U.S. patent application number 09/769094 was filed with the patent office on 2002-09-26 for method for determining photodiode performance parameters.
This patent application is currently assigned to APPLIED OPTOELECTRONICS, INC.. Invention is credited to Johnson, Jeffery L..
Application Number | 20020134908 09/769094 |
Document ID | / |
Family ID | 25084443 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020134908 |
Kind Code |
A1 |
Johnson, Jeffery L. |
September 26, 2002 |
Method for determining photodiode performance parameters
Abstract
One or more photodiode performance parameters for a photodiode
are determined by first determining four data points Iph1, Voc1,
Iph2, and Voc2, where Iph1 is a first short-circuit current, and
Voc1 is a first open-circuit voltage, for the photodiode under a
first illumination condition, and Iph2 is a second short-circuit
current, and Voc2 is a second open-circuit voltage, for the
photodiode under a second illumination condition. Then, at least
one photodiode performance parameter for the photodiode is
determined as a function of said four data points.
Inventors: |
Johnson, Jeffery L.; (Sugar
Land, TX) |
Correspondence
Address: |
APPLIED OPTOELECTRONICS, INC.
13111 JESS PIRTLE BLVD.
SUGAR LAND
TX
77478
US
|
Assignee: |
APPLIED OPTOELECTRONICS,
INC.
Sugar Land
TX
|
Family ID: |
25084443 |
Appl. No.: |
09/769094 |
Filed: |
January 24, 2001 |
Current U.S.
Class: |
250/200 |
Current CPC
Class: |
G01R 31/2635
20130101 |
Class at
Publication: |
250/200 |
International
Class: |
H01J 040/14 |
Claims
What is claimed is:
1. A method for determining one or more photodiode performance
parameters for a photodiode, the method comprising the steps of:
(a) determining, for the photodiode, four data points Iph1, Voc1,
Iph2, and Voc2, where Iph1 is a first short-circuit current, and
Voc1 is a first open-circuit voltage, for the photodiode under a
first illumination condition, and Iph2 is a second short-circuit
current, and Voc2 is a second open-circuit voltage, for the
photodiode under a second illumination condition; and (b)
determining at least one photodiode performance parameter for the
photodiode as a function of said four data points.
2. The method of claim 1, the at least one photodiode performance
parameter comprises the dynamic impedance at zero bias R.sub.0, the
saturation current I.sub.0, and the ideality factor n.
3. The method of claim 1, wherein the at least one photodiode
performance parameter comprises the dynamic-impedance-area product
R.sub.0A of the photodiode and the specific detectivity D* of the
photodiode, where R.sub.0 is the dynamic impedance at zero bias and
A is the junction area of the photodiode.
4. The method of claim 3, wherein the at least one photodiode
performance parameter further comprises the external quantum
efficiency .eta. of the photodiode and step (b) comprises the
further step of determining the external quantum efficiency .eta.
based on Iph1 and Iph2.
5. The method of claim 3, wherein the at least one photodiode
performance parameter further comprises the external quantum
efficiency .eta. of the photodiode and step (b) comprises the
further steps of: determining the external quantum efficiency .eta.
based on Iph1 and Iph2; determining R.sub.0A based on said four
data points; and determining D* based on R.sub.0A and .eta..
6. The method of claim 1, wherein step (a) comprises the steps of:
measuring Iph1 and Voc1 in an environment having a background
photon flux; introducing a known photon flux; and measuring Iph2
and Voc2 in an environment having the background photon flux plus
the known photon flux.
7. The method of claim 6, wherein the known photon flux has a
magnitude from approximately one to ten times the magnitude of the
background flux.
8. The method of claim 1, wherein the at least one photodiode
performance, parameter comprises the dynamic-impedance-area product
R.sub.0A of the photodiode, where R.sub.0 is the dynamic impedance
at zero bias and A is the junction area of the photo diode.
9. The method of claim 1, wherein the at least one photodiode
performance parameter comprises the s specific detectivity D* of
the photodiode.
