High-gain Avalanche Rushing Amorphous Photoconductor Target Research Articles

Double-Gated, Spindt-Type Field Emitter With Improved Electron Beam Extraction

We have developed a highly sensitive, compact image sensor that comprises a field emitter array (FEA) and a high-gain avalanche-rushing amorphous photoconductor target, with the ultimate aim of developing an ultrahighly sensitive, compact, high-definition television camera. Double-gated FEs have an advantage of having a compact electron beam focusing system; however, image intensities reproduced by the sensor were nonuniform due to low electron beam current. Furthermore, the simulated electron beam current disagreed with the measured current. The electron beam current characteristics of two types of double-gated, Spindt-type FEs (both with improved electron beam current extraction) are discussed for possible use within the sensor: convex-structured and volcano-structured. A highly accurate simulation model of the image sensor using a double-gated, Spindt-type FE has been examined; the simulated electron beam currents extracted from the double-gated, Spindt-type FE are in agreement with the measured electron beam currents when the initial electron velocity is assumed, thus suggesting that the simulated anode current–anode voltage characteristic conforms to the measured one. For example, the electron beam currents extracted from the convex-structured and volcano-structured FEs when the focusing electrode is placed 0.2 $\mu \text{m}$ below the gate electrode opening at a focusing electrode voltage of 15 V are, respectively, 1.8 and 1.9 times larger than that extracted from the previously used FEs when the focusing electrode is stacked 1.5 $\mu \text{m}$ above the gate electrode. The results show potential for reducing the degradation of the uniformity of the reproduced image’s intensity, and show that the highly accurate simulation model of the sensor is valid to design the double-gated FEAs for the sensor.

Tellurium doping effect in avalanche-mode amorphous selenium photoconductive film

Amorphous selenium (a-Se) high-gain avalanche rushing amorphous photoconductor (HARP) film has been used for highly sensitive imaging devices. To improve the spectral response of a-Se HARP photoconductive film at a long wavelength, the tellurium (Te) doping effect in an 8-μm-thick a-Se HARP film was investigated. The thickness of the Te-doped a-Se layer in the 8-μm-thick a-Se HARP films was varied from 60 to 120 nm. The signal current increases significantly due to the avalanche multiplication when the target voltage is increased over the threshold voltage. In the 8-μm-thick a-Se HARP film with a Te-doped layer, the spectral response at a long wavelength was improved in comparison with the a-Se HARP film without a Te-doped layer. In addition, the increase of the lag in the 8-μm-thick a-Se HARP target with a Te-doped layer of 120 nm is caused by the photoconductive lag due to the electrons trapped in the Te-doped layer. Based on the current-voltage characteristics, spectral response, and lag characteristics of the 8-μm-thick a-Se HARP targets, the Te-doped layer thickness of 90 nm is suitable for the 8-μm-thick a-Se HARP film.

Avalanche multiplication and impact ionization in amorphous selenium photoconductive target

The avalanche multiplication factor and the hole ionization coefficient in the amorphous selenium (a-Se) high-gain avalanche rushing amorphous photoconductor (HARP) target depend on the electric field. The phenomenon of avalanche multiplication and impact ionization in the 0.4-µm-thick a-Se HARP target is investigated. The hot carrier energy in the 0.4-µm-thick a-Se HARP target increases linearly as the target voltage increases. The energy relaxation length of hot carriers in the a-Se photoconductor of the 0.4-µm-thick HARP target saturates as the electric field increases. The average energy Eav of a hot carrier and the energy relaxation length λE in the a-Se photoconductor of the 0.4-µm-thick HARP target at 1 × 108 V/m were 0.25 eV and 2.5 nm, respectively. In addition, the hole ionization coefficient β and the avalanche multiplication factor M are derived as a function of the electric field, the average energy of a hot carrier, and the impact ionization energy. The experimental hole ionization coefficient β and the avalanche multiplication factor M in the 0.4-µm-thick a-Se HARP target agree with the theoretical results.

