Abstract:
This invention discloses the present invention discloses a junction field effect transistor UFET) device supported on a substrate. The JFET device includes a gate surrounded by a depletion region. As the distance between the gates is large enough, there is a gap between the depletion regions surrounding adjacent gates. Depletion mode JFET transistor which is normally on is provided. The normally on transistors respond to negative bias applied to the gates to shut of the current path in the substrate. The current path in the substrate is normally available with a zero gate bias. As the distance between the gates is reduced, the JFET transistor is normally off because the depletion regions surround the gates shut of the current channel. The depletion region responding to a positive bias applied to the gate to open a current path in the substrate wherein the current path in the substrate is shut off when the gate is zero biased. The normally on and normally off JFET transistors are configured to achieve low voltage drop, low on resistance, high current density and high frequency operations.

Description:
This Application claims a Priority Date of Jan. 6, 1999, benefited from a previously filed Provisional Application No. 60/115,009 filed on Jan. 6, 1999 by a same Inventor of this Patent Application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the structure and fabrication process for manufacturing the junction field effect transistors (JFET). More particularly, this invention relates to a novel device structure and fabrication process for manufacturing normally on or normally off JFET transistors with low on-resistance for low voltage and high current density applications. 
     2. Description of the Prior Art 
     The semiconductor industry now faces a difficult challenge to satisfy the ever increased needs for providing transistors suitable for power management with voltage under three-volts. As the feature sizes of the integrated circuits (ICs) are becoming ever smaller and electronic devices are continuously being miniaturized, the voltages from AC or DC power sources for providing power to these devices are further dropped. Dropping the voltage from five volts to three volts results in a 25:9 reduction in power if the current density is maintained unchanged. At 1.8 volts, the power drops another 60%. However, the transient current loads can be very high. Under such operation conditions, the normal forward voltage drop for a p-n junction typically employed in a rectifier is about 0.9 volts. Most of the power will be consumed in the rectifying process. Power supply systems built with such types of p-n junctions would no longer be suitable for the low voltage applications. Even the Schottky barrier diodes with a forward voltage drop below 0.5 volts would not be a suitable solution to provide rectifiers or power switching devices for operation under the low voltage conditions. 
     The concept of junction field effect transistors (JFETs) has been disclosed after the invention of the bipolar transistors. JFET transistor can be operated at very high frequency with high switching speed because the JFET transistors are operated with majority carriers. The JFET transistor are well known and employed commonly in a naturally on state when the gate bias is zero. Because of the naturally on state, the JFET transistors are not as widely used in the semiconductor industry as the MOSFET, i.e., the metal oxide semiconductor field effect transistors. In order to make the JFET transistors to operate in a naturally off state, the distance between the gates has to be reduced for the depletion regions from the gate to shut off the current conducting paths. Such naturally off JFET transistors are however not very useful in their conventional configurations due to the longer current channel thus limiting the current capacity with a high on-resistance. S. M. Sze in “Physics of Semiconductor Devices” disclosed one example of such configuration cohn Wiley &amp; Son, 1981 Second Edition, page 322). The normally off JFET transistors discussed by Sze are for high speed low power applications. The long current channel and high-on resistance limit the usefulness of JFET transistors particularly the high on-resistance prevents such transistors for application in modern electronic devices operated with extreme low voltages. 
     In U.S. Pat. No. 4,523,111 entitled “Normally-Off Gate-Controlled Electric Circuit with Low On-Resistance”, Baliga disclosed a JFET serially connected to an IGFET. The gate of IGFET is operated as the gate for the serially connected circuit. The gate of IGFET is applied to block the current to flow through a normally on JFET until the IGFET is turned on with a positively biased voltage above an IGFET threshold voltage. The on-resistance is the sum of the JFET resistance and the IGFET resistance. The on-resistance would not be adequate for extreme-low voltage applications required by modern electronic devices as discussed above. A similar invention is disclosed in U.S. Pat. No. 4,645,957 that is entitled “Normally Off Semiconductor Device with Low On-Resistance and Circuit Analogue” by Baliga. The JFET transistor is serially connected to a bipolar transistor to achieve the normally off state. Again, the on-resistance is the sum of the bipolar resistance and the JFET resistance and becomes too high for modern application to modern devices operated with extremely low voltages. 
