Isolated Gate JFET Structure

The present invention relates generally to junction field effect transistors and, more specifically, to an improved thin channel junction field effect transistor.

Junction field effect transistors (JFET) have been used as active devices for many years. More recently, a JFET structure which is compatible with bipolar processing known as a BIFET has been developed. Parallel source and drain regions are formed in a bottom gate region during the base diffusion of the bipolar processing. A common ohmic contact region to the bottom gate region and the to-be-formed top gate region are formed during the emitter diffusion of the bipolar devices. The additional BIFET processing steps are ion implantations of a channel region between the source and drain regions and buried below the surface followed by an ion implantation of the top gate region. Since in most applications, the top and bottom gates are connected together, a single ohmic contact to the bottom and the thin top gate regions is used. Also, the top gate extends outside the channel region and makes direct contact with the bottom gate. Such a prior art device is illustrated in FIG. 1.

The P implant which forms a channel region is about 0.3 microns thick and about 0.3 microns below silicon surface. The two P diffusion contacts which form the source and drain along the two edges provide a path from the channel implant up to the top surface wherein interconnects can be made. The N implant forms the top gate and is very shallow typically about 0.3 microns into the top surface. The N top gate implant is lower in impurity concentration than the surface of the P source and drain diffusion and, consequently, aluminum cannot make an ohmic contact thereto. Also, the top gate is so shallow that aluminum will migrate through it to form a short to the channel region if direct contact were made. To avoid these problems, the gate implant is extended and overlaps the N+ contact diffusion at the end of the channel.

In many applications, it is desirable to isolate the top gate from the bottom gate so they can be connected to separate terminals. This is particularly desirable when gate leakage or AC performance is important. In both cases, the presence of isolation region parasitics on the bottom gate make it desirable to disconnect the bottom gate from the input from the signal input or top gate. Another advantage afforded by an isolated gate structure is that several devices can be built in a common bottom gate isolated island rather than in separate isolated islands. This saves die area and improves match of matched pairs by allowing the members of the pair to be closer to one another.

The basic method used in the prior art to isolate top and bottom gates is to form the top gate as a closed geometry surrounding either the source or drain and the top gate surrounded by the other terminal. The choice of a closed geometry provides termination of gate region across the source and drain contact regions along the entire parameter of the gate. This assures that there is no contact between top and bottom gates along their edge. An example of such a closed geometry JFET is illustrated in U.S. Pat. No. 3,649,385.

Thus, there exists the need for a thin channel junction field effect transistor which has an isolated top and bottom gate.

An object of the present invention is to provide a thin channel junction field effect transistor having isolated top and bottom gates.

Another object of the present invention is to provide a junction field effect transistor compatible with bipolar processing techniques having isolated top and bottom gates.

Still another object of the present invention is to provide a thin channel junction field effect transistor requiring less surface area than prior art devices.

These and other objects of the invention are attained by forming the source as an annulus region with the drain, thin channel and top gate regions formed interior thereto, as well as including, interior to the source an isolation region extending into the bottom gate and a top gate contact region in the isolation region in contact with the thin top gate region which extends over the top of the isolation region. The isolation region isolates the top gate contact region from the bottom gate while the source and channel isolate the top gate region from the bottom gate. The isolation region and the top gate contact region are formed in a portion of the channel and top gate region exterior the region defined on two opposed sides by the lengths of the source and drain regions. The impurity concentration of the isolation region is less than that of the top gate region and the impurity concentration of the top gate contact is greater than that of the top gate region. The depth of the isolation region and the top gate contact region is greater than that of the top gate and thin channel regions. The isolation region may contact and overlap the source region in some applications.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is a perspective cross-sectional view of a BIFET structure of the prior art.

FIG. 2 is a top logical view of a junction field effect transistor in accordance with the principles of the present invention.

FIG. 3 is a cross-sectional view taken along lines 3--3 of FIG. 2.

The improved thin channel junction field effect transistor of the present invention is illustrated in FIGS. 2 and 3 without the top insulative layer and metal contacts. The cross-hatched areas in FIG. 2 illustrate the position of the contact apertures in an oxide layer and are not included in FIG. 3 for sake of clarity. The bottom gate of the JFET is an N- region 10 which may be a dielectrically isolated region or an epitaxial junction isolated region. The impurity concentration of the bottom gate 10 may be in the range of 1.times.10.sup.14 to 1.times.10.sup.17. The thin channel JFET of the present invention is compatible with and may be formed using bipolar processing techniques and, thus, is considered a BIFET. An isolation region 12 is formed in the bottom gate 10 by introducing impurities opposite that of the bottom gate 10 in this example P-. This region may be formed simultaneously with forming the collector of an PNP, a low concentration NPN base region, a low concentration I.sup.2 L base or a special diffusion designed for isolation functions. The surface impurity concentration of the resulting isolation region 12 should be in the range of 5.times.10.sup.15 to 5.times.10.sup.17.

