Semiconductor barrier photo-detector

Abstract

The present invention discloses a photo-detector comprising: an n-type photon absorbing layer of a first energy bandgap; a middle barrier layer, an intermediate layer is a semiconductor structure; and a contact layer of a third energy bandgap, wherein the layer materials are selected such that the first energy bandgap of the photon absorbing layer is narrower than that of said middle barrier layer; wherein the material composition and thickness of said intermediate layer are selected such that the valence band of the intermediate layer lies above the valence band in the barrier layer to create an efficient trapping and transfer of minority carriers from the barrier layer to the contact layer such that a tunnel current through the barrier layer from the contact layer to the photon absorbing layer is less than a dark current in the photo-detector and the dark current from the photon-absorbing layer to said middle barrier layer is essentially diffusion limited and is due to the unimpeded flow of minority carriers, thus reducing generation-recombination (GR) noise of the photo-detector. The principles of the present invention also apply to inverted polarity structures of the form pBp in which all the doping polarities and band alignments described above are reversed.

Claims

1. A photo-detector having pixels comprising: a p-type photon absorbing layer of a first energy bandgap on top of which are located in the following order; a barrier layer; an intermediate layer being a semiconductor structure having a second energy bandgap; and a contact layer of a third energy bandgap; wherein said pixels are fabricated by etching mesas at least through said contact layer and said intermediate layer, each pixel being formed of a mesa having side walls and edges of said intermediate layer in a pixel are exposed at the mesa side walls; wherein layer materials are selected such that the first energy bandgap of the photon absorbing layer is narrower than that of said barrier layer; and wherein the material composition and thickness of said intermediate layer are selected such that the conduction band of the intermediate layer lies below the conduction band in the barrier layer in order to trap minority carriers passing from the barrier layer and transfer the trapped minority carriers to the contact layer, a dark current from the photon-absorbing layer to said barrier layer being diffusion limited and the dark current being due to the unimpeded flow of minority carriers, thus reducing generation-recombination (GR) noise of the photo-detector.

2. A photo-detector according to claim 1, wherein said intermediate layer comprises a sub-band energy level that lies lower in energy than the conduction band of the barrier layer such that minority carriers created in the photon absorbing layer are collected on said sub-band energy level.

3. A photo-detector according to claim 1, wherein the barrier layer is p-type so that the detector is configured and operable to prevent creation of a depletion region in said photon absorbing layer when an operating bias is applied across the detector.

4. A photo-detector according to claim 1, wherein the barrier layer is intrinsic or n-type and the contact layer is p-type so that the detector is configured and operable at close to zero bias whereby a diffusion current is greater than a GR current of minority carriers flowing from the photon absorbing layer to the barrier layer and is also greater than any opposing current due to minority carriers thermally created in the contact layer.

5. A photo-detector according to claim 1, wherein said material composition and thickness of said intermediate layer are selected such that said intermediate layer prevents current flow from said contact layer to said photon absorbing layer while promoting flow of minority carriers from said photon absorbing layer to said contact layer.

6. A photo-detector according to claim 1, wherein said contact layer is made of one of: a Ga.sub.yIn.sub.1yAs.sub.1xSb.sub.x alloy, a Ga.sub.y1In.sub.1y1As.sub.1x1Sb.sub.x1/Ga.sub.y2In.sub.1y2As.sub.1x2Sb.sub.x2 superlattice, a Ga.sub.zAl.sub.1zSb.sub.1wAs.sub.w alloy, and a Ga.sub.z1Al.sub.1z1Sb.sub.1w1As.sub.w1/Ga.sub.z2Al.sub.1z2Sb.sub.1w2As.sub.w2 superlattice with values for the indices which lie in the ranges 0x1, 0.sub.x11, 0.sub.x21, 0y1, 0.sub.y11, 0.sub.y21, 0z1, 0z.sub.11, 0z.sub.21, 0w1, 0w.sub.11, 0w.sub.21.

