Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-01-24
2002-09-10
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S201100, C324S765010
Reexamination Certificate
active
06448547
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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
0
A, the external quantum efficiency &eegr;, the specific detectivity D*, and other photodiode performance parameters.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
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.
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.
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.
The most relevant performance parameters can be assessed according to the ideal diode equation, which for a device under illumination is given by:
I
=
I
0
[
exp
(
qV
nkT
)
-
1
]
-
Iph
(
A
)
,
(
1
)
where I is the photodiode current, I
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.
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.
The most relevant electrical performance parameter is the dynamic impedance-area product R
0
A, which is defined as:
R
0
A
=
A
·
(
ⅆ
I
ⅆ
V
)
V
=
0
-
1
=
nkTA
qI
0
(
Ω
/
cm
2
)
,
(
2
)
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
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
0
A is basically an indication of noise: the higher R
0
A is, the lower the noise. R
0
A 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).
The most relevant optical performance parameter used to characterize the performance of a photodiode is the external quantum efficiency &eegr;. The photocurrent induced in a photodiode of area A due to a background photon flux of Q
BK
can be expressed by:
Iph=&eegr;qAQ
BK
, (3)
The external quantum efficiency &eegr; 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 &lgr;, measuring the photocurrent, and computing the external quantum efficiency from Eq. (3).
As noted above, the performance of a photodiode is related to these two primary photodiode performance parameters. Specifically, the dynamic impedance-area product R
0
A is related to its electrical properties (noise), and the external quantum efficiency &eegr; is related to its optical properties (signal), respectively.
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
0
A is an indication of noise, and the external quantum efficiency &eegr; is an indication of signal, D* may be computed from the primary performance parameters, R
0
A and &eegr;. Specific detectivity D* may be referred to herein as an overall performance parameter, because it is based on these two primary performance parameters.
The specific detectivity D* of a photodiode at zero applied bias is given by the expression:
D
λ
*
=
q
ηλ
hc
2
η
q
2
Q
BK
+
4
kT
R
0
A
(
4
)
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
0
A and the external quantum efficiency &eegr; , so that specific detectivity may be estimated. Additionally, it is sometimes useful to determine the dynamic impedance-area product R
0
A and the external quantum efficiency &eegr; parameters individually. For example, the external quantum efficiency &eegr; 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. Willardson & 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.”
In addition to R
0
A, &eegr;, and D*, the saturation current I
0
, dynamic impedance at zero bias R
0
, and ideality factor n may also be regarded as photodiode performance parameters, because they can be used as figures of merit t
Applied Optoelectronics, Inc.
Kinsella N. Stephan
Pyo Kevin
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