Low-Field Behavior of Source-Gated Transistors

Abstract

-A physical description for low-field behavior of a Schottky source-gated transistor (SGT) is outlined where carriers crossing the source barrier by thermionic emission are restricted by JFET action in the pinch-off region at the drain end of the source. This mode of operation leads to transistor characteristics with low saturation voltage and high output impedance without the need for field relief at the edge of the Schottky source barrier and explains many characteristics of the SGT observed experimentally. Two-dimensional device simulations with and without barrier lowering due to the Schottky effect show that transistors can be designed so that the current is independent of source length and thickness variations in the semiconductor. This feature, together with the fact that the current in an SGT is independent of source-drain separation, hypothesizes the fabrication of uniform current sources and other large-area analog circuit blocks with repeatable performance even in imprecise technologies such as high-speed printing.

Temperature dependence of the current in Schottky-barrier source-gated transistors

“…Regardless of the current injection mechanism at the source 3,5,6 , the intentional use of a source barrier in an otherwise conventional TFT structure leads to major differences in the transistor (output) characteristic: low-voltage saturation 2,3,[7][8][9][10] , flat saturated output characteristics 3,7,9,11 , tolerance to geometrical variations 4,12 , bias stress stability 13 and improved current levels in low mobility materials 14 . Both digital 15 and analog applications 14,[16][17][18][19] could benefit from using these devices, in terms of amplification, energy efficiency, uniformity of electrical performance, and reliability.…”

“…For low barriers and long sources we expect the current to be restricted by the JFET action and by the resistive path through the semiconductor in the bulk region of the source. For high barriers and short sources, the current should be determined by the field dependence of the barrier at the end of the source 5 .…”

“…Thus, at high electric field, the barrier in this region of the source can be modulated, and since the reverse current of the contact is exponentially dependent on effective barrier height 20 , large changes in current (I 1 ) can be effected by changing the potential on the gate. The drain bias, however, has a similar effect on this area of the source, so without purposeful screening of the source edge from drain electric field 11 , output characteristics have a drain voltage dependence of the current in saturation 5,9 .…”

“…The transconductance of the drain current (g m , Figure 2b) shows a definite kink when switching between modes of operation 5 . A longer S allows more current to be injected from the bulk of the source, pushing the gate voltage at which the kink occurs to higher values.…”

“…When I 2 dominates, the flatness of the output characteristics is superior without the need for a specially-designed field relief structure 5,11 . Long source devices should have improved intrinsic gain characteristics.…”