Photovoltaic detector with integrated dark current offset correction

An integrated photovoltaic detector includes a reference photovoltaic detector and an active photovoltaic detector in a series connection. The reference detector produces a dark current that opposes the active detector's dark current. The active detector effectively masks the reference detector from incident illumination so that the active detector produces photocurrent but the reference detector does not. The band gap of the reference detector is preferably matched to the active detector so that their dark currents are substantially matched over a temperature range. As a result, the current read out of the integrated detector at the series connection is approximately equal to the photocurrent generated by the active detector. This improves the detector's SNR, signal resolution, and useful operating temperature range.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to photovoltaic detectors and more specifically to a 
photovoltaic detector with integrated dark current offset correction. 
2. Description of the Related Art 
Photodetector arrays which may include hundreds or thousands of 
photovoltaic detector cells are used to detect intensity patterns in the 
infrared, ultraviolet and visible spectra in applications such as weather 
satellites, earth remote sensing, and industrial processes. A photovoltaic 
detector cell includes a flat photovoltaic wafer made from n-type or 
p-type crystalline semiconductor material that absorbs photons over a 
desired spectrum. The wafer is formed on a substrate that is transparent 
to photons in the desired spectrum. A thin surface layer of the opposite 
conductivity type is formed on the wafer so that the interface between the 
surface layer and the main or bulk region of the wafer defines a 
semiconductor p-n junction. A potential is applied across the cell such 
that the cell operates in reverse bias mode. 
Illumination of the transparent substrate with photons having wavelengths 
in the desired spectrum creates electron-hole pairs in the cell that 
diffuse to and are collected at the p-n junction. This mechanism generates 
a photocurrent that is proportional to the intensity of the incident 
photons. Thermal energy also generates electron-hole pairs in both the 
bulk and depletion regions that diffuse to and are collected at the p-n 
junction. This mechanism generates a dark or leakage current that is 
independent of the illumination intensity and which adds to the 
photocurrent. The dark current increases as the band gap energy of the 
cell is narrowed. For example, in infrared detectors the dark current may 
be 5 to 10 times greater than the photocurrent. The dark current is also 
highly sensitive to changes in temperature and increases exponentially as 
the temperature increases. In high temperature industrial applications the 
dark current may effectively swamp out the photocurrent. In general the 
dark current is insensitive to changes in the potential across the cell. 
However, at low enough temperatures the electron hold pairs in the 
depletion region tend to dominate such that the dark current does change 
with potential. 
As a result, the photovoltaic detector cell generates a detection current 
having a signal component equal to the photocurrent and an offset equal to 
the dark current. In a typical IR focal plane array, a readout capacitor 
integrates the detector current for a preset integration period at which 
point the capacitor is discharged and reset. An A/D converter, suitably 14 
bits, digitizes the voltage signal from the integration capacitor and 
passes it to a calibration circuit. The calibration circuit computes the 
offset and the slope of the cell's current v. photon intensity response 
curve and normalizes the output to a zero offset and reference slope so 
that each cell in the array has the same response. The fabrication of a 
HgCdTe infrared detector of the type described above is described in Reine 
et al. Semiconductors and Semimetals: Volume 18 Mercury Cadmium Telluride, 
Academic Press, Ch 6--Photovoltaic Infrared Detectors, pp. 246-256, 1981. 
The dark current offset reduces the cell's sensitivity to changes in photon 
intensity, which reduces the resolution of the digitized voltage signal. 
First, the signal-to-noise ration (SNR) of the detector current is 
proportional to the square root of the integration period for charging the 
capacitor. The dark current reduces the integration period, and thus 
reduces the SNR. Second, approximately 6 bits of the A/D converter are 
used to digitize the offset. Those bits are effectively wasted, thus 
reducing the resolution of the signal component from 14 bits to 8 bits. 
Due to the strong variation in dark current with temperature, many systems 
have strict requirements on the maximum spatial temperature variation 
across the focal plane array as well on the temperature stability itself. 
This is true even when the dark current is less than the photocurrent. To 
maintain the accuracy of the calibration circuitry, the spatial variation 
is often specified as less than 0.1K variation across the array, and the 
stability is specified as less than 0.1K change over 30 seconds. In 
addition, some systems have strict requirements on the time required to 
cooldown and stabilize the array to the point it can operate accurately. 
These requirements necessitate expensive packaging, cooling, and 
calibration techniques. 
