2.11 Carrier generation and recombination


Table of Contents - Glossary - Study Aids -
In this section:
  1. Introduction - Simple recombination model
  2. Carrier generation due to light
  3. Band-to-Band recombination
  4. Trap-assisted recombination (Shockley-Read-Hall)
  5. Auger recombination
  6. Surface recombination
  7. Recombination mechanisms in a Quantum Well

2.11.1 Introduction - Simple recombination model

Recombination of electrons and holes is a process by which both carriers annihilate each other: the electrons fall in one or multiple steps into the empty state which is associated with the hole. Both carriers eventually disappear in the process. The energy difference between the initial and final state of the electron is given off. This leads to one possible classification of the recombination processes: In the case of radiative recombination this energy is emitted in the form of a photon, in the case of non-radiative recombination it is passed on to one or more phonons and in Auger recombination it is given off in the form of kinetic energy to another electron. Another classification scheme considers the individual energy levels and particles involved. These different processes are further illustrated with the figure below.


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Fig.2.11.1 Carrier recombination mechanisms in semiconductors

Band-to-band recombination occurs when an electron falls from its state in the conduction band into the empty state in the valence band which is associated with the hole. This band-to-band transition is typically also a radiative transition in direct bandgap semiconductors.

Trap-assisted recombination occurs when an electron falls into a "trap", an energy level within the bandgap caused by the presence of a foreign atom or a structural defect. Once the trap is filled it can not accept another electron. The electron occupying the trap energy can in a second step fall into an empty state in the valence band, thereby completing the recombination process. One can envision this process either as a two-step transition of an electron from the conduction band to the valence band or also as the annihilation of the electron and hole which meet each other in the trap. We will refer to this process as Shockley-Read-Hall (SRH) recombination.

Auger recombination is a process in which an electron and a hole recombine in a band-to-band transition, but now the resulting energy is given off to another electron or hole. The involvement of a third particle affects the recombination rate so that we need to treat Auger recombination differently from band-to-band recombination.

Each of these recombination mechanisms can be reversed leading to carrier generation rather than recombination. A single expression will be used to describe recombination as well as generation for each of the above mechanisms.

In addition there are generation mechanisms which do not have an associated recombination mechanism: generation of carriers by light absorption or a high energy electron/particle beam. These processes are also refered to as ionization processes. Impact ionization which is the generation mechanism associated with Auger recombination also belongs to this category. The generation mechanisms are illustrated with the figure below:


generati.gif

Carrier generation due to light absorption occurs if the photon energy is large enough to lift an electron from the valence band into an empty state in the conduction band, generating one electron-hole pair. The photon energy needs to be at least equal to the bandgap energy to satisfy this condition. The photon is absorbed in this process and the excess energy, Eph-Eg is added to the electron and the hole in the form of kinetic energy.

Carrier generation or ionization due to a high energy beam consisting of charged particles is similar except that the available energy can be much larger than the bandgap energy so that multiple electron-hole pairs can be formed. The high-energy particle gradually loses its energy and eventually stops. This generation mechanism is used in semiconductor-based nuclear particle counters. As the number of ionized electron-hole pairs varies with the energy of the particle, one can also use such detector to measure the particle energy.

Finally there is a generation process called impact ionization, the generation mechanism which is the counterpart of Auger recombination. Impact ionization is caused by an electron (hole) with an energy which is much larger (smaller) than the conduction (valence) band edge. The detailed mechanism is illustrated with the figure below:


impactio.gif

The excess energy is given off to generate an electron-hole pair through a band-to-band transition. This generation process causes avalanche multiplication in semiconductor diodes under high reverse bias: As one carrier accelerates in the electric field it gains energy. The kinetic energy is given off to an electron in the valence band, thereby creating an electron-hole pair. The resulting two electrons can create two more electrons which generate four more causing an avalance multiplication effect. Electrons as well as holes contribute to avalanche multiplication.

A simple model for the recombination-generation mechanisms states that the recombination-generation rate is proportional to the excess carrier density. It acknowledges the fact that no recombination takes place if the carrier density equals the thermal equilibrium value. The resulting expression for the recombination of electrons in a p-type semiconductor is given by:

and similarly for holes in an n-type semiconductor: where the parameter t can be interpreted as the average time after which an excess minority carrier recombines.

