Nonequilibrium detector

A partial analogy may be drawn between the electron characteristics of nonequilibrium detectors and semiconductor lasers. Actually, in some aspects of their statistics and carrier transport the nonequilibrium detectors may be described as inverse lasers . To suppress carrier generation-recombination it is necessary to... 

It is only to be expected that some nonequilibrium detector stmctures have their analogs in semiconductor lasers. Exclusion detectors correspond to single-hetero-lasers, extraction devices to double-heterolasers, and magnetoconcentration detectors to lasers with the magnetoelectric photoeffect proposed by Marimoto et al. [331]. This inverse analogy is valid not only in electrical, but also in optical field, where e.g., resonant cavity (RCE) detector structures are connected with VCSEL lasers, and lasers with a PBG cavity with PCE (photonic crystal-enhanced) detectors. 

Nonequilibrium detectors share a number of similar or even identical characteristics, independently on a particular structure or process. This is a consequence of similar basic principles utilized for nonequilibrium operation. All of the Auger-suppressed devices utilize relatively strong external or internal fields to decrease minority carrier concentration in a given volume and thus operate in the mode of large deviations from equilibrium. The degree of suppression is proportional to the intensity of the applied fields. The structures with the decreased concentration of minority carriers are conventional intrinsic photonic detector strucmres. 

In this section, we consider the limits of operating parameters required for the function of a nonequilibrium detector. This is a general consideration in the sense that it does not depend on a particular mechanism of nonequilibrium suppression. 

Minimization of Joule effects is a serious problem in semiconductor microelectronic generally. Among the standard methods to reduce dissipation are the decree of the device volume, the improvement of the contact between active area and the substrate, mounting of a heat sink, optimization of the methods of detector mounting, etc. All of these methods can be utilized for nonequilibrium detectors. For instance, in the case of devices utilizing magnetic fields, one of the ways to reduce dissipation effects is to position a heat sink near the part of the sample with a local current density maximum. 

The goal of the presentation in this section is to pose a generalized model comprising all the types of nonequilibrium detectors presented in the literature until now, and applicable to potential novel devices. When deriving the model we start from the semiconductor equations in their general form (Maxwell s equations and Boltzmann s transport equation.). 

Transport Equation (Current Density in Nonequilibrium Detectors)... 

The literature published until now mentions only the nonequilibrium detectors utilizing the mechanisms 2 and 3, although, analogously to equilibrium devices, larger response speeds (i.e., wider bandwidths) may be expected in nonequUibrium devices utilizing e.g. Schottky junctions. 

In farther text we consider a one-dimensional model with galvanic (i.e., purely electrical) Auger process suppression. Here, we actually continue the consideration presented in this chapter. Since the term of magnetic induction is here equal to zero, carrier transport is described by (3.57) and (3.58). Thus, the generalized model of a nonequilibrium detector is reduced to the van Roosbroeck s model [353]. This well-known model is the basis of programs for simulation of practically all standard semiconductor devices. 

Figure 3.7 shows the electric field distribution in an n v nonequilibrium detector along the device with injected current density and dopant concentration as parameters. Equation (3.86) was used for the calculation. Recombination rate was calculated according to (1.85) and (1.86), assuming that the Auger and SR terms are negligible, which is consistent with the previous approximations. 

Because of the above reasons, the exclusion devices are today completely abandoned as a stand-alone solution for the suppression of Auger g-r processes. And yet, in this consideration they received a lot of attention. The reason for this is purely practical. The principles and approaches connected with this type of devices are directly or indirectly used for all of the other known types of nonequilibrium detectors. Besides this, the exclusion isojunction is one of the two basic elements of the strucmre of all photodiodes with extraction-based suppression of Auger processes, as it will be presented in the next Sect. 3.6. 

Extraction diodes are the most advanced of all nonequilibrium detectors. The fabrication of these detectors is fully compatible with standard narrow-bandgap semiconductor technologies and with standard detector circuitry. Their function does not cause interference with surrounding circuitry, as is the case with magnetoconcentration devices. 

Approximations usual for the calculation of the frequency response of conventional photonic detectors of optical radiation are not applicable to nonequilibrium detectors, since they operate exclusively under high bias levels. The treatment of magnetoconcentration devices is especially complex in this regard because of the requirement for high magnetic fields. Thus, the approach used here is based on direct numerical solution of the full mathematical-physical model of the device, similarly to the approach to the stationary solution. 

The concept of the magnetoconcentration-extraction combination (or, more accurately, magnetoconcentration-extraction-exclusion) is actually similar to the one described in the above section. Basically, one places a nonequilibrium extraction diode into a crossed electric and magnetic field, as shown in Fig. 3.75. This device could potentially offer the best performance of all nonequilibrium detectors, since it further enhances the operation of the extraction photodiode which already furnished superior performance. 

On Application of Nonequilibrium Detectors LWIR Free-Space Optics... 

Special care is dedicated to the consideration of properties common to all types of nonequilibrium devices. Based on this a general model is defined to be utilized in simulation of any nonequilibrium detector, including the novel ones. The model is derived for isothermal processes, but can be extended to nonisotiiermal ones. When deriving the model, special care is dedicated to the limits of the model and the generality of the introduced approximations. 

The use of ACRT at SELEX Sensors and Airborne Systems Infrared Ltd also made possible the fabrication of early 2-dimensional arrays of photodiodes with a high degree of uniformity of response. This type of material was also used to demonstrate nonequilibrium detector operation. The quality and costs of epitaxial material are improving continuously but it will be some years yet before bulk-grown material ceases entirely to be produced, at least for long-wavelength photoconductive applications. 

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