SUMMARY OF THE INVENTION 
In view of the above problems, the present invention provides a 
photovoltaic detector with integrated dark current offset correction 
having improved SNR, signal resolution and temperature independence. 
This is accomplished by integrating a reference photovoltaic detector with 
the active photovoltaic detector to produce a dark current that opposes 
the active detector's dark current. The active detector effectively masks 
the reference detector from incident illumination so that the active 
detector produces photocurrent but the reference detector does not. The 
band gap of the reference detector is preferably matched to the active 
detector so that their dark currents are substantially matched over a 
temperature range. As a result, the current read out of the integrated 
detector is approximately equal to only the photocurrent generated by the 
active detector. This increases the detector's SNR and signal resolution, 
and reduces its temperature dependence which allows less expensive packing 
and cooling techniques to be employed.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, a reference photovoltaic detector cell is 
integrated with the active photovoltaic detector cell to provide an 
opposing dark current that preferably reduces the offset to substantially 
zero. This actually adds a noise component that reduces the SNR by up to 
.sqroot.2. However, if the dark current is more than twice the 
photocurrent the increase in the integration period will more than offset 
the added noise component so that the overall SNR will be improved. 
Furthermore, by eliminating the offset all of the A/D converter's bits are 
used to digitize the signal component of the detector current. In 
addition, by eliminating the detector's temperature dependence, less 
expensive packaging and cooling techniques can be employed. This is a 
significant benefit even when there is no improvement in sensitivity due 
to relatively low levels of dark current. 
Although the invention is applicable to all photodetectors, it is 
particularly applicable to infrared detectors and high temperature 
detectors. Infrared detectors have relatively narrow band gap energies, 
100-250 meV, so that the dark current is several times greater than the 
photocurrent. Furthermore, infrared sources typically have a very low 
contrast, approximately 1 to 2%. As a result, the detectors must have a 
very high SNR. In visible light detectors the photocurrent is typically 
much larger than the dark current. However, in high temperature industrial 
applications the dark current can become significant. 
FIG. 1 is a schematic diagram of a detector cell 8 including an integrated 
photovoltaic detector 10 and a digital signal processing (DSP) circuit 12. 
Photodetector arrays typically include between 64.times.64 and 1k.times.1k 
detector cells 8. The detector 10 includes a series connection of an 
active photovoltaic detector D.sub.act and a reference photovoltaic 
detector D.sub.ref. A pair of bias terminals 14 and 16 are connected to 
the anode 18 and cathode 20 of D.sub.ref and D.sub.act, respectively. Bias 
voltages V.sub.B1 and V.sub.B2 are applied to terminals 16 and 14, 
respectively, to reverse bias D.sub.act and D.sub.ref. A readout terminal 
22 is connected at the series connection of the active detector's anode 24 
and the reference detector's cathode 26 to readout a detector current 
I.sub.det. In this configuration, the reference detector generates a dark 
current I.sub.Dref in opposition to the dark current I.sub.Dact produced 
by the active detector. The detectors'polarities can be reversed, which in 
turn reverses the polarity of I.sub.det. 
The band gap energy of the active detector D.sub.act is selected so that 
the detector responds to incident photons over a desired range of 
wavelengths by generating a photocurrent I.sub.ph. For example, infrared 
detectors having a band gap energy of 100 meV absorb photons up to 
wavelengths of approximately 12 microns and produces a photocurrent in the 
range of 1-100 nA depending on the area of the detector cell. The band gap 
energy of the reference detector D.sub.ref is preferably matched to the 
active detector's band gap energy so that their dark currents are the same 
and respond equally to changes in temperature. Furthermore, the active 
detector masks the reference detector from incident illumination so that 
the reference detector does not produce photocurrent. As a result, the 
detector current I.sub.ph read out at terminal 22 is ideally all 
photocurrent with no offset. 
The DSP circuit 12 includes a readout circuit 28, an A/D converter 30, and 
a calibration circuit 32. The readout circuit 28 typically includes a 
capacitor that integrates the detection current for a preset integration 
period at which point the capacitor is discharged and reset. Eliminating 
the offset component of the detector current increases the integration 
period, which in turn increases he detector's SNR. The SNR of the 
integrated detector is approximately: 
##EQU1## 
where SNR.sub.active is the SNR without the reference detector. Thus, if 
the dark current is more than approximately twice the photocurrent the SNR 
of the integrated detector is better than known detectors. 
