The ideality factor of a diode shows how closely the diode follows the ideal diode equation. Practically, no diode follows the ideal diode equation. However, there should be a parameter or factor that should tell the degree of variation in the diode equation. Thus, the ideality factor is the parameter for knowing the variation of the diode equation from the ideal diode equation.
Recombination Mechanisms
The recombination process has a dominating effect on direct bandgap semiconductors. In this process, the extrinsic semiconductors equilibrate excess charge carriers through the bringing together and destroying of oppositely charged carriers. Thus, the recombination process causes a release of energy in the form of photon or thermal lattice vibrations.
The expression of the ideal voltage equation assumes that the recombination process occurs via band to band or via traps in a large area, and not via the junction. Therefore, as per this concept of recombination, the ideal diode equation with unity ideality factor(n) is given below.
However, the recombination process not only takes place in a larger area but also in other areas as well. This way, the recombination process causes the diode not to follow the ideal diode equation.
In an ideal diode, n=1, the slope is about 18mv per octave (2X) of current change or 60mv per decade (10X) of current change at 27 degree C. whereas, in a non-ideal diode with a n=2, the slope is about 36mv per octave (2X) of current change or 120mv per decade (10X) of current change at 27 degree C.
The ideal diode equation is one that has ideality factor unity. The following table shows the ideality factor of a diode having different recombination processes.
| Recombination Type | Ideality factor | Description |
|---|---|---|
| SRH, band to band (low-level injection) | 1 | Recombination limited by minority carriers. |
| SRH, band to band (high-level injection) | 2 | Recombination limited by both carrier types. |
| Defect assisted (low-level injection) | 1 | Recombination limited by minority carriers. |
| Defect-assisted (high-level injection) | 2 | Recombination limited by both carrier types. |
| Auger | 2/3 | Two majority and one minority carrier are required for recombination. |
| Depletion region (junction) | 2 | two carriers limit recombination. |
Factors affecting the ideality factor of Diode
The following factors affect the ideality factor of the diode.
- Electron Drift
- Diffusion
- Carrier Combination in the depletion region
- Doping Level
- Manufacturing Process
- Purity of the material
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