Power transistor free from back gate bias effect and an integrated circuit device using the same
FIELD OF THE INVENTION 
     This invention relates to a power transistor and, more particularly, to a power transistor free from back gate bias effect and appropriate for scaling down. 
     DESCRIPTION OF THE RELATED ART 
     Power transistors are incorporated in a prior art integrated circuit, and drive high voltage signals ranging from 30 volts to 120 volts. Such a power transistor is expected to withstand large source-to-drain voltage, and the source and drain regions thereof are usually formed in offset regions lower than impurity concentration that the source and drain regions. If a power transistor is expected to withstand 100 volts, the offset regions thereof are doped at 1×10 17  cm -3 , and is 8 microns in a direction of source-to-drain current path of the power transistor. The power transistors are incorporated in various integrated circuits such as, for example, pixel driving circuits of a liquid crystal display system or a plasma display system, and is increasing the application field. 
     A typical example of the power transistor is illustrated in FIG. 1, and forms a part of an integrated circuit device. The integrated circuit device is fabricated on a p-type semiconductor substrate 1 doped with p-type dopant impurity at 1×10 14  cm -3 , and surface portions of the p-type semiconductor substrate 1 is doped with n-type dopant impurity at 1×10 19  cm -3  for serving as source and drain regions 2a, 2b, 2c and 2d. The source and drain regions 2a to 2d are spaced apart from one another, and the drain regions 2b and 2c are formed in lightly doped n-type offset regions 2e and 2f. The source region 2d is also formed in a lightly doped offset region 2g. However, any offset region is not provided for the source region 2a, because the source region 2a is grounded. The p-type semiconductor substrate 1 between the source region 2a and the offset region 2e serves as a channel region 2h of an n-channel enhancement type offset field effect transistor Q1, and the p-type semiconductor substrate 1 between the offset regions 2f and 2g also serves as a channel region 2i of an n-channel enhancement type offset field effect transistor Q2. On the channel regions 2h and 2i are grown thin gate oxide films 3a and 3b of 2000 angstroms which is overlain by patterned gate electrodes 4a and 4b of polysilicon. In the prior art fabrication process, after the patterning stage for the gate electrodes 4a and 4b, the offset regions 2e, 2f and 2g are doped at 1×10 17  cm -3  in a selfaligned manner with the gate electrodes 4a and 4b, and each of the offset regions 2e to 2g extends over 8 microns in the lateral direction of FIG. 1 or a direction of the source-to-drain current path of the offset field effect transistor Q1 or Q2. 
     An inter-level insulating film 5 covers the entire structure, and appropriate constant holes are formed in the inter-level insulating film 5 for integrating the n-channel enhancement type offset field effect transistors Q1 and Q2. 
     As will be better seen from FIG. 2, the source regions 2a and 2d are coupled with a ground voltage line GND and a variable power voltage line PW1, and the drain regions 2b and 2c are coupled with an output signal line C. An input signal line A supplies an input voltage signal to the patterned gate electrode 4b of the n-channel enhancement type field effect transistor Q2, and a constant voltage B is applied to the patterned gate electrode 4a of the n-channel enhancement type the p-type semiconductor substrate 1 is grounded through a heavily doped ohmic contact region 2j. 
     The n-channel enhancement type offset field effect transistor Q1 is turned on in the presence of the constant voltage B, and a current path takes place from the drain regions 2b and 2c through the channel region 2h to the ground voltage line GND. In this situation, if the input voltage signal remains low, the n-channel enhancement type offset field effect transistor Q2 turns off, and the output voltage signal C is delayed to the ground voltage level. However, if the input voltage signal A goes up to high voltage level, the n-channel enhancement type offset field effect transistor Q2 turns on, and the output voltage signal C is regulated to a certain voltage level between the variable power voltage level PW1 and the ground voltage level. Thus, the source region 2d is applied with the source region 2d, however, the source region 2d withstands the variable power voltage level of the order of 100 volts. 
