Source-gated transistors in hydrogenated amorphous silicon

Abstract

Source-gated transistors form a new class of transistors in which the current is controlled entirely by the source. As such, they have properties that are fundamentally different from their nearest relative the field-effect transistor. Instead of using the field-effect to modulate the conductance of a channel it is used to change to electric field at a reverse biased source barrier thereby changing the source current. Saturation of the current occurs when the source is depleted of carriers by the reverse biased barrier. This feature contrasts markedly with saturation in a standard field-effect transistor that occurs when the drain end of the channel is depleted of charge. A model describing the characteristics of a source-gated transistor is outlined and compared with characteristics measured on transistors made using hydrogenated amorphous silicon. Good agreement is found between theory and experiment. It is shown that the saturation voltage of the SGT can be very much smaller than it is in a FET leading to lower voltage operation and power dissipation. Furthermore, the output impedance of the SGT can exceed that of an FET. Transistors covering a wide range of currents have been made by modifying a Schottky barrier source using ion implantation.