Hot-carrier injection
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Hot carrier injection (HCI) is a phenomenon in
Physics
The term “hot carrier injection” usually refers to the effect in
To become “hot” and enter the
The term “hot electron” was originally introduced to describe non-equilibrium electrons (or holes) in semiconductors.
Hot electrons can tunnel out of the semiconductor material, instead of recombining with a hole 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.[4]
Hot electrons arise generically at low temperatures even in degenerate semiconductors or metals.[5] There are a number of models to describe the hot-electron effect.[6] The simplest predicts an electron-phonon (e-p) interaction based on a clean three-dimensional free-electron model.[7][8] Hot electron effect models illustrate a correlation between power dissipated, the electron gas temperature and overheating.
Effects on transistors
In MOSFETs, hot electrons have sufficient energy to tunnel through the thin gate oxide 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 circuits (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, power and delay requirements, and non-scaling of threshold voltage, subthreshold slope, and parasitic capacitance.
As a result, internal electric fields increase in aggressively scaled MOSFETs, which comes with the additional benefit of increased carrier velocities (up to
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
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, electron, X-ray and gamma ray exposure.
HCI and NOR flash memory cells
HCI is the basis of operation for a number of non-volatile memory technologies such as EPROM 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
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 (also time-dependent dielectric breakdown, TDDB)
- Electromigration (EM)
- Negative bias temperature instability(NBTI)
- Stress migration
- Lattice scattering
References
- ^ Keane, John; Kim, Chris H (25 Apr 2011). "Transistor Aging". IEEE Spectrum. Retrieved 21 Jun 2020.
- ^ Conwell, E. M., High Field Transport in Semiconductors, Solid State Physics Supplement 9 (Academic Press, New York, 1967).
- ^ "Hot-Electron Effect in Superconductors and Its Applications for Radiation Sensors" (PDF). LLE Review. 87: 134.
- S2CID 35169618.
- PMID 10032346.
- .
- PMID 10011570.
- S2CID 15241519.
- ISBN 0-8493-0185-8page 578
External links
- An article about hot carriers at www.siliconfareast.com
- IEEE International Reliability Physics Symposium, the primary academic and technical conference for semiconductor reliability involving HCI and other reliability phenomena