CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a filing under 35 U.S.C. 371, which claims priority to International Application Serial No. PCT/US03/03181, filed Feb. 3, 2003, which claims the benefit of U.S. Provisional Application No. 60/353,849, filed Feb. 1, 2002. 


   FIELD OF THE INVENTION 
   The present invention relates to a semiconductor-based photodetector, and in particular to a high-speed, broad bandwidth photodetector having enhanced absorption characteristics. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   There is a well-known tradeoff between high speed and sensitivity in a photodetector. High bandwidth signal detection requires a short transit time of the carriers and thus a thin absorption layer. However, the geometrical constraints on the absorption layer thickness results in a reduced absorption and lower responsivity. 
   One type of semiconductor-based photodetector is termed a p-i-n junction diode, or a PIN diode. This type of structure is generally composed of a number of solid semiconductive sandwiched together in an epitaxial structure. In particular, a p-type semiconductor material and an n-type semiconductor region are separated by an intrinsic semiconductor. 
   In a PIN diode, the depletion layer extends into each side of junction by a distance that is inversely proportional to the doping concentration. Thus, the p-i depletion layer extends well into the intrinsic material, as does the depletion layer of the i-n junction. Accordingly, a PIN diode functions like a p-n junction with a depletion layer that encompasses the entirety of the intrinsic material. The primary advantages inherent to this structure are twofold. First, the addition of the intrinsic layer permits a fractional increase in the amount of light to be captured by the diode. Secondly, due to the extended depletion layer, the PIN diode has a very small junction capacitance and corresponding fast response. 
   Most attempts at increasing the speed of PIN diodes have focused on reducing the capacitance at the junction. At least one proposed design has included an undoped drift region for this purpose, effectively increasing the size of the intrinsic portion of the diode. Although this solution is suitable for decreasing the junction capacitance, it unfortunately increases the transit time for the carriers and thus reduces the response time of the photodetector. As such, there is a need in the art for an improved photodetector that strikes the proper balance between capacitance and response time, while increasing the responsivity of the device. 
   Accordingly, the present invention includes a photodiode having a first p-type semiconductor layer and an n-type semiconductor layer coupled by a second p-type semiconductor layer. The second p-type semiconductor layer has graded doping along the path of the carriers. In particular, the doping is concentration graded from a high value near the anode to a lower p concentration towards the cathode. By grading the doping in this way, an increase in absorption is achieved, improving the responsivity of the device. Although this doping increases the capacitance relative to an intrinsic semiconductor of the same thickness, the pseudo electric field that is created by the graded doping gives the electrons a very high velocity which more than compensates for this increased capacitance. Further embodiments and advantages of the present invention are discussed below with reference to the figures. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an energy band diagram of a pin photodiode in accordance with the present invention. 
       FIG. 2  is a cross-sectional view of a basic configuration of a pin photodiode in a surface illuminated structure in accordance with the present invention. 
       FIG. 3  is a graph representing the relationship between the electric field and the electron velocity according to an aspect of the present invention. 
       FIG. 4  is a graph representing the relationship between the doping concentration and the relative depth of a semiconductor layer of the present invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In accordance with a preferred embodiment of the present invention, an epitaxial structure is provided for photoconductive purposes. The photoconductive structure is a modified PIN diode that is optimized for increased performance through an enhanced layer having a graded doping concentration. The particulars of the structure and method of manufacture of the present invention are discussed further herein. 
   Referring to  FIG. 1 , an energy band diagram of a PIN photodiode  10  shows the relative energy levels of the semiconductor materials that form the photodiode  10 . In particular, the photodiode  10  is comprised of a group of semiconductor materials, including a first p-type semiconductor layer  14 , a second p-type semiconductor layer  16 , and an n-type semiconductor layer  18 . An anode layer  12  is shown adjacent to the first p-type semiconductor layer  14  to collect holes. 
   The first p-type semiconductor layer  14  is selected from a group comprising tertiary semiconductors, or group III–V semiconductors. Accordingly, the first p-type semiconductor layer  14  is either two elements from group III combined with one element from group V or the converse, two elements from group V combined with one element from group III. A table of representative groups of the periodic table is shown below. 
   
     
       
             
             
             
             
           
         
             
                 
             
             
               GROUP II 
               GROUP III 
               GROUP IV 
               GROUP V 
             
             
                 
             
           
           
             
               Zinc (Zn) 
               Aluminum (Al) 
               Silicon (Si) 
               Phosphorus (P) 
             
             
               Cadmium (Cd) 
               Gallium (Ga) 
               Germanium (Ge) 
               Arsenic (As) 
             
             
               Mercury (Hg) 
               Indium (In) 
                 
               Antimony (Sb) 
             
             
                 
             
           
        
       
     
   