     In U.S. Pat. No. 5,321,283 entitled “High Frequency JFET” Cogan et al. disclose a JFET for radio frequency (RF) operation at high frequency. The normally-on JFET transistors disclosed in this patent are operated with high voltage and not suitable to satisfy the requirements of modern portable electronic devices that require extremely low voltage and relatively high current capacity. Similarly, in U.S. Pat. No. 5,618,688 entitled “Method of Forming a Monolithic Semiconductor Integrated Circuit having an N-Channel JFET”, Ruess et al disclose a normally on JFET transistor manufactured with BiCMOS processes. The JFET transistors disclosed in this patent are not suitable for low voltage and high current applications. 
     Therefore, a need still exits in the art of design and manufacture of transistor for low voltage power supply to provide a novel structural configuration and fabrication process that would resolve these difficulties. More specifically, it is preferably that the transistor for low voltage power supply has low on-resistance and high switching speed. It is further desirable to employ a simplified manufacture process to fabricate the power transistors such that highly reliable power transistors can be made available at a reasonably low production cost. 
     SUMMARY OF THE PRESENT INVENTION 
     It is therefore an object of the present invention to provide new device structures and manufacture methods. The conventional JFET transistors, which are normally on with negatively biased off, are now provided with unconventional device structures such that the new JFET transistors are operated with low on-resistance and high current capacity that can be either normally on or normally off by arranging the distance between the gates. The unconventional JFET transistors of this invention provide special advantages for low-voltage low-resistance applications such that aforementioned limitations and difficulties as encountered in the prior art can be overcome. 
     Specifically, it is an object of the present invention to provide a new device structure and manufacture method to provide an unconventional JFET transistor with flexibly on-off adjustment. When the distance between the gates is sufficiently large, the JFET transistors are normally on with zero gate bias. As the distance between the gates is sufficiently small, the depletion regions from the gates fills the conduction paths between the gates thus the conduction paths are shut off and the JFET transistors are normally off with a zero gate bias and turned on with positive gate bias voltage. A positive or negative bias voltage applied to the gate causes the depletion regions surrounding the gate to shrink and open up the channel for current to pass through. Very low resistance current path is provided. Low voltage and high current density can be achieved by applying to the normally-off-positively-biased-on or normally-on-negatively-biased-off JFET transistors. 
     Another object of the present invention is to a new device structure and manufacture method for providing JFET transistors that can be flexibly configured by an unconventional device structure such that the JFET transistors can be either normally-off-positively-biased-on or normally-on-negatively-biased-off. The JFET transistors configured with unconventional structures are formed with low on-resistance, high current capacity and high switching speed. In a preferred embodiment, a horizontal JFET device is manufactured with standard CMOS processes with top and bottom gates controlling the drain to source current. 
     Another object of the present invention is to provide a new device structure and manufacture method for providing JFET transistors that can be flexibly configured by an unconventional device structure such that the JFET transistors can be either normally-off-positively-biased-on or normally-on-negatively-biased-off. In a preferred embodiment, a Schottky barrier is used as the top gate to achieve high switching speed. The barrier height is adjusted by controlling the implant dopant concentration or using different metal silicon or silicide/silicon systems. 
     Another object of the present invention is to provide a new device structure and manufacture method for JFET transistors that can be flexibly configured by an unconventional device structure such that the JFET transistors can be either normally-off-positively-biased-on or normally-on-negatively-biased-off. In a preferred embodiment, a deep implanted gate is formed in a multiple resistivity layers to provide gate control channel and to reduce the resistance of the drain/source ohmic contacts. 
     Another object of the present invention is to provide a new device structure and manufacture method for providing JFET transistors that can be flexibly configured by an unconventional device structure such that the JFET transistors can be either normally-off-positively-biased-on or normally-on-negatively-biased-off. In a preferred embodiment, a vertical gate pillar is used for a horizontal JFET transistor. The gate pillars can control the current flowed between deeper source and drain regions. Greater current density can be controlled without requiring greater silicon areas thus reducing the semiconductor real estate requirement for power transistor used for large current applications. 
     This invention is based on Applicant&#39;s disclosures filed in the United States Patent and Trademark Office under the Disclosure Document Program. The disclosures filed are: 1) “Low On Resistance Transistors and the Method of Making” U.S. Pat. No. 444,899 filed on Sep. 24, 1998 and 2) “Novel Structure of JFETs for Low Voltage Applications” U.S. Pat. No. 444,874 filed on Sep. 17, 1998. This invention provides the advantages of low forward voltage drops between the source and drain. The voltage drops can be reduced to 0.1 volts or less and a current over 100 amperes for large size chips can also be achieved. 
