Hot carrier injection (HCI) is a phenomenon in
solid-state
Solid state, or solid matter, is one of the four fundamental states of matter.
Solid state may also refer to:
Electronics
* Solid-state electronics, circuits built of solid materials
* Solid state ionics, study of ionic conductors and their use ...
electronic devices where an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
or a “
hole
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 ...
” gains sufficient
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
to overcome a
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. ...
necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a
MOS transistor
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
, the switching characteristics of the transistor can be permanently changed. Hot-carrier injection is
one of the mechanisms that adversely affects the
reliability of semiconductors of solid-state devices.
Physics
The term “hot carrier injection” usually refers to the effect in
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s, where a carrier is injected from the conducting channel in the
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
substrate to the
gate dielectric
A gate dielectric is a dielectric used between the gate and substrate of a field-effect transistor (such as a MOSFET). In state-of-the-art processes, the gate dielectric is subject to many constraints, including:
* Electrically clean interface to ...
, which usually is made of
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(SiO
2).
To become “hot” and enter the
conduction 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 w ...
of SiO
2, an electron must gain a kinetic energy of ~3.2
eV. For holes, 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 w ...
offset in this case dictates they must have a kinetic energy of 4.6 eV. The term "hot electron" comes from the effective temperature term used when modelling carrier density (i.e., with a Fermi-Dirac function) and does not refer to the bulk temperature of the semiconductor (which can be physically cold, although the warmer it is, the higher the population of hot electrons it will contain all else being equal).
The term “hot electron” was originally introduced to describe non-equilibrium electrons (or holes) in semiconductors. More broadly, the term describes electron distributions describable by the
Fermi function
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and t ...
, but with an elevated effective temperature. This greater energy affects the mobility of charge carriers and as a consequence affects how they travel through a semiconductor device.
''Hot electron''s can tunnel out of the semiconductor material, instead of recombining with a
hole
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 ...
or being conducted through the material to a collector. Consequent effects include increased leakage current and possible damage to the encasing dielectric material if the hot carrier disrupts the atomic structure of the dielectric.
Hot electrons can be created when a high-energy photon of electromagnetic radiation (such as light) strikes a semiconductor. The energy from the photon can be transferred to an electron, exciting the electron out of the valence band, and forming an electron-hole pair. If the electron receives enough energy to leave the valence band, and to surpass the conduction band, it becomes a hot electron. Such electrons are characterized by high effective temperatures. Because of the high effective temperatures, hot electrons are very mobile, and likely to leave the semiconductor and travel into other surrounding materials.
In some semiconductor devices, the energy dissipated by hot electron phonons represents an inefficiency as energy is lost as heat. For instance, some solar cells rely on the photovoltaic properties of semiconductors to convert light to electricity. In such cells, the hot electron effect is the reason that a portion of the light energy is lost to heat rather than converted to electricity.
Hot electrons arise generically at low temperatures even in degenerate semiconductors or metals. There are a number of models to describe the hot-electron effect. The simplest predicts an electron-phonon (e-p) interaction based on a clean three-dimensional free-electron model. Hot electron effect models illustrate a correlation between power dissipated, the electron gas temperature and overheating.
Effects on transistors
In
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s, hot electrons have sufficient energy to tunnel through the thin oxide gate to show up as gate current, or as substrate leakage current. In a MOSFET, when a gate is positive, and the switch is on, the device is designed with the intent that electrons will flow laterally through the conductive channel, from the source to the drain. Hot electrons may jump from the channel region or from the drain, for instance, and enter the gate or the substrate. These hot electrons do not contribute to the amount of current flowing through the channel as intended and instead are a leakage current.
Attempts to correct or compensate for the hot electron effect in a MOSFET may involve locating a diode in reverse bias at gate terminal or other manipulations of the device (such as lightly doped drains or double-doped drains).
When electrons are accelerated in the channel, they gain energy along the mean free path.
This energy is lost in two different ways:
#The carrier hits an atom in the substrate. Then the collision creates a cold carrier and an additional electron-hole pair. In the case of nMOS transistors, additional electrons are collected by the channel and additional holes are evacuated by the substrate.
#The carrier hits a Si-H bond and break the bond. An interface state is created and the hydrogen atom is released in the substrate.
The probability to hit either an atom or a Si-H bond is random, and the average energy involved in each process is the same in both case.
This is the reason why the substrate current is monitored during HCI stress.
A high substrate current means a large number of created electron-hole pairs and thus an efficient Si-H bond breakage mechanism.
When interface states are created, the threshold voltage is modified and the subthreshold slope is degraded. This leads to lower current, and degrades the operating frequency of integrated circuit.
Scaling
Advances in semiconductor manufacturing techniques and ever increasing demand for faster and more complex
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s (ICs) have driven the associated Metal–Oxide–Semiconductor field-effect transistor (MOSFET) to scale to smaller dimensions.
However, it has not been possible to scale the supply voltage used to operate these ICs proportionately due to factors such as compatibility with previous generation circuits,
noise margin
In electrical engineering, noise margin is the maximum voltage amplitude of extraneous signal (electrical engineering), signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate fr ...
, power and delay requirements, and non-scaling of
threshold voltage
The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
,
subthreshold slope
The subthreshold slope is a feature of a MOSFET's current–voltage characteristic.
In the subthreshold region, the drain current behaviour – though being controlled by the gate terminal – is similar to the exponentially decreasing curre ...