10. An apparatus for determining one or more photodiode performance
parameters for a photodiode, comprising: (a) means for determining,
for the photodiode, four data points Iph1, Voc1, Iph2, and Voc2,
where Iph1 is a first short-circuit current, and Voc1 is a first
open-circuit voltage, for the photodiode under a first illumination
condition, and Iph2 is a second short-circuit current, and Voc2 is
a second open-circuit voltage, for the photodiode under a second
illumination condition; and (b) means for determining at least one
of an electrical performance parameter and the specific detectivity
D* for the photodiode as a function of said four data points.
11. A computer data signal transmitted via a propagation medium,
the computer data signal comprising a plurality of instructions for
determining one or more photodiode performance parameters of a
photodiode, wherein the plurality of instructions, when executed by
a processor, cause the processor to perform the step of: (a)
determining at least one at least one photodiode performance
parameter for the photodiode as a function of four data points
Iph1, Voc1, Iph2, and Voc2, where Iph1 is a first short-circuit
current, and Voc1 is a first open-circuit voltage, for the
photodiode under a first illumination condition, and Iph2 is a
second short-circuit current, and Voc2 is a second open-circuit
voltage, for the photodiode under a second illumination
condition.
12. The computer data signal of claim 11, wherein the at least one
photodiode performance parameter comprises the dynamic impedance at
zero bias R.sub.0, the saturation current I.sub.0, and the ideality
factor n.
13. The computer data signal of claim 11, wherein the at least one
photodiode performance parameter comprises the
dynamic-impedance-area product R.sub.0A of the photodiode and the
specific detectivity D* of the photodiode, where R.sub.0 is the
dynamic impedance at zero bias and A is the junction area of the
photodiode.
14. The computer data signal of claim 13, wherein the at least one
photodiode performance parameter further comprises the external
quantum efficiency .eta. of the photodiode and step (b) comp rises
the further step of determining the external quantum efficiency
.eta. based on Iph1 and Iph2.
15. The computer data signal of claim 13, wherein the at least one
photodiode performance parameter further comprises the external
quantum efficiency .eta. of the photodiode and step (a) comprises
the further steps of: determining the external quantum efficiency
.eta. based on Iph1 and Iph2; determining R.sub.0A based on said
four data points; and determining D* based on R.sub.0A and
.eta..
16. The computer data signal of claim 11, wherein Iph1 and Voc1
have been measured in an environment having a background photon
flux and Iph2 and Voc2 have been measured in an environment having
the background photon flux plus a known photon flux introduced by a
light source.
17. A computer-readable medium having stored thereon a plurality of
instructions for determining one or more photodiode performance
parameters of a photodiode, wherein the plurality of instructions,
when executed by a processor, cause the processor to perform the
step of: (a) determining at least one at least one photodiode
performance parameter for the photodiode as a function of four data
points Iph1, Voc1, Iph2, and Voc2, where Iph1 is a first
short-circuit current, and Voc1 is a first open-circuit voltage,
for the photodiode under a first illumination condition, and Iph2
is a second short-circuit current, and Voc2 is a second
open-circuit voltage, for the photodiode under a second
illumination condition.
18. The computer-readable medium of claim 17, wherein the at least
one photodiode performance parameter comprises the dynamic
impedance at zero bias R.sub.0, the saturation current I.sub.0, and
the ideality factor n.
19. The computer-readable medium of claim 17, wherein the at least
one photodiode performance parameter comprises the
dynamic-impedance-area product R.sub.0A of the photodiode and the
specific detectivity D* of the photodiode, where R.sub.0 is the
dynamic impedance at zero bias and A is the junction area of the
photodiode.
20. The computer-readable medium of claim 19, wherein the at least
one photodiode performance parameter further comprises the external
quantum efficiency .eta. of the photodiode and step (b) comprises
the further step of determining the external quantum efficiency
.eta. based on Iph1 and Iph2.
