Capacitance–voltage Profiling
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Capacitance–voltage profiling (or C–V profiling, sometimes CV profiling) is a technique for characterizing
semiconductor material A semiconductor is a material with electrical conductivity between that of a Electrical conductor, conductor and an Insulator (electricity), insulator. Its conductivity can be modified by adding impurities ("doping (semiconductor), doping") to ...
s and devices. The applied
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
is varied, and the
capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
is measured and plotted as a function of voltage. The technique uses a
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
junction (
Schottky barrier A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifier, rectifying characteristics, suitable for use as a diode. One of the p ...
) or a p–n junctionJ. Hilibrand and R.D. Gold, "Determination of the Impurity Distribution in Junction Diodes From Capacitance-Voltage Measurements", RCA Review, vol. 21, p. 245, June 1960 or a
MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...
to create a
depletion region In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region, or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobil ...
, a region which is empty of conducting
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s and
holes A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
, but may contain ionized donors and electrically active defects or ''traps''. The depletion region with its ionized charges inside behaves like a capacitor. By varying the voltage applied to the junction it is possible to vary the
depletion width In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region, or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
. The dependence of the depletion width upon the applied voltage provides information on the semiconductor's internal characteristics, such as its doping profile and electrically active defect densities. , Measurements may be done at DC, or using both DC and a small-signal AC signal (the ''conductance method'' , ), or using a large-signal transient voltage.


Application

Many researchers use capacitance–voltage (C–V) testing to determine semiconductor parameters, particularly in MOSCAP and MOSFET structures. However, C–V measurements are also widely used to characterize other types of semiconductor devices and technologies, including bipolar junction transistors, JFETs, III–V compound devices, photovoltaic cells, MEMS devices, organic thin-film transistor (TFT) displays, photodiodes, and carbon nanotubes (CNTs). These measurements' fundamental nature makes them applicable to a wide range of research tasks and disciplines. For example, researchers use them in university and semiconductor manufacturers' labs to evaluate new processes, materials, devices, and circuits. These measurements are extremely valuable to product and yield enhancement engineers who are responsible for improving processes and device performance. Reliability engineers also use these measurements to qualify the suppliers of the materials they use, to monitor process parameters, and to analyze failure mechanisms. A multitude of semiconductor device and material parameters can be derived from C–V measurements with appropriate methodologies, instrumentation, and software. This information is used throughout the semiconductor production chain, and begins with evaluating epitaxially grown crystals, including parameters such as average doping concentration, doping profiles, and carrier lifetimes. C–V measurements can reveal oxide thickness, oxide charges, contamination from mobile ions, and interface trap density in wafer processes. A C–V profile as generated on
nanoHUB nanoHUB.org is a science and engineering gateway comprising community-contributed resources and geared toward education, professional networking, and interactive simulation tools for nanotechnology. Funded by the United States National Science F ...
for bulk MOSFET with different oxide thicknesses. Notice that the red curve indicates low frequency whereas the blue curve illustrates the high-frequency C–V profile. Pay particular attention to the shift in threshold voltage with different oxide thicknesses. These measurements continue to be important after other process steps have been performed, including lithography, etching, cleaning, dielectric and polysilicon depositions, and metallization, among others. Once devices have been fully fabricated, C–V profiling is often used to characterize threshold voltages and other parameters during reliability and basic device testing and to model device performance. C–V measurements are done by using capacitance–voltage meters of Electronic Instrumentation. They are used to analyze the doping profiles of semiconductor devices by the obtained C–V graphs.


C–V characteristics of metal–oxide–semiconductor structure

A metal–oxide–semiconductor structure is critical part of a
MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...
, controlling the height of
potential barrier In quantum mechanics, the rectangular (or, at times, square) potential barrier is a standard one-dimensional problem that demonstrates the phenomena of wave-mechanical tunneling (also called "quantum tunneling") and wave-mechanical reflection. ...
in the
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Australia in Queensland and pa ...
via the gate oxide. An ''n''-channel MOSFET's operation can be divided into three regions, shown below and corresponding to the right figure.


Depletion

When a small positive bias voltage is applied to the metal, the
valence band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
edge is driven far from the
Fermi level The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''μ'' or ''E''F for brevity. The Fermi level does not include the work required to re ...
, and holes from the body are driven away from the gate, resulting in a low carrier density, so the capacitance is low (the valley in the middle of the figure to the right).


Inversion

At larger gate bias still, near the semiconductor surface the conduction band edge is brought close to the Fermi level, populating the surface with electrons in an inversion layer or n-channel at the interface between the semiconductor and the oxide. This results in a capacitance increase, as shown in the right part of right figure.


Accumulation

When a negative gate-source voltage (positive source-gate) is applied, it creates a ''p''-channel at the surface of the ''n'' region, analogous to the ''n''-channel case, but with opposite polarities of charges and voltages. The increase in hole density corresponds to increase in capacitance, shown in the left part of right figure.


See also

*
Current–voltage characteristic A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or p ...
*
Depletion region In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region, or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobil ...
*
Depletion width In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region, or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
* Drive Level Capacitance Profiling *
Deep-level transient spectroscopy Deep-level transient spectroscopy (DLTS) is an experimental tool for studying electrically active defects (known as charge carrier traps) in semiconductors. DLTS establishes fundamental defect parameters and measures their concentration in the mater ...
* Metal–oxide–semiconductor structure


References


External links


MOScap simulator on nanoHUB.org
enables users to compute C-V characteristics for different doping profiles, materials, and temperatures. {{DEFAULTSORT:Capacitance Voltage Profiling Semiconductor device fabrication