# What is electron trapping

I am reading an article in which they consider a model for charge transport in dielectric.

The following figure can summarize the different procedures done in this manner:

I am new in the domain of electronics, plus english is not my native language, for that I am facing a problem understanding some scientific expressions, that can not be translated word by word. I try to find some definitions in wikipedea, but for some expressions I failed due to confussion in the domain (same word may have different meaning according to the domain of study).

so would some one help me understand some concepts: What is meaning of :

1. Trapping and detrapping ?
2. why dielectrics have traps ?
3. Charge injection ?
4. Barrier height for injection?

Thank you in advance for any help. Any suggestion for references on the subject is highly appreciated.

• This is more a question about the physics of insulators than about electronics. You should ask at the physics SE. – Janka Sep 29 '17 at 17:35
• In general, those traps correspond to impurities of the material or imperfections of the crystal lattice which increases probability density of electrons. – Janka Sep 29 '17 at 17:41

Very simplified discussion.

Here I deal just with electrons. Similar discussion is valid for holes

The band diagram shows the energy values an electron can assume. You have the forbidden band, the conduction band (first band empty or not completely filled), and the valence band (last band entirely filled by electrons). We call the difference between the bottom of the conduction band and the top of the valence band "energy gap".

Traps, and trapping.

Defects of any kind will introduce new energy levels. If these energy levels are in the energy gap, they will act like traps. In fact, an electron in the conduction band that comes in the proximity (it's called "cross section") of that trap, will probably get captured by that energy level as it has a lower energy (the energy difference will be likely lost by phonon emission: lattice vibration, i.e. heat).

Detrapping:

How long will an electron stay there? It depends on the trap depth (not only though). The shallower the trap, the higher the chance that an electron can gain enough energy (e.g. due to phonon adsorption) to come out of that trap. An electron can also come out by quantum tunneling.

Why insulators (and semiconductors too) have traps:

Insulators and semiconductors may have traps. These are induced by grain to grain boundaries (in polycrystalline materials), lattice imperfections, unsaturated/broken bonds, impurities, etc.

Charge injection:

It means when a contact (or another material) injects electrons/holes to a semiconductor (or even an insulator, as it occurs in floating gate cells). An electron can be injected into a material only if its energy is larger than the minimum energy it can assume on that material. This does not mean that if the conduction band of the "source material" is lower than the conduction band of the "destination" material, injection cannot occur. Thermoionic emission aids injection. Tunneling aids injection. And carriers can become "hot" i.e. they can gain a lot of energy, so their energy will be larger than the conduction energy of the destination material. Hot electron injection is used in NOR flash memories, to inject electrons into the floating gate (through the oxide).

Barrier height for injection:

It is the difference of the "destination material" conduction band level and the "source material" conduction band level. The higher the barrier height, the larger the energy difference, i.e. the less likely that injection will occur.

• Unfortunately i don't have here the reference about traps (especially in the oxide), but for a general introduction, this might be enough: S.M. Sze "Semiconductor Devices, Physics and Technology", John Wiley & sons. – next-hack Sep 30 '17 at 6:58
• Of course they do! Think about organic electronics! Conductivity depends on the mobility, and charge density. SiO2 has an electron mobility quite "decent" (between 20 and 40 $cm^2/(Vs)$ if I recall correctly), but it has a huge band gap (9eV) so the charge density is very low. So it's almost a perfect insulator. But if you manage to inject carriers into it (e.g. using a very low work-function contact, and a suitable injection layer. Or by hot carrier injection) then it conducts. Or if you stress and create a lot of traps, it becomes "slightly" conductive, by trap-assisted tunneling. – next-hack Sep 30 '17 at 7:06
• Electrons in atoms can only assume discrete energy levels. If you join two atoms to form a molecule, they share the electrons, but the energy level split by a small amount. If you join "a lot of" atoms to form a crystal, the levels split, but the difference is so small that you can assume the energy levels are contiguous. However, there will be some forbidden bands. Therefore you have many bands. At 0 K, those bands at the lowest energy are completely filled. Those, with energy larger than the Fermi level, will be empty. The valence band is the last completely filled. (continues) – next-hack Sep 30 '17 at 10:00
• The conduction band is the lowest (in terms of energy) band not completely filled. It has this name, because, since it is not completely filled, conduction (of electrons) can occur. In fact an electron needs a free (not occupied by another electron) energy state to jump into, in order to move. I know this is explained in the "how are you doing" way, but for better and more precise explanation, you should refer to some literature :) – next-hack Sep 30 '17 at 10:04
• Thank you for this thorough explanation, and for your consideration and patience in answering my questions, all the respect. – Nizar Sep 30 '17 at 10:09