Modulated multi-quantum well collector for HgCdTe photodiodes

Abstract

A HgCdTe heterojunction photodiode and array of same has a multilayered, modulated multi-quantum well (MMQW) structure 12 interposed between a radiation absorbing base region 10 and an overlying current collector region 18. The MMQW structure is comprised of a plurality of alternating thin layers of wide bandgap CdTe 14 and narrow bandgap HgTe 16 material which together form a plurality of quantum wells in the conduction band. The width of each of the wells is defined by the physical thickness of a corresponding one of the HgTe layers, the width being modulated or varied across the MMQW structure. This variation in HgTe layer width varies the energy of the quantized electronic ground state of each well, wider width wells being associated with higher-lying, less tightly bound, ground states. The thickness of the HgTe layers, and hence the width of each of the wells, is selected such that the ground energy levels of each of the wells will "line-up" within a range of reverse bias potential. A transmission resonance is thus provided for minority charge carriers at their band edge in the base region while simultaneously blocking the transmission of other charge carriers. This results in the unimpeded flow of photocurrent across the device heterojunction while suppressing the tunnelling and g-r components of the dark current.

Description

FIELD OF THE INVENTION

This invention relates generally to mercury-cadmium-telluride (HgCdTe) infrared radiation responsive photodiodes and, in particular, to a HgCdTe heterojunction photodiode having a modulated multi-quantum well (MMQW) structure interposed between the base and collector regions, the MMQW structure providing a transmission resonance for photo-generated minority charge carriers while suppressing transmission of the tunnelling and g-r components of the dark current.

BACKGROUND OF THE INVENTION

In high performance infrared radiation detector arrays, particularly in long wavelength infrared radiation (LWIR) applications (8-12 micrometers), the maximization of the R.sub.o A product of the detectors is an important consideration. In order to maximize the R.sub.o A product it is necessary to cool the detectors below 77 K. At such low temperatures device performance is limited by tunnelling current. In order to reduce this undesirable tunnelling current another conventional approach involves the fabrication of two layer HgCdTe heterojunction photodiodes by a liquid phase epitaxy (LPE) technique, the photodiodes generally having an n-type radiation absorbing base region and a p-type collecting region in intimate contact with the base. However, it has been found that in this type of LPE-grown photodiode that it is difficult to precisely control the relative positions of the metallurgical and the electrical junctions and the grading of these junctions, resulting in a degradation in device performance and a difficulty in achieving a reproducibility of performance.

It is thus one object of the invention to provide an IR responsive photodiode in which components of the dark current are suppressed while permitting the unimpeded flow of photo-generated current.

It is another object of the invention to provide an IR responsive photodiode which is more readily fabricated than conventional LPE grown photodiodes and which provide for a greater reproducibility of performance.

It is another object of the invention to provide an IR responsive photodiode which includes a modulated multi-quantum well structure interposed between the base and collector regions, the structure impeding the flow of components of the dark current while permitting the unimpeded flow of photo-generated current.

It is another object of the invention to provide a HgCdTe IR responsive photodiode having a barrier region comprised of a plurality of CdTe and HgTe layers interposed between the base and collector regions, the thickness of the HgTe layers and, hence, the width of the quantum wells being modulated across the barrier region.

SUMMARY OF THE INVENTION

The above mentioned problems associated with conventional IR responsive photodiode arrays are overcome and the objects of the invention are realized by a HgCdTe heterojunction photodiode and array of same having a multilayered MMQW structure interposed between a radiation absorbing base region and an overlying charge collector region. The MMQW structure is comprised of a plurality of alternating thin layers of wide bandgap CdTe and narrow bandgap HgTe material which together form a plurality of quantum wells in the conduction band. The width of each of the wells is defined by the physical thickness of a corresponding one of the HgTe layers, the width being modulated or varied across the MMQW structure. This variation in HgTe layer width varies the energy of the quantized electronic ground state of each well, wider width wells being associated with higher-lying, less tightly bound, ground states. The thickness of the HgTe layers, and hence the width of each of the wells, is selected such that the ground energy levels of each of the wells will "line-up", or be substantially equalized, within a range of reverse bias potential. A transmission resonance is thus provided for minority charge carriers at their band edge in the base region while simultaneously blocking the transmission of other charge carriers. This results in the unimpeded flow of photocurrent across the device heterojunction while suppressing the tunnelling and g-r components of the dark current.

