Hot Media Injection

The term “hot carrier injection” usually indicates the application of technology for tunneling accelerated electrons through a thin film of a dielectric in a MOS structure . The technology is used, for example, to change the charge of isolated areas of the semiconductor structure (the so-called pockets), which serve as a storage cell in EEPROM chips and flash memory . Acceleration of electrons is carried out by creating a current in a conducting channel. Channel, dielectric and a pocket form a field effect transistor .

To become “hot” and enter the conduction band of a dielectric made of silicon dioxide SiO ₂ , the electron must receive a kinetic energy of approximately 3.2 eV . For holes, the valence band in this case shifts and dictates the need for an energy of 4.6 eV. The term "hot electrons" comes from the effective temperature used in modeling the density of carriers (that is, with the Fermi-Dirac function) and does not refer to the temperature of the semiconductor (which can be physically cold).

The term “hot electron” was originally introduced to describe nonequilibrium electrons (or holes) in semiconductors. ^[2] In a broader sense, the term describes the distribution of electrons in accordance with the Fermi function , but with an increased effective temperature. This greater energy affects the mobility of charge carriers and, as a result, affects how they pass through a semiconductor device. ^[3]

Hot electrons can tunnel from a semiconductor material instead of recombining with a hole or pass through the material to a collector. The following effects can be: an increase in leakage current and possible damage to the sealing dielectric material if hot carriers violate the atomic structure of the dielectric.

Hot electrons can be created by hitting high-energy photons of electromagnetic radiation (such as light) against a semiconductor. The photon energy can be transferred to the electron, capturing the latter from the valence band and forming an electron-hole pair. If an electron receives enough energy to leave the valence band and go into the conduction band, then it becomes a hot electron. Such electrons are characterized by high effective temperatures. Due to such high effective temperatures, hot electrons are very mobile and are likely to be able to leave the semiconductor to move to other surrounding materials.

In some semiconductor devices, the energy dissipated by hot electrons from photons is inefficient and is lost in the form of heat. For example, some solar cells rely on the photoelectric properties of semiconductors when converting light energy into electricity. In such elements, the effect of hot electrons is the reason that part of the energy of light is lost in the form of heat, and not converted into electrical energy. ^[four]

Mostly, hot electrons occur at low temperatures, even in degenerate semiconductors or metals. ^[5] There are a number of examples to describe the effect of hot electrons. ^[6] The simplest prediction of electron-photon interaction is based on a three-dimensional model of free electrons. ^{[7] [8]} Hot electron effect models illustrate the relationship between power dissipation, electron gas temperature, and overheating.

In MOS transistors, hot electrons have enough energy to tunnel through a thin layer of gate oxide, as seen by the gate current or leakage current on the substrate. Hot electrons can travel from the channel or source region to the gate or substrate.

For example, in MOM transistors, when the gate has a positive charge and the circuit is on, the devices are designed so that the electrons pass through the conducting channel to the source. These hot electrons add nothing to the magnitude of the current flowing through the channel as intended, but instead go into the leakage current.

Attempts to correct or compensate for the effect of hot electrons in MOS transistors may include placing a diode to reverse the gate or other manipulations with the device (for example, using a lightly doped source or double-doped sources).

When electrons are accelerated in a channel, they receive energy along its entire length. This energy is lost in two different ways:

1. A carrier strikes an atom in a substrate. Then the collision creates a cold carrier and an additional pair of electron holes. In the case of nMOS transistors, additional electrons are collected by the channel and additional holes are removed by the substrate.
2. The carrier hits the Si-H bond and breaks the bond. An interface state is created and the hydrogen atom is released from the substrate.

The probability of getting into an atom or Si-H bond is random and the average energy of all elements involved in this process is also random in both cases.

This is the reason why the substrate current is monitored during hot electron injection strokes. A high substrate current means a large number of created electron-hole pairs and, thus, the efficiency of the breaking of Si-H bonds.

When the state of the interface is created, the threshold voltage changes and the subthreshold slope decreases. This leads to a decrease in current and degrades the operating frequency of the integrated circuit.

Advances in the manufacture of semiconductors and the ever-growing demand for faster and more complex integrated circuits (ICs) have led to the appearance of field effect transistors with a MOS structure scaled to small sizes.

However, it was not possible to proportionally scale the power sources used to work with these ICs, due to factors such as compatibility with previous generation circuits, noise levels, power requirements and delays, and the lack of scaling of threshold voltage, pre-threshold tilt, and stray capacitance .

As a result, internal electric fields increase in aggressively scalable MOS transistors, which gives additional advantages with increasing speeds of hot carriers (to saturate the speed characteristics), and, therefore, increasing the switching speed, ^[9] but also poses a serious reliability problem for long-term operation of these devices, since high fields induce the injection of hot carriers, which affects the reliability of the device.

Large electric fields in MOS transistors imply the presence of high-energy carriers called “hot carriers”. These hot carriers have sufficiently high energies and momentum, which allows them to be “introduced” from the semiconductor into the surrounding dielectric films, such as the gate and sidewalls coated with oxides, in the case of MOS transistors with silicon on the insulator .

The presence of mobile carriers in oxide films initiates dielectric degradation processes, which after a long period of time can drastically change the characteristics of the device. Accumulation of damage can lead to improper operation of the circuit due to changes in the key parameters of the transistor (shift of the threshold voltage due to damage to the oxide film). Degradation of the device due to accumulation of damage from the injection of hot carriers was called "degradation from hot carriers".

Thus, the useful life of circuits and integrated circuits with similar MOS structures depends on the life of the MOS structure itself. In order to make sure that integrated circuits made with the minimum geometry of the device do not have violations of their minimum useful life, the degradation of their components due to injection of hot carriers should be well studied. Failure to comply with the technology is precisely characterized by the effects of the lifetime of the components with the injection of hot carriers, and ultimately sloppiness in production affects business expenses such as a manufacturing guarantee, support costs, marketing promises, and estimated supply levels of an integrated circuit manufacturer.

The degradation of components with hot carriers is basically the same as that from exposure to ionizing radiation, known as the total dose of damage in semiconductors, experimentally studied in space systems under the influence of solar protons , electrons, x -rays and gamma rays .

Injection of hot media is the basis for the manufacture of non-volatile memory, similar to the EEPROM memory cell technology. Once the effect of the potential harm of hot carrier injection on the reliability of the circuit was recognized, several fabrication strategies were developed to reduce such harm without compromising circuit performance.

Non-volatile memory uses the principle of injection of hot media by deliberately “stuffing” it through the oxide layer of the shutter to charge the floating shutter . This charge changes the threshold voltage of the MOS transistor to represent the logical state “0”. An uncharged floating gate represents a logical state of “1”. Erasing non-volatile flash memory cells removes the stored charge during the Fowler-Nordheim tunneling process.

Due to the fact that oxide damage occurs during the normal operation of non-volatile memory, injection damage from hot media is one of the factors limiting the number of write-erase cycles. Since the ability to hold a charge and form trap holes in the oxide affects the ability to have different charge states “1” and “0”, the result of turning off non-volatile memory will be the ability to use it for a long time. The number of rewrite cycles at which “1” and “0” can no longer be distinguished, and determines the lifetime of non-volatile memory.

• Electromigration

• Hot Media Article. Archived copy (unopened) . Date of treatment December 8, 2014. Archived on February 13, 2014.
• IEEE International Reliability Physics Symposium - the main scientific and technical conference on the reliability of semiconductors, including the phenomenon of injection of hot carriers (Retrieved December 8, 2014)