We will show for each of the different recombination mechanisms that the recombination rate can be simplified to this form when applied to minority carriers in a "quasi-neutral" semiconductor. The above expressions are therefore only valid under these conditions. The recombination rates of the majority carriers equals that of the minority carriers since in steady state recombination involves an equal number of holes and electrons. As a result the recombination rate of the majority carriers depends on the excess minority carrier density which are the limiting factor in this situation.

Recombination in a depletion region and in situations where the hole and electron density are close to each other can not be described with the simple model and the more elaborate expressions for the individual recombination mechanisms must be used.


2.11.2 Carrier generation due to light absorption

Carriers can be generated in semiconductors by illuminating the semiconductor with light. The energy of the incoming photons is used to bring an electron from a lower energy level to a higher energy level. In the case where an electron is removed from the valence band and added to the conduction band, an electron-hole pair is generated. A necessary condition for this to happen is that the energy of the photon, Eph, is larger than the bandgap energy, Eg. As the energy of the photon is given of to the electron, the photon no longer exists.

Assuming that each absorbed photon creates one electron-hole pair, the electron and hole generation rates are given by:

where a is the absorption coefficient of the material at the energy of the incoming photon.

2.11.3 Band-to-Band recombination

Band-to-band recombination depends on the density of available electrons and holes. Since both carrier types need to be available in the recombination process, the rate is expected to be proportional to the product of n and p. However in thermal equilibrium the recombination rate must equal the generation rate since there is no net recombination or generation. As the product of n and p equals ni2 in thermal equilibrium, the net recombination rate can be expressed as: where b is the bimolecular recombination constant.

2.11.4 Trap-assisted recombination - Shockley-Hall-Read recombination

The net recombination rate for trap-assisted recombination is given by: This expression can be further simplified for p >> n to: and for n >> p to: were

2.11.5 Auger recombination

Auger recombination involves three particles: an electron and a hole which recombine in a a band-to-band transition and give off the resulting energy to another electron or hole. The expression for the net recombination rate is therefore similar to that of band-to-band recombination but includes the density of the electrons or hole which receive the released energy from the electron-hole annihilation: The two terms correspond to the two possible mechanisms.

2.11.6 Surface recombination

Recombination at semiconductor surfaces and interfaces can have a significant impact on the behavior of devices. This is due to the fact that surfaces and interfaces typically contain a large number of recombination centers because of the abrupt termination of the semiconductor crystal which leaves a large number of electrically active dangling bonds. In addition the surfaces and interfaces are more likely to contain impurities since they are exposed during the device fabrication process. The net recombination rate due to trap-assisted recombination and generation is given by: This expression is almost identical to that of Shockley-Hall-Read recombination. The only difference is that the recombination is due to a two-dimensional density of traps, Nts, as the traps only exist at the surface or interface.

This equation can be further simplified for minority carriers in a quasi-neutral region. For instance for electrons in a quasi-neutral p-type region p >> n and p >> ni so that for Ei = Est the expression can be simplified to:

where the recombination velocity vs is given by:

2.11.7 Recombination mechanisms in a Quantum Well

There is conceptually little difference between the recombination and generation mechanisms in bulk material and those occuring in a quantum well. However it is convenient to rewrite the basic equations as a function of the carrier densities per unit area, rather than per unit volume when one deals with devices where the current is perpendicular to the plane of the quantum well. This leads to the following net recombination rates: for band-to-band recombination, for Shockley-Read-Hall recombination and for Auger recombination.

2.11.8 Minority carrier life time in a quasi-neutral region

Even though the net recombination rate can be described using only the simple model, we still need to be able to relate the minority carrier life time to the band-to-band, trap-assisted and Auger recombination constants. Since the total recombination rate equals the some of the recombination rates of the individual mechanisms, we can add the inverse of the life times corresponding to each process, yielding:
2.10 2.12

Bart J. Van Zeghbroeck, 1996, 1997