higher temperature in the semiconductor material intensifies the thermal agitation of the free charge carriers, thereby increasing this frequency-independent thermal noise.

In contrast, the discrete nature of the electric charge crossing a potential barrier within the semiconductor material—whether at a junction or any other element that the net current must traverse one charge carrier at a time—produces shot noise; each individual charge carrier crosses the potential barrier at a random moment of its own, uncorrelated with the moment the next carrier crosses, and it is this very randomness in the crossing times of discrete charge carriers that produces a current fluctuation proportional to the net current itself. Consequently, a higher net current flowing across the potential barrier results in greater shot noise. Thus, this model—building upon the concepts of thermal noise and shot noise—describes a third fluctuation mechanism of empirical origin that is not as well understood as the first two; it depends on surface states and defects present on the semiconductor material's surface. These states and defects trap and release charge carriers with a wide distribution of characteristic time constants, and it is this superposition of time constants that produces flicker noise—noise whose intensity increases as the measurement frequency decreases, unlike thermal and shot noise, both of which remain independent of frequency. Consequently, the higher density of these surface states and defects in the semiconductor material results in greater flicker noise; furthermore, it is precisely at low frequencies that this flicker noise comes to dominate over the thermal and shot noise, thereby dictating the signal-to-noise ratio that these three mechanisms collectively limit.

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