An experimental crystalline-selenium vidicon

The use of polycrystalline Se as the photosensitive target of a vidicon image tube has been demonstrated for the first time. The mode of operation of this target layer is fundamentally different from that of the amorphous-Se vidicon target, which is commonly described as a photosensitive insulator layer. In contrast, the explanation for the crystalline-Se vidicon operation, as proposed here, is based on the utilization of a reverse-biased heterojunction contact between n-type SNO2and p-type crystalline Se. A fabrication process has been developed which yields target layers with an attractive combination of operating characteristics. This new type of vidicon has the following nominal operating parameters: dark current ≃ 5-10 nA, peak sensitivity ≃ 0.270 A/W, peak quantum efficiency ≃ 60 percent, and 50-ms lag ≃ 10 percent. Compared to the standard Sb2S3vidicon, this experimental crystalline-Se vidicon has more than twice the sensitivity and less than half as much lag and dark current.     

Wavelength demultiplexing heterojunction phototransistor

Experimental and theoretical results on a wavelength demultiplexing receiver composed of an AlGaAs/GaAs heterojunction phototransistor (HPT) integrated within a resonant cavity are reported. A high quality factor cavity was formed using a very thin In/sub 0.05/Ga/sub 0.95/As active absorption layer in the collector depletion region of the HPT. Crosstalk attenuations of 15 dB for dual and 12 dB for triple wavelength demultiplexing were demonstrated. The individual HPTs had an optical gain of 500 at the resonant modes. Theoretical calculations predict crosstalk attenuation levels as high as 40 dB with high reflection mirrors on both ends of the cavity.<>     

Electron images of vidicon cathodes

Excessive patchiness of the emission from the oxide-coated cathodes used in cathode-ray tubes, camera tubes and traveling-wave tubes can have adverse effects on their performance or useful life. There is a simple technique for forming an electron image of a CRT cathode directly on the view screen. This technique, which has been known for more than 30 years, may be used to show patchiness when it exists, but it has not been possible in the past to image the cathode of a camera tube because of the lack of a built-in display mechanism (phosphor-coated view screen) and because of the peculiar geometry of the electron guns. A technique has now been devised for displaying the electron images of vidicon cathodes. This technique comprises) 1) priming the optical image-sensing surface with uniform light, 2) projecting a "pinhole snapshot" electron image of the cathode on the image-sensing surface with an unscanned beam and storing it there, and 3) reading out the stored image with the normal scanning beam and displaying it on a CRT monitor. Photographs of electron images of vidicon cathodes obtained in this manner are shown in this paper.     

Transient response of vidicon photoconductors

An expression for the rise and decay times of vidicon photocurrents as a function of incident illumination is derived for a certain class of vidicon photoconductors. The approach taken is semiempirical in that certain parameters in the derived expression are obtained directly from vidicon manufacturer's data sheets. Regults compare well with the manufacturer's published results.     

Hard-vacuum pyroelectric vidicon: suitable materials

A new mode of operation of the pyroelectric Vidicon is reported. This mode does not require a soft vacuum and the tube is operated hard. Requirements for suitable pyroelectric materials are discussed and the results of experiments operating in this mode are presented.     

Mathematical model to describe vidicon operation

A theoretical development is presented that describes the operation of a vidicon. Output current is determined for an arbitrary distribution of intensity and wavelength of illumination falling on the face of the vidicon. The nonlinear behavior of the photoconductor is taken into account. The effect of progressive erasure of the stored charge on the rear of the photoconductor with the motion of the electron beam is included. The readout beam is assumed to be Gaussian with either a circular or elliptical cross section. The effect of the electron beam on lines other than that being scanned is included. Variation in electron beam spot size across the face of the vidicon is also considered. Correlation with published output data is presented. The apparent phase shift of objects, especially at high beam currents, can be significant. Numerical results for the cases of line and sinusoidal targets are discussed in detail.     

Ultrahigh-sensitive AlGaAs-GaAs punchthrough heterojunction phototransistor

An avalanche enhanced AlGaAs-GaAs punchthrough heterojunction phototransistor with an improved guard-ring emitter structure has been proposed. It has demonstrated a record high sensitivity of 5790 A/W and optical conversion gain of 10810 at 400-nW incident optical power level. Its novel guard-ring emitter structure can suppress peripheral recombination current more effectively, provide capability to enhance and tune the optical conversion gain at low-incident optical power and reduce the effective emitter area.     