Next, the source and drain regions 14 and 16 are formed by introducing impurities opposite that of the bottom gate or in this example P type impurities into the bottom gate region. In order to achieve a isolated top and bottom gate, the source region 14 is formed as an annulus or closed geometry. The drain 16 is formed interior to the source 14. It should also be noted that the isolation region 12 is formed interior to the source region 14. The source and drain regions 14 and 16 may be formed simultaneously with the formation of bases of NPN bipolar transistors and have a surface impurity concentration in the range of 5.times.10.sup.17 to 5.times.10.sup.19. The final step which is common with the bipolar processing is the formation of the N+ top gate and bottom gate contacts 18 and 20 formed in the isolation region 12 and the bottom gate 10, respectively. The N+ surface impurity concentration is in the range of 5.times.10.sup.19 to 5.times.10.sup.21.

Next, the thin P channel region is formed by ion implanting P type impurities. The impurities are introduced to form a thin channel region below the surface having thickness in the range of 0.1 to 0.5 microns. The thin channel region overlaps portions of the source region 14 and the drain region 16 and all of the isolation region 12 and top gate contact region 18. The P channel region 22 has a peak impurity concentration in the range of 1.times.10.sup.16 to 1.times.10.sup.18. Next, N type impurities are ion implanted to form the top gate region 24 on top of the channel region 22 and on the top of the isolation region 12. The top gate is formed to have a depth in the range of 0.05 to 0.4 microns and interconnects the source 14, drain 16 and the top gate contact 18. The top gate region has an impurity concentration in the range of 1.times.10.sup.16 to 5.times.10.sup.18. The impurity concentration of the isolation region 12 is selected to be less than that of the top gate region 24 such that during the ion implantation, the ions will change the conductivity type of the isolation region 12 such that the top gate will contact and be connected to the top gate contact 18. Since the ion implantation of the channel region 22 produces a distinct channel region at a specific depth below the surface, it does not affect the impurity concentration at the top of the isolation region 12. The extent of the ion implants is signified by the dotted lined cross-hatched area in FIG. 2.

Contact to the source is at regions 26 and 28, to the drain at region 30, to the top gate at region 32 and to the bottom gate at region 34.

As is evident from FIG. 2, the active channel region between the source contacts 26 and 28 and the drain contact 30 is the area of the channel region defined on opposed sides by the length of the source and drain regions. The isolation region 12 and top contact 18 are formed outside this area and are laterally displaced therefrom. This particular design thus reduces the amount of surface area required for the formation of an isolation top gate contact. Also, by placing the contact outside the active channel region, the thin dimension of the channel and top gate and its unformity is preserved. The depth required for the deep N+ region 18 is compensated for by forming the deep P- isolation region 12. As discussed above, the thickness of the top gate region is in the range of 0.05 to 0.4 and the thickness of the channel region is in the range of 0.1 to 0.5. Typically, the isolation region 12 is formed to a depth of the range of 2 to 15 microns and the top gate contact 18 is formed generally in the range of 1 to 6 microns. Thus, the isolation region 12 and the top gate contact region 18 have a greater depth than the channel region 22 and the top gate region 24.

The closed gemoetry of the source region 14 isolates the top gate layer 24 from the bottom gate 10 laterally while the channel region 22 isolates the top gate region 24 from the bottom gate 10 along its bottom surface. The isolation region 12 isolates the top gate contact 18 from the bottom gate 10.

It should be noted that the annulus shaped source 14 may be the drain and a source would be formed interior thereto. Although the present example has described a P channel junction field effect transistor, it is obvious that the same processing steps may be used introducing opposite impurity concentration to form an N channel junction field effect transitor. Also, in certain applications it may be desirable to have the P- isolation region 12 overlap the source contact 14. This would provide a bias on the isolation region which may be desirable in some applications.

It should also be noted that the present process may be used to form any thin film device which is to be isolated from the substrate area and requiring an ohmic contact. For example, the top gate may represent a thin film diffused resistor with the P source and/or drain regions and the P channel region forming the junction isolation. In these instances, a pair of isolation regions 12 and contact regions 18 would be required. Another structure could be thin film diodes. These are two examples of other types of structures which may be used wherein a ohmic contact to a thin surface layer is required and the top surface layer and contacts are to be isolated from the region in which it is built.

It is evident from the description of the preferred embodiment, that the objects of the invention are obtained. Although the invention is described and illustrated in detail it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention is defined by way of the claims.