Alternate 4-terminal JFET geometry to reduce gate to source capacitance

The present invention relates to Junction Field Effect Transistors (JFET).

In FIGS. 1 and 2 , a four-terminal JFET 10 is shown. Four-terminal JFETs are comprised of the following terminals: a substrate 14 , a gate 11 c , a first channel terminal 13 c , and a second channel terminal 16 c . Usually, the first channel terminal 13 c is the source and the second channel terminal 16 c is the drain. Typically the substrate 14 and the gate 11 c will be made of a first conduction type, such as p, and are usually highly doped. The source 13 c and drain 16 c are usually made of a second conduction type, which is opposite of the conduction type of the gate 11 c , such as n, and are also highly doped. The gate 11 c and source 13 c are usually embedded in an upper layer 15 a - c that is the same conduction type as the source 13 c , such as n, but is usually not highly doped. This upper layer 15 a - c is often an epitaxial layer.

A gate pad 11 a , used for making an external connection to the gate 11 c , may be attached to the gate through a gate trace 11 b . The gate pad 11 a and/or gate trace 11 b may be situated over, and electrically isolated from, the source 13 c . An insulating material 12 , such as an oxide may be used to isolate the source 13 c from the gate pad 11 a and/or gate trace 11 b . Similarly, a source pad 13 a is connected to the source 13 c via a source trace 13 b , and a drain pad 16 a is connected to the drain 16 c via a drain trace 16 b.

The upper layer 15 a - c may be divided into three different sections. There may be no actual separation between the sections, but such a division may aid in description of the device. Section one 15 c of the upper layer is the section that is disposed at least partly beneath the gate 11 c . Section two 15 a of the upper layer is the section that is disposed beneath the gate pad 11 a . Section three 15 b of the upper layer is the section that is disposed at least partly beneath the gate trace 11 b . Section one 15 c may also be disposed between the source 13 c and the gate 11 c , although the source 13 c and the gate 11 c may also touch each other with no such section between.

For optimal JFET performance, it is usually desirable to reduce the capacitance between the gate 11 c and any other component because such capacitance can magnify undesirable noise in the device. The upper layer 15 a - c may reduce the capacitance between the gate, gate trace, and gate pad 11 a - c and the substrate 14 . The insulating layer 12 may reduce the capacitance between the gate, gate trace, and gate pad 11 a - c and the source 13 c . Because the upper layer 15 is of the same conduction type as the source 13 c , this upper layer is not very effective at reducing the capacitance between the source 13 c and the gate pad 11 a.

As noted by dashed lines 17 a - c , there are various capacitance paths between the gate, gate trace, and gate pad 11 a - c and the source 13 c . One capacitance path 17 c is between the gate 13 c and the source 11 c . A second capacitance path 17 b is between the gate trace 11 b and the source 13 c . Another capacitance path 17 a is between the gate pad 11 a and the source 13 c . Because sections two and three 15 a and 15 b of the layer are electrically connected to the source 13 c and because of the large size of the gate pad 11 a , the capacitance through this path is significant. The combined capacitance of these three capacitance paths has a significant effect on noise amplification of the device. It would be beneficial to reduce this capacitance.

It has been recognized that it would be advantageous to reduce the capacitance between the gate, gate trace, and gate pad and the source.

The present invention provides a reduced gate-to-source capacitance JFET device with a substrate having a first conduction type. An upper layer is disposed over the substrate and has a second, opposite, conduction type. A region having a first conduction type is formed in the upper layer such that it separates the upper layer into two sections, including section one and section two. A gate is disposed in section one of the upper layer and is separated from the substrate by the upper layer. The gate has the first conduction type. A first channel terminal, which is usually the source, is disposed in section one of the upper layer and has the second conduction type. A gate pad is disposed over section two of the upper layer and is electrically coupled to the gate, and electrically isolated from the source and the upper layer.

In accordance with a more detailed aspect of the present invention, the JFET device can be part of a radiation detection system with a radiation detector electrically attached to the JFET. An hermetically sealed container can surround the radiation detector and JFET. A widow in the container can allow x-rays to pass into the container and impinge upon the radiation detector.

FIG. 1 is a schematic top view of a prior-art JFET;

FIG. 2 is a schematic cross-sectional side view of the JFET of FIG. 1 taken along line 2 - 2 in FIG. 1 ;

FIG. 3 is a schematic top view of a JFET in accordance with an embodiment of the present invention;

FIG. 4 is a schematic cross-sectional side view of the JFET of FIG. 3 taken along line 4 - 4 in FIG. 3 ; and

FIG. 5 is a schematic cross-sectional side view of a radiation detection system in accordance with an embodiment of the present invention utilizing the JFET of FIGS. 3 and 4 .

Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.

