In the field of semiconductors, diodes are not exactly ideal for cut-off when they are reverse cut-off.When subjected to back pressure, there is some small leakage of current from the cathode to the anode. This current is usually very small and the higher the counter voltage, the higher the leakage current and the higher the temperature, the higher the leakage current. A large leakage current results in large losses, especially in high voltage applications.
The cause: From the internal structure of the semiconductor material, it is the reverse electric field E generated by the applied reverse voltage in the potential barrier region of the PN junction that is greater than the electric field E formed by the diffused charge in the potential barrier region. this results in a reverse leakage current through the PN junction. The thinness of the barrier region, and the magnitude of the applied reverse voltage together determine the magnitude of the leakage current.
In a laser chip, which is also a type of diode, when a forward bias is applied to its terminals, electrons flow from N to the active region, but also some electrons will have enough energy to spill out of the active region and flow to the P region, and these currents that flow to P are called leakage currents. Leakage currents can be divided into two parts, one as described above and the other possessing sufficient thermal energy so as to exceed the potential barrier. The other part is due to a small amount of electrons within the P-energy itself penetrating or drifting into the P-contact region, forming a leakage current. Leakage currents do not contribute to luminescence and only make the internal quantum efficiency of the device less efficient. It is also very sensitive to temperature and the leakage current increases rapidly as the temperature rises.
Also for short wavelength lasers are more prone to leakage than longer wavelength lasers.
As shown above, the energy gap difference between the conductive bands of AlGaIn phosphide chips with a wavelength of 690nm is 400meV, but the difference between AlGaIn phosphide with a wavelength of 650nm is only 320meV, making it easier for electrons to escape. Several ways to reduce leakage in short-wavelength AlGaN phosphide: 1) Increase the doping concentration of the P-cladding. Increasing the conductive energy gap difference makes it more difficult for electrons to cross the potential. 2) Increasing the number of quantum wells makes it possible to accommodate more carriers and reduce current spillover; as the number of quantum wells increases, more current must be injected to generate laser, and thus the critical current increases.
