A. J. Snell

Application of amorphous silicon field effect transistors in addressable liquid crystal display panels

It is shown that thin-film field effect transistors (FETs) made from amorphous (a-) silicon deposited by the glow-discharge technique have considerable potential as switching elements in addressable liquid crystal display panels. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed in some detail. Two important points are stressed: (i) a-Si device arrays can be produced by well-established photolithographic techniques, and (ii) satisfactory operation at applied voltages below 15VV is possible. Small experimental 7×5 transistor panels have been investigated and it is shown that with the present design up to 250-way multiplexing could be achieved. The reproducibility of FET characteristics is good and in tests so far no change has been observed after more than 109 switching operations.

Amorphous silicon analogue memory devices

Amorphous silicon M-p+ ni-M and M-p+ -M memory devices have been prepared. The characteristics are critically dependent on the metal used for the top contact. Devices with Cr top contacts exhibit the fast digital behaviour reported previously, whereas those using V exhibit fast analogue switching. The paper reports results for these and other metals.

The switching mechanism in amorphous silicon junctions

Abstract Extensive new results have been obtained on memory switching in a-Si p + ni junctions. It is shown that the ON-state has its origins in a highly conducting filament less than lμm in diameter. The physical mechanisms that could play a role in the switching operations are discussed.

An investigation of the amorphous-silicon barrier and p-n junction

Abstract With the growing interest in the properties and application of amorphous (a-) Si it is relevant to look in some detail into the formation and electronic properties of the amorphous barrier and p-n junction. In the first part of this paper a series of model calculations based on the experimentally determined density of state distribution in glow-discharge a-Si is presented. The net space charge in the barrier region is calculated as a function of energy for doped and undoped specimens. A step-by-step solution of Poissons equation then leads to the barrier profile showing that the a-barrier for undoped and weakly doped specimens differs basically from its crystalline counterpart. The analysis is then extended to the amorphous p-n junction. In the second part of the paper the differential barrier capacitance, C, is calculated and its dependence on applied potentials and on frequency investigated. It is found that C‒V plots should be critically dependent on the measuring frequency because the latter...

The characteristics and properties of optimised amorphous silicon field effect transistors

A series of experiments aimed at improving the performance of amorphous silicon field effect transistors has been carried out. The dc and dynamic characteristics of the optimised devices are described. Stable devices capable of ON-currents of the order of 100 μA with OFF-currents ≃10−11 A can be fabricated which could, in principle, be used to address more than 1000 lines of a liquid crystal display. The properties of the highly conducting ON-state channel have also been studied. The field effect mobility, 0.3 cm2 V−1 s−1 at room temperature, has an activation energy of 0.1 eV at the higher gate voltages. The possible reasons for the improvement in performance over earlier devices are discussed.

The interpretation of capacitance and conductance measurements on metal-amorphous silicon barriers

Abstract In a recent paper we investigated the formation, profile and capacitance of the metal/a-Si barrier by means of model calculations based on the experimentally determined distribution of localized states. This approach has been developed in the present paper and is used in a detailed comparison with capacitance measurements on Au/a-Si barriers. The a-Si specimens, either undoped or with donor concentrations of up to 2 × 1018 cm−3, were produced by the glow-discharge technique. It has been possible to give a satisfactory interpretation of the observed frequency, bias and temperature dependence of the barrier capacitance. For this two important aspects had to be included in the model calculations : first the electronic behaviour of the barrier, determined by the known space-charge distribution, and secondly the properties of the specimen in the measuring circuit, analysed by means of an equivalent circuit. In the final section of the paper we present current-voltage curves for different donor concent...

Application of amorphous silicon field effect transistors in integrated circuits

The work described in this paper is concerned with the possible applications in integrated circuits of thin-film field effect transistors (FETs) made from glow discharge amorphous (a-) silicon and silicon nitride. The construction and performance of inverter circuits, employing integrated a-Si load resistors, are described in some detail. The extension of this basic circuit to NAND and NOR gates, to a bistable multivibrator and to a shift register is reported. Based on the excellent photoconductive properties of a-Si an integrated addressable photosensing element has been constructed, which could have potential applications in imaging arrays.

The metal-amorphous silicon barrier, interpretation of capacitance and conductance measurements

Abstract Previous model calculations based on the experimentally determined density of states distribution of glow discharge a-Si have been further developed to investigate the capacitance of the metal/a-Si barrier. The observed dependence of barrier capacitance on donor concentration, measuring frequency, applied bias and temperature is explained in terms of the extended model. Finally, the current-voltage characteristics of some of the diodes, and the high field breakdown in reverse bias, are briefly discussed.

The effect of?-irradiation on amorphous silicon field effect transistors

The paper deals with the effect ofγ-radiation from a Co60 source on the electronic properties of amorphous silicon field effect transistors. These thin film devices, deposited by the glow discharge technique, are being developed for addressable liquid crystal displays, logic circuits and other applications. 1 Mrad (Si) and 5 Mrad (Si) doses were used and the transistors were held at gate voltages between −8V and +8V during irradiation. Measurements on irradiated specimens showed shifts in threshold voltage of less than 3 V and a change in transconductance below 10%, both of which could be removed by annealing above 130 °C. These results are compared with presently available “radiation hardened” crystalline silicon device structures and it is concluded that in spite of the thicker gate insulation layer (0.3 μm of silicon nitride) of the amorphous devices, the latter are remarkably radiation tolerant, with little degradation in performance. Measurements on irradiatedα-Si films deposited on glass show pronounced conductivity changes, not observed in the transistors. It is suggested that these effects arise at the Si/glass interface, and are prevented by the presence of the silicon nitride film in the devices.

Recent developments in amorphous silicon p-n junction devices

Abstract Amorphous Si p-n junctions with various doping profiles have been prepared by the glow discharge process to investigate the effect of the barrier profile on the electrical properties of the diodes. The highest current densities, up to 40A/cm 2 , are obtained with n + -ν-p + structures. Under AM-I illumination photovoltaic p + -i-n + cells generate open circuit voltages of 0.7V and short-circuit currents up to 10mA/cm 2 , corresponding to efficiencies between 3 and 4%. The diode quality factors have also been investigated.