Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Heterojunction formed between semiconductor materials which...
Reexamination Certificate
2001-12-13
2003-03-04
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Heterojunction formed between semiconductor materials which...
C438S030000, C438S050000, C438S150000, C438S486000
Reexamination Certificate
active
06528830
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to semiconductor transistors and has particular application to thin film polycrystaline transistors.
2. Description of Related Art
Herein the abbreviation I
D
is used to refer to the transistor drain current, V
G
is used to refer to the transistor gate voltage generally, V
DS
is used to refer to the transistor drain to source voltage, and V
GS
is used to refer to the transistor gate to source voltage. Furthermore, herein the word “on”, such as in the description of one film or layer being “formed on another” is not intended to require direct contact between the two layers. That is, for example, it should not be interpreted as excluding arrangements in which another layer or film is interposed between the one layer which is formed “on” the other.
Unlike the output characteristics (I
D
V
DS
) of single crystal MOSFETs, a saturation regime is not observed, for example, in a polycrystaline silicon thin film transistor. Instead, as shown in
FIG. 1
, when the device.is operating above the so-called pinch-off level, generally when V
DS
>V
GS
, high electric fields are formed near the drain and this results in so-called impact ionisation. The result is an increase in drain current I
D
which is often referred to as the kink effect. This effect increases power dissipation and degrades the switching characteristics in digital circuits, whilst reducing the maximum obtainable gain as well as the common mode rejection ratio in analogue circuits.
The kink effect is also affected by the so-called parasitic bipolar effect, which is well known in silicon-on-insulator (SOI) devices. This occurs when electron-hole pairs are generated with impact ionisation at high electric fields near the drain, resulting in the holes drifting towards the source and causing a potential barrier lowering at the source junction. This effect also occurs in polysilicon thin film transistors and is due to the fact that the thin film active layer acts as the base of a bipolar transistor.
SUMMARY OF THE INVENTION
Against this background and with a view to providing an improved semiconductor transistor, in a first aspect the present invention provides a semiconductor transistor comprising a substrate having an active layer formed thereon, a source and a drain formed in the active layer, a gate insulating layer formed on the active layer and a gate electrode formed on the insulating layer, wherein the active layer has at least one centre which is located between the source and the drain and which extends ate side through the active layer for less than the full depth thereof.
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Lui Basil
Migliorato Piero
Nelms David
Oliff & Berridg,e PLC
Seiko Epson Corporation
Tran Mai-Huong
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