Noise in phototransistor optical isolators

Noise measurements were performed on several commercially available phototransistor optical isolators in order to examine the signal detection limits of typical devices. It was found that, in general, phototransistor optical isolators are very noisy devices exhibiting all of the common types of noise usually found in bipolar junction transistors. A large number of devices exhibited burst noise which dominated their low-frequency noise performance. The noise performance of devices without burst noise was dominated by 1/f noise or flicker noise at low frequencies and by shot noise at high frequencies. Experimental data indicates that in most cases the electrical noise contribution of the LED is negligible and that the dominant source of noise is the phototransistor.     

High-gain wide-gap-emitter Ga1-xAlxAs-GaAs phototransistor

A Ga1-xAlxAs-GaAs heterostructure phototransistor has been manufactured and tested. The device is an n-p-n structure employing the `wide-gap-emitter? effect. High internal current gain (>2000) has been achieved.     

Avalanche InP/InGaAs heterojunction phototransistor

We report on the characteristics of an avalanche InP/InGaAs heterojunction phototransistor. Below the turnover voltage, the gain is bias dependent and avalanching can be used to achieve significant (sim5times) improvement in the gain-bandwidth product. The noise current in this bias region has been measured and is shown to be predominantly shot noise of the photocurrent and the leakage current. Above the turnover voltage, negative resistance is observed and extremely high gains (>10⁴) are achieved. In this mode, the pulse response is a narrow spike (rise time ≃ 20 ns) whose width is independent of the width of the incident optical pulse.     

GaAs-GaAlAs phototransistor/laser light amplifier

A hybrid light amplifier consisting of a GaAs-GaAlAs widegap phototransistor as light detector and a d.h. laser as light emitter has been realised. Amplification up to 25 dB has been achieved. Fastest turn-on and turn-off times were 16 ns and 24 ns, respectively. A monolithically integrated amplifier structure is proposed.     

High light-level blooming in the silicon vidicon

Blooming occurs in silicon-vidicon targets because more hole-electron pairs are created at a given point by the incident light signal than can be removed by the action of the scanning electron beam. The excess carriers move laterally in the target through diffusion and, as a result, the blooming has the following properties- 1) the Slope of the blooming curve on semilog paper does not depend on the size of the incident light spot, i.e., at a given overload intensity, all circular spot sizes add the same excess amount to their radii; 2) the radius of a bloomed spot increases linearly with the log of the incident intensity; 3) the Slope of the blooming curve does not depend on beam current; however, the intensity at which the blooming begins does depend directly on the beam current. At very high incident-light intensities, a dramatic increase in the blooming is observed. This can be attributed to diffusion under conditions where the minority carrier level has become equal to the majority carrier level.     

Quantum-well infrared phototransistor with pHEMT structure

A quantum-well infrared phototransistor with a pseudomorphic high-electron mobility transistor (pHEMT) structure is presented. The proposed phototransistor uses four periods of a GaAs/Al/sub 0.3/Ga/sub 0.7/As (50 /spl Aring//120 /spl Aring/) quantum-well absorption region, as well as an In/sub 0.15/Ga/sub 0.85/As quantum well conducting channel under the absorption layer. The phototransistor shows a large responsivity of 140 A/W around 6 /spl mu/m at 23 K (for a cutoff wavelength of 7.5 /spl mu/m). The relation between the photoconductive gain and the transconductance of the pHEMT structure is also investigated.     

Ramp mode operation of a pyroelectric vidicon

A new mode of operating a pyroelectric vidicon is demonstrated. It uses a linearly rising voltage ramp applied to the target connection to generate the positive bias across the target necessary for efficient tube operation. The theoretical relationship between lag (time response of the tube to a step impulse signal) and readout efficiency is examined. Lag measurements of a tube operating in ramp mode are used to find the readout efficiency under different operating conditions, and the importance of the beam energy spread of the electron gun in determining the operating efficiency is emphasized.     

A positive-reading silicon vidicon for X-ray imaging

A silicon vidicon that uses an n on p silicon diode array for a target was designed. The electron beam landing energy is 600 eV, and all the diodes are charged to a reversed bias by secondary electron emission. With the high electron beam energy, it is possible to sharply focus the beam into a sheet beam, which can be used to simultaneously bombard several separate linear diode arrays in a detector scheme for a bent-crystal X-ray spectrometer. By such a method, the bandwidth requirement of the data-transmission system is reduced by a factor equal to the number of separate linear arrays that are simultaneously bombarded. Data are presented on the diode-charging mechanism, with emphasis on the method used to determine the potential of the diode n-surface. Examples are shown of the tube being used to detect 25-keV X-rays.     

A phototransistor optical isolator noise model

A noise model for the phototransistor optical isolator is presented and used to predict optical isolator noise performance. Results are shown to agree with existing experimental data on phototransistor optical isolator noise. The model presented includes burst noise, flicker noise and shot noise.