GB2111197A - Photoconductive detector bias - Google Patents

Abstract

It is a problem extracting the photosignal component from detector output, to the exclusion of pedestal bias response. To overcome this, a time varying bias signal is applied to the detector. The duration of the time varying bias signal, or if a periodic signal, the signal period, is chosen as long compared to photocarrier lifetime and the signal amplitude is large enough to range over a non-linear portion of the responsivity characteristic of each element. The bias signal contains a d.c. component so that the bias signal ranges about a point of operation-a point of asymmetry lying on the responsivity characteristic. The photosignal component of the output signal may be removed by time averaging or by harmonic separation. Alternatively the alternating component of the bias signal may be amplitude modulated at a lower frequency, and the photosignal component extracted by detection of demodulated signal. <IMAGE>

Description

SPECIFICATION A method of biassing a photoconductive detector, and detector apparatus therefor Technical field This invention concerns photoconductive detector biassing and detector apparatus; in particular detector apparatus including: a detector comprising at least one photoconductive element; a bias source, connected to the detector, for applying bias to each element of the detector; and, an output circuit, connected to each element, responsive to electrical output signal from each element, to extract from each electrical output signal a photosignal dependent on the intensity of radiation incident upon each element; and, in particular a method of biassing a photoconductive detector wherein bias is applied to each element of the detector and a photosignal dependent on radiation intensity is extracted from the output signal developed by each element.

Photoconductive detectors, particularly those sensitive to infra-red radiation, have been considered for use in imaging applications. They may also find application in future laser communication and laser rangefinding systems.

Background art Conventional photoconductive detectors comprise one or more square elements of photosensitive material, each element having a pair of spaced bias contacts. For imaging applications, such a detector is placed in the image plane of an optical assembly and is usually shielded to reduce the incidence of background illumination upon the detector. The detector is usually mounted on a cold stage and is cooled to enhance signal-over-noise discrimination. In one form of conventional detector apparatus using intrinsic photoconductive elements responsive to the middle and far-infra-red region of the spectrum, a steady direct current (DC) bias, from a constant current source, is applied to each element. There is thus developed across each detector element a bias pedestal voltage, a voltage dependent on bias current magnitude and element resistance.When radiation of appropriate wavelength is incident upon the detector elements, photo-signals-in this case photovoltages-are developed and these increment the voltage provided by each element.

The incremental photosignal voltage is, for normal radiation intensities, of magnitude several orders smaller than the magnitude of the bias pedestal, and it is usual to back-off each element voltage by subtracting DC voltage to allow extraction and amplification of the photosignal. However, to be wholly effective the back-off voltage applied, in each case, must follow changes in the pedestal voltage. Such changes may occur, for example, as a result of cold stage temperature drift, of change in ambient temperature, of change of average background illumination, and of bias current drift.

Such pedestal voltage changes are in general also orders of magnitude higher than the photosignal increment. Furthermore the pedestal voltage and the change of this voltage will vary from element to element. In general the resistance of each element will differ, since material resistivity and element dimensions vary within manufacturing tolerance. Because of non-uniformity in the bias pedestal, it is in the very least difficult, if not impractical, in unscanned, so called "staring" systems, to back off element voltage satisfactorily so that the wanted illumination dependent photosignal can be extracted without the introduction of an unacceptable degree of fixed pattern noise. It is also possible to operate these detectors using constant voltage drive bias instead of constant current in which case device current is measured.This too requires bias compensation, and this likewise introduces fixed pattern noise.

Because of these difficulties, progress in photoconductive detector development is impeded and this development is giving way to the alternative development of photovoltaic detectors, albeit this latter involves a more complex, generally more expensive and less far advanced technology.

Alternative to the use of DC bias, experimental use of microwave frequency alternating current bias has been reported in the literature (A. S.

