Lateral silicon carbide transistor
BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to semiconductor devices, and more particularly, to silicon carbide semiconductor devices. 
     In the past, the semiconductor industry has utilized silicon carbide to form a variety of transistors including vertical transistors, lateral transistors, metal semiconductor field effect transistors (MESFETs), and metal oxide semiconductor field effect transistors (MOSFETs). One problem with the prior lateral MOSFETs, is the breakdown voltage. Typically, prior lateral MOSFETs have breakdown voltages less than approximately 100 volts. Such low breakdown voltages prevent utilizing the prior lateral MOSFETs in high voltage applications such as power devices for motor controls and power supplies. These type of applications typically require breakdown voltages of at least 150 volts. 
     Additionally, prior lateral MOSFETs that have a breakdown voltage greater than about 20 volts generally have high on-resistances. These on-resistances are greater than the less than 5 milliohm-cm 2  desired for power devices for motor controls and power supplies. 
     Accordingly, it is desirable to have a lateral silicon carbide MOSFET that has an on-resistance less than 5 milliohm-cm 2 , and a breakdown voltage greater than 150 volts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE illustrates an enlarged cross-sectional portion of a transistor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE illustrates an enlarged cross-sectional portion of a silicon carbide transistor 10 that has a high breakdown voltage and a low on-resistance. Transistor 10 includes a silicon carbide substrate 11 on which other portions of transistor 10 are formed. In the preferred embodiment, transistor 10 is a N-channel accumulation mode transistor and substrate 11 is heavily doped P-type silicon carbide. Substrate 11 could also be lightly doped N-type silicon carbide. A sub-channel layer 12 is epitaxially formed on substrate 11. Subsequently, a channel layer 13 is epitaxially formed on sub-channel layer 12. Layers 12 and 13 are formed by epitaxial techniques that are well known to those skilled in the semiconductor art. Sub-channel layer 12 has a first or P-type doping and channel layer 13 has a second or N-type doping in order to form a P-N junction at the interface of layers 12 and 13. The doping concentration and thickness of layers 12 and 13 are selected to provide a reduced surface field effect (RESURF) that lowers the on-resistance for a desired breakdown voltage of transistor 10. In the preferred embodiment, layer 12 has a P-type doping concentration of approximately 2×10 15  to 8×10 15  atoms/cm 3  and a thickness of at least approximately five microns while layer 13 has an N-type doping concentration of approximately 0.8×10 16  to 2×10 16  atoms/cm 3  and a thickness of approximately 0.3 to 0.8 microns in order to provide the RESURF effect. 
     A channel insert 14 is formed on a top surface of channel layer 13 in order to provide transistor 10 an on-resistance less than approximately five milliohm-cm 2 . Insert 14 has an N-type doping concentration that is higher than the N-type doping concentration of channel layer 13. The thickness or depth of insert 14 typically is at least half the thickness of layer 13 and can extend all the way into layer 12. In the preferred embodiment, insert 14 has a doping concentration of approximately 1×10 17  to 5×10 17  atoms/cm 3  and depth of approximately 0.3 to 0.8 microns in order to facilitate providing an on-resistance no greater than approximately three milliohm-cm 2 . Additionally, the thickness and doping is chosen to facilitate the RESURF effect for a desired breakdown voltage. 
     A source region 16 is formed on the surface of channel layer 13 and is spaced a first distance 31 from insert 14. A source electrode 32 on source region 16 facilitates electrical contact to region 16. A modulated channel region 18 extends laterally from source region 16 to channel insert 14 near the surface of channel layer 13. A gate dielectric 19 is formed on the top surface of channel layer 13 and overlays region 18. Dielectric 19 also has one edge that substantially aligns with an edge of channel insert 14 in order to ensure that region 18 is electrically connected to channel insert 14. A gate electrode 22 is formed on the surface of gate dielectric 19 and has an edge that substantially aligns with the edge of channel insert 14. In operation, at zero gate-to-source bias or potential, region 18 is resistive and a small current flows from source region 16 to drain region 17, thus, a negative gate-to-source bias is required to prevent current flow. When a positive gate-to-source bias is applied, an accumulation region is formed in modulated channel region 18. The accumulation region accumulates electrons in order to provide a low resistance path for electron flow between source region 16 and insert 14 and subsequently to a drain region 17. It should be noted, that doping levels and gate materials can be selected that prevent current flow at zero gate-to-source bias. 
     Drain region 17 is formed on channel layer 13 and within channel insert 14. Drain region 17 is formed a sufficient distance from modulated channel region 18 to provide a high breakdown voltage for transistor 10. Additionally, the width of region 18 is sufficient to support the desired breakdown voltage. In the preferred embodiment, drain region 17 is at least approximately six microns from region 18, and region 18 is approximately one to four microns wide to ensure that transistor 10 can be substantially turned-off. 
     A thicker field dielectric 21 abuts gate dielectric 19 so that an edge of dielectric 19 substantially aligns with the edge of channel insert 14. Dielectric 21 also extends laterally across the surface of channel layer 13 toward drain region 17, and may overlap a portion of drain region 17. Dielectrics 19 and 21 can be a variety of materials such as silicon dioxide, silicon nitride, or aluminum nitride including multilayer conbinations thereof. 
     A gate field plate 23 abuts gate electrode 22 and extends a second distance 33 onto field dielectric 21 in order to terminate electric fields within transistor 10 and increases the breakdown voltage of transistor 10. Similarly, a drain field plate 24 has one end electrically connected to drain region 17 to form a drain electrode 26, and extends a third distance 34 onto field dielectric 21 in order to form an electrical field plate that terminates electric field lines and increases the breakdown voltage of transistor 10. The distance between plates 23 and 24 also affects the breakdown voltage of transistor 10, and is chosen to provide the desired breakdown voltage. In the preferred embodiment, distances 33 and 34 each are approximately five to ten microns, and the distance between plates 23 and 24 is approximately five to fifteen microns in order to achieve a breakdown voltage of at least approximately six hundred volts. 
     A high resistivity layer 29 is formed on dielectric 21 and extends between plates 23 and 24, and is electrically connected thereto. Layer 29 increases the breakdown voltage of transistor 10. Layer 29 also assists in creating a second accumulation layer within insert 14 and underlying both layer 29 and plates 33 and 34 thereby decreasing the on-resistance of transistor 10. Layer 29 can be formed from a variety of high resistance materials including, but not limited to, polysilicon and metal oxides such as titanium oxide. In the preferred embodiment, layer 29 is polysilicon having a resistivity of approximately ten to one thousand megohms/square. 
     An isolation plug 28 isolates transistor 10 from other devices on substrate 11 and provides electrical connection to layer 12. Plug 28 is a P-type region that typically is formed by implantation techniques. A contact 30 provides electrical contact to plug 28, and is typically connected to electrode 32. 
     By now it should be appreciated that there has been provided a silicon carbide transistor having both a high breakdown voltage and a low on-resistance. Utilizing gate and drain field plates and providing an optimal distance between the field plates provides the transistor with a high breakdown voltage. The channel insert facilitates reducing the transistor&#39;s on-resistance. Also, forming a P-N junction that facilitates RESURF effects also reduces the on-resistance. Forming an accumulation layer in the modified channel region results in an enhancement transistor that is easily controlled by external circuitry. Such a transistor is suitable for use in high voltage applications such as power devices for motor controls and power supplies.