     Since the power voltage level on the line PW1 is variable, the offset region 2g allows the source region 2d to withstand break-down. However, a problem is encountered in high threshold voltage of the n-channel enhancement type offset field effect transistor Q2. In detail, the p-type semiconductor substrate 1 per se serves as the channel region 2i of the n-channel enhancement type offset field effect transistor Q2, and is coupled through the ohmic contact region 2j with the ground voltage line GND. This means that the n-channel enhancement type offset field effect transistor Q2 is left under the influence of the back gate bias phenomenon, and the ground voltage level lifts the threshold voltage of the n-channel enhancement type offset field effect transistor. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide a power transistor which is free from the problem inherent in the prior art power transistor. 
     To accomplish the object, the present invention proposes to fabricate a double diffused MIS type power transistor in a well reversely biased with respect to a semiconductor substrate. 
     In accordance with one aspect of the present invention, there is provided a power transistor fabricated on a semiconductor substrate of a first conductivity type, comprising: a) a well of a second conductivity type formed in a surface portion of the semiconductor substrate, and reversely biased with respect to the semiconductor substrate the second conductivity type being opposite to the first conductivity type; b) a first impurity region of the first conductivity type formed in a predetermined surface portion of the well, and partially used as a channel region; c) a gate insulating film covering the channel region and another surface portion of the well adjacent to the predetermined surface portion; d) a gate electrode formed on the gate insulating film; e) a source region of the second conductivity type formed in yet another surface portion of the well adjacent to the aforesaid another surface portion, and having a boundary substantially aligned with an edge of the gate electrode; f) a drain region of the second conductivity type formed in a predetermined surface portion of the first impurity region, and having a boundary substantially aligned with the opposite edge of the gate electrode; and g) a second impurity region of the first conductivity type formed in another surface portion of the first impurity region, and supplying a certain bias voltage to the first impurity region for controlling back gate biassing phenomenon in the channel region. 
     In accordance with another aspect of the present invention, there is provided an integrated circuit device fabricated on a semiconductor substrate of a first conductivity type, comprising: a) an enhancement type offset transistor having a conductive channel of a second conductivity type opposite to the first conductivity type, and comprising a-1) a heavily doped source region of the second conductivity type formed in a first surface portion of the semiconductor substrate, and coupled with a constant voltage source, a-2) a lightly doped offset region of the second conductivity type formed in a second surface portion of the semiconductor substrate spaced apart from the first surface portion, a-3) a heavily doped drain region of the second conductivity type formed in a surface portion of the lightly doped offset region, and spaced apart from a periphery of the heavily doped source region, the heavily doped drain region being coupled with an output node for an output voltage signal, a-4) a gate insulating film covering a channel region between the heavily doped source region and the lightly doped offset region, and a-5) a gate electrode formed on the gate insulating film, and applied with a constant voltage signal, the gate electrode having edges substantially aligned with the periphery of the lightly doped offset region and with a periphery of the heavily doped source region; b) an ohmic contact region formed in a third surface portion of the semiconductor substrate, and coupled with the constant voltage source; and c) an enhancement type switching transistor having a conductive channel of the second conductivity type, and comprising c-1) a lightly doped well of the second conductivity type formed in a fourth surface portion of the semiconductor substrate, and reversely biased with respect to the semiconductor substrate, c-2) a first impurity region of the first conductivity type formed in a predetermined surface portion of the lightly doped well, and partially used as a channel region; c-3) a gate insulating film covering the channel region and another surface portion of the lightly doped well adjacent to the predetermined surface portion; c-4) a gate electrode formed on the gate insulating film of the enhancement type switching transistor, and is applied with an input voltage signal, c-5) a source region of the second conductivity type formed in yet another surface portion of the lightly doped well adjacent to the another surface portion, and having a boundary substantially aligned with an edge of the gate