The widths of the CdTe layers are also predetermined such that substantially all of the electric field required to accommodate the electrochemical potential difference between the base and collector contacts appears across the CdTe layers. An increase in the number of quantum wells in the MMQW structure, that is an increase in the number of alternating HgTe/CdTe layers, increases the effective width of the transmission resonance and also increases the reverse bias range over which the photocurrent can be collected.

The MMQW structure of the invention advantageously improves the performance of LWIR photodiodes operated at temperatures below 77 K since the tunnelling component of the dark current is strongly suppressed. Operation at higher temperatures, that is at 77 K and above, is also improved since the MMQW structure acts to confine the space charge region to the CdTe layers. This suppresses the production of thermally generated pairs in the space charge region with a consequent reduction in the g-r component of the dark current.

The invention also provides LWIR photodiodes which may be fabricated by a combination of a LPE and a molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) technique and also solely by a MBE or MOCVD technique. Thus, fabrication costs are reduced, reproducibility is improved, and device performance is enhanced over conventional LPE fabricated devices.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects of the invention will be made more apparent in the following Detailed Description of a Preferred Embodiment read in conjunction with the accompanying Drawing wherein:

FIGS. 1a, 1b, 1c, 1d and 1e are a series of cross-sectional views, not to scale, of a photodiode constructed in accordance with the invention, the Figure also illustrating the steps of a method which is one aspect of the invention;

FIG. 2a and 2b are energy band diagrams which illustrate the equilibrium and the reversed biased, illuminated states, respectively, of a n+/p homojunction photodetector device, the Figure being provided to facilitate the description of the MMQW photodetector of the invention;

FIG. 3a and 3b are energy band diagrams which illustrate the equilibrium and the reversed biased, illuminated states, respectively, of a first type of n+/MMQW/p heterojunction photodetector device constructed and operated in accordance with the invention; and

FIG. 4a and 4b are energy band diagrams which illustrate the equilibrium and the reversed biased, illuminated states, respectively, of a second type of n+/MMQW/p heterojunction photodetector device constructed and operated in accordance with the invention, the device of FIG. 4 having a wider bandgap collector material than the device of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description of presently preferred embodiments will be made in the context of a LWIR n on p heterojunction HgCdTe mesa-type photodiode. It should be realized that the teaching of the invention is applicable to photodiodes adapted for the detection of other than LWIR radiation, such as photodiodes responsive to SWIR and MWIR radiation. The teaching of the invention is also applicable to other than mesa-type photodiodes such as planar-type photodiodes. The invention is also applicable to p on n heterojunction devices, isotype heterojunction devices and to Schottky devices. Thus, the following description is not intended to be limiting but is provided by example only.

Referring now to FIGS. 1a-1e there is illustrated an exemplary MMQW photodiode constructed in accordance with the invention; FIGS. 1a-1e also illustrating a method of the invention of fabricating a HgCdTe heterojunction photodiode having a MMQW barrier region interposed between a base and collector region.

In FIG. 1a there is shown a base layer 10 which may be grown by a conventional LPE technique. Base layer 10 is comprised of p-type HgCdTe radiation absorbing material and may be adapted for the absorption of LWIR radiation, that is, radiation within the range of approximately 8-12 micrometers. Referring to FIG. 1b there is shown a multilayered structure which is formed upon an upper surface of the base layer 10. The multilayered structure is the MMQW barrier region 12 which can be seen to be comprised of a plurality of layers 14 and 16. Layers 14a, 14b, 14c and 14d are comprised of a high energy bandgap binary compound such as CdTe. Layers 16a, 16b and 16c are comprised of a lower energy bandgap binary compound such as HgTe. In accordance with the invention the HgTe layers 16 are modulated, or varied, in thickness such that the thinnest layer is closest to the surface of the base layer 10 while the thickest layer 16c is furthest removed from the base layer 10. Each of the HgTe layers 16 can be seen to be interposed between two CdTe layers 14, the CdTe layers 14 typically being of a substantially constant thickness. By example, each of the CdTe layers 16 may have a thickness of approximately 20 to 200 angstroms, 100 angstroms being a typical value. The thickness of the HgTe layers 16 may be modulated, or varied, over a range of from approximately 20 angstroms to approximately 200 angstroms.

