Browse Prior Art Database

IMPROVED HIGH VOLTAGE THIN FILM TRANSISTOR

IP.com Disclosure Number: IPCOM000026657D
Original Publication Date: 1993-Feb-28
Included in the Prior Art Database: 2004-Apr-06
Document File: 4 page(s) / 204K

Publishing Venue

Xerox Disclosure Journal

Abstract

The present invention relates to high-voltage thin film transistors (referred to as "HVTFT"s). A goal of the present invention is to increase the carrier injection level in the ungated region of an a-Si lateral action HVTFT. This is accomplished by increasing the capacitance between the drain electrode and the gated region of the transport layer. By increasing the number of carriers that the carrier transport layer of the device is capable of supporting for a given drain bias (by increasing the capacitance between the drain electrode and the gated region of the transport layer), an improved HVTM' or similar device having reduced saturation voltage, and thus reduced loss, having increased gain, and having reduced switching speed is provided.

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Page 1 of 4

XEROX DISCLOSURE JOURNAL

Proposed Classification

IMPROVED HIGH VOLTAGE THIN FILM TRANSISTOR US. C1.357/002
H.
Tuan Int. C1. HOll45/00

M. Hack

70\

56 -

I

54

66 68

Fig. I

XEROX DISCLOSURE JOURNAL - Vol. 18, No. 1 Januaryflebruary 1993 69

[This page contains 1 picture or other non-text object]

Page 2 of 4

IMPROVED HIGH VOLTAGE THIN FILM TRANSISTOR(Cont'd)

The present invention relates to high-voltage thin film transistors (referred to as "HVTFT"s). A goal of the present invention is to increase the carrier injection level in the ungated region of an a-Si lateral action HVTFT. This is accomplished by increasing the capacitance between the drain electrode and the gated region of the transport layer. By increasing the number of carriers that the carrier transport layer of the device is capable of supporting for a given drain bias (by increasing the capacitance between the drain electrode and the gated region of the transport layer), an improved HVTM' or similar device having reduced saturation voltage, and thus reduced loss, having increased gain, and having reduced switching speed is provided.

In one embodiment, the present invention comprises an a-Si HVTFT or similar lateral action device including a source portion laterally offset from a drain portion, the source portion at least partially overlaying a gate portion, the source and gate portions being spaced apart in a direction normal to the direction of lateral spacing of the source and drain portions, a carrier transport layer extending between and making physical and electrical contact with the source and drain portions and extending between the source and gate portions, all formed on a substrate. The carrier transport layer is therefore divided into two regions, a gated region above the gate portion, and an ungated region between the drain portion and the gated region. A region of dielectric material having a dielectric constant at least equal to the dielectric constant of the substrate at least partially overlays the ungated region and extends laterally at least part-way to the source portion.

According to one aspect of the present invention, the dielectric region is formed at least over the ungated region. If the proper thickness of the overcoat dielectric region is maintained, and if the dielectric material has a high enough dielectric constant, then for a given Vds the device is capable of supporting a larger number of carriers in the carrier transport layer than without such a region. Thus, according to this embodiment of the invention, the dielectric region has a dielectric constant at least equal to the dielectric constant of the substrate, and a thickness appropriate to increase the number of carriers that the transport layer is capable of supporting. If r is the ratio of the dielectric constant of the overlayer to that of the substrate, and s the ratio of the overlayer thickness to the length of the ungated region, then it is concluded that device performance is improved...