7. A photo-detector according to claim 1, wherein said photon absorbing layer is made of one of a Ga.sub.yIn.sub.1yAs.sub.1xSb.sub.x alloy or a Ga.sub.y1In.sub.1y1As.sub.1x1Sb.sub.x1/Ga.sub.y2In.sub.1y2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x.sub.11, 0x.sub.21, 0y1, 0y.sub.11, 0y.sub.21.

8. A photo-detector according to claim 1, wherein said barrier layer is made of one of Ga.sub.zIn.sub.yAl.sub.1zySb.sub.1wAs.sub.w alloy or a Ga.sub.z1In.sub.y1Al.sub.1z1y1Sb.sub.1w1As.sub.w1/Ga.sub.z2In.sub.y2A1.sub.1z2y2Sb.sub.1w2As.sub.w2 superlattice with values for the indices of 0z1, 0z.sub.11, 0z.sub.21, 0y1, 0y.sub.11, 0y.sub.21, 0w1, 0w.sub.11, 0w.sub.21.

9. A photo-detector according to claim 1, wherein said intermediate layer is made ofIn.sub.1stGa.sub.tAl.sub.sAs.sub.vSb.sub.1v, with 0s1, 0t1, 0v1.

10. A photo-detector according to claim 1, wherein the thickness of said intermediate layer is selected to be in the range of about 50-200 A.

11. A photo-detector according to claim 1, wherein each of said photon absorbing layer and said contact layer has a thickness in the range of about 0.1-10 m.

12. A photo-detector according to claim 1, wherein the photon absorbing layer has a doping in the range of 510.sup.14 cm.sup.3 <p<510.sup.16 cm.sup.3 and the contact layer has a doping in the range of 510.sup.14 cm.sup.3<p<510.sup.18 cm.sup.3.

13. A photo-detector according to claim 1, wherein said barrier layer has a thickness of between 0.05 and 1 m.

14. A photo-detector according to claim 1, wherein said barrier layer is p-type and is doped in the range of p<110.sup.17 acceptors cm.sup.3.

15. A photo-detector according to claim 1, wherein said barrier layer is n-type and is doped in the range of with n<110.sup.16 cm.sup.3.

16. A photo-detector according to claim 1, wherein the contact layer has a doping in the range of 510.sup.14 cm.sup.3<p<510.sup.18 cm.sup.3.

17. A photo-detector according to claim 1, wherein the contact layer is n-type such that the transfer of minority carriers from the intermediate layer to the contact layer is performed by tunneling.

18. An array of detectors in which each detector is in accordance with claim 1 and which is connected to a silicon readout circuit by an indium bump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1a-1b show exemplary energy band diagrams of a standard nB.sub.nn photo-detector and C.sub.pB.sub.nn photo-detector respectively;

(3) FIGS. 1c-1d show an external Quantum Efficiency (QE) plot for devices with each of the energy band diagrams of FIGS. 1a-1b respectively;

(4) FIG. 2 illustrates in a schematic cross-section form, a possible structural arrangement of an embodiment of the photo-detector of the present invention;

(5) FIG. 3A shows a band diagram of the photo-detector of the present invention according to the embodiment described in FIG. 2 with an n-type barrier;

(6) FIG. 3B is a general knowledge illustration showing the valence band offset as a function of lattice constant for a number of binary and ternary semiconductor materials including InAs, GaSb, AlSb.sub.0.92As.sub.0.08, and InAs.sub.0.91As.sub.0.09;

(7) FIG. 3C shows a band diagram of the photo-detector of the present invention according to the embodiment in FIG. 3A in which the polarity is reversed;

(8) FIG. 4A shows a band diagram of the photo-detector of the present invention according to the embodiment described in FIG. 2 having a p-type barrier layer when the photo-detector is operated at zero bias;

(9) FIG. 4B shows a band diagram of the photo-detector of the present invention according to the embodiment described in FIG. 2 having a p-type barrier layer when the photo-detector is biased; and;

(10) FIG. 4C shows a band diagram of the photo-detector of the present invention according to the embodiment of FIG. 4A in which the polarity is reversed;

(11) FIG. 5A shows a band diagram of the photo-detector of the present invention according to another embodiment of the present invention; and;

(12) FIG. 5B shows a band diagram of the photo-detector of the present invention according to the embodiment of FIG. 5A in which the polarity is reversed.

