Optically-addressed two-terminal multicolor photodetector
E. H. Steenbergen, M. J. DiNezza, W. H. G. Dettlaff, S. H. Lim, and Y.-H. Zhang
Citation: Applied Physics Letters 97, 161111 (2010); doi: 10.1063/1.3505137
View online: http://dx.doi.org/10.1063/1.3505137
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APPLIED PHYSICS LETTERS 97, 161111 共2010兲

Optically-addressed two-terminal multicolor photodetector
E. H. Steenbergen, M. J. DiNezza, W. H. G. Dettlaff, S. H. Lim, and Y.-H. Zhanga兲
Center for Photonics Innovation and School of Electrical, Computer and Energy Engineering,
Arizona State University, Tempe, Arizona 85287, USA

共Received 22 September 2010; accepted 2 October 2010; published online 22 October 2010兲
A two-terminal multicolor photodetector that is most advantageous for greater than two bands is
proposed. This two-terminal design is particularly significant for focal plane arrays as it maximizes
the fill factor and simplifies the readout integrated circuits. Individual color detection is realized
with appropriate optical biasing. This concept is demonstrated experimentally using a three-color
photodetector and biasing light emitting diodes. The measured linear dynamic range is greater than
four orders of magnitude, making it a practical device for a broad range of applications. © 2010
American Institute of Physics. 关doi:10.1063/1.3505137兴
Multicolor photodetectors and focal plane arrays
共FPAs兲 are desirable for a variety of applications, including
defense, imaging, environmental monitoring, communications, and spectroscopy.1–3 Three-terminal monolithic,
dual-band photodiodes2 and two-terminal, voltage-biasswitchable, back-to-back dual-band photodiodes4,5 are examples of existing two-color photodetectors. For FPAs to
have more than two colors, additional terminals must be
added for each pixel,6–8 which greatly complicates the FPA
layout and device processing, decreases the fill factor, and
increases the readout integrated circuit 共ROIC兲 complexity.6
This paper proposes an optically-addressed two-terminal
multicolor photodetector that maximizes the fill factor and
simplifies the ROIC design. This photodetector makes FPAs
with greater than two bands and only two terminals per pixel
possible through the use of appropriate optical biasing to
realize individual color detection.
The photodetector structure and system operating principles are as follows. The proposed two-terminal photodetector, as shown in Fig. 1, consists of multiple photodiodes with
different cutoff wavelengths monolithically connected in series with tunnel diodes between adjacent photodiodes. Due to
the series connection, the photodiode with the smallest current dictates the output of the device. To optically-address
one of the photodiodes, all of the photodiodes except one,
the active detector, are optically biased using a set of light
emitting diodes 共LEDs兲 or laser diodes with output wavelengths within the spectral response range of the individual
photodiodes. The photodiode’s spectral response range spans
the energies from the photodiode’s band gap to the next highest band gap. The optically-biased photodiodes operate in the
photovoltaic mode with their forward voltages determined by
the operating current of the entire device.9 The optical bias
creates a reverse voltage bias on the active photodiode, causing it to operate in the photoconductive mode. The device
then has a very small current 共the dark current of the active
photodiode兲 when the input signal is not within the active
photodiode’s spectral response range, and thus the device is a
single-color detector with the spectral response of the active
photodiode only. Such single-color detection is maintained as
long as the photogenerated current due to the signal is
smaller than the photogenerated currents from the opticallya兲

Electronic mail: yhzhang@asu.edu.

biased photodiodes. A multiplexing control algorithm can determine which combination of biasing LEDs are on to cycle
through the available bands, thus enabling multicolor detection. The integration of multijunction photodetectors and the
optical biasing scheme allows the number of detecting bands
to increase without raising the number of contacts above two
per pixel.
To demonstrate this device concept, a commercial
InGaP/InGaAs/Ge triple-junction solar cell 共2 ⫻ 2 cm2兲 is
used as the multicolor photodetector because the solar cell
structure is almost identical to the proposed multicolor photodetector design. Three LEDs with center wavelengths 共470,
780, and 940 nm兲 within the response ranges of the photodiodes are used as the optical biasing sources. The “dark”
current density versus voltage 共J-V兲 curves 共Fig. 2兲 of the
three photodiodes are measured with matching light bias
photon flux 共1.0⫻ 1016 photons/ cm2 / s兲 on the inactive photodiodes and no light bias or signal on the active photodiode.
The voltage is across the entire device, not just the active
photodiode. The spectral responsivity 共Fig. 3兲 of the active
photodiode is also measured with equal optical bias photon
flux illuminating the inactive photodiodes. In addition, a
2.1 V bias across the entire device is used for the Ge photo-

FIG. 1. 共Color online兲 Schematic of the optically addressed, two-terminal,
multicolor photodetector. The detector structure consists of multiple photodiodes with different cutoff wavelengths connected in series with tunnel
diodes between adjacent photodiodes. The LEDs optically bias the inactive
photodiodes in the detector to enable single color detection.

