An optical system for bilateral recombination-radiation diagnosticsof the carrier redistribution in switching power devices

An optical scanning system is developed for the detection of recombination radiation from power devices. The instrument facilitates two-dimensional characterization of the excess-carrier content in a device. A bilateral technique enables measurements along various perpendicular surfaces of the devices under investigation. Hence, measurements of processes not present, or less dominant, in one direction may be observed in another. Examples taken from gate turn-off (GTO)-thyristor investigations are presented. All stages of the switching cycles as well as during the ON-state can be investigated with the instrument. Detailed three-dimensional carrier maps are produced for the visualization of measurement results, and sequences of maps are used to observe time-dependent phenomena involved in the transient operation of a device. Features associated with placement of the optical focus are explained, and possibilities of tomographic mapping of power devices are discussed     

Post-traumatic hemiplegia in a patient with fibromuscular dysplasia of the carotid artery

A 44-year-old patient, without remarkable medical history, was admitted with a head trauma with initial loss of consciousness and a thoracic trauma. The initial treatment included the insertion of a chest drain for evacuation of a pneumothorax and intrapleural analgesia with bupivacaine. The day after admission, the patient experienced a generalized epileptic crisis, without prodomes. Later, a left proportional hemiplegia with aphasia was recognized. The CT scan obtained immediately after the crisis, as well as the carotid Doppler ultrasonography and echocardiography were normal. The bilateral carotid angiography showed an image of fibromuscular dysplasia of the extracranial segment of the right internal carotid artery. The migration of a carotid thrombus initiated by the trauma was hypothetized. A treatment with a platelet aggregation inhibiting drug was started and associated 20 days later with low molecular weight heparin. The patient recovered a normal motility within 10 days; only the aphasia remained. Trauma of the carotid artery is not a frequent cause of cerebrovascular accident. The occurrence of the latter is favoured by a pre-existing lesion of this artery. This case demonstrates that in a trauma patient not all central nervous system manifestations are initiated by a head trauma.     

A study of design influence on anode-shorted GTO thyristor turn-onand turn-off

Anode-shorted GTO thyristor samples were investigated by means of the free-carrier absorption (FCA) technique. Both the turn-on and turn-off processes were investigated as regards the two-dimensional carrier distribution for different stages of the transient cycles. The results are presented as carrier-map sequences, i.e., 3-D pictures of measured 2-D carrier distributions. Samples were formed as unit segments cut out from large-area devices, and associated with different degrees of anode shorting, silicon thickness, and lifetime treatment. During investigation, the samples were inductively anode loaded, and as regards the turn-off process they were operated near the safe-operation limit. The measurements clearly illustrate the way carriers are transported in the sample when firing the device, and the turn-on process is visualized in steps by means of carrier-map sequences. These measurements are supported by computer simulations. The turn-off process is also visualized in carrier-map sequences measured from two perpendicular directions, and the maps show the critical electric-field expansion which always precedes a turn-off failure due to dynamic breakdown mechanisms. Further on, the effect of design-parameter variations, e.g., anode shorting pattern and carrier lifetime reductions, on destructive GTO turn-off phenomena are discussed     

A time-resolved optical system for spatial characterization of thecarrier distribution in a gate turn-off thyristor (GTO)

The measurement of two-dimensional (2-D) excess-carrier distribution in a gate GTO by a time-resolved infrared-absorption technique is discussed. The optical scanning system employs a wide memory digital oscilloscope for data acquisition and a computer system for control, data processing, and display. Maps of the carrier distribution in the active GTO device are produced under steady-state conditions as well as during turn-on and turn-off operation. The maps are presented as three-dimensional (3-D) views produced from 2-D measurements     

Measurements of failure phenomena in inductively loadedmulti-cathode GTO thyristors

The free-carrier absorption (FCA) technique is used for mapping of the carrier content in a dual-cathode gate turn-off thyristor (GTO) at different stages of the turn-off cycle. The FCA technique outputs 3-D maps based on 2-D measurements of local carrier concentration. The measurements are time resolved, thus making a transient response converted into a corresponding sequence of carrier maps. The dual-cathode device is shown to be sufficient for determining the transient behavior in a multicathode structure. The destruction mechanism and reasons for turn-off failure are investigated. The GTO device is inductively loaded and asymmetrically gate contacted in order to emulate a realistic mode of operation. The gate-driving conditions are altered, and the importance of the turn-off gain for turn-off failure is established. The case of equal ON-state cathode currents in both segments is particularly highlighted. The influence of snubber circuits is also discussed     