DETAILED DESCRIPTION OF EMBODIMENTS

(13) Reference is made to FIG. 2 illustrating in a schematic cross-section form, a possible structural arrangement of an embodiment of the photo-detector. The photo-detector 100 comprises a first n-type photon absorbing layer 102 of a first energy bandgap, a doped middle barrier layer 104, an intermediate layer 106 being a semiconductor structure with a second energy bandgap and an n-type contact layer 108 of a third energy bandgap. The layer materials are selected such that the energy bandgaps of the photon absorbing layer and contact n-type layers are narrower than that of the middle barrier layer. In the detector of the present invention the tunnel current from the contact layer to the photon absorbing layer is less than a dark current in the photo-detector and the dark current from the photon-absorbing layer to the middle barrier layer which is (the dominant contribution to the total dark current of the photo-detector and which is) essentially diffusion limited, is due to the unimpeded flow of minority carriers, thus reducing generation-recombination (GR) noise of the photo-detector and reducing noise due to charge build-up in the barrier layer.

(14) In some embodiments, then-type photon-absorbing layer 102 is made of one of: Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x11, 0x21, 0y1, 0y11, 0y21, 0q1, 0q11, 0q21. In a specific and non-limiting example, the n-type photon-absorbing layer 102 is made of InAs.sub.1xSb.sub.x alloy, or an InAs.sub.1x1Sb.sub.x1/InAs.sub.1x2Sb.sub.x2 superlattice. The doping is typically in the range of n<210.sup.16 cm.sup.3 and the thickness is typically in the range 1-10 m. The use of InAs.sub.1xSb.sub.x based alloys or superlattices enables operation in the MWIR atmospheric transmission window (3-5 m). The photon absorbing layer may be buried at a finite depth below the contact layer i.e. typically to a depth of 0.1-10 m. The contact layer 108 is made of Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x.sub.11, 0x.sub.21, 0y1, 0y.sub.11, 0y.sub.21, 0q1, 0q11, 0q21. In a specific and non-limiting example, the contact layer 108 is made of n-type InAs.sub.1xSb.sub.x or an InAs.sub.1x1Sb.sub.x1/InAs.sub.1x2Sb.sub.x2 superlattice with typical values of doping in the range 510.sup.14 cm.sup.3<n<110.sup.18 cm.sup.3 and thickness>0.1 m. The barrier layer 104 is made of Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x.sub.11, 0x.sub.21, 0y1, 0y.sub.11, 0y.sub.21, 0q1, 0q11, 0q21. In a specific and non-limiting example, the barrier layer 104 is made of AlSb.sub.1yAs.sub.y alloy, with thickness typically in the range of 0.05-1 m. The barrier layer 104 is n-type with a typical doping range of 110.sup.15 cm.sup.3n<110.sup.17 cm.sup.3. The n-type doping of the barrier layer prevents the creation of a depletion region in the photon absorbing layer when an operating bias is applied across the detector. The intermediate layer 106 is made of a pseudomorphic semiconductor structure meaning that the layer grows with all of the in-plane lattice spacings in register and equal to each other. The intermediate layer 106 is made of In.sub.1stGa.sub.tAl.sub.sSb.sub.1vAs.sub.v with 0s1, 0t1, 0v1. In some embodiments, the intermediate layer is made from a thin single semiconducting material with no use of metals or insulators. In a specific and non-limiting example, the intermediate layer 106 is made of GaSb and has a thickness in the range of about 50-200 A. The intermediate layer 106 should preferably be undoped.