, and
parasitic capacitance
Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages a ...
.
As a result, internal electric fields increase in aggressively scaled MOSFETs, which comes with the additional benefit of increased carrier velocities (up to
velocity saturation Saturation velocity is the maximum velocity a charge carrier in a semiconductor, generally an electron, attains in the presence of very high electric fields. When this happens, the semiconductor is said to be in a state of velocity saturation. Cha ...
), and hence increased switching speed,
[Richard C. Dorf (ed) ''The Electrical Engineering Handbook'', CRC Press, 1993 {{ISBN, 0-8493-0185-8 page 578] but also presents a major
reliability
Reliability, reliable, or unreliable may refer to:
Science, technology, and mathematics Computing
* Data reliability (disambiguation), a property of some disk arrays in computer storage
* High availability
* Reliability (computer networking), a ...
problem for the long term operation of these devices, as high fields induce hot carrier injection which affects device reliability.
Large electric fields in MOSFETs imply the presence of high-energy carriers, referred to as “hot carriers”. These hot carriers that have sufficiently high energies and momenta to allow them to be injected from the semiconductor into the surrounding dielectric films such as the gate and sidewall oxides as well as the buried oxide in the case of
silicon on insulator
In semiconductor manufacturing, silicon on insulator (SOI) technology is fabrication of silicon semiconductor devices in a layered silicon–insulator–silicon substrate, to reduce parasitic capacitance within the device, thereby improving perfo ...
(SOI)
MOSFETs
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
.
Reliability impact
The presence of such mobile carriers in the oxides triggers numerous physical damage processes that can drastically change the device characteristics over prolonged periods. The accumulation of damage can eventually cause the circuit to fail as key parameters such as threshold voltage shift due to such damage. The accumulation of damage resulting degradation in device behavior due to hot carrier injection is called “hot carrier degradation”.
The useful life-time of circuits and integrated circuits based on such a MOS device are thus affected by the life-time of the MOS device itself. To assure that integrated circuits manufactured with minimal geometry devices will not have their useful life impaired, the life-time of the component MOS devices must have their HCI degradation well understood. Failure to accurately characterize HCI life-time effects can ultimately affect business costs such as warranty and support costs and impact marketing and sales promises for a foundry or IC manufacturer.
Relationship to radiation effects
Hot carrier degradation is fundamentally the same as the
ionization radiation effect known as the
total dose damage to semiconductors, as experienced in space systems due to solar
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
, electron,
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
and
gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
exposure.
HCI and NOR flash memory cells
HCI is the basis of operation for a number of
non-volatile memory
Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data.
Non-volatile memory typic ...
technologies such as
EPROM
An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power s ...
cells. As soon as the potential detrimental influence of HC injection on the circuit reliability was recognized, several fabrication strategies were devised to reduce it without compromising the circuit performance.
NOR
flash memory
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...
exploits the principle of hot carriers injection by deliberately injecting carriers across the gate oxide to charge the
floating gate
The floating-gate MOSFET (FGMOS), also known as a floating-gate MOS transistor or floating-gate transistor, is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) where the gate is electrically isolated, creating a floating no ...
. This charge alters the MOS transistor threshold voltage to represent a
logic '0' state. An uncharged floating gate represents a '1' state. Erasing the NOR Flash memory cell removes stored charge through the process of
Fowler–Nordheim tunneling
Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emissio ...
.
Because of the damage to the oxide caused by normal NOR Flash operation, HCI damage is one of the factors that cause the number of write-erase cycles to be limited. Because the ability to hold charge and the formation of damage
trap
A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research.
Trap or TRAP may also refer to:
Art and entertainment Films and television
* ''Trap'' (2015 film), Fil ...
s in the oxide affects the ability to have distinct '1' and '0' charge states, HCI damage results in the closing of the non-volatile memory logic margin window over time. The number of write-erase cycles at which '1' and '0' can no longer be distinguished defines the endurance of a non-volatile memory.
See also
*
Time-dependent gate oxide breakdown Time-dependent gate oxide breakdown (or time-dependent dielectric breakdown, TDDB) is a kind of transistor aging, a failure mechanism in MOSFETs, when the gate oxide breaks down as a result of long-time application of relatively low electric field ...
(also ''time-dependent dielectric breakdown'', TDDB)
*
Electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direc ...
(EM)
*
Negative bias temperature instability
Negative-bias temperature instability (NBTI) is a key reliability issue in MOSFETs, a type of transistor aging. NBTI manifests as an increase in the threshold voltage and consequent decrease in drain current and transconductance of a MOSFET. The d ...
(NBTI)
*
Stress migration Stress migration is a failure mechanism that often occurs in integrated circuit metallization (aluminum, copper). Voids form as result of vacancy migration driven by the hydrostatic
Fluid statics or hydrostatics is the branch of fluid mechanic ...
*
Lattice scattering
Lattice scattering is the scattering of ions by interaction with atoms in a lattice. This effect can be qualitatively understood as phonons colliding with charge carriers.
In the current quantum mechanical picture of conductivity the ease with wh ...
References
External links
An articleabout hot carriers a
www.siliconfareast.com*
IEEE
The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operation ...
br>
International Reliability Physics Symposium the primary academic and technical conference for semiconductor reliability involving HCI and other reliability phenomena
Integrated circuits
Semiconductors
Semiconductor device defects
Charge carriers
Electric and magnetic fields in matter