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The Fermi level does not necessarily correspond to an actual energy level (in an insulator the Fermi level lies in the band gap), nor does it require the existence of a band structure. Nonetheless, the Fermi level is a precisely defined thermodynamic quantity, and differences in Fermi level can be measured simply with a voltmeter.
Depending on the material and the degree of detail desired, a variety of energy levels will be plotted against position: E F or μ: Although it is not a band quantity, the Fermi level (total chemical potential of electrons) is a crucial level in the band diagram. The Fermi level is set by the device's electrodes.
The Fermi energy is only defined at absolute zero, while the Fermi level is defined for any temperature. The Fermi energy is an energy difference (usually corresponding to a kinetic energy), whereas the Fermi level is a total energy level including kinetic energy and potential energy.
To understand how band structure changes relative to the Fermi level in real space, a band structure plot is often first simplified in the form of a band diagram. In a band diagram the vertical axis is energy while the horizontal axis represents real space. Horizontal lines represent energy levels, while blocks represent energy bands.
When a semiconductor is in thermal equilibrium, the distribution function of the electrons at the energy level of E is presented by a Fermi–Dirac distribution function. In this case the Fermi level is defined as the level in which the probability of occupation of electron at that energy is 1 ⁄ 2. In thermal equilibrium, there is no need to ...
is the w:conduction band, indicates the quasi-w:fermi energy levels, is the intrinsic Fermi level of the undoped semiconductor, and is the w:valence band. This band alignment is due to the biasing conditions that correspond with forward-active mode; forward bias on the emitter-base junction and reverse bias on the base-collector junction.
Energy band diagram of a simple NPN w: ... is the w:conduction band, is the device w:fermi energy level, is the intrinsic Fermi level of the undoped ...
Band edge diagram of a basic HEMT. Conduction band edge E C and Fermi level E F determine the electron density in the 2DEG. Quantized levels form in the triangular well (yellow region) and optimally only one of them lies below E F. Heterostructure corresponding to the band edge diagram above.