   In the preferred embodiment, the first p-type semiconductor layer  14  is InAlAs. However, it is understood that the first p-type semiconductor layer  14  may be any tertiary semiconductor that provides the necessary bandgap for optimized operation of the photodiode  10 . 
   The n-type semiconductor layer  18  is also selected from a group comprising tertiary semiconductors, or group III–V semiconductors. As before, the n-type semiconductor layer  18  is either two elements from group III combined with one element from group V or the converse, two elements from group V combined with one element from group III. In the preferred embodiment, the n-type semiconductor layer  18  is InAlAs. However, it is understood that the n-type semiconductor layer  18  may be any tertiary semiconductor that provides the necessary bandgap for optimized operation of the photodiode  10 . 
   The second p-type semiconductor layer  16  is also selected from a group comprising tertiary semiconductors, or group III–V semiconductors. In the preferred embodiment, the second p-type semiconductor layer  16  is InGaAs with a graded doping concentration. However, it is understood that the second p-type semiconductor layer  16  may be any tertiary semiconductor that provides the necessary low bandgap for optimized operation of the photodiode  10 . 
   In order to achieve a graded doping concentration, the second p-type semiconductor layer  16  is not doped in a typical manner. In general, a p-type semiconductor is fabricated by using dopants with a deficiency of valence electrons, also known as acceptors. The p-type doping results in an abundance of holes. For example, in a type III–V semiconductor, some of the group III atoms may be replaced with atoms from group II, such as Zn or Cd, thereby producing a p-type material. Similarly, as group IV atoms act as acceptors for group V atoms and donors for group III atoms, a group IV doped III–V semiconductor will have an excess of both electrons and holes. 
     FIG. 2  is a cross-sectional view of a basic configuration of a photodiode  10  in a surface illuminated structure designed in accordance with the present invention. A substrate layer  20  is provided for growing the semiconductor structure. The n-type semiconductor layer  18  is deposited upon the substrate. The first p-type semiconductor layer  14  and the second p-type semiconductor layer  16  are deposited in a manner such that the second p-type semiconductor layer  16  is directly adjacent to the n-type semiconductor layer  18 . As before, an anode layer  12  is deposited on the first p-type semiconductor layer  14  for collecting holes. Also shown is a cathode layer  22 , or n-type contact layer, for collecting electrons. 
   As noted, it is a feature of the second p-type semiconductor layer  16  that it includes a graded doping concentration. The presence of dopants in the second p-type semiconductor layer  16  is controlled in order to optimize the performance of the photodiode. A first concentration  15  is located near the first p-type semiconductor  14 , and a second concentration  17  is directly adjacent to the n-type semiconductor  18 . Preferably, D is between 800 and 1,000 angstroms deep, i.e. the dimension parallel to the travel of the carriers. 
   In the preferred embodiment, the first concentration  15  is greater than the second concentration  17 . In particular, the first concentration  15  is located at a position x o  and defines a dopant concentration P o . A preferred doping concentration gradient is governed by the following equation: 
                 p   =       p   o     ⁢     ⅇ       -   x     D                 (   1   )               
over the depth D of the second p-type semiconductor layer  16  for all x and D greater than zero. A generic representation of the dopant concentration P is shown in  FIG. 4 .
 
   The graded doping structure of the second p-type semiconductor layer  16  results in improved performance of the photodiode  10 . During operation, incident light is absorbed in the second p-type semiconductor layer  16  of the photodiode  10 . The light that is absorbed in the second concentration  17  part of the second p-type semiconductor layer  16  produces electrons and holes which drift to the anode  12  and cathode  22  under the influence of the large drift electric field. Although this is the usual situation in standard uniformly low doped absorber PIN photodetectors, in the present invention, the photoresponse of the carriers is more complex. 
   The electrons generated in the second concentration  17  part of the second p-type semiconductor layer  16  reach the cathode with their saturation velocity and are collected. The holes generated in the second concentration  17  part of the second p-type semiconductor layer  16  travel to the anode  12 , thus entering the first concentration  15  where the concentration of dopants is relatively high and where they are collected, thus ending their transit time. 
   By way of comparison, the light that is absorbed in the first concentration  15  part of the second p-type semiconductor layer  16  also produces electrons and holes. In this case however, the holes are readily collected in the first concentration  15  and thus do not add substantially to the transit time of the carriers or reduce the bandwidth of the photodiode  10 . Accordingly, insofar as the holes are concerned, the graded doping concentration of the photodiode  10  does not add to their transit time or reduce the detector bandwidth in either in the first concentration  15  or the second concentration  17 . 
   Another aspect of the graded doping concentration of the second p-type semiconductor layer  16  is the creation of a pseudo-electric field. The electrons generated in the first concentration  15  region are subject to this pseudo-field shown below as 
                   E   =       -     (     kT   q     )       ⁢       ⅆ   p       ⅆ   x       ⁢     1   p         ,           (   2   )               
where k is Boltzman&#39;s constant, T is the temperature, q is the charge of an electron, and the value
 
             ⅆ   p       ⅆ   x           
is the doping concentration gradient.
 
   The pseudo-field E produces an “overshoot” electron velocity, i.e. the electron velocity is potentially many times faster than the saturation velocity. A typical electron saturation velocity is on the order of 5×10 6  cm/sec. However, the exponential gradient shown in Equation (1) with D=1,000 angstroms yields a field E=2.5 kV/cm, which corresponds to an electron overshoot velocity as large as 3×10 7  cm/sec. A graph depicting the relationship between the magnitude of the pseudo-field E and the electron velocity is shown in  FIG. 3 . 
   As described, the present invention improves upon the state of the art in photodiodes by implementing a graded doping concentration. In such a manner, the net absorption of a photodiode can be increased without substantially reducing the overall bandwidth of the device. It is further understood that it may be advantageous to optimize the overall speed by adjusting the doping concentration, the capacitance of the device, and the total thickness of the absorption region. It should be apparent to those skilled in the art that the above-described embodiments are merely illustrative of but a few of the many possible specific embodiments of the present invention. Numerous and various other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention as defined in the following claims.
The present invention includes a photodiode having a first p-type semiconductor layer and an n-type semiconductor layer coupled by a second p-type semiconductor layer. The second p-type semiconductor layer has graded doping along the path of the carriers. In particular, the doping is concentration graded from a high value near the anode to a lower p concentration towards the cathode. By grading the doping in this way, an increase in absorption is achieved, improving the responsivity of the device. Although this doping increases the capacitance relative to an intrinsic semiconductor of the same thickness, the pseudo electric field that is created by the graded doping gives the electrons a very high velocity which more than compensates for this increased capacitance.