21. The computer-readable medium of claim 19, wherein step (a)
comprises the further steps of: determining the external quantum
efficiency .eta. based on Iph1 and Iph2; determining R.sub.0A based
on said four data points; and determining D* based on R.sub.0A and
.eta. of determining the external quantum efficiency .eta. based on
Iph1 and Iph2, further wherein Iph1 and Voc1 have been measured in
an environment having a background photon flux and Iph2 and Voc2
have been measured in an environment having the background photon
flux plus a known photon flux introduced by a light source.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention pertains to methods for manufacturing and
testing semiconductor photodetector devices and, in particular, to
methods for determining photodiode performance parameters including
the dynamic impedance-area product R.sub.0A, the external quantum
efficiency .eta., the specific detectivity D*, and other photodiode
performance parameters.
[0003] 2. Description of the Related Art
[0004] The following descriptions and examples are not admitted to
be prior art by virtue of their inclusion within this section.
[0005] It is desirable to employ photodetectors to convert
electromagnetic radiation, such as infrared (IR) radiation, into
electrical signals. Such photodetectors may be used in a variety of
applications, including thermal imaging and transmission of
information using signals having infrared wavelengths. One type of
photodetector is the junction photodetector, or photodiode, which
has a semiconductor p-n junction that produces electrical current
under illumination with electromagnetic radiation. When properly
biased, therefore, the photodiode thus produces a current related
in a known manner to the electromagnetic radiation incident
thereon.
[0006] The performance of a photodiode may be predicted, to varying
degrees of certainty, from various photodiode performance
parameters. These performance parameters indicate various
properties or characteristics of the photodiode, e.g. its
electrical and optical properties. Performance parameters, e.g.
normalized performance parameters, may be used as figures of merit,
e.g. to compare the operation and characteristics of the device to
certain thresholds or to other devices. The terms "figure of merit"
and "performance parameter" may be used interchangeably herein.
[0007] It is desirable to determine these performance parameters,
so as to be able to determine the overall performance of the
photodiode or to determine its performance with respect to a
particular characteristic. For example, knowledge of the
photodiode's performance may be used for testing a fabricated
photodiode during or after manufacture.
[0008] The most relevant performance parameters can be assessed
according to the ideal diode equation, which for a device under
illumination is given by: 1 I = I 0 [ exp ( qV nkT ) - 1 ] - Iph (
A ) , ( 1 )
[0009] where I is the photodiode current, I.sub.0 is the saturation
current, V is the applied bias, n is the ideality factor, k is the
Boltzman factor, and T is the operation temperature of the device.
As can be seen, the total photodiode current consists of two
components. The exponential term represents current contributions
arising from diffusion processes in a semiconductor p-n junction,
which is sometimes referred to as the dark current. The second
term, Iph, is the photocurrent induced under illumination. Because
the photocurrent Iph is related to the radiation incident on the
photodiode, the total photodiode current I is also related to this
radiation. Thus, measuring the current I can provide an indication
of the intensity of local radiation.
[0010] Referring now to FIG. 1, there is shown a plot of the
typical current verses voltage (I-V) curve 100 for an ideal
photodiode (not shown). As shown in FIG. 1, under illumination, a
zero bias photocurrent, Iph, flows at zero bias. Thus, the
short-circuit current of a photodiode is equal to the induced
photocurrent Iph. The open circuit voltage, Voc, is the point in
forward positive bias where diffusion (dark) current equals the
photocurrent so that no net current flows in the device.
[0011] The most relevant electrical performance parameter is the
dynamic impedance-area product R.sub.0A, which is defined as: 2 R 0
A = A ( I V ) V = 0 - 1 = nkTA qI 0 ( / cm 2 ) , ( 2 )
[0012] where I is the total diode current from Eq. (1), q is the
electron charge, A is the junction area of the device, and R.sub.0
is the dynamic impedance at zero bias (i.e., the exponential
derivative term in Eq. (2), which is multiplied by area A). This
performance parameter embodies the essential elements of the
diffusion process in the photodiode junction, and is an industry
standard for comparing the electrical performance of photovoltaic
structures. R.sub.0A is basically an indication of noise: the
higher R.sub.0A is, the lower the noise. R.sub.0A is typically
found by measuring the current as a function of voltage (I-V) and
calculating the derivative, at V=0, according to Eq. (2).