It should be realized that the number of layers shown in FIG. 1 is exemplary only and that for a given application more or less than this number of MMQW layers may be provided. For example, a device having one HgTe layer interposed between two CdTe layers represents a minimal configuration. However, this minimal configuration does not permit the thickness modulation of the HgCd layers within the MMQW region 12, which is an aspect of the invention. Typically, the MMQW region 12 will comprise a plurality of HgTe layers 16, each of which is interposed between two CdTe layers 14, thereby defining a plurality of varying width quantum wells in the conduction band of the base 10. The total thickness of the MMQW region 12 may range from approximately 0.1 micrometer to approximately one micrometer, a thickness which is substantially less than the typical thickness of the radiation absorbing base region 10. The individual layers 14 and 16 are preferrably grown by a metal organic chemical vapor deposition (MOCVD) technique or by a molecular beam epitaxy (MBE) technique.

Referring now to FIG. 1c there is shown an n-type collector layer 18 which overlies the MMQW region 12. Collector layer 18 may be grown in the same growth apparatus and during the same growth run as the underlying MMQW region 12. It can be seen that n-type collector layer 18 is separated from the p-type base layer 10 by the intervening MMQW barrier region 12. Due to the relative thinness of the layers 14 and 16 the MMQW region 12 defines a plurality of quantum wells which are interposed between the heterojunction defined by opposing surfaces of the layers 10 and 18. The operation of the MMQW region 12 will be described in detail hereinafter in reference to the energy band diagrams of FIGS. 2, 3 and 4.

At this point in the fabrication of the device the n-type collector layer 18 is typically differentiated into a plurality of individual photodiodes, the individual photodiodes typically forming a regular two dimensional array of photodiodes suitable for focal plane applications. One method of differentiating the n-type collector layer 18 is to photolithographically etch a plurality of intersecting V-shaped grooves thereby forming upstanding mesa structures, one of which is shown in FIG. 1d. The V-shaped grooves are etched through the collector layer 18, through the underlying MMQW region 12 and into the p-type base layer 10.

Referring now to FIG. 1e the upper surface of the mesa structure may be coated with a surface passivation layer 20 and also an insulating dielectric layer 22. A window may then be opened through the layers 20 and 22 and a metal contact 24 may be deposited to make an ohmic contact to the collector layer 18 of each of the mesa structures. An ohmic contact may also be made to the p-type base layer 10 in order that each photodiode of the array may be coupled, during operation, to a source of reverse bias potential (V.sub.bias) which is schematically illustrated as a battery 26.

In order to provide a fuller understanding of the operation of the MMQW barrier region 12 shown in FIG. 1 reference will now be made to the energy band diagrams shown in FIGS. 2-4. In these diagrams there is illustrated the valence band, conduction band and the vacuum level. The dashed lines lying between the valence and conduction bands are provided to illustrate the quasi-fermi energies of both electrons and holes. For example, in FIG. 2b the bifurcated dashed lines lying to the left of the potential barrier between the base and collector layers illustrate the quasi-fermi energy levels for electrons and holes in the base layer in the region closely adjacent to the homojunction. The upper extension represents the level for electrons and the lower extension represents the level for holes. These two quasi-fermi levels meet at a point within the bulk material of the base layer where the electrons and holes may be considered, due to recombination, to be in equilibrium.