electrode of the enhancement type switching transistor, the heavily doped source region of the enhancement type switching transistor being coupled with a power voltage source, c-6) a heavily doped drain region of the second conductivity type formed in a predetermined surface portion of the first impurity region, and having a boundary substantially aligned with the opposite edge of the gate electrode, and c-7) a second impurity region of the first conductivity type formed in another surface portion of the first impurity region, and coupling the output node with the first impurity region for controlling back gate biassing phenomenon in the channel region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the power transistor and the integrated circuit according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross sectional view showing the structure of the prior art integrated circuit device partially formed by the power transistors; 
     FIG. 2 is an equivalent circuit diagram showing the prior art integrated circuit device; 
     FIG. 3 is a cross sectional view showing the structure of an integrated circuit device according to the present invention; 
     FIG. 4 is an equivalent circuit diagram showing the integrated circuit device shown in FIG. 3; 
     FIG. 5 is a cross sectional view showing the structure of another integrated circuit device according to the present invention; and 
     FIG. 6 is an equivalent circuit diagram showing the integrated circuit shown in FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 3 of the drawings, an integrated circuit device embodying the present invention is fabricated on a p-type semiconductor substrate 11 doped with p-type dopant impurity at 6×10 14  cm -3 , and largely comprises an n-channel enhancement type offset transistor Q11 and an n-channel enhancement type switching transistor Q12. A lightly doped n-type well 12 is defined in a surface portion of the p-type semiconductor substrate 11, and the lightly doped n-type well 12 is doped with n-type dopant impurity at 5×10 15  cm -3 . The n-type enhancement type offset transistor Q11 and the n-channel enhancement type switching transistor Q12 are fabricated on a surface portion of the p-type semiconductor substrate 11 and the n-type well 12, respectively, and the n-type enhancement type switching transistor Q12 is constructed into a double diffused MIS structure as described hereinbelow. 
     The n-channel enhancement type offset transistor Q11 comprises a heavily doped n-type source region 11a formed in the p-type semiconductor substrate 11, a lightly doped n-type offset region 11b also formed in the p-type semiconductor substrate 11, a heavily doped n-type drain region 11c formed in the lightly doped n-type offset region 11b, a gate insulating film 13a formed on a channel region between the source region 11a and the offset region 11b, and a gate electrode 14a provided on the gate insulating film 13a. The gate electrode 14a is self-aligned with the heavily doped n-type source region 11a and with the lightly doped n-type offset region 11b. 
     The source region 11a is doped with n-type dopant impurity at 1×10 19  cm -3 , and the drain region has dopant concentration at 1×10 19  cm -3 . However, the offset region 11b is doped at 5×10 16  cm -3 , and allows the drain region 11c to be spaced apart from the channel region by 7.5, microns. The gate insulating film 13a is of the order of 2000 angstroms thick, and the gate electrode is formed of doped polysilicon. A heavily doped p-type ohmic contact region 11d is formed in the semiconductor substrate 11, and has dopant concentration at 1×10 19  cm -3 . The p-type ohmic contact region 11d supplies a certain voltage to the semiconductor substrate 11. 
     On the other hand, the n-channel enhancement type switching transistor Q12 comprises a lightly doped p-type region 12a formed in the n-type well 12, an n-type source region 12b slightly spaced from the lightly doped p-type region 12a, a heavily doped ohmic contact region 12c formed in the source region 12b, a heavily doped n-type drain region 12d formed in the lightly doped p-type region 12a, a heavily doped p-type ohmic contact region 12e also formed in the lightly doped p-type region 12a, a gate insulating film 13b covering a conductive channel area in the lightly doped p-type region 12a, and a gate electrode 14b provided on the gate insulating film 13b. 
     In this instance, the p-type region 12a, the source region 12b, the ohmic contact region 12c, the drain region 12d and the ohmic contact region 12e are respectively doped at 2×10 16  cm -3 , 5×10 16  cm -3 , 1×10 19  cm -3 , 1×10 19  cm -3  and 1×10 19  cm -3 . The gate insulating film 13b is 2000 angstroms in thickness, and the gate electrode 14b is formed of doped polysilicon. The n-channel enhancement type switching transistor Q12 thus constructed in the double diffused MIS structure is advantageous over the prior art n-channel enhancement type offset transistor Q2 in transistor size, and, accordingly, occupies a small amount of real estate rather than the prior art offset transistor Q2. 
