Patent Publication Number: US-6034397-A

Title: Silicon-on-insulator body- and dual gate-coupled diode for electrostatic discharge (ESD) applications

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 08/731,941 filed Oct. 22, 1996, now U.S. Pat. No. 5,811,857 of Assaderaghi et al. for Silicon-On-Insulator Body-Coupled Gated Diode for Electrostatic Discharge (ESD) and Analog Applications. 
    
    
     BACKGROUND OF THE INVENTION 
     1. TECHNICAL FIELD 
     The invention relates generally to diodes, and more specifically, to silicon-on-insulator (SOI) diode circuits. 
     2. BACKGROUND ART 
     Silicon-On-Insulator (SOI) technology, which is becoming of increasing importance in the field of integrated circuits, deals with the formation of transistors in a relatively thin layer of semiconductor material overlying a layer of insulating material. Devices formed on SOI offer many advantages over their bulk counterparts, including: higher performance, absence of latch-up, higher packing density, low voltage applications, etc. However, SOI circuits, like other electronic circuits, are: First, susceptible to electrostatic discharge (ESD), a surge in voltage (negative or positive) that occurs when a large amount of current is applied to the circuit; and second, in need of providing an ideality (a constant voltage swing of 60 mV/decade over several decades of current) for analog applications, such as in phase-locked-loop circuits, voltage regulators, and band gap reference circuits. 
     For ESD applications, to discharge ESD impulses, ESD protection schemes need a low voltage turn-on and a high current drive (the ability to generate or sink a large amount of current before a large amount of negative or positive voltage is developed). Traditional bulk overvoltage protection schemes, such as diode circuits, do not work well on SOI because of the presence of the SOI buried oxide. That is, conventional diodes on SOI have small current drivability because the current is carried laterally (limited by the thickness of the semiconductor material). Thus, developing a new approach or a new type of diode was necessary for adequate ESD protection for SOI circuits. 
     Some approaches for protecting SOI circuitry from ESD are found in the following U.S. Patents: U.S. Pat. No. 4,889,829 &#34;Method for Producing a Semiconductor Device having a Silicon-On-Insulator Structure,&#34; issued December 1989 to Kawai; and U.S. Pat. No. 4,989,057, &#34;ESD Protection for SOI Circuits,&#34; issued January 1991 to Lu. In the Kawai reference, ESD protection circuits, such as diodes, are made from a non-SOI substrate to protect SOI circuits. A findamental disadvantage with this approach is the circuit as disclosed in the Kawai reference creates a non-planar surface structure during fabrication, leading to many process related difficulties. For example, there would be difficulty in removing residual contaminant metal during the cleansing process, which would create large metal steps during metallization, leading to metal thing and cracking. Thus, in general, manufacturing such a circuit would not be feasible. 
     The Lu reference discusses a gated diode, which could be used for ESD designs. The gated diode consists of a floating-body SOI transistor, with the gate connected to a signal pad. Although the diode disclosed in the Lu reference could provide some ESD protection, the diode does not allow for the desired trait of ideal characteristics, as discussed above. Some reasons preventing ideal diode characteristics with the diode in the Lu reference and with conventional diodes in general include: 1) alignment tolerances of the substrate cause large process-induced variations; and 2) the conventional diode structure may be a polysilicon diode, which receives extension and halo implants (implants normally utilized in deep submicron MOSFETs) that degrades the ideal diode characteristics on SOI. Thus, the diode in the Lu reference may be used for ESD protection, but is not appropriate for use with analog finctions. 
     Accordingly, a need has developed in the art for an SOI diode that will not only provide ESD protection, but will provide compactability. 
     SUMMARY OF THE INVENTION 
     It is thus an advantage of the present invention to provide a diode for SOI technology with ESD protection. 
     It is also an advantage of the present invention to provide a compact diode for SOI technology. 
     The foregoing and other advantages of the invention are realized by a SOI body and dual gate-coupled diode formed from an SOI field-effect transistor (FET) structure. The source of the SOI FET structure forms the first terminal of the diode. The drain of the SOI FET structure forms the second terminal of the diode. The SOI FET structure includes two gates, which are tied to the body of the SOI FET structure. An SOI circuit comprising at least one body and dual gate coupled diode formed from the SOI FET structure provides electrostatic discharge (ESD) protection. 
    