Inasmuch as the operation of the MMQW barrier region 12 may best be described as an evolution of a homojunction type device reference is made first to FIG. 2. FIG. 2a shows a schematic energy band diagram of a n+/p homojunction photodiode in an equilibrium condition. FIG. 2b shows the homojunction device of FIG. 2a under reverse bias and illuminated conditions. In that FIG. 2 illustrates a homojunction device, the incident radiation is absorbed on both sides of the diode junction thereby creating excess electron-hole pairs. The resulting photocurrent consists of the minority carriers, from either side of the junction, which reach the space charge region and are swept across to the opposite side of the junction. This photocurrent is comprised of three components. A first component comprises electrons which are generated in the p-type base layer and which cross the junction to the n-type collector. A second component comprises holes which are generated in the n-type collector layer and which cross the junction into the p-type base layer. The third component comprises carriers of both types which are generated in the space charge region. Generally, in HgCdTe photodetecting devices only the base layer has a thickness which significantly exceeds the radiation absorption wavelength, resulting in the first of the above mentioned photocurrent components being dominant; that is, electrons generated in the base layer.

The dark current component is comprised of four components. A first component comprises thermally generated electrons in the base region which reach the junction by diffusion. A second dark current component is comprised of thermally generated holes in the junction. A third component is comprised of thermally generated electron-hole pairs in the space charge region (g-r current). The fourth component of the dark current comprises electrons which tunnel from the valence band of the base to the conduction band of the collector. In order to improve the performance of such a photodetector it is necessary to suppress the components of the dark current while preserving the components of the photocurrent.

In accordance with the invention, this improvement in performance is accomplished by the interposition of the MMQW barrier region 12 between the base layer 10 and the collector layer 18. FIGS. 3a and 3b illustrate the energy band diagrams for a photodetector having a MMQW region 12 constructed in accordance with the device shown in FIGS. 1a-1e; FIGS. 3a and 3b illustrating an equilibrium condition and a reverse biased, illuminated condition, respectively. The energy band diagram of FIG. 3 includes three basic assumptions. A first assumption is that a negligible valence band offset exists between the CdTe layers 14 and the HgTe layers 16. A second assumption is that the HgCdTe base layer 10 is a p-type layer such that excess minority carriers generated therein by the absorption of radiation are electrons in the conduction band. A third assumption is that substantially all of the potential difference between the collector 18 and the base 10 appears across the CdTe layers 14 within the MMQW barrier region 12. In FIG. 3b it can be seen that the space charge layer of the photodiode is constrained to coincide with the MMQW barrier region 12. These assumptions simplify the description of device operation, but all can be relaxed while preserving the essence of the device function.

Each of the positive excursions seen in the conduction band energy coincides with a respective one of the CdTe layers 16 of FIG. 1. Similarly, each of the wells coincides with a respective one of the HgTe layers 16. It should be noted that a quantum well nearer the collector layer 18 is wider than a preceding well, the increase in width being a function of the thickness of the corresponding layer 16. This results in the reduction of the g-r component of the dark current and also results in the creation of a barrier in the conduction band which blocks the tunneling component of the dark current. The major component of photocurrent, that component due to injection of electrons into the base which diffuse to the space charge region and which are swept across into the collector, is unimpeded at the reverse bias or range of reverse bias where the ground state energy levels in the MMQW structure line up to permit resonant transmission. The component of photocurrent which results from the injection of holes into the n-type collector is also unimpeded. However, this component is generally minimal and may be suppressed by employing, in accordance with another embodiment of the invention, a wider band gap HgCdTe material for the collector layer 18 than for the base layer 10. This condition of a wider band gap collector is illustrated in the energy band diagrams of FIGS. 4a and 4b. It can be seen in FIG. 4b that no excess electron-hole pairs are injected into the collector by the absorption of radiation therein.