     Briefly, in a preferred embodiment, the present invention discloses a junction field effect transistor UFET) device supported on a substrate. The JFET device includes a gate surrounded by a depletion region. For a normally on JFET transistor, the depletion regions respond to a negative bias applied to the gate to turn off the current path. For a normally off JFET transistor, the depletion region responds to a positive bias applied to the gate to open a current path in the substrate wherein the current path in the substrate is shut off when the gate is zero biased. Special configurations are disclosed for normally on and normally off JFET transistors to shorten the current channels and to achieve low on-resistance, high current density and high switching speed. 
     The present invention also discloses a method for manufacturing a junction field effect transistor (JFET) device. The method includes the steps of a) forming a plurality of gates separated by a distance W wherein a depletion region surrounds each gate. And, b) adjusting a threshold voltage for the JFET transistor by gradually increasing a channel doping concentration near the depletion regions surrounding the gates for gradually decreasing the threshold voltage for the JFET transistor. In a preferred embodiment, the step of gradually increasing a channel doping concentration near the depletion regions surrounding the gates for gradually decreasing the threshold voltage is a step of making the JFET transistor with a threshold voltage greater than zero for providing a normally on JFET transistor. In a different preferred embodiment, the step of gradually increasing a channel doping concentration near the depletion regions surrounding the gates for gradually decreasing the threshold voltage is a step of making the JFET transistor with a threshold voltage equal or less than zero for providing a normally on JFET transistor. The distance, W, between gates can be determined by lithography, etch, and/or thermal process and can be reduced by RTA thermal process for threshold voltage adjustment prior to metalization. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are respectively a top view and a cross sectional view of a conventional junction field effect transistor (JFET); 
     FIG. 1C is an equivalent circuit and FIG. 1D is a current voltage diagram of the JFET transistor of FIGS. 1A and 1B; 
     FIG. 2 shows a cross sectional view of a JFET device for low voltage applications that is either normally-off-positively-biased-on or normally-on-negatively-biased-off JFETdevice; 
     FIG. 3 is a cross sectional view of horizontal JFET device of this invention; 
     FIGS. 4A to  4 E are a series of cross sectional views for illustrating the processing steps for manufacturing a JFET device of FIG. 3; 
     FIGS. 5A to  5 E are a series of cross sectional views for illustrating the processing steps for manufacturing a JFET device with Schottky barrier functioning as top gate; 
     FIGS. 6A to  6 F are a series of cross sectional views for illustrating the processing steps for manufacturing a JFET device by using high energy implant method to form buried gates; 
     FIGS. 7A to  7 F are a series of cross sectional views for illustrating the processing steps for manufacturing a vertical JFET device with pillar gates by using via/trench processes; 
     FIGS. 8A to  8 E are a series of cross sectional views for illustrating the processing steps for manufacturing an alternate JFET device by using the via/trench processes; and 
     FIG. 9 is a cross sectional view of a vertical JFET transistor with a dielectric sleeve; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purpose of providing background information of this invention, FIGS. 1A to  1 D of conventional configuration and operational characteristics are explained first. The junction field effect transistor (JFET) first proposed by Shockley in 1952 as a depletion mode device. The transistor is normally in an on-state, i.e., a freely conducting state. Applying a voltage of suitable polarity, i.e., a reverse bias, to the gate turns off the transistor. The reverse voltage biases the gate-channel pn-junction and expands the depletion layer. The cross sectional area of the effective conducting channel is reduced and the resistance is increased. For an n-channel transistor as that shown in FIG. 1B, a negative voltage applied to the gate will pinch off the conduction channel between the source and drain. Conversely, in a p-channel transistor, a positive voltage applied to the gate will pinch off the conduction channel. 