[0013] The most relevant optical performance parameter used to
characterize the performance of a photodiode is the external
quantum efficiency .eta.. The photocurrent induced in a photodiode
of area A due to a background photon flux of QBK can be expressed
by:
Iph=.eta.qAQ.sub.BK, (3)
[0014] The external quantum efficiency .eta. is a measure of
electrical carriers collected per incident photon, and thus is an
indication of signal, ranging from 1 (best) to 0 (worst). It is
typically measured by exciting the device under test (DUT) with a
known photon flux within a narrow band around a specified spectral
wavelength .lambda., measuring the photocurrent, and computing the
external quantum efficiency from Eq. (3).
[0015] As noted above, the performance of a photodiode is related
to these two primary photodiode performance parameters.
Specifically, the dynamic impedance-area product R.sub.0A is
related to its electrical properties (noise), and the external
quantum efficiency .eta. is related to its optical properties
(signal), respectively.
[0016] Another important performance parameter is the specific
detectivity, D*, which is an overall photodiode performance
parameter that indicates the signal-to-noise ratio (SNR) for the
photodiode. D* is normalized with respect to detector area A and
electrical bandwidth. Because the dynamic impedance-area product
R.sub.0A is an indication of noise, and the external quantum
efficiency .eta. is an indication of signal, D* may be computed
from the primary performance parameters, R.sub.0A and .eta..
Specific detectivity D* may be referred to herein as an overall
performance parameter, because it is based on these two primary
performance parameters.
[0017] The specific detectivity D* of a photodiode at zero applied
bias is given by the expression: 3 D * = q hc 2 q 2 Q BK + 4 kT R 0
A ( 4 )
[0018] where h is Planck's constant and c is the speed of light.
This overall performance parameter is the most widely accepted
comparative parameter for specifying the detector's characteristics
and performance. It can therefore be useful to accurately and
easily determine the dynamic impedance-area product R.sub.0A and
the external quantum efficiency .eta., so that specific detectivity
may be estimated. Additionally, it is sometimes useful to determine
the dynamic impedance-area product R.sub.0A and the external
quantum efficiency .eta. parameters individually. For example, the
external quantum efficiency .eta. of a given device may be compared
to that of other devices or to a benchmark or threshold value.
Background information regarding photodiodes and related
performance parameters may be found in: Thomas Limperis &
Joseph Mudar, "Detectors," Ch. 11 in The Infrared Handbook, rev'd
ed., William L. Wolfe & George J. Zissis, eds. (Infrared
Information analysis (IRIA) Center, Environmental Research
Institute of Michigan, 1985); Semiconductors and Semimetals, vol.
18: Mercury Cadmium Telluride, R. K. Willlardson & Albert C.
Beer, eds. (New York: Academic Press, 1981), esp. ch. 6,
"Photovoltaic Infrared Detectors," by M. B. Reine, A. K. Sood &
T. J. Tredwell; and John David Vincent, Fundamentals of Infrared
Detector Operation and Testing (New York: John Wiley & Sons,
1990), esp. ch. 2, "Detector Types, Mechanisms, and Operation."
[0019] In addition to R.sub.0A, .eta., and D*, the saturation
current I.sub.0, dynamic impedance at zero bias R.sub.0, and
ideality factor n may also be regarded as photodiode performance
parameters, because they can be used as figures of merit to compare
the performance of the photodiode. For example, the ideality factor
n is an electrical performance parameter, and the saturation
current I.sub.0 is an electrical performance parameter embodying
material characteristics. The dynamic impedance at zero bias
R.sub.0 is also an electrical performance parameter.
