Charge injection and radiative recombination

CHARGE INJECTION AND RADIATIVE RECOMBINATION

Electrons and holes can be injected into the conduction and valence band in a number of way. The light incident on the material and the absorption of photons creates electron-hole pairs.

We also use an external battery bias in a p-n diode also inject electrons and holes.

The electrons and holes will recombine with each other and the electron in the conduction band will return to the valence band.

This recombination process can be made in two processes. They are (i) radiative processes and (ii) non-radiative processes.

In the radiative process the e-h pair recombines and a photon is emitted. The is the inverse of the photon absorption process.

Electron-hole pairs can also recombine without emitting light. Instead, they may emit (i) heat or (ii) a photon or (iii) a long-wavelength photon together with a phonon. Such processes are non-radiative processes.

As the electrons and holes are "pumped" into the semiconductor they re-combine through the process of spontaneous emission. This process does not require photons to be present for the photon emission process to occur.

The spontaneous recombination rate is quite important for both electronic and optoelectronic devices.

Types of carrier Injections

(i) Minority Carrier Injection

If n >> p and the sample is heavily doped n-type, recombination rate is proportional to hole density.

Thus the recombination rate is proportional to the minority carrier density (holes in the case).

(ii) Strong Injection

This case is important when a high density of both electrons and holes is injected.

The rate of recombination proportional to majority charge carrier.

(iii) Weak Injection

In this case, the rate of recombination is very low.

(iv) At low injection, the electrons have a low probability to find a hole with which to recombine.