As has been previously stated, inasmuch as the MMQW region 12 is comprised of a plurality of alternating thin layers 14 and 16 of CdTe and HgTe, there is created the series of quantum wells in the conduction band. The width of each of the wells is related to the physical thickness of each of the HgTe layers 16, the width being modulated or varied from the top to the bottom of the MMQW structure. This variation in HgTe layer width varies the energy of the quantized electronic ground state of each well; wider width wells being associated with higher-lying, less tightly bound ground states. The thickness of each of the HgTe layers 16, and hence the width of each of the quantum wells, is selected such that the ground energy levels of each of the quantum wells are substantially equalized one to another under a given magnitude range of reverse bias potential. As can be seen in FIGS. 3b and 4b the ground energy levels assume magnitudes which are equal to or greater than the energy of an electron in the conduction band of the base 10. A transmission resonance is thus provided for minority charge carriers at their band edge in the base layer 10 while simultaneously blocking the transmission of other charge carriers. This results in the unimpeded flow of photocurrent across the device heterojunction while suppressing the tunnelling and g-r components of the dark current.

The widths of the CdTe layers 14 are selected such that substantially all of the electric field required to accommodate the electrochemical potential difference between the base and collector, due to bias source 26, appears across the relatively high resistance CdTe layers 14a-14d. An increase in the number of quantum wells in the MMQW structure, that is an increase in the number of alternating HgTe/CdTe layers, increases the effective width of the transmission resonance and also increases the range of reverse bias potential over which the photocurrent can be collected.

The MMQW barrier region 12 of the invention advantageously improves the performance of LWIR photodiodes operated at temperatures below 77 K since the tunnelling component of the dark current is strongly suppressed. Operation at higher temperatures, that is at 77 K and above, is also improved since the MMQW structure acts to confine the space charge region to the relatively thin, compared to the base and collector layers, MMQW region 12. This consequently suppresses the production of thermally generated pairs in the space charge region with a consequent reduction in the g-r component of the dark current.

The invention also provides LWIR photodiodes which may be fabricated by the LPE technique in combination with MBE or MOCVD, or to solely MBE or MOCVD techniques. Fabrication costs are thereby reduced, reproducibility is improved, and device performance is enhanced over conventional LPE fabricated devices.

In general, the assumption that there is no significant valence band offset between the CdTe layers 14 and the HgTe layers 16 within the MMQW barrier region 12 is not essential to the operation of the invention. It is made only for convenience in the simplification of the energy band diagrams of FIGS. 2-4. Even a significant valence band offset between the CdTe and HgTe layers would not significantly alter the operation of the photodetector. Furthermore, the assumption of a p-type base is also not essential to the operation of the invention. If the valence band offset is significant it is within the scope of the invention to employ an n-type base region having an overlying MMQW barrier region 12 having quantum barriers and wells in the valence band which are optimized for the resonant transmission of holes from the valence band of the base layer to the valence band of the collector layer.

Based upon the foregoing description of preferred embodiments of the invention those having skill in the art may derive modifications to these embodiments. Thus, the invention is not intended to be limited to only these embodiments but is instead to be limited only by the scope of the following claims.

Claims

1. An infrared radiation responsive photodiode comprising:

a radiation absorbing base region comprised of HgCdTe, said base region being operable for generating a photocurrent from the absorbed radiation;

a photocurrent collector region comprised of HgCdTe and forming a heterojunction with said base region; and

a quantum well region interposed between said base and said collector regions, said quantum well region comprising at least two layers of CdTe and at least one layer of HgTe disposed between said CdTe layers.

2. A photodiode as defined in claim 1 wherein said base region and said collector region each have an energy bandgap which are substantially equal in magnitude one to another.

3. A photodiode as defined in claim 1 wherein said base region and said collector region each have an energy bandgap, the energy bandgap of said collector region being wider than that of said base region.

