The semiconductor devices that utilize the effect of carrier recombination to produce the electromagnetic radiation are widely used in electronics and optoelectronics. Typically, to achieve efficient carrier recombination, the portions of semiconductor materials are brought in contact that provide two different general conductivity types, known as electron and hole conductivity types. Due to the mutual carrier penetration through the said contact caused by diffusion and/or drift in the externally applied electric field, the two types of carriers are simultaneously present in the region of the semiconductor device adjacent to the said contact, also referred to as active region, where they can recombine with each other under the emission of energy, in a form of either electromagnetic radiation (photon) or elastic crystalline lattice vibration (phonon). For a large class of devices, the said photon generation is the useful output.
In most of the cases, the photons are produced most effectively if the electron and hole interact and recombine as “free” carriers, resulting in so-called band-to-band recombination. This recombination leads to the generation of a photon with energy of (slightly higher than) the electronic bandgap of the portion of semiconductor comprising the said active region of the semiconductor device. This photon can be effectively absorbed by the semiconductor material having the same or lower bandgap as/than the said active region of the device. To prevent this absorption, efficient photon generating semiconductor devices are usually designed in such a way that the active region comprises the semiconductor material of the lowest bandgap all over the said device. The semiconductor device comprising the regions of different semiconductor materials is usually referred to as a heterostructure. Thus, effective photon generation device is usually a heterostructure device.
Exactly one electron and one hole contribute to a single act of recombination with generation of a photon. The probability of the recombination is proportional to both electron and hole concentrations at a given position within the active region of the semiconductor device. The steady-state concentration of the carriers during the device operation is achieved by detailed equilibrium of the electron and hole concentrations vanishing due to recombination and the electron and hole electric currents into and from the said active region.
The narrower bandgap semiconductor material usually has both conduction band and valence band within the energy range of the bandgap of the surrounding higher bandgap semiconductor material. Thus, the active region of a properly designed photon generating semiconductor device normally comprises one or several potential wells for both types of carriers. This helps maintain the carrier confinement within the said active region, increasing the photon generation efficiency of the device. However, the ability of the narrow band gap material comprising the active region to confine the carriers becomes weak with increasing carrier concentrations. This happens mainly because higher electric currents are needed to produce higher steady state carrier concentrations; these higher currents require higher electric fields in and near the active regions; carriers receive extra energy from the higher electric fields and obtain higher probability to drift into higher bandgap regions, thus leaving or passing through the active region. Typically, due to lower effective mass, the electrons gain more energy from the electric fields and are more likely to leave the active region. Due to current continuity, the number of the electrons and holes entering the active region in unit time is equal; as a result of the electrons having higher probability to escape, the symmetry of carrier concentration in the active region vanishes, and the efficiency of photon generation degrades. In addition, at high injection levels, the effect of a limited rate of radiative recombination through any centers involved in the desired radiative recombination may become relevant. This for example includes the considerations of a dipole matrix element reduced by the presence of an electric field within the quantum wells. It also may include the finite rate of excess carrier relaxation and thermalization processes required for carriers to reach the centers of desired radiative recombination.
In order to prevent the carrier escape from the active region, additional barrier regions are sometimes introduced adjacent to the said active region. Those barrier regions are usually designed to block the electron escape more effectively and, therefore, are usually positioned within the portion of the said semiconductor device having hole conductivity type. The pay-off of the barrier regions introduction is usually the degradation of the hole injection into the active region and increase in the electric potential drop across the device necessary to achieve the desired level of photon generation. In addition, as a result of the hole injection degradation, the active region becomes flooded with the electrons and, therefore, is turned into the electron (n-) conductivity type, at least for the conditions corresponding to the photon generation.
In spite of the barrier regions introduction, the carriers' leakage out of the active region still becomes severe at high carrier concentrations in the said active region. It happens mainly because the external electric potential applied to the device drops mostly across the less conductive regions of the device, which are in the case described above the barrier regions. At high enough applied electric potentials, the barriers become punched through for the electrons escape. This limits the efficiency of the photon generation by the semiconductor device at high currents and even leads to the saturation of the total number of photons that the said device is able to produce.
The present invention solves the problems of the electrons leakage from the active region of the photon generating semiconductor device and of balancing the electron and hole concentrations in the active region(s) of the said device by taking independent control of the electric potential applied between the active region of the device and the regions of the said device of different conductivity types.