Abstract:
A semiconductor having a an n-type material and a p-type material, wherein the n-type material and p-type material are joined to form a space-charge-free p-n junction. The energy of the Fermi-level of the n-type material is equal to the energy of the Fermi-level of the p-type material. This allows for the pre-alignment of the Fermi-levels of the n-type and the p-type materials. The semiconductor has minimal or no g-r noise. The semiconductor can be operated at T BLIP  in the range of about 220° to about 240° K.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/916,678, filed May 8, 2007, the subject matter of which is hereby incorporated by reference in its entirety. 
    
    
     This invention was made with government support under UR Award Number 048783-003 from the Office of Naval Research. The government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductors, and more particularly to p-n junctions having improved signal-to-noise ratio. Even more particularly, the invention is directed to space-charge-free p-n junctions in semiconductor devices, materials and methods associated therewith, and applications thereof. 
     BACKGROUND 
     Semiconductors that detect light or other electromagnetic energy are called photo-detectors. There are several varieties including, but not limited to, optical detectors, chemical detectors, photoresistors, photovoltaic cells, photodiodes, and phototubes. Photo-detectors are used in a variety of applications including imaging. A specific type of photo-detector sensitive to the infra-red (IR) wavelengths of light is also known as an IR detector. 
     IR detectors are used in a variety of applications, and in particular in the military field where they are used as thermal detectors in night vision equipment, air borne systems, naval systems and missile systems. 
     The p-n junction is the foundation of semiconductors. It has been used as the basic building block for a wide variety of electronic and optoelectronic devices, such as diodes, transistors, optical detectors, and semiconductor lasers. It has been thought that a fundamental characteristic of the p-n junction is that it requires a depletion region, also known as a space charge region. The space charge region of conventional mid-IR detectors is a dominant source of defect-produced noise, also known as Shockley-Read-Hall (SRH) noise or generation-recombination (g-r) noise. This noise is produced by the creation of current carriers via thermal excitation through mid-gap defects in depletion regions. Fluctuations in g-r current are often the dominant source of noise in infrared photodiodes. G-r noise determines the amount of cooling required to reach background-limited-infrared-photodetection (BLIP) conditions in certain photodetectors. G-r noise can be reduced to minimal levels by cooling, but cooling is inconvenient and expensive, adds weight and bulk to the detector, and reduces the detector lifetime. If g-r noise could be eliminated, BLIP temperatures (T BLIP ) of various photodetectors could be increased. An example of a T BLIP  for a conventional photodetector is in the range of about 100° K. to about 120° K. 
     It is a primary object of the invention to provide a space-charge-free p-n junction in semiconductors. It is another object of the invention to provide p-n junctions having minimal or no g-r noise. It is a further object of the invention to provide p-n junctions that can function with increased T BLIP . 
     SUMMARY OF THE INVENTION 
     These and other objects and advantages are accomplished by a semiconductor having a an n-type material and a p-type material, wherein the n-type material and p-type material are joined to form a space-charge-free p-n junction. In one embodiment of the invention, prior to the joining of the n-type material and p-type material at a p-n junction, the Fermi-levels are pre-aligned. In one aspect of this embodiment, the energy of the Fermi-level of the n-type material is equal to the energy of the Fermi-level of the p-type material. This allows for the pre-alignment of the Fermi-levels of the n-type and the p-type materials. The semiconductor has minimal or no g-r noise. The semiconductor can be operated at T BLIP  in the range of about 220° K. to about 240° K. 
     In another embodiment of the invention, a method of making a semiconductor having a space-charge-free p-n junction is provided. In one aspect of the method, an n-type material is selected. The Fermi energy of the n-type material is known. A p-type material is then selected such that the Fermi energy of the p-type material can be adjusted or “tuned” to equal the Fermi energy of the n-type material. 
     In yet another embodiment of the invention, a semiconductor having a p-i-n junction is provided. In one aspect of this embodiment, an intrinsic material is inserted between the n-type and p-type materials to prevent or suppress leakage current. The energy level of the valance band of the intrinsic material is not lower than the energy level of the valence band of the n-type material. 
