Capacitively coupled field effect transistors for...

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1996-12-03

2001-01-16

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

C349S043000, C257S360000

Reexamination Certificate

active

06175394

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the provision of electrostatic discharge (ESD) protection for flat panel displays incorporating thin film transistors (TFTs) in the pixel drive circuitry.
2. Description of the Related Art
Recently, there has been growing commercial utilization of flat panel displays both as a replacement for the conventional CRT display and as a display for applications in which a CRT display is impractical, such as in portable computers and in viewers for video cameras. There are a variety of flat panel display technologies, including liquid crystal displays (LCDs), plasma displays and field emission displays. Of these, LCD technology has found the greatest level of acceptance in view of the comparative simplicity of the technology and the comparative ease of production. Flat panel displays are conventionally manufactured by forming electronic circuitry on an optically transparent substrate (e.g., glass) using photolithographic processing techniques (deposition, mask definition and etching) to form semiconductor thin film transistors (TFTs) and the other circuitry used for addressing and driving the pixels of the display. Generally, an electrically excitable medium is provided in close proximity to the driving electronics, and signals provided to the electrically excitable medium by the driving electronics cause the electrically excitable medium to produce a modulated optical signal. In the case of plasma displays, the electrically excitable medium itself generates the modulated optical signal. In other cases, such as twisted nematic liquid crystal displays (TN-LCDs), a light source is provided adjacent the electronically excitable medium (i.e., the liquid crystal) and the electronically excitable medium is used as a graduated light valve to modulate the light passing through the liquid crystal.
Typically, flat panel displays are configured as a matrix array of addressable pixel locations, with corresponding drive circuitry provided to drive each pixel independently. In an exemplary configuration of a TN-LCD, one thin film transistor (TFT) is provided adjacent each pixel for locally exciting the liquid crystal material which serves as the electrically excitable medium of the TN-LCD. The TFT is coupled to a pixel electrode, a column signal line and a row signal line. Signals provided to the TFT over the column signal line and the row signal line determine whether the TFT causes the pixel electrode to be at a high potential which excites the liquid crystal, or at a low potential which does not excite the liquid crystal.
FIG. 1
illustrates a small portion of the drive circuitry formed on the backplane of a TN-LCD. The backplane may be formed, for example, on a glass substrate
10
and the illustrated circuitry can be used to excite the liquid crystal material. Row select signals are provided at row contact pads
12
and through row bus lines
16
; column drive signals are provided at column contact pads
14
and through column bus lines
18
. Thin film transistors (TFTs)
20
adjacent the intersections between the row and column lines act as switching elements for coupling drive signals to individual pixel locations. When the TFTs are turned on, charge can flow to the pixel electrodes
22
to charge the capacitor formed by the pixel electrode
22
and a ground electrode formed on the front substrate of the display (not shown). Liquid crystal fills the gap of this capacitor, so that the charge flowing through the thin film transistor generates an electrical field across the adjacent liquid crystal. The thin film transistors
20
are typically formed from amorphous silicon on the surface of the glass substrate
10
or on one or more thin film layers deposited on the glass substrate
10
.
A common failure mode for the circuitry illustrated in
FIG. 1
involves the destruction of an insulation layer, such as the silicon nitride gate insulator often used in TFTs, as a result of electrostatic discharge (ESD). ESD might generate a large transient potential between a column line and a row line of a TN-LCD, in turn causing the breakdown of the silicon nitride of the TFT at the intersection between the column line and the row line. After the silicon nitride undergoes dielectric breakdown, the silicon nitride is no longer an effective insulator and the affected TFT acts as a short-circuit between the corresponding column line and row line. This failure mode has been recognized, for example, in U.S. Pat. No. 5,019,002 to Holmberg. The Holmberg patent states that this electrostatic discharge mode can be avoided by providing a guard ring around the active elements of the display, with the guard ring coupled to the row and column lines of the display. This guard ring structure is schematically illustrated in FIG.
1
. Guard ring
24
is typically formed from a low resistivity conductor, which might be a metal such as aluminum, and the guard ring is coupled to the row and column lines
16
,
18
adjacent the contact pads
12
,
14
by protection circuit elements
26
. According to the Holmberg patent, the protection circuit elements
26
might be a thin film transistor without a gate, that is, each of the protection circuit elements
26
might consist of two amorphous silicon diodes in a back to back configuration except that the diodes are coupled by an amorphous silicon resistor.
There are substantial drawbacks to the structure described in the Holmberg patent. To provide effective ESD protection, guard ring
24
must remain connected to the backplane's drive circuitry throughout the assembly and test of the display. Under some circumstances, it is desirable to leave the guard ring connected to the drive circuitry after completion of the display so that the ESD protection is provided for the completed display. Whether for testing purposes or in the course of the normal operation of the display, it is necessary to provide signals to the thin film transistors to drive the pixels of the display. As such, the protection circuit elements
26
could drain away driving current from a point immediately adjacent the contact pads for the display. Accordingly, it is necessary to ensure that the protection circuit elements have a substantial resistance, at least to relatively low frequency drive signals. To this end, a high resistance, intrinsic amorphous silicon (a-Si) resistor is typically incorporated in the protection circuit element
26
used to form the ESD protection structure of the Holmberg patent. The resistance of such an intrinsic a-Si resistor is very high (>G&OHgr;) and causes the protection circuit element
26
to provide only a weak coupling between the guard ring
24
and the row and column lines
16
,
18
, even for the large transients associated with ESD. As such, the ESD protection circuit illustrated in the Holmberg patent provides inadequate protection from ESD damage for the TN-LCD.
An improvement to the Holmberg patent's ESD protection circuit is described in U.S. Pat. No. 5,220,443 to Noguchi. In the Noguchi patent's ESD protection circuit, illustrated in
FIG. 2
, a pair of thin film field effect transistors
40
,
42
(TFTs) form an ESD protection circuit element
26
that may be used in the circuitry illustrated in FIG.
1
. The gate of transistor
40
is connected to both the drain of transistor
40
and the source of transistor
42
, and the gate of transistor
42
is connected to both the drain of transistor
42
and the source of transistor
40
. As such, the source of transistor
40
is connected to the drain of transistor
42
and the drain of transistor
40
is connected to the source of transistor
42
. The common node consisting of the drain of transistor
40
and the source of transistor
42
forms an input terminal for the
FIG. 2
protection circuit element. The common node between the drain of transistor
42
and the source of transistor
40
forms an output terminal for the
FIG. 2
circuit. Significantly improved ESD protection is obtained when the
FIG. 2
protection ci

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