Hot Carrier Energy and Energy Relaxation Length in a-Se Photoconductive Target

The a-Se HARP (High-gain Avalanche Rushing amorphous Photoconductor) target was prepared, and the avalanche characteristics were investigated. In this paper, to study avalanche multiplication at a high electric field, the lucky-drift model was used. In addition, the energy-and field-dependent energy relaxation length was considered. The avalanche multiplication factor was obtained from the current-voltage characteristic of the a-Se HARP target. The threshold field of the 0.4-μm-thick a-Se HARP target was 0.88×108V/m. The hot carrier energy at the threshold field for the avalanche multiplication in the 0.4-μm-thick a-Se HARP target was 0.21 eV. The hot carrier energy in the a-Se layer increases linearly as the electric field increases. The hot carrier energy also saturates as the avalanche multiplication factor increases. In addition, the energy relaxation length between the inelastic scattering events in the a-Se layer saturates as the hot carrier energy and the avalanche multiplication factor increase.

Drift Velocity of Hot Carriers in a-Se Photoconductive Target

Avalanche multiplication of the 0.4-μm-thick a-Se HARP (High-gain Avalanche Rushing amorphous Photoconductor) target was obtained at a high electric field. To study the drift velocity of hot carriers in the a-Se layer, the energy-and field-dependent energy relaxation length was considered in the lucky-drift model. The impact ionization energyEIof 2.0 eV and the optical phonon energyћωof 31 meV for a-Se were used to obtain the impact ionization parameters in the a-Se layer. The drift velocity of hot carriers at 1×108V/m in the 0.4-μm-thick a-Se HARP target was obtained as 1.87×106cm/s. The drift velocity of hot carriers saturates as the electric field and the avalanche multiplication factor increase. In the 0.4-μm-thick a-Se HARP target, the relaxation length ratioλE/λand the relaxation time ratioτE/τsaturate as the avalanche multiplication factor increases. In addition, the relaxation length ratioλE/λand the relaxation time ratioτE/τat 1×108V/m were 2.75 and 14.66, respectively.

イメージセンシング技術とその応用 ＦＥＡ‐ＨＡＲＰにおけるブルーミング抑制駆動法

We have been developing an image sensor, FEA-HARP, which consists of a field emitter array (FEA) and high-gain avalanche rushing amorphous photoconductor (HARP). This sensor will be used in an ultrahigh-sensitivity compact HDTV camera. We propose new operation methods for the FEA-HARP that maintain picture quality even when strong light enters the sensor. Experimental results show that blooming generation was suppressed by removing excess charges from the HARP target during horizontal blanking intervals.

640 × 480 pixel active-matrix Spindt-type field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 640×480 pixel field emitter array (FEA) image sensor with a high-gain avalanche rushing amorphous photoconductor (HARP) target was fabricated and tested as a step toward the development of ultrahigh-sensitivity compact image sensors for high-definition television cameras. For this sensor, an active-matrix Spindt-type FEA integrated with scanning circuits and a magnetic electron focusing system were designed, and a 15-μm-thick-HARP target was used. The experimental results revealed that the prototype sensor had enough resolution for its pixel number and high sensitivity due to the electrons emitted from the active-matrix FEA being focused onto the HARP target.

Dependence of Thickness on Avalanche Characteristics of Te-Doped Amorphous Selenium Photoconductive Target

The thickness dependence of the avalanche characteristics of a tellurium (Te)-doped amorphous selenium (a-Se) high-gain avalanche rushing amorphous photoconductor (HARP) target is investigated. To improve the quantum efficiency of the a-Se HARP photoconductive target, a Te-doped a-Se photoconductive layer is sandwiched within a-Se HARP target. The avalanche multiplication factor and hole ionization coefficient of the a-Se HARP target are obtained using the result of photocurrent measurement. The multiplication factor in the avalanche mode exponentially increases with increasing electric field by avalanche multiplication phenomena over the threshold field. The quantum efficiency of the 8-µm-thick a-Se HARP target in the avalanche mode is higher than that of the thin HARP target below 2 µm thickness. Also the spectral response, decay lag, and light-transfer characteristics are studied.

Development of a super-high-sensitivity image sensor using 640×480 pixel active-matrix high-efficiency electron emission device

An extremely high-sensitivity image sensor has been developed with a combination of an active-matrix high-efficiency electron emission device (HEED) array and a high-gain avalanche rushing amorphous photoconductor (HARP) target. Following the description of the requirements for the use of the electron emitter as an image-sensing probe, the characteristics of a fabricated 640×480 pixel active-matrix HEED image sensor with a HARP target are presented by image-pickup experiments under a dark condition. The emission current density of the experimental HEED under an active-matrix drive was 4 A/cm2 corresponding to a practical level. A clear image observed in the prototype compact camera demonstrates that the surface-emitting HEED is useful for image sensing based on HARP with high sensitivity and sufficient definition.

