K. Fynn

A discrete element model of laser beam induced current (LBIC) due to the lateral photovoltaic effect in open-circuit HgCdTe photodiodes

The non-destructive optical characterization technique of Laser-Beam-Induced-Current (LBIC) imaging has proven useful in qualitatively assessing electrically active defects and localized non-uniformities in HgCdTe materials and devices used for infrared photovoltaic arrays. To further the development of a quantitative working model for LBIC, this paper focuses on the application of the technique to photovoltaic structures that are represented by a discrete element equivalent circuit. For this particular case the LBIC signal arises due to the lateral photovoltaic effect in non-uniformly illuminated open-circuit photodiodes. The outcomes of the model predict all of the experimentally observed geometrical features of the LBIC image and signal. Furthermore, the model indicates that the LBIC signal has an extremely weak dependence on the p-n junction reverse saturation current, and shows a linear dependence with laser power. This latter feature map be useful for non-contact measurement of the quantum efficiency of individual photodiodes within a large two-dimensional focal plane array. The decay of the LBIC signal outside the physical boundary of the p-n junction is of the same form as the roll-off in the short circuit photoresponse and, therefore, can be used to extract the diffusion length of minority carriers. Experimental data is obtained from an arsenic implanted p-on-n junction fabricated on MBE grown Hg/sub 1-x/Cd/sub x/Te material with an x-value of 0.3. The p-on-n diode is shown to be uniform and of high quality with an R/sub 0/A product of 1/spl times/10/sup 8/ /spl Omega//spl middot/cm/sup 2/ at 77 K. The validity of the simple model developed in this paper, is confirmed by the excellent agreement with experimental results. Consequently, the LBIC technique is shown to be an appropriate diagnostic tool for non-contact quantitative analysis of semiconductor materials and devices. >

Improved device technology for epitaxial Hg1-xCdxTe infrared photoconductor arrays

The performance of Hg1-xCdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg1-xCdxTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg1-xCdxTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m2. We describe in detail for the first time a device technology which enables the fabrication of Hg1-xCdxTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg1-xCdxTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg1-xCdxTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg1-xCdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

MOCVD-grown wider-bandgap capping layers in long-wavelength infrared photoconductors

The use of MOCVD-grown wider-bandgap as a capping layer for long-wavelength infrared (LWIR) photoconductors has been studied using both theoretical and experimental results. A device model is derived which shows that in the presence of a suitable energy barrier between the infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usually present at the semiconductor/passivant interface is prevented from having a significant effect on device performance. The energy barrier, which repels photogenerated minority carriers from the semiconductor surface, is introduced by employing an n-type wafer which consists of a wider-bandgap capping layer that is grown in situ by MOCVD on an LWIR absorbing layer. The derived model allows the responsivity to be calculated by taking into account surface recombination at both the front and back interfaces, thickness of capping and absorbing layers, recombination at the heterointerface, and variations in equilibrium electron concentration. Calculations show that for an absorbing layer, the optimum capping layer consists of and a thickness of the order of 0.1 to 0.2 . Experimental results are presented for x = 0.22 n-type conventional single-layer LWIR photoconductors, and for heterostructure photoconductors consisting of an LWIR absorbing layer of x = 0.22 capped by an n-type layer of x = 0.31. The model is used to extract the recombination velocities at the heterointerface and the semiconductor/substrate interface, which are determined to be and respectively. The experimental data clearly indicate that the use of a heterostructure barrier between the overlaying passivation layer and the underlying LWIR absorbing layer produces detectors that exhibit much higher performance and are insensitive to the condition of the semiconductor/passivant interface.

A model for phase noise generation in amplifiers

In this paper, a model is presented for predicting the phase modulation (PM) and amplitude modulation (AM) noise in bipolar junction transistor (BJT) amplifiers. The model correctly predicts the dependence of phase noise on the signal frequency (at a particular carrier offset frequency), explains the noise shaping of the phase noise about the signal frequency, and shows the functional dependence on the transistor parameters and the circuit parameters. Experimental studies on common emitter (CE) amplifiers have been used to validate the PM noise model at carrier frequencies between 10 and 100 MHz.

The impact of transmission line terminations on radiated emissions

Terminating transmission lines at either the source or the load side with a damping resistor is often used to improve the signal integrity of clock or control lines on PCBs. While a number of termination variants may produce similar results in respect to signal integrity the EMC performance can differ due to a different current and voltage distribution along the line. This paper investigates the impact of transmission line terminations on radiated emissions.

Shielding performance of a metallic enclosure with slots - numerical modelling and measurements

The shielding performance of a rectangular metallic enclosure with two slots is analysed using a fullwave electromagnetic simulation software based on the method of moments. Measurements are conducted in a 3m semi-anechoic chamber to validate the results. A good agreement between the results gives confidence in the simulation. Thus. simulation tools can be used to predict and optimise the shielding performance of metallic enclosures.

Computer simulation and experimental validation of a metallic enclosure with slots

Predicting and understanding the electromagnetic behaviour of metallic enclosures is an important issue when designing products for EMC compliance. The proposed paper investigates a rectangular box with two slots excited by monopole antennas, and compares various results obtained from computer simulations and measurements. A good agreement between measurement and simulation was found for most scenarios. Results for comparison include S- and Z-parameters and electric far field values.

Using N-port models for the analysis of radiating structures

Radiating structures with distributed parameters, such as PCBs, are considered as linear N-port networks and correlations between port currents and field strength values at arbitrary observation points are calculated. Knowing the S-parameters for all accessible ports the port currents can be re-calculated for other source and load scenarios. Field strength values are then obtained as a linear combination of contributions due to the individual port currents. The effect of changing load and source parameters, e.g. varying de-coupling impedances on a PCB, or changing the internal impedance of sources, on radiated fields can then be investigated in a very efficient way without repeating full wavefield simulations.

Rational approximation of transfer functions for radiating structures

The interpolation of transfer functions in the spectral domain is an important issue in the EMC simulation of complex systems. Rational functions provide a good approximation for a wide range of results such as input impedance, induced voltages or field strength values when investigating radiating structures. The estimation whether an approximation is accurate and where to place additional sample frequencies, if required, is given special emphasis.

Prediction of RF radiation from a PCB enclosed in a metallic box with an aperture

The radiation fields, both near and far fields, from a PCB enclosed in a rectangular metallic box with an aperture are predicted using method of moments. Validation measurements were conducted. Good agreement is achieved in both cases. The results indicate that MoM shows promise for predicting the EMC behaviour of such a PCB enclosed by a shielding box with a slot.