Hot carrier injection

Hot carrier injection

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Encyclopedia
Hot carrier injection is a phenomenon in solid-state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 electronic devices where an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 or a “hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

” gains sufficient kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which 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 acceleration, the body maintains this kinetic energy unless its speed changes...

 to overcome a potential barrier 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
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

, 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
Reliability (semiconductor)
Reliability of semiconductor devices can be summarized as follows:# Semiconductor devices are very sensitive to impurities and particles. Therefore, to manufacture these devices it is necessary to manage many processes while accurately controlling the level of impurities and particles...

 of solid-state devices.

Physics


The term “hot carrier injection” usually refers to the effect in MOSFETs, where a carrier is injected from the conducting channel in the silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 substrate to the gate dielectric
Gate dielectric
A gate dielectric is a dielectric used between the gate and substrate of a field effect transistor. In state-of-the-art processes, the gate dielectric is subject to many constraints, including:...

, which usually is made of silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 (SiO2).

To become “hot” and enter the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...

 of SiO2, an electron must gain a kinetic energy of 3.3 eV. For holes, the valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....

 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 actual temperature of anything. That is, high temperatures caused by the effect are unrelated to the phrase "hot electron effect".

The hot electron effect occurs in semiconductor devices where electrons are excited to energy levels higher than those associated with the semiconductor’s conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...

. These hot electrons can tunnel out of the semiconductor material—instead of recombining with a hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

 or being conducted through the material to a collector. Consequent effects of this phenomenon include heating of the device, and increased leakage current. Because hot electrons generally give off their excess energy as phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

s, a common manifestation of the hot electron effect is an increase in the heat of the semiconductor device.

The term “hot electrons” was originally introduced to describe non-equilibrium electrons (or holes) in semiconductors. More broadly, the term describes electron distributions describable by the Fermi function, but with an elevated effective temperature. This has implications for where, within a semiconductor device, electrons may travel because the mobility of carriers (here, electrons) depends on their effective temperature.

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.

The tendency of hot electrons to travel across material boundaries and to emit phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

s (as opposed to traveling to an electron acceptor/ terminal and contributing to the generation of current) explains why the hot electron effect often manifests as an increase in the temperature of a device. In some semiconductor devices, this 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
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

s, hot electrons have sufficient energy to tunnel through the thin oxide gate to show up as gate current, or as substrate leakage current. The hot electrons may jump from the channel region or from the drain, for instance, and into the gate or the substrate.

For instance, in a MOSFET, when a gate is positive, and the switch is on, the device is designed with the intent that electrons will flow through the conductive channel to the drain. 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:
  1. The carrier hit an atom in the substrate. Then the collision create 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.
  2. The carrier hit 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
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

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
Noise margin
In electrical engineering, noise margin is the amount by which a signal exceeds the minimum amount for proper operation. It is commonly used in at least two contexts:...

, power and delay requirements, and non-scaling of threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...

, subthreshold slope
Subthreshold slope
The subthreshold slope of 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 increasing current of a forward biased diode...

, and parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-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...

.

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
Velocity saturation
In semiconductors, when a strong enough electric field is applied, the carrier velocity in the semiconductor reaches a maximum value, saturation velocity. When this happens, the semiconductor is said to be in a state of velocity saturation...

), and hence increased switching speed, but also presents a major reliability
Reliability (semiconductor)
Reliability of semiconductor devices can be summarized as follows:# Semiconductor devices are very sensitive to impurities and particles. Therefore, to manufacture these devices it is necessary to manage many processes while accurately controlling the level of impurities and particles...

 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
Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...

 (SOI) MOSFETs
SOI MOSFET
In electronics, an SOI MOSFET semiconductor device is a Silicon on Insulator MOSFET structure in which a semiconductor layer, e.g. silicon, germanium or the like, is formed above an insulator layer which may be a buried oxide layer formed in a semiconductor substrate. SOI MOSFET devices are...

.

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 same as the ionization radiation effect known as the total dose damage to semiconductors, as experienced in space systems due to solar proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

, electron, X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 and gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 exposure.

HCI and NOR flash memory cells


HCI is the basis of operation for a number of non-volatile memory
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...

 technologies such as Electrically Erasable Programmable Read-Only Memory
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 (EEPROM) 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
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

 exploits the principle of hot carriers injection by deliberately injecting carriers across the gate oxide to charge the floating gate. This charge alters the MOS transistor threshold voltage to represent a logic '0' state
Boolean-valued function
A boolean-valued function, in some usages is a predicate or a proposition, is a function of the type f : X → B, where X is an arbitrary set and where B is a boolean domain....

. An uncharged floating gate represents a '1' state. Erasing the NOR Flash memory cell removes stored charge through the process of Fowler–Nordheim tunneling.

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 traps 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
    Time-dependent gate oxide breakdown is 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
    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 direct current densities are used, such as in...

     (EM)
  • Negative bias temperature instability (NBTI)
  • Stress migration
    Stress migration
    Stress Migration is a failure mechanism that often occurs in IC metallization . Voids form as result of vacancy migration driven by the hydrostatic stress gradient. Large voids may lead to open circuit or unacceptable resistance increase that impededs the IC performance...

  • lattice scattering
    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....


External links

  • An article about hot carriers at www.siliconfareast.com
  • IEEE
    Institute of Electrical and Electronics Engineers
    The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

    International Reliability Physics Symposium, the primary academic and technical conference for semiconductor reliability involving HCI and other reliability phenomena