The A/D converter 30 digitizes the voltage signal V.sub.c. Because the 
offset has been substantially removed from the detector current all of the 
A/D converter's bits, typically 14, are used to digitize the signal 
component. This greatly improves the resolution of the signal. 
Alternately, a lower resolution (8 to 10 bits), and thus cheaper, A/D 
converter can be used. The calibration circuit 32 computes the slope of 
the cell's response curve and normalizes it to a reference curve so that 
all of the cells 8 in the array have the same response. The dark current 
will vary by as much as a factor of two across the array. Therefore, it is 
important that each cell have its own calibration circuitry. 
FIG. 2 is an I-V plot 34 for the active detector D.sub.act. In the reverse 
bias mode, the detector operates on the right side of the plot. As shown, 
the dark current I.sub.Dact is approximately 5 to 10 times the 
photocurrent I.sub.ph. The reference detector preferably has the same dark 
current characteristics but does not produce photocurrent. 
FIG. 3 is an I-V plot 36 for the active detector D.sub.act illustrating the 
sensitivity of the dark current I.sub.Dact to temperature changes. The 
dark current increases exponentially as the temperature increases with 
temperature changes as small as one-tenth of a degree producing noticeable 
changes in dark current. As a result, it is preferable that the active and 
reference detectors have the same band gap energy to match their 
temperature responses. Furthermore, the beneficial effect of providing the 
reference detector is greater in high temperature applications. 
As shown in FIG. 4, the integrated photovoltaic detector 10 is preferably a 
four-layer heterojunction structure 38 on a transparent substrate 40. The 
substrate material is selected so that it has a lattice constant that 
matches the heterojunction structure 38 and has a band gap energy wide 
enough to pass the desired wavelengths. In an infrared detector a cadmium 
telluride (CdTe) or cadmium zinc telluride (CdZnTe) substrate having a 
thickness of 500-600 microns is typical. 
The active detector includes a bulk N-type layer 42 on the substrate 40 and 
a thin P-type layer 44 on or in the surface of the bulk layer 42. The bias 
voltage terminal 16 and read out terminal 22 are connected to the N and P 
type layers, respectively. The layers are suitably indium (In) doped 
mercury cadmium telluride (HgCdTe). Alternately, the layers can be 
aluminum gallium arsenide (AlGaAs) or indium antimonide (InSb). The 
detector's band gap energy is set by adjusting the doping levels and the 
volume of material in each cell. The N-type layer is doped with 
approximately 10.sup.15 -10.sup.16 atoms/cm.sup.3 of In and P-type layer 
is doped with approximately 10.sup.17 -10.sup.18 atoms/cm.sup.3. Cell 
sizes range from 25.times.25 microns to 100.times.100 microns on a side 
with P and N layer thicknesses of approximately 2 and 15 microns, 
respectively. The P-type layer 44 preferably has a wider band gap than the 
N-type layer to inhibit dark current in the depletion layer. The two 
layers together must be thick enough to absorb all of the photons in the 
IR spectrum and thereby effectively mask the reference detector. Otherwise 
some photons will penetrate to the reference detector and generate a 
photocurrent in opposition to the active detector. 
The reference detector includes a bulk N-type layer 46 on P-type layer 44 
and a thin P-type layer 48 on or in the surface of the bulk layer 46. The 
bias voltage terminal 14 is connected to the P-type layer 48. The layers 
in the reference detector are preferably formed from the same material as 
the active layers. The layers' doping and volume are preferably controlled 
so that the band gap of N-type layer 46 is matched to that of N-type layer 
42 and the band gap of P-type layer 48 is matched to that of P-type layer 
44 so that the active and reference detectors' dark currents are matched 
over a temperature range. 
The four-layer heterojunction structure is preferably fabricated using 
molecular beam epitaxy (MBE) to grow the layers 42 through 48 on the 
substrate 40. A reactive ion etch is preferably used to delineate the 
individual cells. Etching undercuts the upper layers so that the mesas 
between cells are angled. As a result, N-type layer 46 has to be thicker 
than N-type layer 42 to have the same volume. Alternately, the doping of 
layer 46 can be adjusted to vary the thermal carrier generation rate. This 
allows the leakage currents to be matched despite different volumes. 
While several illustrative embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art. For example, the conductivity types shown in 
FIG. 4 can be reversed. Such variations and alternate embodiments are 
contemplated, and can be made without departing from the spirit and scope 
of the appended claims.