Abstract:
An avalanche photodiode having a reduced capacitance is provided. The avalanche photodiode includes a wide band gap layer in its depletion region. The width of the wide band gap layer increases the extent of the depletion region, thereby reducing the capacitance while minimizing the impact on the dark current.

Description:
TECHNICAL FIELD 
   This invention relates generally to photodiodes, and more particularly to a low capacitance avalanche photodiode. 
   BACKGROUND 
   An intrinsic semiconductor possesses relatively poor conductivity. The vast majority of the electrons in an intrinsic semiconductor are in the valence band whereas relatively few electrons are in the conduction band. The band gap between the valence and conduction bands presents an energy barrier which inhibits the movement of electrons from the valence band to the conduction band. When exposed to light, however, electrons may be excited from the valence band to the conduction band. The promoted electron and the resulting positive charge left behind in the valence band form an electron-hole pair (EHP). As more and more EHPs are created, the conductivity will increase substantially. Solid-state optical detectors (photodetectors) exploit this increase in conductivity, denoted as photoconductivity, to detect and/or measure optical excitation. Photodiodes form a particularly useful group of solid-state optical detectors, providing a rapid and sensitive response to optical or high-energy radiation. 
   In contrast to other types of solid-state optical devices, photodiodes possess a single p-n junction. Should a photodiode be used as a photodetector, the p-n junction is typically reverse biased, thereby forming a relatively wide depletion region. The current through the reverse-biased p-n junction will be a function of the optically-generated electron-hole pairs. Carriers resulting from an electron-hole pair formation in the depletion region are swept out by the electric field resulting from the reverse bias, thereby providing a rapid response. With the proper doping and reverse biasing, the carriers resulting from an electron-hole formation cause impact ionization of other carriers in a multiplication region, a process denoted as avalanche breakdown. Photodiodes configured for such breakdown are denoted as avalanche photodiodes. Because a single optically-induced carrier may produce many additional avalanche-ionized carriers, the resulting photocurrent gain makes avalanche photodiodes very sensitive and high-speed optical detectors. 
   The avalanche breakdown process, being stochastic, is inherently noisy. To minimize this noise, the doping profile and position of the multiplication region are typically configured to favor the carrier type having a larger ionization efficiency. Another problem avalanche photodiodes must minimize is the dark current, which is the generation current across the p-n junction resulting from thermally-created carriers in the absence of any light excitation. As implied by the name, separate absorption and multiplication (SAM) avalanche photodiodes separate the absorption region from the multiplication region to reduce the dark current. SAM avalanche photodiodes are typically heterojunction semiconductor devices comprised of different III–V materials such as InGaAs and InP. A low band gap material such as InGaAs is used within the absorption region whereas a high band gap material such as InP forms the multiplication region. Because of this arrangement, the electric field is much higher in the multiplication region as compared to the absorption region. Low band gap materials will typically have greater tunneling current (dark current). However, because the electric field is relatively low across the low band band gap material, tunneling dark current is suppressed. 
   Regardless of whether the absorption region and the multiplication region are separated, avalanche photodiodes (APDs) may reduce noise by preferentially injecting carriers having a higher ionization efficiency (holes or electrons) into the multiplication region. High-speed photodetectors using APDs require electronic amplifiers that set the noise floor and limit sensitivity. The current gain provided by APDs alleviates the sensitivity limitation. However, the noise suffers if the APD capacitance and/or the dark current are too large. 
   One approach to minimize APD capacitance is to reduce the device area. Each pixel in an APD array must be isolated from adjacent pixels such that there is a fixed area around each pixel that cannot detect light. To minimize the dead space between pixels, it is desirable to increase the area that collects and amplifies the light signal. However, as the pixel size is decreased to reduce APD capacitance, the “fill factor” or ratio of light receiving to total-array size decreases. To prevent this undesirable tradeoff, a “microlens” may be used to focus light from a relatively large area such as 100×100 um onto a relatively small area such as 20×20 um to achieve a high fill factor while having a small pixel dimension. However, such an approach requires significant additional processing on the backside of a fragile semiconductor wafer as well as demanding relatively tight alignment tolerances between the front side APD pixel pattern and the backside microlens pattern. 