X-ray phase imaging of biological soft tissue using a direct-sensing x-ray HARP tube camera

A HDTV camera having a direct-sensing x-ray high-gain avalanche rushing amorphous photoconductor (HARP) tube was used, for the first time, to acquire x-ray phase maps. The tube can achieve a high sensitivity as a result of the avalanche multiplication process in the HARP target. A beryllium plate, rather than a glass plate, was used as the face plate of the tube to minimize the loss of x-rays due to absorption, and a 15 µm thick HARP target was directly formed on it. In the experiment, the x-ray phase shifts produced by a rat liver were measured using synchrotron x-rays (λ = 0.0766 nm) and a triple Laue-case (LLL) x-ray interferometer. Interference patterns produced by the sample were observed with the direct-sensing x-ray HARP tube camera. A voltage of 1300 V was applied to the HARP target to give an output signal gain of two. The camera was operated in 1125 scanning-line mode, and real-time images were stored on a workstation at a rate of 30 images/s with an image format of 960 (H) × 1100 (V) pixels. A phase-map image of the sample was successfully obtained using the fringe scanning method and phase unwrapping. The observed phase shifts ranged from 50° to 200°. Trees of blood vessels in the rat liver were clearly depicted without using a contrast agent. The spatial resolution of the x-ray camera was estimated to be better than 35 µm in the vertical direction and 100 µm in the horizontal direction.

Fabrication of active-matrix high-efficiency electron emission device and its application to high-sensitivity image sensing

Practical advantages of an advanced high-efficiency electron emission device (HEED) (low-voltage operation, relatively simple structure, and availability for large-area applications) are further confirmed by its compatibility with active-matrix drive configuration. The fabricated HEED array has been applied to a high-sensitivity image sensing tube with a target of high-gain avalanche rushing amorphous photoconductor (HARP). It is demonstrated that a prototype 256×192pixel active-matrix HEED array operates well as an effective probe for high-resolution image pickup in combination with a HARP target. The advanced HEED array is promising for development of next-generation image sensor with ultrahigh sensitivity and high definition.

磁界集束系を備えた１インチ２５６×１９２画素冷陰極ＨＡＲＰ撮像板

We have been developing a new image sensor with a field emitter array and a high-gain avalanche rushing amorphous photoconductor (HARP) target, with the aim of using it in ultrahigh-sensitivity, high-resolution compact TV cameras. We fabricated and tested 256 × 192 pixel sensor with a pixel size of 50 × 50μm and its electron-beam focusing system consisting of permanent magnets. Experiments showed that the prototype sensor has both enough resolution for its pixel size and high sensitivity due to the HARP target, thus demonstrating its potential as a practical image sensor.

256×192 pixel field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 256×192 pixel field emitter array (FEA) image sensor with a highly sensitive high-gain avalanche rushing amorphous photoconductor (HARP) target was fabricated and tested as a step toward the development of a practical image sensor for ultrahigh sensitivity and ultrahigh-definition compact TV cameras. The experimental results showed that the prototype had an adequate resolution corresponding to the spot size of the scanning electrons on the HARP target which was estimated by the simulation. The prototype also had a wide dynamic range and high sensitivity, demonstrating its potential as a next generation practical image sensor and offering new application of FEAs.

冷陰極ＨＡＲＰ撮像板

An image sensor has been developed that consists of an X-Y-matrix field emitter array (FEA), a mesh electrode, and a high-gain avalanche rushing amorphous photoconductor (HARP) target, each of which is set closely facing one another. Investigation of its fundamental driving characteristics in a vacuum chamber showed that TV images are produced when the matrix FEA consists of 50×50 elements, that images are picked up with high sensitivity due to the HARP target, and that the output signal current is not affected by fluctuations in the emission current from the FEA under ordinary lighting conditions. Applying appropriate voltages to the mesh electrode improved the resolution. This image sensor will enable the development of super-high-sensitivity and ultrahigh-definition TV cameras.