Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2003-03-18
2004-05-25
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S190000, C257S196000, C257S458000, C257S656000
Reexamination Certificate
active
06740908
ABSTRACT:
TECHNICAL FIELD
The invention relates to photodiodes. In particular the invention relates to photodiodes having high bandwidth-efficiency products used in optical network receivers.
BACKGROUND ART
Semiconductor photodetectors, most notably various forms of photodiodes, absorb incident light in the form of photons and convert the absorbed photons into an electric current. The current within a lattice of the semiconductor is often represented in terms of ‘free carriers’ or simply ‘carriers’. In particular, when a photon with sufficient energy interacts with an atom of the semiconductor lattice, an electron associated with the atom moves across an energy band gap from a valence shell or band to a conduction shell or band of the semiconductor. Movement of the electron across the band gap creates a negative carrier, i.e., the electron, and leaves behind a positive carrier known as a ‘hole’. After carrier generation through photon absorption, a carrier transport mechanism within the semiconductor-based photodetector separates the generated holes and electrons, thereby creating an electric current known generally as a photocurrent. In general, both the electron and the hole may act as carriers within the semiconductor and contribute to the photoelectric current. The photocurrent thus created enables' the photodetector to interact in various ways with an external circuit or system. Among other things, photodiodes find wide-scale application in optical receivers used for optical communication networks.
Photodetector performance is often summarized in terms of bandwidth, efficiency, maximum current output, and optical wavelength range. Bandwidth is a measure of a speed of response of the photodetector to changes in an incident optical signal or light source. Efficiency indicates how much of the incident optical signal is converted into carriers. Maximum current output is typically determined by a saturation condition within the semiconductor of the photodetector while optical wavelength range is a function of certain material properties of the photodetector among other things. In general, photodetector performance is limited by a combination of material properties of constituent materials of the photodetector and a structural characteristic of the photodetector associated primarily with a type and/or structure of a given photodetector.
For example,
FIG. 1A
illustrates a cross sectional view of a conventional positive-intrinsic-negative (PIN) photodiode
10
. The PIN photodiode
10
comprises an intrinsic or lightly doped semiconductor layer
14
(i-layer) sandwiched between a p-type semiconductor layer
12
(p-layer) and an n-type semiconductor layer
15
(n-layer). The i-layer
14
is often referred to as a photoactive or a light absorption layer
14
since ideally, photon absorption is primarily confined to the i-layer
14
of the PIN diode
10
. Typically a deposited metal, such as aluminum (Al), or another conductive material, such as heavily doped polysilicon, form a pair of ohmic contacts
17
a
,
17
b
, that provide an electrical connection between the PIN photodiode and an external circuit.
The ohmic contact
17
a
connected to the p-layer
12
is an anode contact
17
a
while the ohmic contact
17
b
connected to the n-layer
15
is a cathode contact
17
b
. Typically, the PIN photodiode
10
is formed on and structurally supported by a semi-insulating substrate
19
.
FIG. 1B
illustrates a band diagram
20
of the PIN photodiode
10
illustrated in FIG.
1
A. The band diagram
20
depicts energy levels as electron-volts (eV) in a vertical or y-direction and physical length or distance along a conduction path within a device in a horizontal or x-direction. Thus, the band diagram
20
illustrates a valence band energy level
21
and a conduction band energy level
22
separated by a band gap
23
for each of the layers of the PIN photodiode
10
. When a hole
30
and electron
32
are separated by absorption of a photon by the photoactive i-layer
14
, the hole
30
moves in the i-layer
14
toward the p-layer
12
under the influence of an electric potential gradient formed by an inherently lower energy level of the p-layer
12
for holes. Once the hole reaches the p-layer
12
, the hole combines at the anode ohmic contact
17
a
with an electron supplied by the external circuit (not illustrated). Similarly, the electron
32
moves in the i-layer
14
toward the n-layer
15
under the influence of an electric potential gradient formed by the inherently lower energy level of the n-layer
15
for electrons. Electrons in the n-layer
15
enter the cathode contact
17
b
. The drift or movement of electrons
32
and holes
30
in the i-layer
14
drives or creates an electric current in the n-layer
15
, the p-layer
12
, and the external circuit.
Among the performance limitations associated with the conventional PIN photodiode is a bandwidth limitation due to the time required for the transport of holes
30
and electrons
32
within the i-layer
14
. In particular, holes
30
are known to have a much slower transport velocity than that of electrons
32
. The slower transport velocity of holes
30
results in a transport time for the holes
30
that is much longer than a transport time of the electrons
32
. The longer hole transport time normally dominates and ultimately limits an overall response time or bandwidth of the PIN photodiode
10
.
Accordingly, it would be advantageous to have a photodiode that overcomes the bandwidth-efficiency product limitations associated with conventional PIN photodiodes. Such a photodiode would solve a longstanding need in the area of photodiodes for optical networking.
SUMMARY OF THE INVENTION
The present invention provides an extended drift heterostructure (EDH) photodiode with enhanced electron response. In particular, the present invention is an enhanced EDH (EEDH) photodiode that employs an additional p-type light absorption layer. The additional p-type light absorption layer promotes unidirectional photo-generated minority carrier (e.g., electron) drift or motion within the photodiode according to the present invention. The unidirectional electron carrier motion effectively enhances an electron contribution to a device photocurrent without degrading an overall device bandwidth.
In an aspect of the invention, an enhanced extended drift heterostructure (EEDH) photodiode is provided. The EEDH photodiode comprises a first layer comprising a semiconductor having a first doping concentration that maintains a charge neutrality condition in at least a portion of the first layer. The EEDH photodiode further comprises a second layer adjacent and interfaced to the first layer. The second layer comprises a semiconductor having a second doping concentration that is lower than the first doping concentration, such that a non-neutral charge condition is maintained. The first and second layers comprise respective first and second band gap energies that facilitate light absorption by the first and second layers. The EEDH photodiode further comprises an ohmic anode contact directly or indirectly interfaced to the first layer and a cathode contact directly or indirectly interfaced to the second layer. A characteristic of one or more of the layers in addition to the second layer directs a movement of photo-generated electrons away from the ohmic anode contact.
In some embodiments, the characteristic that directs the movement of the photo-generated electrons is manifested in a carrier block layer adjacent and interfaced to the first layer on a side opposite to the second layer. The carrier block layer comprises a semiconductor having a block band gap energy that is greater than the first and second band gap energies, such that a block energy barrier is created between the first layer and the carrier block layer to so direct the electron movement. In other embodiments, the characteristic that so directs the electron movement is either further manifested in the first layer or alternatively manifested in the first layer. The first layer has the first band ga
Agilent Technologie,s Inc.
Ngo Ngan V.
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