     FIG. 2 shows a novel vertical JFET device  100  of this invention. The JFET device  100  is formed on a N+ substrate  105 . The N+ substrate  105  has a resistivity ranging from 0.005 to 0.01 ohm/cm. A two-layer epitaxial structure includes a N-epitaxial layer  110  with a doping concentration ranging from 5X10E13/cm3 to 5X10E17/cm3 and a N-epitaxial layer  115  with a doping concentration ranging from 1X10E13/cm3 to 5X10E16/cm3 is supported on the N+ substrate  105 . The range of the doping concentration depends on the device structure and the tradeoff between the forward voltage drop and the reverse blocking voltage. Applying a blank phosphorous implant with an ion flux 10E14to 10E15/cm2 of 40 Kev forms a top N layer  120 . The N-layer  120  has a doping concentration about 10E16/cm3 and an the thickness of the layers  110 ,  115 , and  120  can range between 200 Angstroms to four microns depending on the design and the application of the device. An oxidation layer is then formed on top of the N-layer  120  either by thermal oxidation or a chemical vapor deposition (CVD) silicon dioxide deposition process. Then, a plurality of implant windows are opened by employing a photo-lithography and etch processes. A boron implant with ion flux of 10E13to 10E15/cm2 of ion energy ranging from 125 Kev to 1.0 Mev is carried out through these implant windows to form an array of p-regions  125 . A distance X is provided between these p-regions. The p-regions  125  are formed by a deep boron implant and implant annealing. 
     When X is sufficient large, the device is a normally on JFET transistor. If the X is a sufficiently small, when the gate has a zero bias, the depletion region of the gate shuts off the electrode. Under a forward bias voltage that is above the threshold voltage VT, the gate starts to open and the current flow between the electrodes under extremely low resistance. When the dopant concentration of the N-region  115  is 10E16/cm3, and the silicon region  115  has a resistivity around 0.5 ohm-cm, under normal zero bias condition, the depletion region from each side of the p-region  125  is 0.337 micron. If the distance between the gate is 0.62 micron, the depletion region will shut off the channel current flowing through the N-regions  115 . Under this circumstance, the channel will be opened when the depletion regions shrink from both sides of the gates by a distance of 0.07 micron. A forward bias of 0.6 volt at the gate will open a distance of 0.24 micron with a forward current density at the gate around one-amper/cm2. The resistivity between the electrodes is around 0.5X10E-4 ohm. A bias of 0.1 volt between the electrodes will provide a current density of 2X10E3 amperes/cm2. A current gain of over 1000 can be achieved. By lowering the distance between the gates and by adjusting the doping concentration of the region  115 , the forward bias voltage can be even reduced. The threshold voltage VT is dependent on the doping concentration of the region  115  and the distance X. For normally on JFET, the current gain is even higher for shutting off to currents. 
     According to FIG.  2  and above description, this invention discloses a normally off JFET device supported on a substrate. The JFET device includes a gate surrounded by a depletion region. The depletion region responding to a positive bias applied to the gate to open a current path in the substrate wherein the current path in the substrate is shut off when the gate is zero biased for the normally off JFET. For normally on JFET, the depletion region responding to a negative bias applied to the gate to shut off the current. For a normally on JFET transistor, the depletion regions respond to a negative bias applied to the gate to shut off the current channel. 
     Refer to FIG. 3 for a horizontal JFET transistor  200  of this invention. The JFET transistor  200  is manufactured by applying basic processing steps commonly applied for making CMOS integrated circuit (IC). FIGS. 4A to  4 E are a series of cross sectional views for illustrating the processing steps to manufacture the transistor  200 . In FIG. 4A, a thick oxide layer  210  is first grown on a p-type substrate  205 . In FIG. 4B, an active mask (not shown) is employed to define the active area by etching the thick oxide layer  210 . An N-well implant is carried out on P-type silicon substrate  205  with a phosphorous ion flux with a flux density of 2X10E11 to 10E15/cm2. The implant flux has an energy level of 120 Kev to 1.0 Mev. Then, an implant anneal is performed to form deep N-well  215  with a depth from 0.75 to 2 microns in the substrate  205 . Referring to FIG. 4C, after the N-well implant, trench mask (not shown) is used to etch a plurality of trenches  220  followed by depositing N-type polysilicon into the trenches and then etching back to form the N-type source and drain regions  220 ′. The trenches may be filled with tungsten metal to lower the source/drain terminal resistance. After stripping the thin oxide layer formed during implant anneal and after stripping the thick oxide layer, a blank phosphorous implant with flux between 1X10E12 to 1X10E14/cm2 at an energy of about 60 Kev is applied to form the control gate  225 . 