     The n-channel enhancement type offset transistor Q11 and the n-channel enhancement type switching transistor Q12 are integrated as shown in FIG. 4. The source node or the source region 11a is coupled with a ground voltage line GND, and an output node C is coupled with the drain nodes 11c and 12d as well as the ohmic contact region 12e. The source node 12c is coupled with a power voltage line PW11, and variable power voltage on the line PW11 swings its voltage level between 1.0 volts and 80 volts. A constant voltage signal B of 5 volts is applied to the gate electrode of the n-channel enhancement type offset transistor 11c, and the n-channel enhancement type switching transistor Q12 is controlled with an input voltage signal A variable between 0 volts and 80 volts. In other words, the n-channel enhancement type switching transistor Q12 turns on and off depending upon the input voltage signal A, and causes the output node C to change the output signal between 0 volts and 77 volts. 
     The constant voltage signal B allows the n-channel enhancement type offset transistor Q11 to turn on at all times, and provides predetermined resistance against current flowing into the ground voltage line GND. 
     As described hereinbefore, the output voltage signal is applied through the ohmic region 12e to the lightly doped p-type region 12a, and the conductive channel takes place in the lightly doped p-type region 12a. This means that the back gate biasing phenomenon at the conductive channel of the switching transistor Q12 is independent from that of the n-channel enhancement type offset transistor Q11, and the positive output voltage signal eliminates the undesirable built-in potential under the back gate biasing phenomenon from the channel area. As a result, the threshold level of the n-channel enhancement type switching transistor Q12 is smaller than that of the prior art offset transistor Q2. 
     Second Embodiment 
     Turning to FIG. 5 of the drawings, an operational amplifier circuit embodying the present invention is integrated on a p-type semiconductor substrate 21. Although wirings are deleted from FIG. 5, the operational amplifier circuit is arranged in a current mirror configuration as shown in FIG. 6, and comprises two p-channel enhancement type load transistors Q23 coupled with a power voltage line PW21, two n-channel enhancement type amplifier transistors Q22 respectively coupled with the p-channel enhancement type load transistors Q23 and an n-channel enhancement type current source transistor Q21 coupled between the source nodes of the n-channel enhancement type amplifier transistors Q22 and a ground voltage line GND. An input differential voltage signal is applied to the gate electrodes of the n-channel enhancement type amplifier transistors Q22, and a constant voltage source 22 allows the n-channel enhancement type current source transistor Q21 to turn on at all times. An output voltage signal takes place at the common drain node DN of the load transistor Q23 and the amplifier transistor Q22. However, the circuit behavior of the current mirror circuit is well know to those skilled in the art, and no further description is incorporated hereinbelow. 
     Turning back to FIG. 5, the n-channel enhancement type current source transistor Q21 and the n-channel enhancement type amplifier transistor Q22 respectively correspond to the n-channel enhancement type offset transistor Q11 and the n-channel enhancement type switching transistor Q12, respectively, and regions and films of the transistors Q21 and Q22 are labeled with the same references corresponding to those of the transistors Q11 and Q12 without any detailed description. 
     Each of the p-channel enhancement type load transistors Q23 is fabricated on an n-type well 23, and comprises a lightly doped p-type offset region 23a, a heavily doped p-type drain region 23b, a heavily doped p-type source region 23c opposed through a channel region to the lightly doped p-type offset region 23a, a heavily doped n-type ohmic contact region 23d coupled with the power voltage line PW21, a gate insulating film 24 over the channel region and a gate electrode 25 on the gate insulating film 24. 
     Since the n-type well 12 is approximately equal in voltage level to the source node thereof, the n-channel enhancement type amplifier transistor Q22 is free from the back gate biasing phenomenon. 
     Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.