Sommers Jr., Microwave biassed photoconductive detector, Chapter 11 (page 435) Semiconductors and Semimetals Vol. 5, Eds Willardson and Deer (Academic Press) 1970). The DC responsivity (ie voltage increment for unit intensity of radiation of appropriate wavelength) of a photoconductive element is limited by photocarrier recombination losses occurring at the bias contacts. However using very high frequency alternating bias as reported it is possible to reduce these forced recombination losses, since the flow direction of the photocarriers can be reversed before many of the photocarriers reach the bias contacts. In this case the photocarrier density is limited instead by natural recombination losses in the element material bulk, these carriers recombining within a natural average lifetime. Much higher linear responsivity (AC) is claimed to be attained.

However, as reported, the detector is biassed in the microwave field of a tuned microwave resonant cavity. Such apparatus is complex, difficult to set up accurately, and is expensive. So far as is known, such apparatus has not been applied commercially. The principle merit is that it allows the photoconductor to be biassed at high fields without suffering the consequential effects of carrier loss at the contacts. The developed photosignal and bias response however are both linear and suffer the same fixed pattern noise problem.

Description of the invention This invention is intended to provide a remedy; a method of biassing-as also detector apparatus-both enabling the extraction of useful illumination dependent photosignal from element signal.

According to the invention there is provided a method of biassing a photoconductive detector characterised in that the bias applied is of time varying amplitude and has such peak amplitude as to range over a substantially non-linear portion of the responsivity characteristic of each element of the detector, the bias amplitude varying in a time that is significantly longer than the photocarrier lifetime.

It is convenient to use as bias, an alternating bias or a bias having an alternating component~ eg an alternating current applied at a finite dc current level, or an alternating voltage superimposed on a finite dc voltage.

The useful photosignal component of the resulting element signal may be extracted by harmonic separation. However, provided that the bias alternates about a point of operation lying on the responsivity characteristic of each element above and below which point the responsivity changes by different degree and this is so if the bias applied has both an alternating component and a finite dc component-the photosignal component of the element signal may instead be extracted by rectification.

In further accordance with the invention there is provided detector apparatus for performing the method above described, the apparatus being characterised by a bias source arranged to apply a cyclic bias having both an alternating component and a finite dc component, the bias having such peak amplitude as to range over a substantially non-liner portion of the responsivity characteristic of each element of the detector, and the bias having a cycle period significantly longer than the photocarrier lifetime.

Useful signal may be extracted by means of a filter arranged to pass dc signal-the dc signal induced by the non-linear function of the detector-itself induced by incident illumination.

Alternatively, useful signal may be extracted as an harmonic of the frequency of the alternating bias, using a high pass filter or phase sensitive circuit. The filtered harmonic can also be subsequently converted to a dc signal by simple rectification.

In preference to the foregoing, however, it can be advantageous to apply as the alternating component of the bias, an amplitude modulated alternating bias, and to extract the wanted signal as a demodulation signal, using a low pass filter to block signal at the higher alternating frequency (ie the carrier frequency of the bias) and to pass signal at the modulation frequency. Since modulation and carrier frequencies with well chosen separation can be utilised, the extraction filter design can accordingly be much simplified.

The detector may be used to receive modulated radiation, and accordingly the filter design can be chosen appropriately.

As discussed in the description that follows hereinafter, when using current bias, the preferred value of DC bias current may be somewhat lower than that of DC current bias ordinarily applied, and is such as to produce a bias field E a factor between 0.5 and 0.7 of the sweep-out bias field Eo ordinarily applied~ 0.5Eo < EA0.7Eo and preferably E#0.6E0 where the ordinary bias field Eo is given by the relation: ,ualEoTil= 1 ssa: being the photocarrier ambipolar mobility; T: z: the natural lifetime of the photocarriers; and, I: the contact-to-contact spacing of each element.

The preferred value of AC bias current peak amplitude also optimally corresponds to a peak amplitude bias field É a factor between 2.0 and 4.0 of the DC bias field E: 2.0E < Ê < 4.0E and preferably E#3.0E.

In this way the peak envelope power dissipation necessary to produce best performance can be minimised.