     Referring to FIG. 4D, a thermal oxide or CVD oxide layer  230  is grown to cover the entire top surface. A contact mask (not shown) is employed to open the gate and source/drain contact opening  235  followed by performing a p+ implant to form the p+ region and implant annealing to form a gate-contact p+ region  240  to reduce the ohmic contact resistance for the gate metal. Then a metal layer is formed on top followed by applying a metal mask to etch and pattern the metal layer to form the source/drain metal  245  and gate metal  250 . 
     Referring to FIGS. 5A to  5 E for a series of cross sectional view to show the processing steps for manufacturing another preferred embodiment of this invention. In FIG. 5A, a thick oxide layer  310  is thermally grown on a p-type substrate  305 . In FIG. 5B, an active mask (not shown) is employed to define the active area by etching partially the thick oxide layer  310  in the active area thus forming a thin oxide layer  310 ′ in the active layer after an implant anneal. An N-well implant is carried out on the P-type silicon substrate  305  with a phosphorous ion flux with a flux density of 2X10E11 to 1X10E15/cm2 at an energy of 120 Kev to 1 Mev. Then, an implant anneal is performed to form deep N-well  315  with a depth from 0.5 to 2 microns in the substrate  305 . The depth of the N-well is a function of the N-well drive-in temperature and time. Referring to FIG. 5C, after the N-well implant, a trench mask is used to etch a plurality of trenches  320  deeper than the N-well  315  followed by depositing N-type polysilicon into the trenches and then etching back to form the N-type source and drain regions. The thin oxide layer  310 ′ is stripped. Referring to FIG. 5D, a thermal or a CVD oxide layer is formed to cover the entire top surface. A contact mask (not shown) is employed to open the gate and source/drain contact opening  335  followed by performing a p+ implant to form the p+ region and implant annealing to form a gate-contact p+ region  340  to reduce the ohmic contact resistance for the gate metal. Then a thermal oxide layer is grown. A Schottky mask is used to open a plurality of Schottky contact openings. A metal layer is formed on top followed by applying a metal mask to etch and pattern the metal layer to form the source/drain metal  345  and Schottky gate metal  350 . The Schottky barrier is formed for the top gate  350  for faster switching speed. The Schottky barrier height can be increased by performing a low energy boron or boron silicon implant at 5 to 10 Kev with an ion flux of 1X10E10 to 1X10E11/cm2 prior to metal deposition. It can also be achieved by implanting through a layer of oxide with thickness between 100 to 500 Angstroms by using a higher energy boron beam. Different metal/silicon or silicide/silicon systems can also be used to increase the barrier height. 
     Referring to FIGS. 6A to  6 F for a series of cross sectional views to illustrate the processing steps for manufacturing a power transistor of this invention. An N-N- double-epitaxial layer, i.e., epitaxial layers  410  and  415 , are formed on top of a N+ substrate  405  with N-type dopant with resistivity below 0.015 ohm-cm. The N epitaxial layer  410 , with a thickness of approximately 0.5 to 2 microns, is doped with N-type dopant with doping concentration of 1X10E16 to 5X10E17/cm3. The N-epitaxial layer  415 , with a thickness of approximately 1 to 5 microns, is doped with N-type dopant with doping concentration of 1X10E14 to 5X10E17/cm3. In FIG. 6B, a phosphorous implant is carried out to form an N layer  420 . In FIG. 6C, an oxide  425  is grown and patterned with an implant mask (not shown) to open a plurality of implant windows  430 . Referring to FIG. 6C, a high-energy boron implant with ion flux of 1X10E12 to 5X10E15/cm2 is carried out with ion energy of 250 Kev to over 1 Mev to form an array of gate array  435  in the N- layer  415 . A connection implant of P-type is carried out for providing a connection-doping region  440  from the gate grid array  435  to the top surface above the N layer  420 . An oxide layer  445  is then formed and a N+ mask (not shown) is employed to etch the oxide layer  445 . A shallow low-energy high-dose phosphorous N+ implant is performed to form a shallow N+ layer  450 . Applying a surface diffusion source such as POC13 can also form this shallow N+ layer  450 . A contact mask (not shown) is applied to open a plurality of contact openings followed by carrying out a metallization to form a top metal layer  455  and bottom metal layer  460 . A metal mask is then used to pattern the top metal layer  455  to form a plurality of gate metals  465  and source metals  470  where the gate metal  465  is in electrical contact with the buried gate regions  435  through the connection region  440 . 
     Referring to FIGS. 7A to  7 F for a series of cross sectional view for illustrating the processing steps to manufacture a vertical JFET power transistor with pillar gates of this invention. 