[0020] There are, however, difficulties in determining these
photodiode performance parameters using standard techniques. First,
for high-volume production of photodetectors, the amount of
experimental data required to extract R.sub.0A from I-V
measurements is prohibitively large and time-consuming to produce.
Second, the electrical and optical properties are typically
determined in separate measurements, e.g. the external quantum
efficiency must be determined under controlled conditions.
[0021] Another, simpler approach, which is not admitted to be prior
art by virtue of its inclusion within this section, is to employ an
analysis using only two points on the characteristic I-V curve. One
of these points is the short-circuit current (i.e., the current
measured at zero bias), which is the simply the photocurrent, Iph,
produced by the unspecified background photon flux present during
the I-V measurement. The other point is the voltage under forward
bias for which the diffusion current equals the photocurrent so
that no net current flows in the device When the total photodiode
current I is zero, the open circuit voltage can be defined from Eq.
(1) as: 4 Voc = nkT q ln ( Iph I 0 + 1 ) ( 5 )
[0022] For unity ideality factors (n=1), the saturation current
I.sub.0 can also be determined from Eq. (1), as follows: 5 I 0 =
Iph [ exp ( qVoc nkT ) - 1 ] ( 6 )
[0023] and R.sub.0A can be estimated directly by definition in Eq.
(2).
[0024] There are two primary difficulties with this simple
two-point analysis. First, the ideality factor, n, is unknown for a
given device, and can range from one to three depending on the
actual dark current mechanisms present. The assumption that n=1 is
not always correct. Because the ideality factor is in the
exponential, substantial errors can be made in the estimation of
R.sub.0A if n is not known or imprecisely estimated. Second, the
background flux present during the I-V measurement is often not
controlled and can vary from measurement to measurement.
[0025] The foregoing drawbacks of conventional performance
parameter measuring techniques can limit the ability to perform
high throughput screening of photodetector performance at low cost.
There is, therefore, a need for improved methods for quickly and
accurately estimating the primary photodiode performance
parameters, which are required for determining the specific
detectivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Other objects and advantages of the invention will become
apparent upon reading the following detailed description and upon
reference to the accompanying drawings in which:
[0027] FIG. 1 is a plot of the typical current versus voltage (I-V)
behavior of a photodiode;
[0028] FIG. 2 is a plot showing two illustrative I-V curves
corresponding to two measurement conditions for performing the
current and voltage measurements used to determine the photodiode
performance parameters, in accordance with an embodiment of the
present invention;
[0029] FIG. 3 is a schematic illustration of a photodiode testing
system for testing a photodiode 310 in accordance with the present
invention; and
[0030] FIG. 4 is a flow chart illustrating the photodiode testing
method of the present invention.
[0031] While the present invention is susceptible to various
modifications and alternative forms, specific embodiments thereof
are shown by way of example in the drawings and will herein be
described in detail. It should be understood, however, that the
drawings and detailed description thereto are not intended to limit
the invention to the particular form disclosed, but on the
contrary, the intention is to cover all modifications, equivalents
and alternatives falling within the spirit and scope of the present
invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention provides a method by which performance
parameters for a photodiode can be efficiently determined, to a
higher degree of accuracy than in conventional techniques, and in a
single measurement session. As described in further detail below,
in the present invention, two primary performance parameters--the
dynamic-impedance-area product R.sub.0A and the quantum efficiency
.eta.--are uniquely determined by using only four measured data
points. Further, an estimate of the specific detectivity D*,
another important, overall performance parameter, can be derived
directly from the two primary performance parameters. Other
photodiode performance parameters, such as saturation current
I.sub.0, dynamic impedance at zero bias R.sub.0, and ideality
factor n, may also be determined using these four data points.
[0033] Referring now to FIG. 2, there is shown a plot 200 of two
illustrative I-V curves I1, I2, corresponding to two measurement
conditions for performing the current and voltage measurements used
to determine the photodiode performance parameters, in accordance
with an embodiment of the present invention. In a preferred
embodiment, as illustrated in FIG. 2, two separate I-V
characterizations are performed during the same measurement
session, to measure the four data points of interest.