     In a further embodiment of the invention, a semiconductor having a space-charge-free p-n junction is provided wherein the Fermi levels of the p-type and n-type materials are not initially aligned. The initial misalignment of the Fermi levels can be compensated by the application of a voltage, thereby producing a space-charge-free condition under an applied voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an energy band profile of a conventional p-n junction prior to the n-type and p-type materials being joined; 
         FIG. 2  is an energy band profile of a conventional p-n junction after the n-type and p-type materials have been joined; 
         FIG. 3  is an energy band profile of a space-charge-free p-n junction prior to the n-type and p-type materials being joined, according to an exemplary embodiment of the invention; 
         FIG. 4  is an energy band profile of a space-charge-free p-n junction after the n-type and p-type materials have been joined, according to an exemplary embodiment of the invention; 
         FIG. 5  is a graphical representation of dominant noise sources of conventional photodiodes with respect to reciprocal temperature; 
         FIG. 6  is a graphical representation of dominant noise sources of space-charge-free photodiodes with respect to reciprocal temperature, according to an embodiment of the invention; 
         FIG. 7  is a graphical representation of the Fermi energy of AlGaAsSb; 
         FIG. 8  is an energy band profile of a space-charge-free p-n junction exhibiting leakage current; and 
         FIG. 9  is an energy band profile of a space-charge-free p-i-n junction limiting leakage current, according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention provides a p-n junction having no space charge region or depletion layer. This space-charge-free p-n junction is useful in a variety of semiconductor devices. 
     Reference is made to  FIG. 1 , which shows the transfer of electrons that occurs in conventional or prior art p-n junctions. An n-type material  10  is shown positioned next to a p-type material  12 . Each material is shown with its conduction band E c , valence band E v , and Fermi level E f . In order to equalize the Fermi levels at the p-n junction, a transfer of elections is necessary. The arrows show movement of the electrons from the n-type material  10  to the p-type material  12 .  FIG. 2  illustrates the joined n-type and p-type materials, wherein a space charge region  14  is created by the movement of electrons. Band-bending of the conduction and valence bands also occurs in order to align the Fermi levels of the two materials. The space charge region  14  created by the shift of electrons has been known to create noise, thereby degrading the device performance. 
       FIG. 3  illustrates the materials used to fabricate the p-n junction according to an exemplary embodiment of the invention. The Fermi level of the n-type material is equal to the Fermi level of the p-type material. Since the Fermi levels of the materials are equal and aligned prior to actual joining of the materials, the creation of the space-charge region is prevented.  FIG. 4  illustrates the p-n junction of the joined materials with the absence of a space-charge region. The Fermi level is consistent from the n-type region to the p-type region. The energy levels of the valence band edge and conduction band edge of the n-type region  16  are lower than the energy levels of the valence band edge and conduction band edge of the p-type region  18 . Furthermore, there is no bending of the valence and conduction bands, since the Fermi levels are pre-aligned. 
       FIGS. 5 and 6  show dominant noise sources with respect to reciprocal temperature.  FIG. 5  shows temperature dependence of detector noise of a prior art photodiode. As shown in the graph, SRH or g-r noise is present from the existence of a depletion region. 
       FIG. 6  illustrates temperature dependence of detector noise of a apace-charge-free photodiode according to an exemplary embodiment of the invention. The noise produced in the photodiode of  FIG. 6  increases the T BLIP  in comparison to the prior art photodiode, which has a lower T BLIP . 
