P.K. Weimer

The TFT A New Thin-Film Transistor

A thin-film transistor, TFT, fabricated by evaporation of all components on to an insulating substrate has been developed. Operation is based upon the control of injected majority carriers in a wide-band-gap semiconductor by means of an insulated control gate. Experimental units using microcrystalline layers of cadmium sulfide have yielded voltage amplification factors greater than 100, transconductances greater than 10,000 , μmho, input impedances greater than 106 Ω shunted by 50 pf and gain-bandwidth products greater than 10 Mc. Switching speeds of less than 0.1 μsec have been observed. Simple evaporated thin-film circuits incorporating the TFT have been built. Direct coupling between stages is permitted since the insulated gate electrode can be biased positively as well as negatively without drawing appreciable gate current. Modified forms of the TFT have been built for use as a flip-flop, an AND gate and a NOR gate in computer applications.

Electron Mobility Studies in Surface Space‐Charge Layers in Vapor‐Deposited CdS Films

Hall and field‐effect surface mobility has been measured in polycrystalline films of CdS of thickness 1000–4000 A. At low surface potential, the mobility increases with surface potential. The Hall mobility was observed to increase exponentially with temperature from −80° to 120°C. These effects can be explained using a polycrystalline film model. The effects of diffuse surface scattering were not observed at low surface potential due to the small mean free path of electrons. However, when the channel thickness was comparable to the electron mean free path a decrease in mobility was observed.

A self-scanned solid-state image sensor

An experimental television camera incorporating a completely integrated self-scanned solid-state image sensor has been built. The integrated sensor includes a photosensitive array having 32 400 picture elements, two 180-stage shift register scan generators and associated video coupling transistors. This large-scale integration of more than 100 000 components was carried out in the laboratory entirely by evaporated thin-film techniques. Each element of the photosensitive array comprises one or two photoconductors of CdS or CdS-CASe mixture, each in series with a diode. The 180-stage scan generators utilize 540 CdSe TWFs deposited upon a glass snbstrate. Center-to-center spacing of elements in the array and in the scan generators is 2 mils. The array is scanned at conventional television scan rates permitting the picture to be displayed upon a commercial television receiver. The camera is connected to the receiver either through a cable or through a UHF link with camera and transmitter powered by a self-contained battery. Camera circuits other than the integrated sensor employ conventional transistors and integrated components. Progress toward the development of an improved sensor having more picture elements is outlined.

A 180-stage integrated thin-film scan generator

Experimental models of a 180-stage, thin-film scan generator suitable for digital scanning of an array of elements have been produced by evaporation of all components upon a glass substrate. The generator is a modified shift register, comprising 360-thin-film transistors (CdSe TFTs), 180 field-effect diodes, 360 resistors, and 180 capacitors. Output voltage pulses of two to four volts are obtained, with clock frequencies ranging from 12 kc/s up to 2 Mc/s. The complete circuit, including all active and passive elements, was deposited during one pump-down of the vacuum system. A wire grill having 480 wires per inch was used in combination with movable masks to define the patterns. Reproducibility of fabrication under laboratory conditions has been excellent. Seven out of eighteen recent experimental units were operable throughout all 180 stages. Units have been operated continuously on life test for more than 5000 hours at 85°C without failure of any stages. Successful operation of the entire generator requires all 1080 thin-film components to be free of short circuits and have operating characteristics within the range of tolerance of the circuit.

Integrated circuits incorporating thin-film active and passive elements

The most spectacular achievements in integrated electronics undoubtedly fall in the category of Devices and Structures. The realization of useful circuit functions through combination of the new materials technology, new process technology, and the exploration of functional circuit needs above the discrete device level represents the first departure from conventional electronics using solid-state components. The use of the word integrated as the modifier for this new form of electronics signifies the combination of the previously separate disciplines of materials, devices, and circuits into a single discipline; not, as is sometimes erroneously assumed, just the combination of previously discrete components into a single function entity. Of the many problems facing the integrated circuit technologist, none is more intriguing or elusive than the practical realization of active as well as passive circuit elements in thin-film form. The following invited paper by Dr. P. I(. LYeimer, and his associates, describes a significant approach to the subject.-The Editors Summary-The thin-film field-effect transistor (TFT) provides the active element for complex integrated circuits deposited upon an insulating substrate. N-type transistors are obtained with evaporated layers of cadmium sulfide or selenide and *type transistors with evaporated tellurium. Switching speeds of less than 4 nsec and gainbandwidth products of greater than 30 Mc are observed with polycrystalline films of cadmium sulfide. Oscillations at frequencies up to 74 Mc have been noted. Significant improvements in the life and stability of the coplanar-electrode TFT have been obtained by encapsulation. A 30-stage completely integrated thin-film scan generator incorp+ rating 60 TFT’s, 30 diodes, 60 resistors and 30 capacitors has been designed. The novel circuit, whose operating characteristics resemble those of a shift register, is deposited by evaporation using movable metal masks controlled from outside the vacuum system. In one unit 28 consecutive stages were operated for nearly 700 hours. Laboratory models of the scan generator are being used to drive the address strips in experimental solid-state image-sensor panels.