   Accordingly, there is a need in the art for avalanche photodiodes having reduced capacitance. 
   SUMMARY 
   In accordance with one aspect of the invention, a reduced-capacitance avalanche photodiode (APD) is provided. The APD includes: an absorption region; a multiplication region having a larger band gap than the absorption region, the avalanche photodiode being configured such that the absorption and multiplication regions are included in a depletion region when the avalanche photodiode is reverse-biased; and an undoped wide band gap layer within the depletion region to reduce the capacitance of the avalanche photodiode without increasing the dark current. 
   In accordance with another aspect of the invention, an avalanche photodiode is provided that includes: a cap layer having a first conductivity type; a substrate having a second conductivity type opposite to that of the first conductivity type, wherein the avalanche photodiode is configured such that a depletion region extends between the cap layer and the substrate if the avalanche photodiode is reverse-biased; and a relatively-thick wide band gap layer within the depletion region. 
   In accordance with another aspect of the invention, an avalanche photodiode is provided that includes: a substrate; a buffer layer overlaying the substrate; a contact layer overlaying the buffer layer; a multiplication layer overlaying the buffer layer; a charge layer overlaying the multiplication layer; a grading layer overlaying the charge layer; an absorption layer overlaying the grading layer; a relatively-thick layer overlaying the absorption layer, the relatively-thick layer having a wider band gap than a band gap possessed by the absorption layer; and a cap layer overlaying the relatively-thick layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a prior art separate absorption and multiplication avalanche photodiode (SAM APD). 
       FIG. 2  is a cross-sectional view of a SAM APD with an undoped wide band gap layer in its depletion region to reduce the capacitance according to one embodiment of the invention. 
       FIG. 3  is a cross-sectional view of a SAM APD with a wide band gap layer in its depletion region to reduce the capacitance according to another embodiment of the invention. 
   

   Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
   DETAILED DESCRIPTION 
   The present invention will be described with respect to a separate absorption and multiplication (SAM) avalanche photodiode (APD). However, the present invention may be widely applied to non-SAM avalanche photodiodes as well. Turning now to the Figures, the present invention may be better understood through comparison to a conventional SAM APD  10  as shown in cross-sectional view in  FIG. 1 . SAM APD  10  includes an n+InP substrate  15  upon which a p− InAlAs multiplication layer  20  and an intrinsic (or p−) InGaAs absorption layer  25  are deposited using, for example, epitaxial techniques. As discussed previously, SAM APDs are typically configured to favor the injection of one carrier type into multiplication layer  20 , which in this embodiment would be an electron injection. Optically-induced electrons in absorption layer  25  are swept by the reverse-biased electric field into multiplication layer  20 . As can be seen from the electric field profile also shown in  FIG. 1 , the field is considerably increased in multiplication layer  20  as compared to the field in absorption layer  25  through the use of a wide band gap charge layer  50 . Thus, whereas little tunneling (dark) current flows through absorption layer  25 , an electric field sufficient to achieve optical gain is formed in multiplication layer  20 . Optically-induced electrons that drift into multiplication region  20  can thus release other electrons through energetic collisions with host atoms. The p-n junction  22  may be formed using a heavily doped n+ InAlAs contact layer  30 . An n+ buffer layer  35  of InP lies between contact layer  30  and substrate  10 . An ohmic contact is provided by a p+ InGaAs layer  40 . To reduce electron trapping, an intrinsic grading layer  45  lies between absorption layer  35  and charge layer  50 . Suitable materials for charge layer  50  include p− InAlAs whereas grading layer  45  may be formed using InGaAlAs. Because layers  20 ,  25 ,  45 , and  50  are lightly doped or intrinsic as compared to contact layer  30  and cap layer  40 , the depletion region W extends across these layers and only minimally into layers  30  and  40 . Absorption layer  25  has a relatively narrow band gap as compared to the relatively wider band gap possessed by multiplication layer  20  and charge layer  50 . 