     In FIG. 7A, a thick oxide layer  510  is thermally grown on a p-type substrate  505 . In FIG. 7B, an active mask (not shown) is employed to define the active area by etching the thick oxide layer  510  in the active area and a thin oxide layer  510 ′ is formed after the implant and drive-in processes in the active layer. An N-well implant is carried out on the P-type silicon substrate  505  with a phosphorous ion flux with a flux density of 1X10E12 to 5X10E15/cm2 at an energy of 120 Kev to more than 1 Mev. Then, an implant anneal is performed to form deep N-well  515  with a depth from 0.5 to 2.0 microns in the substrate  505 . The depth of the N-well is a function of the N-well drive-temperature and drive-in time. Referring to FIG. 7C, after the N-well implant, a trench mask (not shown) is used to etch a plurality of trenches  520  deeper than the N-well  515  followed by depositing N+ type polysilicon into the trenches and then etching back to form the N-type source and drain regions  520 ′. For large current density operation, the trenches may be filled with tungsten or other types of metallic materials after a thin layer of doped polysilicon deposition. The thin oxide layer  510 ′ is stripped. A thermal oxide layer or CVD oxide layer  530  is formed on the top surface. Referring to FIG. 5D, a contact mask (not shown) is employed to open the gate contact opening  535  followed by performing a p+ implant to form the p+ region and implant annealing to form a gate-contact p+ region  540  to reduce the ohmic contact resistance for the gate metal. A via mask (not shown) is employed to etch a plurality of vias  545  with aspect ration higher than 20:1. These vias  545  that are deeper than the N-well  515  are then filled with P+ polysilicon  545 ′. Then a thermal oxide layer  550  is grown. A contact mask (not shown) is used to etch a plurality of contact openings. A metal layer is formed on top followed by applying a metal mask to etch and pattern the metal layer to form the source/drain metal  555  and gate metal  560 . The power transistor  500  has deep vertical gate to save the real estate of the silicon wafer and meantime, a high current density can be achieved. 
     Referring to FIGS. 8A to  8 E for a series of cross sectional views for illustrating the processing steps to manufacture a vertical JFET power transistor with pillar gates of this invention. In FIG. 8A, an p-type epitaxial layer  607  layer is formed on a p+ substrate  605 . A N-type epitaxial layer  610  is formed over the top surface of the P-type epitaxial layer  607 . The depth of the N-type epitaxial layer  610  ranges from 0.5 to 20 microns. In FIG. 8B, an oxide layer  615  is grown over the N-type epitaxial layer  610 . A trench mask (not shown) is employed to open a plurality of trenches  620  with the depth of the trenches greater than the depth of the N-type epitaxial layer  610 . In FIG. 8C, the trenches are filled with polysilicon. The trench mask and the top oxide layer are stripped and the polysilicon are etched back to form the source or drain regions  620 ′. Another of oxide layer  622  is grown on the top surface. In FIG. 8D, a via mask (not shown) is applied to etch a plurality of vias  625 . The vias  625  are then filled with P+ polysilicon to form a plurality of P+ gates  625 ′. A contact mask (not shown) is then used to open a plurality of contact openings. A layer of metal is deposited and then a metal mask is employed to pattern the metal layer to function as drain and source metal  630  and gate metal  635 . A metal layer  640  is also formed on the backside of the substrate. The distance between the gates  625 ′ ranging from 0.3 to 5 microns which is a function of the doping concentration of the N-type epitaxial layer  610 . The doping concentration of the N-type epitaxial layer  610  can have range between 1X10E15 to 5X10E16/cm3. For transistors employed for very high current density operation, the trenches may be filled with a thin layer of doped polysilicon then filled with tungsten or other types of metallic materials. 
     Referring to FIG. 9 for a diagram for illustrating the functional relationship between the threshold voltage VTh for turning on a JFET device of this invention as a function of the distance W between the gates and the channel doping concentration. The threshold voltage VTh for turning on the JFET transistor decreases as the width W between the gates becomes larger. Conversely, for a constant width W between the gates, the threshold voltage VTh increases as the channel doping concentration C is decreased. A normally off JFET transistor with constant channel doping concentration C will become a normally on device when the width between the gates increases and the threshold voltage VTh decreases below zero. Similarly, for a constant width W between the gates, a normally off JFET transistor will become a normally on device by increasing the channel doping concentration C. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.