(15) It should be understood that the selection of InAs.sub.1xSb.sub.x alloy or an InAs.sub.1x1Sb.sub.x1/InAs.sub.1x2Sb.sub.x2 superlattice for the n-type photon-absorbing layer is expected to confer a longer minority carrier lifetime in this layer in comparison with a layer made from a Type II superlattice (InAs/GaSb) as described for example in E. H. Steenbergen et al, Appl. Phys. Lett. 99, 251110, (2011). The hole mobility is also expected to be higher in the alloy and in certain configurations of the InAs.sub.1x1Sb.sub.x1/InAs.sub.1x2Sb.sub.x2 superlattice. However, when InAs.sub.1xSb.sub.x alloy lattice matched to a GaSb substrate is selected to be the material used for the n-type photon-absorbing layer, a (lattice matched) barrier material giving an appropriate band alignment with the valence band (as required in an ideal nB.sub.nn structure) does not exist. The closest is AlSb.sub.0.91As.sub.0.09, but it has an offset of about 150 meV (i.e. it is about 150 meV too high), and this can lead to a blockage between the barrier layer and the contact layer for minority carriers i.e. holes. The present invention enables the removal of the blockage between the barrier layer and the contact layer for minority carriers as well as the operation of the photo-detector at a low bias.

(16) The semiconductor layers are usually grown by modern semiconductor epitaxy methods such as Liquid Phase Epitaxy (LPE), Molecular Beam Epitaxy (MBE), Metal-Organic Vapour Phase Epitaxy (MOVPE), or any of their derivatives, onto a semiconductor substrate [e.g. see Klin et al, Progress with Antimonide Based Detectors at SCD, Proc. Infrared Technology and Applications XXXV, SPIE 7298, 7298-OG (2009)].

(17) After growth, the wafer is etched into a mesa structure, after which the sides are passivated with a suitable chemical treatment and/or with the application of a suitable insulating or dielectric layer and electrical contacts are then applied (though vias opened up in the insulating or dielectric layer). The depth of the mesa can vary but it should be etched at least up to the beginning of the barrier layer (which is fully depleted and therefore insulating) to provide suitable isolation between the mesa and other devices. The passivation performs one or more of the following roles: (i) protection of any exposed surfaces from attack by oxygen (especially those of Aluminium containing compounds such an AlSb.sub.1yAs.sub.y barrier layer) (ii) controlling the surface potential of any exposed surfaces (to avoid surface accumulation or inversion, which would lead, for example, to shorting between neighbouring devices). After the wafer is etched into a mesa structure, only the edges of the intermediate semiconducting layer are exposed (e.g. a region of height 50-200 A). These edges are depicted at the ends of the layer 106 in FIG. 2. The intermediate layer 106 is not connected to any external potential source and does not absorb minority carriers but transfers them. No contact wire is attached to the intermediate layer 106. The energy bands of the intermediate layer 106 must have the correct discontinuous energy band alignment with those of the surrounding materials in order to achieve efficient carrier transfer from the barrier layer 104 to the contact layer 108 thereby conferring a number of benefits including operation at lower bias and lower noise due to no charge build up in the barrier as described above.

(18) The substrate is usually thinned or removed to allow tight to pass without significant losses due to free carrier absorption, and also to avoid possible cracking or other stress related problems at low temperatures.

(19) FIG. 3A shows a band diagram of the photo-detector of the present invention with an n-type barrier layer according to the embodiment described in FIG. 2.

