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.