   If the depletion region W can be enlarged, the capacitance per unit area for SAM APD  10  would be reduced. However, adding material to an APD&#39;s depletion region W will typically increase the dark current. In the present invention, a wideband gap layer is inserted into the depletion region W. Advantageously, the capacitance per unit area is reduced while the deleterious impact on dark current is minimized because a wide band gap material has less dark current generation per unit volume than, for example, narrow band gap absorption layer  25 . Referring now to  FIG. 2 , a cross-sectional view of a SAM APD  200  in accordance with an exemplary embodiment of the invention is illustrated. SAM APD  200  includes layers  15  through  50  as discussed with respect to  FIG. 1 . However, between absorption layer  25  and cap layer  40  is a wide band gap layer  205 . The thickness of wide band gap layer  205  is denoted as ΔX whereas the depletion region for SAM APD  200  is denoted as W′. Assuming the common layers in depletion regions W ( FIG. 1 ) and W′ have the same thickness, it may be seen that the depletion layer W′ is increased with respect to W by the ΔX thickness of wide band gap layer  205 . The thickness ΔX of wide band gap layer  205  may be varied to produce the desired reduction in capacitance per unit area. For example, whereas the depletion region width W is typically 2 to 3 micrometers in conventional SAM APDs, wide band gap layer  205  will have thickness ΔX greater than 1 micrometer, thereby making the depletion width W′ for SAM APD  200  approximately 3 to 4 micrometers. Typically, the material forming layer wide band gap layer  205  will have a band gap greater than 1 eV and more preferably closer to 1.5 eV. 
   As known in the arts for heterojunction APDs, the various layers in SAM APD  200  are lattice matched to the InP substrate  15 . To minimize any effects on dark current generation, wide band gap layer  205  should have a background doping concentration of less than 10 15 /cm 3 . Thus, wide band gap layer  205  may be undoped, although a light concentration of p-type dopants may occur through diffusion from p+ cap layer  40 . A wide variety of lattice-matched semiconductor substances having the desired band gap may be used such as InP, InAlAs, InAlGaAs, and InGaAsP. The latter two materials are quaternaries that can form a continuum of band gaps between InGaAs (0.73 eV) and either InAlAs (1.45 eV) or InP (1.35 eV) Because of the additional capacitance provided by wide band gap layer  205 , SAM APD  200  may also be denoted as a separate absorption multiplication and low-capacitance (SMAL-C) avalanche photodiode. During an epitaxial deposition of the various layers, it may be sent that layer  205  will be deposited fairly late into the fabrication of the semiconductor stack that forms SMAL-C APD  200 . By depositing wide band gap layer  205  earlier in the semiconductor stack fabrication process, the exposure of charge layer  50  to the high temperatures used to grow, for example, wide band gap layer  205  may be minimized. Turning now to  FIG. 3 , a cross-sectional view of a SMAL-C APD  300  in accordance with an exemplary embodiment of the invention is illustrated, wherein wide band gap layer  205  is deposited earlier in the fabrication process. In SMAL-C APD  300 , wide band gap layer  205  is deposited on contact layer  30 . This layer would be typically undoped and result in a lightly-doped n-type layer. To prevent wide band gap layer  205  from being exposed to the high fields present at p-n junction  27 , an additional n− InAlAs charge layer  305  separates wide band gap layer  205  from p-n junction  27 . Charge layer  305  reduces the electric field across wide band gap layer  205  to prevent further avalanche multiplication from occurring in layer  205  as well as to minimize dark current generation. 
   Those of ordinary skill in the art will appreciate that many modifications may be made to the embodiments described herein. For example, silicon-based APDs that have a SAM architecture may also be modified to include a wide band gap layer in their depletion. Accordingly, although the invention has been described with respect to particular embodiments, this description is only an example of the invention&#39;s application and should not be taken as a limitation. Consequently, the scope of the invention is set forth in the following claims.