[0034] Referring now to FIG. 3, there is shown a schematic
illustration of a photodiode testing system 300 for testing a
photodiode 310 in accordance with the present invention. As
illustrated, photodiode 310 has terminals 311, 312 to which are
connected suitable testing equipment such as photodiode I-V tester
320. Tester 320 can measure short circuit current (i.e. Iph) and
open-circuit voltage Voc under given illumination conditions. For
example, tester 320 can vary the bias voltage V applied across
photodiode 310 until zero current I is measured, to determine Voc.
Likewise, tester 320 can short circuit terminals 311, 312 and
measure the resulting short-circuit current Isc-Iph. As
illustrated, background flux Q.sub.BK is always present, and a
known photon flux Q.sub.B2 may be selectively introduced using any
light source having a known flux confined within a well-defined
spectral band. For example, a laser 330 may be employed, which is
under the control of tester 320. Alternatively, a black-body
radiation source plus fitter may be employed. In an embodiment, all
light (including background flux Q.sub.BK and known photon flux
Q.sub.B2) is filtered with a passband filter (not shown) before
reaching photodiode 310.
[0035] Referring once more to FIG. 2, the second I-V curve I2
measurement is performed under illumination by controlled photon
flux Q.sub.B2 (plus the background flux Q.sub.BK). In an
embodiment, the controlled photon flux Q.sub.B2 is selected so that
the resulting Iph2 and Voc2 are larger enough than Iph1 and Voc1,
respectively, so that meaningful differences between them may be
measured. Thus, in one embodiment, the controlled photon flux
Q.sub.B2 is approximately equal to or within an order of magnitude
greater than the background flux Q.sub.BK. In a preferred
embodiment, Q.sub.B2 is within a range of approximately one to ten
times the magnitude Q.sub.BK. The four points of interest from the
I-V curves are Iph1 and Voc1, from the first I-V curve I1, and Iph2
and Voc2, from the second I-V curve I2. These points can be
measured electronically using minimal experimental data. In an
embodiment, the dynamic-impedance-area product R.sub.0A is
determined from these four data points. The quantum efficiency
.eta. can be determined using only two of these points, i.e. Iph1
and Iph2. Therefore, in the course of measuring the data points
necessary to determine electrical performance parameter R.sub.0A,
data points needed to compute the optical performance parameter l
are already gathered. The specific detectivity D* can be derived
directly from the two primary performance parameters.
[0036] Referring now to FIG. 4, there is shown a flow chart 400
illustrating the photodiode testing method of the present
invention. In the method of the present invention, first, under
background illumination Q.sub.BK only (i.e., laser 330 is off),
photocurrent Iph1 and open-circuit voltage Voc1 are measured with
tester 320. I.e., the open-circuit voltage Voc1 and short-circuit
current Iph1, which are two specific points of the first curve I1,
are measured under the background photon flux of measurement
environment, denoted as Q.sub.BK (step 401). This radiation
represents an uncontrolled background photon flux, which is always
present and to which the measurement apparatus is subject. Q.sub.BK
does not need to be determined in order to determine the primary
performance parameters.
[0037] In the same measurement session, controlled photon flux
Q.sub.B2 is introduced (step 402), so that the total illumination
incident on photodetector 300 is increased from Q.sub.BK to
(Q.sub.BK+Q.sub.B2). Photocurrent Iph2 and open-circuit voltage
Voc2, which are points of second curve I2, are then measured under
the new illumination condition (step 403). Thus, in a preferred
embodiment, as illustrated in FIG. 2, two separate I-V
characterizations are performed during the same measurement
session, to measure the four data points of interest. As will be
appreciated, the order of measurement of the four data points may
vary in alternative embodiments. For example, as described above,
the I-V measurements Iph1 and Voc1 for background photon flux
Q.sub.BK (curve I1) may be performed, followed by the I-V
measurements Iph2 and Voc2 made after photon flux Q.sub.B2 is
introduced. Alternatively, I measurements Iph1 and Iph2 may be made
first, followed by V measurements for Voc1 and Voc2.