Brief description of the drawings In the accompanying drawings.~ Figure 1: is a schematic illustration of detector apparatus, the detector included in this apparatus having a single photoconductive element; Figure 2: is a graph showing element responsivity as a function of applied bias level; Figure 3: is a graph showing the relationship between AC bias peak amplitude and DC bias level for constant signal peak amplitude and constant peak envelope power; and, Figures 4 and 5: are illustrations of modulated bias and filtered demodulated photosignal waveforms, respectively.

Description of the preferred embodiment An embodiment of the invention will now be described, by way of example only, with reference to the drawings. The detector apparatus shown in figure 1 comprises a single element photoconductive detector 1, the element being of rectangular, in fact square, form. This element is of n-type cadmium mercury telluride material, suitable as a detector of infra-red radiation in the 8 to 14 y window band of the electromagnetic spectrum and having a sensitivity peaked at about 10 y wavelength. The element is of conventional size, approximately 50 y square between gold metal contacts 3 and 5. The resistance of the element lies between 30 and 40 ohms. The element contacts are connected to a high impedance current bias source 7 which provides alternating current bias at a finite DC level. Since the source is of high impedance, both AC peak amplitude and DC bias level are relatively insensitive to changes of element impedance.

Signal voltage developed across the biassed element is relayed to an output filter 11, which serves to extract the photosignal component of the output signal voltage at a frequency other than the bias frequency.

For operation, the detector 1 is cooled to liquid nitrogen temperature and is located at the image plane of an optical assembly (not shown).

To give reasonable photosignal output at a modest peak power level the DC level is set at approximately 2.0 mA magnitude and the AC peak amplitude a factor 3.0 times higher, at 6.0 mA peak. The alternating bias current frequency, which is not critical, is chosen as 10 kHz and this alternating bias is amplitude modulated at somewhat lower frequency, a modulation frequency of approximately 1 kHz. To give largest demodulated signal output, the modulation is set at 100%. The waveform of this alternating bias is shown in figure 4.

The responsivity characteristic of the element is shown in figure 2. As can be seen from this characteristic, at low values of bias field, the responsivity magnitude increases linearly with increasing magnitude of bias. This linear region corresponds to a regime where photocarrier recombination in the material bulk predominates.

However as the bias field is increased to higher values, photocarriers recombine at the bias contact and this loss mechanism begins to predominate. The responsivity increase becomes less pronounced and a saturation value of the responsivity, Rmax, is approached asymptotically.

Over this region the characteristic is markedly non-linear. It is also noted that the response characteristic has a symmetry (in fact it is antisymmetric) about the axes centre 0, the point of zero bias.

The bias level of 2.0 mA corresponds to the point Op marked on the characteristic, this corresponding to a bias field E of 0.6 times the sweep-out field value Eo E=0.6 EO; MEOT/I=1 About this operation point Op, the alternating bias current swings between a maximum positive excursion +8.0 mA (#Ei/l=2.4) and a maximum negative excursion~4.0 mA (#ETA=-1 .2). In the absence of illumination, the detector appears as a simple constant resistance. In the presence of illumination the resultant output signal shows a significant degree of distortion due to the saturation of the responsivity to radiation at high bias levels.There is a partial rectification corresponding to a demodulation of the bias and the demodulation signal can then be removed by AC coupling, further filtered to extract a signal varying at the modulation frequency, and rectified to give a DC output level proportional to incident illumination intensities.

The criteria for choosing the point of operation and peak AC bias magnitude can be ascertained from figure 3. Contours are drawn for different parameter values of output signal peak amplitude:-factorsO.05, 0.06, . . ., 0.11 of standard amplitude, and have been calculated for an ideal detector. The signal strength for a fully swept out detector (saturated responsivity) is 0.5 units. (For comparison with a conventional DC biased detector, this standard is taken as the photosignal amplitude for unit intensity of radiation for a DC bias field E0(,uE0TA=1) applied to the same detector for which the signal is 0.37 in the same units). These are shown in bold outline.Contours are also shown for different parameter values of peak envelope power:~0.5, 1.0 . . ., 2.5 units of standard power. [Standard power is taken as the power dissipated for the DC bias field E,. ] These are shown in broken outline.

