Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2003-06-18
2004-06-08
Tran, Minh-Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S189000, C257S438000
Reexamination Certificate
active
06747296
ABSTRACT:
BACKGROUND
1. Field
Implementations of the present invention relate generally to the field of photodiodes and more particularly to an avalanche photodiode (i.e., APD) with a reduced excess noise factor and an increased gain-bandwidth product.
2. Brief Description of an Illustrative Environment and Related Art
Avalanche photodiodes are incorporated in many high performance optical communications, imaging and sensing applications because they enable high signal to noise ratio and high-speed operation. An optical receiver including an APD has a signal to noise ratio that is a function of several parameters including the APD's excess noise factor. The excess noise factor, in turn, depends on the ratio of electron and hole ionization rates in the multiplication region of the APD. The ratio of the rates at which electrons and holes impact ionize in the multiplication region is expressed in terms of the rate of impact ionization by “undesirable” carriers over the rate of impact ionization by “desirable” carriers. For instance, if a particular APD is designed such that electrons are the charge carriers of choice, which is typical, then the impact ionization rate ratio is expressed in terms of the impact ionization rate of holes (undesirable) over the impact ionization rate of electrons (desirable). Conversely, in an APD in which holes are the carriers of choice, the impact ionization rate ratio is expressed in terms of the impact ionization rate of electrons (undesirable) over the impact ionization rate of holes (desirable). A high signal to noise ratio typically requires a low ionization rate ratio. The gain-bandwidth product also improves as the ionization rate ratio is reduced. In short, the optimization of the performance of an avalanche photodiode is largely focused on devising materials and device architecture that, when subjected to a predetermined reverse-bias voltage, result in impact ionization by a substantially greater quantity of desired carriers in one direction than undesired carriers in the opposite direction.
A typical ionization rate ratio exhibited by a silicon APD is about 0.02. However, silicon is not well suited for optical detection of wavelengths outside the range of about 0.35 to 1.0 microns. Accordingly, outside this range, other materials are typically used for detection. Alternative materials, however, are of limited utility due to less desirable impact ionization characteristics. For instance, InGaAs/InP hetero-junction APDs absorb well in the range of 1.0 to 1.65 microns, but are of limited utility because the impact ionization rate ratio is about 0.4.
Accordingly, a need exists for an avalanche photodiode with an impact ionization rate ratio better than that exhibited by other-than-silicon APDs (e.g., InGaAs/InP) and an optical sensitivity over a wavelength range exceeding that attainable with silicon APDs.
SUMMARY
In various implementations, an avalanche photodiode structure includes a substrate, an optical absorption region and a charge-carrier multiplication region. The optical absorption region and the multiplication region are situated between a p-doped region (i.e., the anode of the APD) and an n-doped region (i.e., the cathode of the APD) that are, in order to introduce a reverse bias, placed in electrically conductive engagement with, respectively, the cathode and anode of an external energy source.
The charge-carrier multiplication region is adapted to facilitate (a) the generation, through impact ionization, of charge-carrier pairs in which each charge-carrier pair includes first and second oppositely charged charge carrier types (e.g., a negatively charged electron and a positively charged hole), and (b) the movement of charge carriers of the first type in a first direction and of charge carriers of the second type in a second direction anti-parallel to the first direction. Impact ionization by one of the first and second charge carrier types is preferred over impact ionization of the opposite charge carrier type. Typically, the preferred charge carrier type (i.e., for impact ionization) is an electron, but devices in which holes are preferred carriers are within the scope and contemplation of the invention.
Typically embodied, the charge-carrier multiplication region comprises at least one period of lattice structure comprising a first crystalline region having first and second sides and a second crystalline region having first and second ends and joined, at its first end, to the second side of the first crystalline region at a first region-second region interface. The first crystalline region is fabricated from a first material having a first impact ionization threshold and the second crystalline region is fabricated from a second material having a second impact ionization threshold. The second impact ionization threshold is lower than the first impact ionization threshold thereby rendering the second material a higher impact ionization rate material than the first material. In various embodiments, it is desirable for the interface to be as “abrupt” as possible. That is, in such embodiments, it is desirable to achieve as clear a delineation as practicable during fabrication between the materials of the first and second regions.
Alternative embodiments include a “transition” between the first and second regions of a period that is intentionally “step-graded” or “continuously graded” resulting in more gradual charge-carrier movement between the first and second materials. The differences between an idealized abrupt interface, a step-graded transition and a continuously graded transition may be conceptualized in terms of a selected manner of descent from a higher floor to a lower floor in a building. Crossing an ideal “interface” is analogous to freefalling to the lower floor through an elevator shaft; moving through a step-graded transition is analogous to descending a staircase and moving through a continuously graded transition is akin to descending a slide. In various embodiments, a continuously graded transition is a region exhibiting a continuous grade in energy gap and ionization threshold. A step-graded transition is a region of at least one discrete sub-region exhibiting a gap energy and ionization threshold between those of the first and second materials of the first and second regions.
A representative, non-exhaustive list of materials suitable, in various implementations, for use as the first material includes InAlAs, AIGaAs, InP, InAlGaAs, and InGaAsP. An analogous list of materials suitable for use as the second material includes InGaAs, InAlGaAs, GaAs, InGaAsP, and Si.
In various embodiments, the first and second materials are intrinsic except that the first crystalline region includes first and second oppositely doped layers separated by an intrinsic sub-region of the first crystalline region. These oppositely charged layers create a localized electric field in the sub-region between the first and second oppositely doped layers. Moreover, the magnitude of the localized electric field is elevated with respect to the “background” electric field resulting from the reverse-bias potential difference applied across the various regions of the APD from an external energy source.
Persons of ordinary skill in the art understand the term “intrinsic” to include “unintentionally doped.” That is, because it is extremely difficult to obtain completely “pure” crystal outside of regions that are intentionally doped, practitioners of the relevant art understand that, in various applications, “intrinsic” implies “as un-doped as practicable” under the design and fabrication circumstances. Accordingly, the term “intrinsic” as used throughout the specification and claims should not be construed so narrowly as to be limited to the nearly unattainable condition of pure, totally un-doped crystal and, in any event, should be construed at least broadly enough to include “unintentionally doped.”
The first and second oppositely doped layers are arranged such that each charge carrier of a set of charge carriers of the preferred type having a travel pat
Franco Louis J.
Solid State Scientific Corporation
Tran Minh-Loan
LandOfFree
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