Thin-film circuits for scanning image-sensor arrays

Integrated circuits for scanning a photosensitive array at standard television scan rates have been fabricated by evaporated thin-film techniques. These include: 1) A 264-stage parallel-output complementary shift register with a driver stage at each output. 2) A 256-output transistor decoder with video coupling circuits for scanning an array with line storage. A single line of 256 photoconductordiode elements has been included on the same substrate with the decoder. The complementary shift register has been demonstrated to operate at clock frequencies up to 100 kHz. The integrated decoder and line sensor, when driven by two external 16-stage shift registers, has been operated successfully with line storage at frequencies up to 4.8 MHz.

Television Camera Tubes: A Research Review

Publisher Summary This chapter presents a research review of television camera tubes. In an ideal tube, the sensitivity, resolution, and contrast discrimination would be limited only by the statistical fluctuations in the number of pho- tons comprising the image. The tube itself should introduce no limitations in the process of generating the video signal. The use of a low-velocity scanning beam eliminates the spurious shading and loss of efficiency caused by the redistribution of secondary electrons in the Iconoscope. Since the electron beam is decelerated just prior to striking the target a strong electric field in front of the target draws all photoelectrons and reflected beam electrons away from the target. A low-velocity electron beam from a gun whose cathode is held within a few volts of the target mesh scans the reverse side of the target depositing electrons in the areas corresponding to the bright, parts of the scene. The two-sided target has been a major fabrication problem as well as a source of some of the most desirable performance characteristics found in the Image Orthicon. The camera tubes based on photoconductivity are also elaborated.

A historical review of the development of television pickup devices (1930—1976)

A personalized account of the development of television pickup devices during the period 1930-1976 is related by a research worker active in the field since 1942. The story begins with the iconoscope and image dissector tubes and concludes with solid-state sensors of the CCD and CID variety. While the development is technical, the details have been minimized to provide a more comprehensive view.

Image Sensors for Solid State Cameras

Publisher Summary This chapter describes the various aspects of image sensors for solid state cameras. The function of an image sensor is to generate a time-varying video signal corresponding to the spatial variations in the incident optical image. The continuous photoconductive layer in a vidicon target can be represented as an array of isolated photoconductive elements each shunted by a capacitor. A major advance in image sensing occurred with the development of the silicon diode array target for the vidicon. Multiplexed scanning of solid state arrays is directly analogous to electron beam scanning in camera tubes. In each case, the picture charge at the element is discharged through a local switch into a common electrode connected to the output amplifier. The elemental capacitor shunting each diode should be large compared to the stray capacitance shunting the transistor. The elemental capacitor may consist largely of the depletion layer capacitance between photodiode and the substrate, or it may be formed of the oxide capacitance to an overlying electrode. It is found that although the derivation of video signal from the substrate permits a simplification of the scan generators and peripheral circuits, the relatively large capacitance of the output to the ground will degrade the signal-to-noise ratio at low light levels.

Self-scanned image sensors based upon bucket-brigade scanning

Image sensors employing the charge-transfer principles of the bucket-brigade delay line for internal scanning offer potential advantages over the usual x-y addressed sensors in simplicity of fabrication. A design for a self-scanned bucket-brigade sensor that can be fabricated upon a monolithic silicon wafer with a single diffusion and one layer of metallization is proposed. An experimental array has been built comprising 15 rows of 32 series-connected MOS transistors whose sources and drains serve as photodiodes. These transistors are interconnected to adjacent horizontal buses so that each row can function independently as a shift register. In operation, each register remains inactive for a period of time during which an image charge pattern is built up on the photodiodes. The application of the horizontal clock pulses to that row causes the charge pattern to be transferred stepwise to the edge of the array where it is converted into a video signal. Although in the test sample the gating of the horizontal clock pulses to successive rows was accomplished by means of an external vertical scan generator, this could be replaced by an integrated vertical register formed on the same chip with the sensor.