The term “shot” in shot noise derives from using shots, like those found in shotgun shells (tiny lead or steel balls), in the analogy instead of marbles.
Shot noise is a phenomenon that arises in semiconductor devices due to the discrete movement of electric charge across a potential barrier, such as in a diode. To grasp the concept of shot noise, imagine dropping marbles onto a table. As these marbles move across the table, some inevitably fall to the floor, and the rate at which they do so appears random, even if you consistently release marbles onto the table. This randomness in the rate of marbles falling represents the essence of shot noise.
In the context of a diode, when current flows (akin to dumping marbles), the movement of electric charge across the depletion region becomes random, analogous to marbles falling. For instance, if we consider a majority carrier, like an electron (representing a marble on the table), moving into the space-charge region (falling off the table), the electric field propels it across the junction (akin to gravity causing the marble to fall to the floor). On the p-type side (comparable to the floor), where it transforms into a minority carrier, it diffuses until it eventually recombines (similar to the marble moving until it stops).
Differentiate between Shot noise and Thermal noise
It’s essential to differentiate shot noise from thermal noise, despite both being characterized by a white noise Power Spectral Density (PSD). In thermal noise, carriers can randomly move either into or out of a resistive material due to thermal fluctuations, even in the absence of current flow. In contrast, for shot noise to manifest, two conditions must be met: there must be a potential barrier, and there must be a current flowing. Shot noise involves a random movement of carriers in one direction across the barrier, resembling marbles not hopping back onto the table.
Shot noise is not observed in long-channel Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). However, it is present in short-channel MOSFETs (where the oxide thickness, tox, is less than 20 nm) due to gate tunneling current. In long-channel MOSFETs operating in the saturation region, the discrete movement of charge into the depletion region results from thermal variations in the MOSFET’s channel resistance, rather than carriers crossing a potential barrier and being swept up by an electric field
In which types of electronic devices is shot noise typically not present, and under what conditions does it become significant?
Shot noise is typically absent in long-channel MOSFETs. However, it becomes significant in short-channel MOSFETs (with channel lengths below 20 nm) due to gate tunneling current.
How can the discrete movement of charge related to shot noise affect the behavior of electronic components like diodes or MOSFETs?
The discrete movement of charge associated with shot noise can introduce randomness and fluctuations in the behavior of electronic components. It can affect the performance of diodes and short-channel MOSFETs, impacting their characteristics and operation.
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