     In another aspect of the invention, a method of fabricating the space-charge-free p-n junction is provided. The first step is to select the starting material for either the n-type material or the p-type material. The next step is to determine and select the second material based on the first material selected. The following non-limiting example illustrates an exemplary embodiment of the invention. In this example, the n-type material is selected first. Examples of n-type materials include, but are not limited to, InAs, InSb and HgCdTe. In this example, InAs is chosen to make a mid-wave infrared (MWIR) detector. Next, a candidate for the p-type material is determined. In this case, alloy semiconductors are considered so that the Fermi energy can be modified or “tuned” by varying the composition. It may be advantageous that the lattice of the p-type material matches the lattice of the n-type material in order to maintain the crystal structure of the material. A quaternary compound is chosen so that the composition can be varied to simultaneously tune the Fermi energy while maintaining a lattice-match to the InAs. It was found that the compound, Al z Ga 1-z As 1-y Sb y  was a suitable match for InAs.  FIG. 5  illustrates the Fermi level of the p-material, Al z Ga 1-z As 1-y Sb y  with respect to varying the composition of the material. As shown in the graph, the composition consisting of Al 0.79 Ga 0.21 As 0.14 Sb 0.86  has a Fermi level equal to the Fermi level of InAs. The value of z can range from about 0.7 to about 0.9 and the value of y can range from about 0.85 to about 0.87. 
     Electron leakage may occur because the top of the valence band on the p-side is immediately adjacent to the bottom of the conduction band on the n-side. In an effort to suppress leakage current that may occur at the p-n junction, it was found that the insertion of an intrinsic layer at the p-n junction to provide a p-i-n junction proved beneficial.  FIG. 8  illustrates leakage current at the p-n junction.  FIG. 9  illustrates a p-i-n junction with an intrinsic layer  20 . Intrinsic layer  20  is selected so that there is near-zero valence band offset with respect to the n-type region. The intrinsic layer  20  separates the top of the valence band on the p-side from the bottom of the conduction band on the n-side. The intrinsic layer is chosen with band edge alignments that do not impede the movement of the minority carriers from the active side across the junction. The requirements of the undoped intrinsic layer will vary with device applications. For example, for a photodiode that is designed so that the light-absorbing layer is the n-side, the intrinsic layer will be designed with band alignments to allow the n-side minority carriers (holes on the n-side) to flow across the device. If the photodiode is designed so that the light-absorbing layer is the p-side, then the intrinsic layer will be designed to allow p-side minority carriers (electrons on the p-side) to flow across the device. 
     In many applications of p-n junctions, such as in photodiodes, a second deleterious effect in conventional p-n junctions is leakage current along the surfaces. Surface leakage currents are more severe problems in semiconductors with low energy bandgaps. The space-charge-free p-i-n junction uses both low bandgap and high bandgap semiconductor materials. The presence of the high bandgap semiconductor will significantly reduce the surface leakage, compared to conventional p-n junctions. Accordingly, the space-charge-free p-i-n junction combines the benefits of small bandgap semiconductors (detection of long wavelength light) with the benefits of large bandgap semiconductors (reduction of surface leakage current). 
     In yet another embodiment of the invention, a space-charge-free p-n junction is provided wherein the Fermi levels of the p-type and n-type materials are not initially aligned. The initial misalignment of the Fermi levels can be compensated by the application of a voltage, thereby producing a space-charge-free condition under an applied voltage. For example, if, before joining of the p-type and n-type materials, the Fermi level of the p-type material is higher than that of the n-type material by an amount, E f,p −E f,n , then application of a reverse voltage equal to 
                 E     f   ,   p       -     E     f   ,   n         e         
would create the space-charge-free condition, wherein
 
     F f,p  is the Fermi level of the p-type material; 
     E f,n  is the Fermi level of the n-type material; and 
     e is the charge on an electron. 
     The p-n and p-i-n junction embodiments of the present invention are useful in a variety of devices including, but not limited to, short-wave, mid-wave, long-wave, very long-wave detectors, type II (strained) superlattice infrared detectors, photovoltaic devices, and thermal imagers (aka infrared cameras). Applications may advantageously include, but are not limited to, devices for the Department of Defense and the Department of Homeland Security. The devices using the p-n and p-i-n junctions herein will be lighter, less expensive, and longer lasting than provided by current technology. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended embodiments.