As can be seen from figure 3, for a given value of signal amplitude, the minimum of peak envelope power is dissipated for a bias of AC peak magnitude a factor of approx. 3.0 times the value of DC bias-as illustrated by the trace line of slope=3.0. The peak envelope power increases only marginally for factors between say 2.0 and 4.0 as shown by the bounding lines slope 2.0 and 4.0. The operating point x (DC=0.6, AC=1.8) is shown. This corresponds to a signal amplitude a little in excess of 0.10 standard units, a peak power of 1.9 standard, and mean power 0.9 standard. Thus for these conditions the mean power demand is a little less than standard.

However, it is the value of peak envelope power and in particular of peak power heating, that limits operation to below high values of DC bias.

Thus to confine operation within reasonable bounds, at least from the figure, it appears that the DC bias field is best limited to a field value 0.7Eo maximum to avoid excessive peak power dissipation and to a field value 0.5Eo minimum (not so critical) to achieve reasonable signal amplitude.

The contours illustrated have been drawn for an ideal element and no account has been taken of contact resistance or other factors. In practice, therefore, the limits given may require modification and optimization.

AC or DC signal back-off as appropriate may be used to set the level of the demodulated signal. It may be chosen to provide zero signal level for background illumination, and thus as a means of enhancing image signal contrast and allowing more effective use of following amplifiers.

Claims (11)

Claims

1. A method of biassing a photoconductive detector wherein bias is applied to each element of the detector and a photosignal dependent on radiation intensity is extracted from the output signal developed by each element, characterised in that the bias applied is of time varying amplitude and has such peak amplitude as to range over a substantially non-linear portion of the responsivity characteristic of each element of the detector, the bias amplitude varying in a time that is significantly longer then the photocarrier lifetime.

2. A method of biassing as claimed in claim 1 wherein the bias is alternating or has an alternating component.

3. A method of biassing as claimed in either claim 1 or claim 2 above wherein photosignal is extracted by harmonic separation.

4. A method of biassing as claimed in claim 2 wherein the bias has both an alternating component and a d.c. component, and photosignal is extracted by rectification of the output signal developed by each element.

5. A method of biassing a photoconductive detector performed substantially as described hereinbefore with reference to figures 1 to 5 of the accompanying drawings.

6. Detector apparatus for performing the method as claimed in claim 1 above, including:~ a detector comprising at least one photoconductive element; a bias source, connected to the detector, for applying bias to each element of the detector; and, an output circuit, connected to each element, responsive to electrical output signal from each element, to extract from each electrical output signal a photosignal dependent on the intensity of radiation incident upon each element; the apparatus being characterised in that the bias source is arranged to apply a cyclic bias having both an alternating component and a finite d.c.

component, the bias having such peak amplitude as to range over a substantially non-linear portion of the responsivity characteristic of each element of the detector, and the bias having a cycle period significantly longer than the photocarrier lifetime.

7. Apparatus as claimed in claim 6 wherein each output circuit includes a filter arranged to extract photosignal as a d.c. signal.

8. Apparatus as claimed in claim 6 wherein each output circuit includes a high pass filter or phase sensitive detector such that photosignal may be extracted at an harmonic of the bias frequency.

9. Apparatus as claimed in claim 6 wherein the bias source is arranged to apply a cyclic bias having as alternating component an amplitude modulated bias, and each output circuit includes a low pass filter for blocking signal at the carrier frequency of the bias and for passing signal at the modulation frequency.

10. Apparatus as claimed in any one of preceding claims 6 to 9, wherein: the amplitude E of the field for the dc bias lies in the range 0.5Eo6E < O-7Eo where Eot as defined in terms of the photocarrier ambipolar mobility jua, the natural lifetime of the photocarriers z and element length from contact to contact I, is given byt juaEOt/I=1; and, the peak amplitude of the a.c. bias field E lies in the range.~ 2.0E6E64.0E

11. Apparatus for performing the method as claimed in claim 1 above, constructed, arranged and adapted to operate substantially as described hereinbefore with reference to and as shown in figures 1 to 5 of the accompanying drawings.