(20) As described with respect to FIG. 2, the photo-detector 100A comprises two n-type narrow bandgap layers 102 and 108 surrounding a middle barrier layer 104 that has an energy bandgap significantly larger than that of the two surrounding layers and which allows the unimpeded diffusion of minority carriers from the photon absorbing layer 102 to the barrier layer 104. An intermediate layer 106 is located between the barrier layer 104 and the contact layer 108. The n-type photon absorbing layer 102 has a narrow energy bandgap selected for its cut-off wavelength and absorbs the radiation impinged on the photo-detector. The middle barrier layer 104 prevents tunneling of electrons from the conduction band (and at sufficiently high bias also from the valence band) of the n-type contact layer 108 to the conduction band of the photon-absorbing layer 102. The contact layer 108 has a narrow bandgap and acts as a contact for biasing the device. The contact layer 108 is biased negative with respect to the photon absorbing layer 102.

(21) In FIG. 3A, .sub.1 indicates the valence band offset between the barrier layer 104 and the active photon-absorbing layer 102 (positive when the valence band of the barrier layer is highest in energy). .sub.2 is the valence band offset between the contact-layer 108 and the barrier layer 104 (positive when the valence band of the contact layer is lowest in energy). A hole sub-band energy level exists in the intermediate layer 106 which is close in energy to the conduction band of the contact layer. Minority carrier holes in the barrier layer 104 illustrated in the figure by circles, collect on this sub-band energy level and then pass easily into the contact layer 108, thereby preventing charge build up in the barrier layer. Thee material composition and thickness of the intermediate layer are selected such that the valence band 110 of the intermediate layer 106 lies close to or above the conduction band 112 in the contact layer 108.

(22) When the photo-detector 100A of the invention is biased to its maximum operating bias, slightly above V.sub.1, with an externally applied voltage, the bands in the photon absorbing layer 102 are flat right up to the barrier layer 104 and minority carriers (holes) can pass freely by diffusion from the photon absorbing layer 102 into the barrier layer 104. The photo-detector will also work at slightly lower bias values, V.sub.1, when the edge of the photon absorbing layer 102 next to the barrier layer 104 can become accumulated. During operation, the flat part of the valence band edge of the photon absorbing layer 102 never lies above the flat part of the valence band edge of the contact layer 108. The edge of the contact layer 108 next to the barrier layer is accumulated both at maximum bias and at lower biases. During operation, a depletion region is allowed only in the barrier layer 104 but not in the active photon-absorbing layer 102. The doping of the barrier layer 104 (and to a lesser degree that in the n-type contact layer 108) is selected according to the present invention to adjust the operating bias to a desirable value.

(23) As will be explained in detail further below with respect to FIGS. 4A and 4B, although the barrier is usually doped n-type, another implementation of the device can be made which operates at a small or even zero bias in which low (typically p<510.sup.15cm.sup.3) p-type doping is used in the barrier layer, due to the asymmetric nature of the layer structure, whereby the intermediate layer acts to enhance the transfer of photoexcited carriers originating in the photon absorbing layer into the contact layer, while preventing back flow.

(24) Some techniques have been developed to provide specific designs of barrier photo-detectors with high performance. Barrier infrared detector with absorber materials having selectable cutoff wavelengths have been developed as described for example in US Patent Publication 2010/072514. A GaInAsSb absorber layer is grown on a GaSb substrate layer formed by mixing GaSb and InAs.sub.1xSb.sub.x by an absorber mixing ratio. A GaAlAsSb barrier layer is then grown on the barrier layer formed by mixing GaSb and AlSb.sub.1yAs.sub.y by a barrier mixing ratio. The technique described in US Patent Publication 2010/072514 proposes to eliminate the offset .sub.2 by using a quaternary GaInAsSb contact layer. However, the growth of a quaternary GaInAsSb material for the contact layer is much harder to control than the growth of a ternary InAs.sub.1xSb.sub.x material, and any segregation of the quaternary into InAs.sub.1xSb.sub.x and GaSb could cause a local type II band alignment with a much smaller bandgap, and thereby lead to a substantial variation in the bandgap.