2-D characterization of dynamic charge distribution in MOScontrolled thyristors: experiment and simulation

The MOS-controlled thyristor (MCT) is well suited for very high power applications. The maximum capability of turn-off currents for p-channel MCT's was so far limited to low values (<10 A at 1000 V snubberless switching) due to strong sublinear current scaling with respect to the device area. In this letter we investigate this phenomenon by full device scale transient carrier distribution measurements and simulations. For the first time the degradation of the switching performance for highly structured and shorted base MOS thyristors is related by measurement to an inhomogeneous current distribution in the device during switching. These measurements confirm predictions of the effect by device simulation     

The effect of emitter shortings on turn-off limitations and devicefailure in GTO thyristors under snubberless operation

The influence of the anode shorting on the turn-off failure of GTO thyristors is investigated. Two types of shorting patterns, viz., “finger”and “ring”-type anode designs, are compared with a non-shorted design. The inductively loaded GTO unit cells are measured under snubberless operation close to the safe-operating area (SOA) limit. The critical anode blocking voltage was obtained for a constant anode current density by varying the turn-off gain, G. The investigation has shown that the turn-off voltage capability of both types of anode-shorted GTO's is deteriorated for high G values. The “ring”-type anode design, however, shows better voltage blocking capability than the “finger” type. This behavior is attributed to the formation of a cylindrical high drift-current density filament at the end of the storage and beginning of the fall-time periods. The electric field in this region, which is referred to as a quasi space-charge region (QSC), is strongly G dependent. Measurements based on the time-resolved free-carrier absorption (FCA) technique are used to map the carrier-density redistribution inside the unit cell during the turn-off switching cycle. For the first time, 3-D simulations of the GTO cell are also presented in order to support the model of dynamic punch-through failure at high G values, which was proposed in earlier papers     

Alterations of autonomic cardiac control in anorexia nervosa

BACKGROUND: Interindividual differences in the structure and expression of the dopamine receptor genes affect dopamine availability and may be the genetic basis for variation in vulnerability to tobacco smoking. In this study, prevalences of polymorphisms in the TaqIA allele (A1 and A2) and the TaqIB allele (B1 and B2) of the D2 dopamine receptor gene in 157 lung cancer case patients and 126 control subjects were determined to assess whether individuals homozygous or heterozygous for the less common A1 and B1 alleles are more vulnerable to nicotine addiction. METHODS: Case and control subjects were accrued from an ongoing epidemiologic study. Blood samples were collected from them and subjected to molecular genetic analyses. Subjects were interviewed to obtain relevant information. Current and former smokers were administered a questionnaire to quantify their addiction to nicotine. RESULTS: The combined B1B2 genotypes appeared to be more prevalent in ever smokers than in never smokers among case patients (30.3% versus 13.3%; two-sided P = .233) and among control subjects (30.9% and 0%; two-sided P = .02); statistically significant differences were not observed among those with A1 genotypes. Statistically significant correlations between the presence of the A1 and B1 alleles were observed (r = .73 for case subjects and r = .76 for control subjects; two-sided P<.001). Individuals with rarer genotypes reported having been substantially younger at the time of smoking initiation (statistically significant for both A1 and B1) and having attempted to quit smoking fewer times (statistically significant for only A1). CONCLUSION: Variant alleles of the D2 dopamine receptor gene may play a role in determining nicotine addiction, although the associations between the at-risk genotypes and measures of nicotine addiction were not entirely consistent.     

Bulk silicon technology for complementary MESFETs

A technology for fabrication of complementary silicon MESFETs on bulk silicon substrates has been developed. The technology is similar to CMOS technology, and utilises n-silicon substrates. P-wells are used for the n-channel devices. Device isolation was achieved by trench etching. The silicides of Er and Pt were used as gate Schottky contacts. P- and n-channel characteristics are presented together with subthreshold behaviour and preliminary results regarding radiation hardness. Also, results from two-dimensional simulations of the devices are presented.<>