4. An infrared radiation responsive heterojunction HgCdTe photodiode comprising:

a radiation absorbing base region comprised of HgCdTe having a first type of electrical conductivity, said base region being operable for generating a photocurrent, including minority charge carriers, from absorbed radiation;

a photocurrent collector region comprising HgCdTe having a second type of electrical conductivity and forming a heterojunction with said base region; and

a multilayered region interposed between said base and said collector regions and having a total thickness which is substantially less than a thickness of either said base region or said collector region, said multilayered region having a plurality of layers comprised of CdTe, pairs of which have a layer comprised of HgTe disposed therebetween, a thickness of each of the HgTe layers being sufficiently narrow so as to define a plurality of quantum wells at least in the conduction band between said base region and said collector region, a width of each of said quantum wells being a function of the thickness of a corresponding layer of HgTe.

5. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 4 wherein:

the width of individual ones of the HgTe layers is varied across the multilayered region such that, at a predetermined voltage potential across said multilayered region, a magnitude of a ground state energy level of each of the plurality of quantum wells is made substantially equal one to another, the magnitudes of the ground state energy levels being at least equal to or greater than a conduction band energy level of minority carriers in said base region at an edge of the conduction band such that the minority carriers cross to said collector region.

6. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 5 wherein said photodiode is a mesa-type photodiode having outwardly sloping sidewalls which extend through said collector region, through said multilayered region and into said base region.

7. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 5 wherein:

the ground state energy levels of each of said quantum wells is greater than an energy level of a tunnelling component of a dark current such that the tunnelling component of the dark current is prevented from crossing to said collector region.

8. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 7 wherein:

an electrical field related to the voltage potential across said multilayered region is developed across substantially only said plurality of CdTe layers such that a space charge region of said photodiode is constrained to exist substantially only within said relatively thin multilayered region.

9. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 8 wherein:

a component of the dark current associated with electron-hole pairs which are thermally generated within the constrained space charge region is reduced.

10. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 9 wherein:

the thickness of said multilayered region varies within a range of approximately 0.1 micrometer to approximately 1.0 micrometer.

11. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 10 wherein:

the thickness of each of said CdTe layers is approximately 20 angstroms to approximately 200 angstoms.

12. An infrared radiation responsive heterojunction HgCdTe photodiode as defined in claim 10 wherein:

the thickness of each of said HgTe layers is approximately 20 angstroms to approximately 200 angstroms.

13. In an infrared radiation responsive heterojunction HgCdTe photodiode, a method of substantially eliminating a tunnelling component of a dark current, comprising the steps of:

fabricating a multilayered structure between a radiation absorbing base layer and a collector layer, the structure having a total thickness which is substantially less than a thickness of either the base layer or the collector layer, the multilayered structure being fabricated by forming a plurality of layers comprised of CdTe, pairs of which have an intervening layer comprised of HgTe formed therebetween, a thickness of each of the HgTe layers being sufficiently narrow so as to define a plurality of quantum wells at least in the conduction band between the base region and the collector region;

varying the width of individual ones of the quantum wells by varying, during the forming of the plurality of layers, the thickness of the layers of HgTe such that a thickest layer is disposed nearest the collector layer and a thinnest layer is disposed nearest the base layer; and

reverse biasing the photodiode with a reverse bias potential having a magnitude selected such that a magnitude of a ground state energy level of each of the plurality of quantum wells is made substantially equal one to another, the magnitudes of the ground state energy levels being at least equal to or greater than a conduction band energy level of minority carriers in the base layer at an edge of the conduction band such that the minority carriers cross to the collector region; wherein

the ground state energy levels of each of the quantum wells is greater than an energy level of a tunnelling component of a dark current such that the tunnelling component of the dark current is suppressed from crossing to the collector region.

14. A method as set forth in claim 13 wherein:

each of the HgTe layers is formed to have a thickness of between approximately 20 angstroms to approximately 200 angstroms.

15. In an infrared radiation responsive heterojunction HgCdTe photodiode, a method of reducing a width of a space charge region to reduce a magnitude of a g-r component of a dark current generated in the space charge region, comprising the steps of:

fabricating a multilayered structure between a radiation absorbing base layer and a collector layer, the multilayered structure being fabricated by forming a plurality of thin layers comprised of CdTe, pairs of which have an intervening thin layer comprised of HgTe formed therebetween, a thickness of each of the HgTe layers being sufficiently narrow so as to define a plurality of quantum wells at least in the conduction band between the base region and the collector region;

reverse biasing the photodiode with a reverse bias potential; and

constraining the space charge region to occupy substantially only the thickness of the multilayered structure thereby generating the g-r components substantially only within the constrained space charge region, wherein the step of constraining is accomplished by developing the reverse bias voltage potential across substantially only the CdTe layers within the multilayered structure.