(25) Moreover, the use of a GaAlAsSb barrier does not eliminate the offsets, .sub.1 and .sub.2, when both the photon absorbing and contact layers are made from the ternary material, InAs.sub.1xSb.sub.x or an InAs.sub.1x1Sb.sub.x1/InAs.sub.1x2Sb.sub.x2 superlattice, for a wide range of useful compositions, x or x.sub.1 and x.sub.2. This may be seen for example in FIG. 3B being the FIG. 11 on page 5855 of the Review article by Vurgaftman et al, Journ. of Appl. Phys., Vol. 89, pp 5815-5875. This figure shows that the valence bands of AlSb.sub.0.92As.sub.0.09 and GaSb lie approximately 150 and 500 meV above the valence bands of InAs.sub.0.91As.sub.0.09 or InAs and that the first three materials have the same lattice parameter as GaSb. Thus an alloy of GaSb and AlSb.sub.0.92As.sub.0.08 that is lattice matched to a GaSb substrate is expected to exhibit a valence band energy that is between 150 and 500 meV above that of lattice matched InAs.sub.0.91As.sub.0.09.

(26) A mixture of the two structures shown in FIGS. 1A and 1B could be used with an appropriate mixing ratio to achieve valence band alignment between the AlSb.sub.0.92AS.sub.0.08 barrier layer and a GaInAsSb contact layer, thereby eliminating .sub.2. The contact layer would then be a GaInAsSb quaternary material which would be hard to grow, and the uniformity of whose bandgap would be hard to control (as already discussed above) over the approximately 1 cm1 cm area of a Focal Plane Array detector.

(27) The combination of barrier layer and absorber must be carefully selected to yield optimal results. Building an optimal nBn or CBn (general name: XBn) infrared detector requires a compatible set of absorber, contact and barrier materials with the following conditions: (1) their valence band edges must be aligned to allow unimpeded hole flow, while their conduction band edges should have a large difference to form an electron barrier, (2) they must have substantially similar lattice constants, and (3) their lattice constants should also match closely to that of a readily available semiconductor substrate material (or a relatively dislocation free buffer layer grown on a readily available substrate) that they are grown on in order to ensure high material quality and low defect density. When criterion (1) is not perfectly fulfilled, so that there is some impediment to the flow of holes due to the valence band of the barrier layer having a higher energy than that for perfect alignment, this impediment can be overcome with the present invention, which therefore provides a simple technique to enable efficient transfer of the minority carriers to the contact layer without blockage, while also effectively preventing the majority carriers in the contact layer from tunneling into the active layer.

(28) FIG. 3C exemplifies a band diagram of the photo-detector of the present invention, in which the doping polarities of the photon absorbing, barrier and contact layers have been reversed compared with the embodiment in FIG. 3A. The photo-detector 100B comprises a photon absorbing p-type layer 102 of a first energy bandgap; a doped middle barrier layer 104; an intermediate layer 106 with a second energy bandgap; and a contact p-type layer of a third energy bandgap 108. The layer materials are selected such that the first and third energy bandgaps of the photon absorbing 102 and contact 108 p-type layers are narrower than that of the middle barrier layer 104. The material composition and thickness of the intermediate layer 106 are selected such that the conduction band 114 of the intermediate layer lies below the valence band 116 in the contact layer 108 to allow efficient transfer of minority carrier (electrons) between the barrier layer and the contact p-type layer.