A reduced capacitance, four-terminal JFET 30 is shown in FIGS. 3 and 4 . The JFET 30 includes the following terminals; a substrate 14 , a gate 11 c , a first channel terminal 13 c , and a second channel terminal 13 c . Usually, the first channel terminal 13 c is the source and the second channel terminal 16 c is the drain. In this application, for simplicity, “source” is used to describe the first channel terminal and “drain” is used to describe the second channel terminal, but the invention is not limited to the source being the first channel terminal and the drain being the second channel terminal. The alternative configuration could be used with the drain being the first channel terminal and the source being the second channel terminal.

The substrate 14 and the gate 11 c can be made of a first conduction type and can be highly doped. The source 13 c can be made of a second conduction type, which is opposite of the first conduction type, and can also be highly doped. The gate 11 c and source 13 c can be embedded in an upper layer 15 a - c that is the same conduction type as the source 13 c , but is usually not highly doped. The upper layer 15 a - c can be disposed over the substrate. The gate 11 c can be placed in the upper layer 15 such that the upper layer 15 a - c separates the gate 11 c from the substrate 14 . The source 13 c may extend down such that it contacts the substrate 14 or the upper layer 15 a - c may separate the source 13 c from the substrate 14 . This upper layer 15 a - c can be an epitaxial layer. The first conduction type can be p, and the second conduction type n, or the first conduction type can be n, and the second conduction type can be p.

A gate pad 11 a , used for making an external connection to the gate 11 c , may be attached or electrically coupled to the gate through a gate trace 11 b . The gate pad 11 a and/or gate trace 11 b may be situated over, and electrically isolated from, the source 13 c . An insulating material 12 , such as an oxide, may be used to isolate the source 13 c from the gate pad 11 a and/or gate trace 11 b . Similarly, a source pad 13 a is connected to the source 13 c via a source trace 13 b and a drain pad 16 a is connected to the drain 16 c via a drain trace 16 b.

For optimal JFET performance, it is usually desirable to reduce the capacitance between the gate 11 c and any other component because such capacitance can magnify undesirable noise in the device. The upper layer 15 a - c may reduce the capacitance between the gate, gate trace, and gate pad 11 a - c and the substrate 14 . The insulating layer 12 may reduce the capacitance between the gate, gate trace, and gate pad 11 a - c and the source 13 c . Because the upper layer 15 a - c is of the same conduction type as is the source 13 c , this upper layer is not very effective at reducing the capacitance between the source 13 c and the gate pad 11 a.

As noted by dashed lines 17 a - c , there are various capacitance paths between the gate, gate trace, and gate pad 11 and the source 13 c . One capacitance path 17 c is between the gate 13 c and the source 11 c . A second capacitance path 17 b is between the gate trace 11 b and the source 13 c . Another capacitance path 17 a is between the gate pad 11 a and the source 13 c . The combined capacitance of these three capacitance paths affects noise amplification of the device.

In order to reduce the noise amplification caused by the capacitance between the gate pad 11 a and the source 13 c , a region 31 , having a first conduction type, is formed in the upper layer and separates the upper layer into two sections, section one 15 c and section two 15 a . Because the region 31 is of the first conduction type, it creates a low capacitance region between the gate pad 11 a and the source 13 c , resulting in reduced overall capacitance between the gate, gate trace, gate pad 11 a - c and the source 13 c . Because the region 31 is of the same conduction type as the substrate and the gate, it can have an undesirable effect of an increased capacitance between the gate and the substrate. To minimize such increased gate-to-substrate capacitance, the region 31 can be placed beneath the low surface area gate trace 11 b and not beneath the large surface area gate pad 11 a . The overall effect of such placement is a significant reduction in gate 11 c to source 13 c capacitance with minimal adverse effect on the gate 11 c to substrate 14 capacitance. Note that if the source 13 c extends down to the substrate, and region 31 is not situated adjacent to the source 13 c , then section one 15 c can be divided into two subsections. Region 31 may be situated adjacent to the source 13 c or there may be upper layer 15 c between region 31 and the source 13 c . The region 31 can physically separate the upper layer, or sections one and two 15 c and 15 a , under the trace. The region can fill a space between the insulating layer 12 and the substrate 14 .

How to Make

The reduced capacitance 4-terminal JFET can be made by standard semiconductor manufacturing processes and can be made of standard semiconductor materials such as silicon, germanium, gallium arsenide, etc. The dopants can be standard doping materials. For example, boron or boron difluoride may be used for p type doping and phosphorous or arsenic may be used for n type doping.

The region 31 between section one 15 c and section two 15 a of the upper layer can be formed by depositing an oxide over the entire surface of the device, then depositing a photoresist mask over the surface which allows the oxide over the region to be selectively etched away. A dopant can be diffused into the material to a sufficient depth that it makes electrical contact with the substrate. The oxide that was not etched way can prevent the dopant from diffusing into other regions.

Referring to FIG. 5 , the above described reduced capacitance 4-terminal JFET 30 can be utilized with a radiation detection system 100 . The radiation detection system can include a radiation detector 120 electrically attached to the JFET 30 . An hermetically sealed container 124 can surround the radiation detector 120 and JFET 30 . A widow 128 in the container 124 can allow x-rays, represented by line 132 , to pass into the container and impinge upon the radiation detector.

It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.