16. A method as defined in claim 15 wherein the step of fabricating forms the multilayered structure with a thickness which varies within a range of approximately 0.1 micrometer to approximately 1.0 micrometer.

17. A method as defined in claim 16 wherein the step of fabricating forms the thickness of each of the CdTe layers within a range of approximately 20 angstroms to approximately 200 angstoms.

18. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes comprising the steps of:

providing a radiation absorbing base region comprised of HgCdTe having a first type of electrical conductivity, the base region being operable for generating a photocurrent, including minority charge carriers, from absorbed radiation;

forming a multilayered region upon the base region, the multilayered region including a plurality of layers comprised of CdTe, pairs of which have a layer comprised of HgTe disposed therebetween;

forming a photocurrent collector region upon the multilayered region, the collector region comprising HgCdTe having a second type of electrical conductivity and forming a heterojunction, through the multilayered region, with the base region; wherein

a thickness of each of the HgTe layers is formed to be sufficiently narrow so as to define a plurality of quantum wells at least in the conduction band between the base region and the collector region, a width of each of the quantum wells being a function of the thickness of a corresponding layer of HgTe; and

differentiating the collector region and heterojunction into a plurality of collector regions and heterojunctions for defining the individual photodiodes of the array.

19. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of differentiating is accomplished by forming a plurality of mesa structures each of which has side walls which extend downwards through the collector region, through the multilayered region and into the base region.

20. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of forming at least the multilayered region is accomplished by MOCVD.

21. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of forming at least the multilayered region is accomplished by MBE.

22. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of providing a base region is accomplished by providing a base region having a first energy band gap and wherein the step of forming a collector region is accomplished by forming a collector having a second energy band gap, the second energy band gap being substantially equal in magnitude to the first energy band gap.

23. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of providing a base region is accomplished by providing a base region having a first energy band gap and wherein the step of forming a collector region is accomplished by forming a collector having a second energy band gap, the second energy band gap being wider than the first energy band gap.

24. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 18 wherein the step of forming a multilayered region includes the step of:

varying the width of individual ones of the HgTe layers across the multilayered region such that, at a predetermined voltage potential across the multilayered region, a magnitude of a ground state energy level of each of the plurality of quantum wells is made substantially equal one to another, the magnitudes of the ground state energy levels being at least equal to or greater than a conduction band energy level of minority carriers in the base region at an edge of the conduction band such that the minority carriers cross to the collector region.

25. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 24 wherein the step of varying the width of individual ones of the HgTe layers is accomplished such that the ground state energy levels of each of the quantum wells is greater than an energy level of a tunnelling component of a dark current such that the tunnelling component of the dark current is prevented from crossing to the collector region.

26. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 25 wherein the step of forming the multilayered region is accomplished such that an electrical field related to the voltage potential across the multilayered region is developed across substantially only the plurality of CdTe layers such that a space charge region of the photodiode is constrained to exist substantially only within the relatively thin multilayered region.

27. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 26 wherein a component of the dark current associated with electron-hole pairs which are thermally generated within the space charge region is reduced.

28. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 27 wherein the step of forming the multilayered region is accomplished by forming the thickness of the multilayered region within a range of approximately 0.1 micrometer to approximately 1.0 micrometer.

29. A method of fabricating an array of infrared radiation responsive heterojunction HgCdTe photodiodes as defined in claim 28 wherein the step of forming the multilayered region is accomplished by forming the thickness of each of the CdTe layers within a range of approximately 20 angstroms to approximately 200 angstoms and by forming the thickness of each of the HgTe layers within a range of approximately 20 angstroms to approximately 200 angstroms.