Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-06-23
2001-01-23
Tran, Minh Loan (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S184000, C385S132000
Reexamination Certificate
active
06177686
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to photodiodes, and more particularly to a photodiode with improved distributed absorption capabilities.
Waveguide photodiodes are used in the digital analog and RF optical links of communication systems to convert optical power into photocurrent. Waveguide photodiodes which operate at high optical power and at a high frequency and do so efficiently within the linear range of the photodiode are critical to a high powered optical link. These high power, high efficiency photodiodes are necessary to provide a communication system which has reduced RF insertion loss, increase signal-to-noise ratio and increased linearity for the communication system.
Referring to
FIG. 1
, a typical waveguide photodiode
10
includes a light absorption layer
12
sandwiched between a top cladding layer
14
and a bottom cladding layer
16
. Light
17
is directed into the absorption layer
12
through one side of the photodiode
10
. The cladding layers
14
,
16
are configured to guide the light
17
in the photodiode
10
in a preselected optical mode. Two contact layers
18
,
20
are positioned on opposite sides of the cladding layers
14
,
16
and are positively and negatively polarized, respectively, by a voltage supply (not shown). The light
17
propagates into the absorption layer
12
which absorbs the light
17
and photogenerates carriers therefrom in the form of pairs of positively charged holes and negatively charged electrons. The voltage supply produces an electric field which provides the stimulus to cause the positively charged holes to travel towards the negatively charged contact layer
16
, and the negatively charged electrons to travel towards the positively charged contact layer
14
. The electric field strength inside the absorption layer
12
of a photodiode
10
determines the speed at which the holes and electrons travel to their respective contact layers, it is therefore very important to maintain a high electric field strength level over the entire length, x, of the photodiode
10
in order to provide an efficient photodiode
10
. The contact layers
14
,
16
collect the electrons and holes that form the photocurrent.
One measure of the performance of a photodiode
10
is the efficiency. Efficiency of a photodiode is a measure of how much of the input light signal is photogenerated into carriers that subsequently form the photocurrent. Other measures of the performance of an RF optical link are the gain and noise figure of the link. High gain and low noise figure are very important to an analog RF optical link of a communications system using the photodiode
10
, particularly in those links which have external modulation. The gain of the RF optical link is proportional to the square of the photocurrent and the noise figure is inversely proportional to the photocurrent making both the gain and the noise figure dependant on the efficiency of the photodiode
10
. Thus, it is desirable to maximize the efficiency of a photodiode
10
to maximize the gain and minimize the noise figure.
To do so in a typical Indium-Phosphate (InP) photodiode
10
operating in the 1.3-1.55 &mgr;m wavelength range, the absorption layer
12
is formed of an InGaAs material which is lattice-matched to InP. InGaAs is typically used for the absorption layer
12
because of its superior light absorbing properties. The absorption coefficient (&agr;) is a measure of the light absorbing capability of a material. For InGaAs, the &agr; is approximately 0.8-1.0 &mgr;m
−1
measured at a wavelength of 1.55 &mgr;m.
An absorption layer
12
having a high absorption coefficient such as 0.8-1.0 &mgr;m
−1
provides for good absorption of the input light signal
17
; however, as shown in
FIG. 2
, the absorption and generation of photocurrent occurs primarily in the first few micrometers of the photodiode
10
(FIG.
1
). This results in a large percentage of the carriers being photo-generated in the first few micrometers (&mgr;m) of the absorption layer
12
which can result in an electric field collapse in the beginning region of the absorption layer
12
.
The collapse of the electric field has the undesirable effect of reducing the velocity of the carriers and inhibiting the collection of the carriers by the contact layers
14
,
16
. In addition, photogenerating a large number of carriers in the beginning region of the photodiode
10
has the undesirable effect of increasing the harmonic frequency generated by the photodiode
10
as well as generating a large amount of heat in the beginning portion of the photodiode
10
. This can lead to a thermal failure of a communications link which contains the photodiode
10
.
To solve this problem, photodiodes which distribute the absorption of light in a more uniform manner across the length (x) of the photodiode
10
have been developed. One such photodiode is the velocity-matched photodiode
30
shown in FIG.
3
. The velocity matched photodiode includes a non-absorbing waveguide core
32
which is sandwiched between two cladding layers
34
,
35
. Coplanar strips
40
are positioned on top of one of the cladding layers
34
. Several interdigitated MSM photodiodes
36
are placed on top of one of the cladding layers
34
and are coupled together through an optical waveguide
38
. Additional information on interdigitated MSM photodiodes
36
can be found at “InGaAs Metal-Semiconductor-Metal Photodetectors for Long Wavelength Optical Communications,” by J. B. D. Stoole, et al., IEEE J. of Quantum Elec., Vol. 27, No. 3, March 1991.
The cladding layers
34
,
35
guide the light
37
in the photodiode
30
. Each MSM photodiode
36
contains an absorbing layer and is operative to couple a portion of the light
37
propagating in the photodiode
30
into the absorbing layer within each of the MSM photodiode
36
.
The MSM photodiodes
36
generate photocurrent from the coupled light.
Since only a portion of the light
37
is converted into photocurrent by each MSM photodiode
36
, the absorption of the light
37
is spread over the length (x) of the photodiode
30
. The MSM photodiodes
36
are configured and positioned in preselected locations across the length, x, of the photodiode
30
so as to match the group velocity of the light
37
in the waveguide core
32
with the group velocity of the photocurrent microwave signal in the coplanar strips
40
. This matching of velocities is needed for the light absorbed by each MSM photodiode
36
to be added together in phase in the optical waveguide
38
. To have high efficiency, a high number of MSM photodiodes is required. This makes the velocity-matched photodiode very long.
In addition, the velocity-matched photodiode
30
can be difficult to fabricate since electron-beam lithography is generally required to pattern the MSM photodiodes
36
. It can also be lossy since the velocity-matched photodiode
30
requires an impedance coupled to the MSM photodiodes
36
to terminate the coplanar waveguide
38
. This is undesirable since a termination impedance reduces the amount of photocurrent delivered to the next stage in the communications link which is typically an amplifier.
Referring to
FIG. 4
, another photodiode which distributes the absorption of light
48
in a more uniform manner across the length, x, of the photodiode
50
is an expanded optical mode photodiode
50
. Referring to
FIGS. 4 & 5
, a comparison of a side sectional view of a portion of the typical prior art photodiode
10
of
FIG. 1 and a
sectional view of a portion of the expanded mode photodiode
50
shows that the expanded optical mode photodiode
50
provides a thinner absorbing layer
52
and thicker cladding layers
54
,
55
than that provided by the typical photodiode
10
. In the typical photodiode
10
, the optical mode
56
occurs mostly in the absorbing layer
12
whereas in the expanded optical mode photodiode
50
, the optical mode
58
occurs mostly in the cladding layers
54
,
55
which do not absorb light. Thus, a smaller amount of the light
48
is absorbed per
Thousand Connie M.
Tran Minh Loan
TRW Inc.
Yatsko Michael S.
LandOfFree
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