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161111-2

Appl. Phys. Lett. 97, 161111 共2010兲

Steenbergen et al.

FIG. 2. 共Color online兲 Dark current densities vs. voltage. The J-Vs were
measured with matching optical bias photon flux on the inactive photodiodes and no input signal to the active photodiode. The voltage is across the
entire device, not just the active photodiode.

FIG. 4. 共Color online兲 Linear dynamic range of the InGaP photodiode with
matching light bias photon flux on the InGaAs and Ge photodiodes. The
detector output current saturates after the photogenerated current due to the
signal photon flux is larger than the photogenerated current from the light
bias photon flux. The dashed lines are guides for the eye.

diode responsivity to reduce measurement artifacts.9,10 The
spectral response is measured using a Newport QE/IPCE system, with the spectral range limited by the monochromator
gratings. The linear dynamic range measurements 共Fig. 4兲
use modulated laser diodes with wavelengths of 405, 780,
and 980 nm as signal sources, while the inactive photodiodes
are under equal optical bias flux. Neutral density filters are
used to achieve signal intensities over five orders of magnitude, and the resulting current is measured with a lock-in
amplifier.
The “dark” current densities versus voltage 共J-V兲 curves
are shown in Fig. 2. The forward and reverse bias regions of
the individual active photodiodes’ J-Vs are clearly discernable. The magnitude of the dark current increases as the photodiode’s band gap decreases, as expected. The operating
point of the active photodiode on the reverse bias portion of
its J-V curve depends on the voltage and light bias conditions, which can be selected to minimize the dark current.
The responsivity, as shown in Fig. 3, of the three photodiodes clearly confirms that optical biasing can address one
photodiode at a time in a multicolor detector with only two-

terminals. When the InGaAs and Ge photodiodes are optically biased, the entire detector response is that of the InGaP
photodiode only, with zero response above 650 nm. The InGaAs photodiode shows a response from 650 to 900 nm,
while the Ge photodiode responds at greater than 900 nm.
The crosstalk between the InGaP and InGaAs photodiodes
and between the InGaAs and Ge photodiodes is a result of 共i兲
luminescence coupling,11,12 共ii兲 photon flux leakage through
the InGaP and InGaAs photodiodes, as they may not be
optically thick, and 共iii兲 shunts in one or more of the
photodiodes.9,10 In both cases, the crosstalk responsivity is
less than ten percent of the responsivity in the photodiode’s
intended response range, and this can be further reduced using design modifications.
The linear dynamic range of the InGaP photodiode covers four orders of magnitude, as shown in Fig. 4, under two
light bias conditions. The detector current increases linearly
as the input signal increases until the photogenerated current
due to the signal is larger than the photogenerated current
from the light bias. After this point, the detector output saturates due to one of the inactive photodiodes limiting the current. The intensity of the biasing LEDs can limit the upper
end of the dynamic range before the detector itself begins to
saturate. This is the case in Fig. 4, as increasing the light bias
by an order of magnitude allows the detector current to continue to increase. The lower detection limit is determined
mainly by the noise of the active photodiode.
In summary, an optically-addressed two-terminal multicolor detector has been proposed, and a two-terminal threecolor detector has been experimentally demonstrated. The
measured dark J-V, responsivity, and linear dynamic range
prove the viability of this optically-addressed two-terminal
photodetector concept. Using only two-terminals and incorporating optical biasing allows a FPA with pixels consisting
of a large number of monolithic photodiodes to detect, in
principle, an almost unlimited number of colors using existing single-color ROICs. Simplified FPA processing and
FIG. 3. 共Color online兲 The spectral responsivity curve of each photodiode.
ROIC architectures, along with the increased FPA fill factor,
The responsivity was measured with matching photon flux on the inactive
aretomajor
advantages
of this multicolor detector. Downloaded to IP:
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161111-3

The authors wish to acknowledge Emcore for donating
the triple-junction solar cells, Newport for collaborating on
the QE/IPCE Measurement system, Dr. David Zelmon for
the use of his equipment at the Air Force Research Laboratory, and Dr. Ding Ding for his original design of Fig. 1. We
also acknowledge the financial support of the Science Foundation Arizona fellowship for E.H.S. and M.J.D., the DOD
SMART scholarship for E.H.S., and the NSF GRFP for
M.J.D. The work done at ASU is partially supported by
AFOSR under Grant No. FA9550-10-1-0129.
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