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Noise in Strain-Engineered Devices
Characterisation of the low-frequency noise in electronic devices gives
important information of the device physics and reliability, such as scattering processes, traps, and defects.
6.2 Fundamental Noise Sources
The total output current, I(t), of a device can be written as the sum of bias current (I bias ) and the randomly fluctuating noise current (i n (t)) as I(t) = I bias + i n (t).
The external sources that cause fluctuations in the current are not considered
in this case. There are some fundamental physical processes, which act as the
sources of noise, that can generate the random fluctuations in the current (or
voltage) in a device. These noise sources are discussed below and described
in terms of the power spectral density (PSD) of the noise current.
6.2.1 Thermal Noise
Thermal noise (also known as Nyquist, Johnson, diffusion, velocity fluctuation, or white noise) originates from the random thermal movement of
electrons, and is present at all frequencies with a flat frequency response
(PSD). The phenomenon of thermal noise can be thought of as the thermal
excitation of the carriers in a resistor. Due to scattering the velocity of the
electron changes randomly. At a particular time instant, there could be more
electrons moving in a certain direction than electrons moving in the other
directions in a random manner, resulting in a small net current. This current
fluctuates randomly in strength and direction, with the average over (long)
time being zero. If a piece of material with resistance R and temperature T is
considered, the PSD of the thermal noise current is found to be
S f
kT R
S f
kTR
( ) 4 / or
( ) 4
I
V
=
=
(6.3)
where k is Boltzmann’s constant. The thermal noise exists in every physical
resistor and resistive part of a device and sets a lower limit on the noise in
an electric circuit. In bipolar transistors, thermal noise can be modeled as
originated from base resistance and the collector impedance. In field-effect
transistors, the existence of thermal noise comes from the physical channel
resistance between the drain and gate (when the device is on and conducting
current).
6.2.2 Shot Noise
The current flowing across a potential barrier, like the p-n junction, fluctuates due to the random movement of the electronic charge (electrons). The
current across a barrier is given by the number of carriers, each carrying the
charge q, flowing through the barrier during a period of time. A shot noise
Noise in Strain-Engineered Devices
Characterisation of the low-frequency noise in electronic devices gives
important information of the device physics and reliability, such as scattering processes, traps, and defects.
6.2 Fundamental Noise Sources
The total output current, I(t), of a device can be written as the sum of bias current (I bias ) and the randomly fluctuating noise current (i n (t)) as I(t) = I bias + i n (t).
The external sources that cause fluctuations in the current are not considered
in this case. There are some fundamental physical processes, which act as the
sources of noise, that can generate the random fluctuations in the current (or
voltage) in a device. These noise sources are discussed below and described
in terms of the power spectral density (PSD) of the noise current.
6.2.1 Thermal Noise
Thermal noise (also known as Nyquist, Johnson, diffusion, velocity fluctuation, or white noise) originates from the random thermal movement of
electrons, and is present at all frequencies with a flat frequency response
(PSD). The phenomenon of thermal noise can be thought of as the thermal
excitation of the carriers in a resistor. Due to scattering the velocity of the
electron changes randomly. At a particular time instant, there could be more
electrons moving in a certain direction than electrons moving in the other
directions in a random manner, resulting in a small net current. This current
fluctuates randomly in strength and direction, with the average over (long)
time being zero. If a piece of material with resistance R and temperature T is
considered, the PSD of the thermal noise current is found to be
S f
kT R
S f
kTR
( ) 4 / or
( ) 4
I
V
=
=
(6.3)
where k is Boltzmann’s constant. The thermal noise exists in every physical
resistor and resistive part of a device and sets a lower limit on the noise in
an electric circuit. In bipolar transistors, thermal noise can be modeled as
originated from base resistance and the collector impedance. In field-effect
transistors, the existence of thermal noise comes from the physical channel
resistance between the drain and gate (when the device is on and conducting
current).
6.2.2 Shot Noise
The current flowing across a potential barrier, like the p-n junction, fluctuates due to the random movement of the electronic charge (electrons). The
current across a barrier is given by the number of carriers, each carrying the
charge q, flowing through the barrier during a period of time. A shot noise