[0038] By definition, the photocurrents Iph1 and Iph2 are given
by:
Iph1=.eta.qA.sub.OPTQbk
Iph2=.eta.qA.sub.OPT(Qbk+Qb2) (7)
[0039] The difference between these photocurrents (.DELTA.Iph)
represents the photocurrent contributions arising from the
controlled radiation flux Q.sub.BK during the second measurement,
because the contribution to the photocurrent from the uncontrolled
photon flux of the measurement cancels out if the two photocurrents
are subtracted from one another. Thus, because the radiation flux
of the second measurement, Q.sub.B2, is known, the external quantum
efficiency .eta. can be computed from the difference in the
photocurrent using the optical area A of the device: 6 = Iph qAQ B2
where Iph = Iph2 - Iph1 ( 8 )
[0040] In this manner, the optical properties (i.e., .eta.) of the
DUT can be determined in the course of measuring the electrical
properties. Therefore, the next step of the method of the present
invention is to determine quantum efficiency .eta. based on the
measured data points Iph1 and Iph2 (step 402), and also based on
area A and known photon flux Q.sub.B2.
[0041] Next, electrical properties (i.e., R.sub.0A) are determined
using the open-circuit voltages Voc1 and Voc2 as well as the short
circuit photocurrents Iph1 and Iph2. With these values
experimentally determined, the diode equations for the respective
measurements are coupled by only two unknowns: the ideality factor
n and the saturation current I.sub.0. By using two measurements of
the current-voltage characteristics, either the ideality factor n
or the saturation current can be uniquely determined, in accordance
with the following transcendental equations and measurements: 7 1 n
= kT qVoc1 ln ( Iph1 I 0 + 1 ) = kT qVoc2 ln ( Iph2 I 0 + 1 ) ( 9 )
I 0 = Iph2 [ exp ( qVoc2 nkT ) - 1 ] = Iph1 [ exp ( qVoc1 nkT ) - 1
] ( 10 )
[0042] In particular, either n or I.sub.0 may be determined by
ascertaining the realistic intersection point of the curves
described by the above Eqs. (9) or (10) (step 403).
[0043] Saturation current I.sub.0 may thus be determined, given a
value for n, using either the curve I1 or I2 measured I-V data
points (step 404). Alternatively, other suitable mathematical
techniques may be used to determine n and I.sub.0, given Iph1,
Iph2, Voc1, and Voc2. For example, the Eqs. (9) containing the
I.sub.0 term may be solved for I.sub.0 using transcendental
techniques or equivalently Eqs. (10) containing the ideality factor
term n term may be solved. Once either I.sub.0 or n is determined,
it may be plugged back into either equation to solve for n or
I.sub.0 respectively.
[0044] Once the saturation current I.sub.0 is known, R.sub.0A is
determined from Eq. (2) above (step 405). As will be appreciated,
the reliability of the estimate for R.sub.0A is enhanced by the
experimental measurement of the ideality factor n. Thus, in the
course of determining the primary electrical performance parameter
R.sub.0A in accordance with the present invention, other
performance parameters are, or may be, also determined. For
example, saturation current I.sub.0 and ideality factor n are
calculated in order to calculate R.sub.0A. Once n and I.sub.0 are
determined, or R.sub.0A is calculated, dynamic impedance R.sub.0may
be easily determined, employing Eq. (2), or dividing R.sub.0A by
A.
[0045] Finally, if desired, once external quantum efficiency .eta.
and R.sub.0A are determined, the specific detectivity D* may be
calculated, using Eq. (4). Therefore, using only four data points
extracted from two separate measurements of the I-V characteristics
of a photodiode., in the same measurement session, the primary
performance parameters .eta. and R.sub.0A, as well as the specific
detectivity D*, of the photodiode 300 may be calculated. In an
alternative embodiment, because .eta. and R.sub.0A are a function
of the four data points Iph1, Voc1, Iph2, Voc2, instead of
computing D* based on l and R.sub.0A, D* may be computed directly
from the four data points, without computing R.sub.0A and .eta. as
intermediate results. Other photodiode performance parameters, such
as I.sub.0, n, and R.sub.0may also be determined in the course of
determining R.sub.0A.