(29) In some embodiments, the p-type photon-absorbing layer 102 is made of one of: Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x11, 0x21, 0y1, 0y11, 0y21, 0q1, 0q11, 0q21. In a specific and non-limiting example, the p-type photon-absorbing layer 102 is made of an InAs/GaSb superlattice. The doping is typically in the range of 510.sup.14 cm.sup.3<p<510.sup.16 cm.sup.3 and the thickness is typically in the range 1-10 m. The use of InAs/GaSb superlattices enables operation in the MWIR (3-5 m) or the LWIR (8-12 m) atmospheric transmission windows. The photon absorbing layer may be buried at a finite depth below the contact layer i.e. typically to a depth of 0.1-10 m. The contact layer 108 is made of Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2 superlattice with values for the indices of 0x1, 0x.sub.11, 0x.sub.21, 0y1, 0y.sub.11, 0y.sub.21, 0q1, 0q11, 0q21. In a specific and non-limiting example, the contact layer 108 is made of an InAs/GaSb superlattice with typical values of doping in the range 510.sup.14 cm.sup.3<p<510.sup.18 cm.sup.3 and thickness>0.1 m. The barrier layer 104 is made of Ga.sub.qIn.sub.yAl.sub.1yqAs.sub.1xSb.sub.x alloy or a Ga.sub.q1In.sub.y1Al.sub.1y1q1As.sub.1x1Sb.sub.x1/Ga.sub.q2In.sub.y2Al.sub.1y2q2As.sub.1x2Sb.sub.x2/Ga.sub.q3In.sub.y3Al.sub.1y3q3As.sub.1x3Sb.sub.x3/ Ga.sub.q4In.sub.y4Al.sub.1y4q4Sb.sub.x4 superlattice with values for the indices of 0x1, 0x.sub.11, 0x.sub.21, 0x.sub.31, 0x.sub.41, 0y1, 0y.sub.11, 0y.sub.21, 0y.sub.31, 0y.sub.41, 0q1, 0q.sub.11, 0q.sub.21, 0q.sub.31, 0q.sub.41. Therefore, in a specific and non-limiting example, the structure may be a two layer structure, a three layer structure, or a four layer structure. In a specific and non-limiting example, the barrier layer 104 is made of an InAs/AlSb superlattice, with thickness typically in the range of 0.05-1 m. The barrier layer 104 is p-type with a typical doping range of 110.sup.15 cm.sup.3p<110.sup.17 cm.sup.3. The p-type doping of the barrier layer prevents the creation of a depletion region in the photon absorbing layer when an operating bias is applied across the detector. The intermediate layer 106 is made of a pseudomorphic semiconductor structure meaning that the layer grows with all of the in-plane lattice spacings in register and equal to each other. The intermediate layer 106 is made of In.sub.1stGa.sub.tAl.sub.sSb.sub.1vAs.sub.v with 0s1, 0t1, 0v1. In some embodiments, the intermediate layer is made from a thin single semiconducting material with no use of metals or insulators. In a specific and non-limiting example, the intermediate layer 106 is made of InAs or InAsSb alloy and has a thickness in the range of about 100-200 A. The intermediate layer 106 should preferably be undoped.

(30) As will be explained in detail further below with respect to FIG. 4C, although the barrier is usually doped p-type, another implementation of the device can be made which operates at a small or even zero bias in which low (typically n<510.sup.15cm.sup.3) n-type doping is used in the barrier layer, due to the asymmetric nature of the layer structure, whereby the intermediate layer acts to enhance the transfer of photoexcited carriers originating in the photon absorbing layer into the contact layer, while preventing back flow.

(31) Reference is made to FIGS. 4A-4C showing band diagrams of the photo-detector according to some embodiments of the present invention. The photo-detectors 200A-20013-200C comprise a first photon absorbing layer 202 of a first energy bandgap, a doped middle barrier layer 204, an intermediate layer 206 and a contact layer 208. More specifically, the photo-detectors 200A and 200B illustrated in FIG. 4A and in FIG. 4B have a p-type barrier layer 204 and are operated at zero bias and are biased respectively. FIG. 4C shows a band diagram of the photo-detector according to the embodiment of FIG. 4A in which the polarity is reversed and the photo-detector 200C is operated at zero bias. As clearly shown in FIG. 4B, when the photo-detector 200B is biased beyond its small operating bias V.sub.1 such that the quasi-Fermi level 207 in the photon absorbing layer 202 near the barrier layer 204 lies sufficiently far below the conduction band of the photon absorbing layer 202, an unwanted G-R dark current is generated. Therefore the bias must be below V.sub.1 so that the quasi-Fermi level in the photon absorbing layer 202 near the barrier layer 204 lies sufficiently close to the conduction band of the photon absorbing layer 202 to ensure that the dark current from the photon-absorbing layer 202 to the middle barrier layer 204 is essentially diffusion limited.

