Multi colour photon detectors

1. An electromagnetic radiation detector responsive to n discrete wavelength ranges, where n is odd and greater than or equal to 3, characterised as λ 0 -λ 1 , λ 1 -λ 2 , . . . , λ n−1 -λ n within a total wavelength range λ 0 -λ n where λ 0 <λ 1 <λ 2 < . . . λ n−1 <λ n , the detector comprising a plurality of layers of semiconductor material providing:

a substrate substantially transparent to electromagnetic radiation within said wavelength range λ 0 -λ n ;

a first sequence of layers, each doped to provide a first type of electrical conductivity, and each having a different bandgap selected for absorbing radiation up to selected first wavelengths of λ (y+1) , where y=0 or is an even number between 0 and n, in accordance with the value of y;

a second sequence of layers, interspersed with layers of said first sequence, each doped to provide a second type of electrical conductivity, and each having a different bandgap selected for absorbing radiation up to selected second wavelengths of λ (z+1) , where z is an odd number from 1 to n−1, in accordance with the value of z, layers of the first sequence alternating with layers of the second sequence;

a third sequence of layers, each doped to provide the second type of electrical conductivity, interspersed between said first and second sequence of layers, each of said third sequence of layers, having a bandgap substantially greater than its adjacent layer of said second sequence of layers; and

terminals electrically coupled to each layer in said first sequence of layers to apply a bias to said first and second sequence of layers to permit detection of radiation in selected ones of the wavelength ranges λ 0 -λ 1 to λ n−1 -λ n , the number of terminals being determined by (n+1)/2.

2. A detector according to claim 1 , wherein the first sequence of layers is n-type material and said second sequence of layers is p-type material.

3. A detector according to claim 1 , wherein n=3 and detection of radiation is allowed in selected ones of the wavelength ranges λ 0 -λ 1 , λ 1 -λ 2 , λ 2 -λ 3 , λ 0 -λ 2 and λ 1 -λ 3 .

4. A detector according to claim 1 , wherein n=5 and detection of radiation is allowed in selected ones of the wavelength ranges λ 0 -λ 1 , λ 0 -λ 2 , λ 1 -λ 2 , λ 1 -λ 3 , λ 2 -λ 3 , λ 2 -λ 4 , λ 3 -λ 4 , λ 3 -λ 5 and λ 4 -λ 5 .

5. A detector according to claim 1 , wherein the semiconductor material comprises Group II-VI semiconductor material.

6. A detector according to claim 5 , wherein the semiconductor material comprises cadmium mercury telluride (Hg 1−x Cd x Te).

7. A detector according to claim 1 , wherein the substrate comprises gallium arsenide, gallium arsenide on silicon, cadmium telluride, cadmium zinc telluride, cadmium telluride on silicon or cadmium telluride on sapphire.

8. A detector according to claim 1 , further comprising an anti-reflection coating disposed on a surface of the substrate, the substrate surface being a radiation-admitting surface of the detector.

9. A detector according to claim 1 , wherein a lower limit λ 0 of the first wavelength range is modified by the composition of a layer in the detector.

10. A detector according to claim 1 , wherein a lower limit λ 0 of the first wavelength range is modified by an optical filter.

11. A detector according to claim 1 , wherein the electromagnetic radiation detector is a photodiode.

12. A detector according to claim 2 , wherein n=3 and detection of radiation is allowed in selected ones of the wavelength ranges λ 0 -λ 1 , λ 1 -λ 2 , λ 2 -λ 3 , λ 0 -λ 2 and λ 1 -λ 3 .

13. A detector according to claim 2 , wherein n=5 and detection of radiation is allowed in selected ones of the wavelength ranges λ 0 -λ 1 , λ 0 -λ 2 , λ 1 -λ 2 , λ 1 -λ 3 , λ 2 -λ 3 , λ 2 -λ 4 , λ 3 -λ 4 , λ 3 -λ 5 and λ 4 -λ 5 .