Drive transistor with fold gate

Details Drive transistor with fold gate Drive transistor with fold gate

2000-01-14

2001-08-14

Chaudhuri, Olik (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Junction field effect transistor

C257S401000, C257S287000, C438S277000, C438S284000, C347S059000

Reexamination Certificate

active

06274896

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally directed to integrated circuits for ink jet print heads. More particularly, the present invention is directed to drive transistors for ink jet heating elements in an ink jet print head.
BACKGROUND OF THE INVENTION
Ink jet printers form images on paper by ejecting ink droplets from an array of nozzles on an ink jet print head. In thermal ink jet print heads, a heating element, such as a resistor, is associated with each nozzle. The heating element heats adjacent, thereby causing formation of a rapidly expanding bubble of ink. The expanding bubble causes a droplet of ink to be ejected from the nozzle.
Generally, each heating element is activated by a corresponding switching device, such as a MOSFET drive transistor, that is connected electrically in series with the heating element. Since these transistors must handle relatively high current levels, they also generate heat that is directly related to their on-resistance. The heat generated by the drive transistors can significantly affect the temperature of the print head chip, and can cause temperature gradients across the chip. Variations in print head temperature cause variations in ink droplet mass, which, in turn, degrade print quality. Therefore, it is desirable to keep the on-resistance of the drive transistors as low as possible.
As the state of the art advances, the spacing between nozzles in ink jet print heads decreases, thus allowing higher print resolution. As nozzle density increases, so does the density of heating elements and drive transistors associated with the nozzles. As the width of drive transistors decreases to accommodate high-density packaging, maintaining low on-resistance becomes much more challenging.
Therefore, a drive transistor that accommodates high-density packaging while maintaining low on-resistance and high-current carrying capability is needed.
SUMMARY OF THE INVENTION
The foregoing and other needs are met by a drive transistor for an ink jet print head that includes a semiconductor substrate having a serpentine channel of semiconductor material. The channel, which has a first type doping, includes first and second serpentine channel portions that are substantially parallel. The first and second serpentine channel portions define an inner region therebetween and an outer region disposed outside the first and second serpentine channel portions. The substrate also includes a drain made of semiconductor material having a second type doping. The drain is disposed within the inner region, and has drain fingers defined by the serpentine channel. The substrate further includes a source made of semiconductor material having the second type doping. The source is disposed within the outer region, and has source fingers defined by the serpentine channel. Due to the serpentine nature of the channel, the source fingers are inter-digitated with the drain fingers. The transistor also has a serpentine gate that substantially overlies the serpentine channel.
An elongate drain conductor, which tapers from a wide drain conductor end to a narrow drain conductor end, at least partially overlies a portion of the drain and the serpentine channel. Distributed along the drain conductor are drain conductor contacts for electrically connecting the drain conductor to the drain.
An elongate source conductor has two tapered source conductor portions that at least partially overly the source and the serpentine channel. The two source conductor portions have wide source conductor ends that are connected together and narrow source conductor ends that are spaced apart. One source conductor portion is disposed to one side of the drain conductor, and the other source conductor portion is disposed to the other side of the drain conductor. The wide source conductor ends are adjacent the narrow drain conductor end, and the narrow source conductor ends are adjacent the wide drain conductor end. Distributed along the source conductor are source conductor contacts for electrically connecting the source conductor to the source.
As explained in more detail hereinafter, the serpentine gate is continuous, thereby completely surrounding or “trapping” the drain. This “trapped drain” design significantly reduces leakage currents as compared to open-ended drain devices. This design also provides for stacking transistors closely together to accommodate a higher packaging density than was previously achieved in ink jet print head chips.
In another aspect, the invention provides a method for forming a drive transistor for an ink jet print head. The steps for forming the transistor include providing a semiconductor substrate having a first type doping. The substrate is doped to form a drain having a second type doping, where the drain has an outer perimeter at least partially defined by a serpentine channel having the first type doping. The channel comprises substantially parallel first and second serpentine channel portions, where the first and second serpentine channel portions define an inner region disposed between the first and second serpentine channel portions and an outer region disposed outside the first and second serpentine channel portions. The drain is formed such that it is disposed within the inner region and has drain fingers defined by the serpentine channel. The method includes doping the substrate in the outer region to form a source having the second type doping. The source is formed such that it has source fingers defined by the serpentine channel, where the source fingers are interdigitated with the drain fingers. A serpentine gate is formed which substantially overlies the serpentine channel. The method further includes forming an elongate drain conductor that at least partially overlies the drain and the serpentine channel. The drain conductor is formed to taper from a wide drain conductor end down to a narrow drain conductor end. According to the method, drain conductor contacts are formed which are distributed along the drain conductor for electrically connecting the drain conductor to the drain. An elongate source conductor is also formed which comprises two source conductor portions that at least partially overly the source and the serpentine channel. The two source conductor portions are tapered from wide source conductor ends that are connected together down to narrow source conductor ends that are spaced apart. The source conductor portions are formed such that one source conductor portion is disposed to one side of the drain conductor, and the other source conductor portion is disposed to the other side of the drain conductor, where the wide source conductor ends are adjacent the narrow drain conductor end, and the narrow source conductor ends are adjacent the wide drain conductor end. The method also includes forming source conductor contacts that are distributed along the source conductor for electrically connecting the source conductor to the source.


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