(32) The device can also be made by introducing an intermediate GaSb layer into a pBn device, between the p-type contact layer and the barrier layer. In this case the p-doping in the contact layer should be quite high to ensure that minority holes illustrated by circles which are collected on the sub-band energy level of the intermediate layer 306 can tunnel efficiently to the valence band of the contact layer 308. An example is to shown in FIG. 5A.

(33) The photo-detector 300A comprises a p-type contact layer 308 and an n-type photon-absorbing layer 302 surrounding a middle barrier layer 304. The n-type photon absorbing layer 302 has a narrow energy bandgap selected for its cut-off wavelength and absorbs the radiation impinged on the photo-detector. The energy bandgap of the n-type photon absorbing layer 302 is narrower than that of the middle barrier layer 304. The middle barrier layer 304 prevents tunneling of electrons from the contact layer 308 to the conduction band of the photon-absorbing layer 302. The contact layer 308 acts as a contact for biasing the device. The contact layer 308 is biased negative with respect to the photon absorbing layer 302. The valence band 312 of the intermediate layer 306 lies higher in energy than the valence band 310 of the barrier layer 304 in order to create an efficient collection of minority carriers illustrated by circles and transfer by tunneling to the p-type contact layer 308 such that a tunnel current from the contact layer 308 to the photon absorbing layer 302 is less than a dark current in the photo-detector, and the dark current from the photon-absorbing layer 302 to the middle barrier layer 304 which is (the dominant contribution to the total dark current of the photo-detector and which is) essentially diffusion limited, is due to the unimpeded flow of minority carriers, thus reducing generation-recombination (GR) noise of the photo-detector and reducing noise due to charge build-up in the barrier layer.

(34) FIG. 5B exemplifies a band diagram of this photo-detector of the present invention, in which the doping polarities of the photon absorbing, barrier 302 and contact layers 308 have been reversed compared with the embodiment in FIG. 5A, and where most of the band offset between the barrier layer and photon absorbing layer is now in the valence band. In this case minority electrons, illustrated by circles, are collected on the sub-band energy level of the intermediate layer 306 and tunnel efficiently to the conduction band of the contact layer 308.

(35) The photo-detector 300B comprises an n-type contact layer 308 and a p-type photon-absorbing layer 302 surrounding a middle barrier layer 304. The p-type photon absorbing layer 302 has a narrow energy bandgap selected for its cut-off wavelength and absorbs the radiation impinged on the photo-detector. The energy bandgap of the p-type photon absorbing layer 302 is narrower than that of the middle barrier layer 304. The middle barrier layer 304 prevents tunneling of carriers from the contact layer 308 to the photon-absorbing layer 302. The contact layer 308 acts as a contact for biasing the device. The contact layer 308 is biased positive with respect to the photon absorbing layer 302. The conduction band 312 of the intermediate layer 306 lies lower in energy than the conduction band 310 of the barrier layer 304 in order to create an efficient collection of minority carriers illustrated by circles and transfer by tunneling to the n-type contact layer 308 such that a tunnel current from the contact layer 308 to the photon absorbing layer 302 is less than a dark current in the photo-detector and the dark current from the photon-absorbing layer 302 to the middle barrier layer 304 which is (the dominant contribution to the total dark current of the photo-detector and which is) essentially diffusion limited, is due to the unimpeded flow of minority carriers, thus reducing generation-recombination (GR) noise of the photo-detector and reducing noise due to charge build-up in the barrier layer.