[0046] The method of the present invention may be used to
expediently monitor devices for compliance to performance
benchmarks, and may be used to determine photodetector performance
in a high-volume manufacturing environment. For example, in one
application, the method of the present invention may be employed to
screen processed devices before proceeding to the next stage of
photodiode manufacturing.
[0047] In an embodiment, therefore, the present invention provides
a method for determining one or more photodiode performance
parameters for a photodiode, including the primary optical
performance parameter (quantum efficiency .eta.), the primary
electrical parameter (dynamic-impedance-area product R.sub.0A), and
the overall performance parameter (specific detectivity D*). The
method involves first obtaining (e.g., by measuring) the four data
points Iph1 and Voc1 (under a first illumination condition) and
Iph2 and Voc2 (under a second illumination condition). Then, either
R.sub.0A or D* are determined as a function of function of the four
data points. External quantum efficiency .eta. can also be
determined as a function of the four data points or, to be more
precise, as a function of only two of them, i.e. Iph1 and Iph2. D*
can be computed based on .eta. and R.sub.0A, or it may be computed
directly from the four data points, without computing R.sub.0A and
.eta. as intermediate results.
[0048] In an embodiment, the four data points are obtained by
measuring these points under suitable illumination conditions with
suitable open-circuit voltage and short-circuit current measuring
and testing equipment. In one embodiment, the first illumination
condition is simply the uncontrolled background illumination of the
photodiode's local environment, and the second illumination
condition is the background illumination plus the illumination
introduced by an illumination laser introducing a known photon
flux.
[0049] One advantage of the present invention is that a simple
two-step (laser ofi; laser on) process, in the same measurement
session, is sufficient to measure the four data points from which
can be derived the photodiode performance parameters described
above. There is no need to employ more complicated devices such as
a chopper device, which is often utilized to produce a square wave
light signal, the AC component of which is related to the known
photon flux and the DC level of which is related to the background
flux.
[0050] The present invention can also be embodied in the form of
computer-implemented processes and apparatuses for practicing those
processes. The present invention can also be embodied in the form
of computer program code embodied in tangible media, such as floppy
diskettes, CD-ROMs, hard drives, or any other computer-readable
storage medium, wherein, when the computer program code is loaded
into and executed by a computer, the computer becomes an apparatus
for practicing the invention. The present invention can also be
embodied in the form of computer program code, for example, whether
stored in a storage medium, loaded into and/or executed by a
computer, or transmitted as a propagated computer data or other
signal over some transmission or propagation medium, such as over
electrical wiring or cabling, through fiber optics, or via
electromagnetic radiation, or otherwise embodied in a carrier wave,
wherein, when the computer program code is loaded into and executed
by a computer, the computer becomes an apparatus for practicing the
invention. When implemented on a general-purpose microprocessor
sufficient to carry out the present invention, the computer program
code segments configure the microprocessor to create specific logic
circuits to carry out the desired process.
[0051] The present invention, therefore, is well adapted to carry
out the objects and attain the ends and advantages mentioned, as
well as others inherent therein. While the invention has been
depicted and described and is defined by reference to particular
preferred embodiments of the invention, such references do not
imply a limitation on the invention, and no such limitation is to
be inferred. The invention is capable of considerable modification,
alteration and equivalents in form and function, as will occur to
those ordinarily skilled in the pertinent arts. The depicted and
described preferred embodiments of the invention are exemplary only
and are not exhaustive of the scope of the invention. Consequently,
the invention is intended to be limited only by the spirit and
scope of the appended claims, giving full cognizance to equivalents
in all respects.
* * * * *