    
     The foregoing and other advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THIE DRAWINGS 
     The preferred exemplary embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and: 
     FIG. 1 is a cross-sectional view of the structure of an SOI n-channel field-effect transistor (NFET) of a body-coupled gated diode; 
     FIG. 2 is the circuit schematic of the body-coupled gated diode of FIG. 1; 
     FIG. 3 is a symbol representing the circuit schematic of FIG. 2; 
     FIG. 4 is a cross-sectional view of the structure of an SOI p-channel field-effect transistor (PFET) of a body-coupled gated diode; 
     FIG. 5 is the circuit schematic of the body-coupled gated diode of FIG. 4; 
     FIG. 6 is the symbol representing the circuit schematic of FIG. 5; 
     FIG. 7 is a graph of measured characteristics of the gate/body/drain voltage vs. the current of FIG. 1; 
     FIG. 8 is a graph of measured characteristics of the threshold voltage vs. body bias of FIG. 1; 
     FIG. 9 illustrates a block diagram of an ESD protection device using a body-coupled gated diode of FIGS. 1 and/or 4; 
     FIGS. 10, 11, 12 and 13 illustrate exemplary circuits for the device of FIG. 9; 
     FIG. 14 is another exemplary circuit of the ESD protection device of FIG. 9; 
     FIG. 15 is a block diagram of a mixed voltage interface device using the ESD protection device of FIG. 9; 
     FIGS. 16, 17, and 18 are exemplary circuits for the mixed voltage interface device of FIG. 15; 
     FIG. 19 is a block diagram of a mixed voltage interface device using the ESD protection device of FIG. 9 having two power supplies; 
     FIGS. 20, 21, and 22 are exemplary circuits for the mixed voltage interface device of FIG. 19; 
     FIG. 23 is a block diagram of a voltage interface device having at least one body-coupled gate diode of FIGS. 1 and 4 used between two voltages; 
     FIGS. 24 and 25 are exemplary circuits for the device of FIG. 23; 
     FIG. 26 illustrates a block diagram of an analog device using the body-coupled gated diode of FIGS. 1 and 4; 
     FIG. 27 is a schematic of a body-coupled gated diode in accordance with a second embodiment; 
     FIG. 28 illustrates a level-shifted body-coupled gated diode using the diode schematic of FIG. 27; 
     FIGS. 29 and 30 illustrate two exemplary circuits of the level shifting device of FIG. 28; 
     FIG. 31 illustrates a level-shifted body-coupled gated diode of FIG. 28 including a rise time network; 
     FIG. 32 illustrates an exemplary circuit of the device of FIG. 31 including an ESD clamp; 
     FIG. 33 is a cross-sectional view of an SOI n-channel field-effect transistor (NFET) structure of a body- and dual gate-coupled diode in accordance with a preferred embodiment of the present invention; 
     FIG. 34 is a cross-sectional view of an SOI n-channel field-effect transistor (NFET) structure of a body- and dual gate-coupled diode in accordance with a second embodiment of the present invention; 
     FIG. 35 is the circuit schematic of the body- and dual gate-coupled diode of FIGS. 33 and/or 34; 
     FIG. 36 is a symbol representing the circuit schematic of FIG. 35; 
     FIG. 37 is the circuit schematic of the body- and dual gate-coupled diode of FIGS. 33 and/or 34 with an additional gate diode; 
     FIG. 38 is a symbol representing the circuit schematic of FIG. 37; 
     FIG. 39 illustrates an exemplary circuit diagram of an ESD protection device using a body- and dual gate-coupled diode of FIG. 35; 
     FIG. 40 illustrates an exemplary circuit diagram of an ESD protection device using a body- and dual gate-coupled diode of FIG. 37; 
     FIG. 41 illustrates an exemplary circuit diagram of an ESD protection device using two body- and dual gate-coupled diodes of FIG. 35; 
     FIG. 42 is an exemplary circuit diagram of a voltage interface device having at least one body- and dual gate-coupled diode of FIG. 35 used between two voltages; and 
     FIG. 43 is a schematic of a body- and dual gate-coupled diode in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring to FIGS. 1-6, an NFET (FIGS. 1-3) and PFET (FIGS. 4-6) configuration of the body-coupled gated (BCG) diode are shown. As depicted in FIG. 1, an NFET BCG diode 10 is formed from an SOI MOSFET 30 having an isolation region 24, buried oxide 12, and silicon substrate 14. Two N+ regions are formed, a source region 16 and a drain region 17, on a P-type body region 18. Gate electrode 22 overlies gate insulator 21 and defines the gate of the MOSFET 30. Surface channel 23 lies below gate insulator 21 and on the surface of the P-type region 18, wherein the P-type region is also known as the channel. The source 36, drain 34, body 38 and gate 32 terminals are affixed to the source region 16, drain region 17, body node region 18 and gate electrode 22, respectively. Terminal A, coupled to the body 38, drain 34, and gate 32 terminals, and terminal B, coupled to the source terminal 36, form the input and output of an N+/P type BCG diode 10. Although not shown, the connections of the terminals can be easily done at any of the metal layers of MOSFET 30. 
     FIG. 2 illustrates the circuit schematic of NFET BCG diode 10. As in FIG. 1, FIG. 2 shows BCG diode 10 comprising a source 36, drain 34, body 38, and gate 32 terminals. BCG diode 10 comprises two terminals, A and B. As aforementioned, terminal B is coupled to the source terminal 36, and terminal A is coupled to the body 38, gate 32 and drain 34 terminals being tied together. When terminal A is greater in voltage than terminal B, the BCG diode 10 will turn on, thus providing ESD protection as discussed in greater detail below. BCG diode 10 of FIG. 2 is represented by the diode symbol 10 as shown in FIG. 3. 
     Similarly, as shown in FIG. 4, a PFET BCG diode 20 is formed from an SOI MOSFET 30 having an isolation region 24, buried oxide 12, and silicon substrate 14. Two P+ regions are formed, a source region 26 and a drain region 27, on the N-type body region 28. As aforementioned, gate electrode 22 overlies gate insulator 21 and defines the gate of the MOSFET. The source 36, drain 34, body 38 and gate 32 terminals are affixed to the source region 26, drain region 27, body node region 28 and gate electrode 22, respectively. Terminal A, coupled to the body 38, drain 34, and gate 32 terminals, and terminal B, coupled to the source terminal 36, form the input and the output of a P+/N type BCG diode 20. 
     FIG. 5 illustrates the circuit schematic of PFET BCG diode 20. As in FIG. 4, FIG. 5 shows BCG diode 20 comprising a source 36, drain 34, body 38, and gate 32 terminals. BCG diode 20 comprises two termninals, A and B. As aforementioned, terminal B is coupled to the source terminal 36, and terminal A is coupled to the body 38, gate 32 and drain 34 being tied together. When terminal B is greater in voltage than terminal A, the BCG diode 20 will turn on, the operation being discussed in greater detail below. BCG diode 20 of FIG. 5 is represented by the diode symbol 20 as shown in FIG. 6. Since the present invention comprises both polarities of diode transistors (N+/P and P+/N), a greater flexibility in circuit operation and design is available for SOI technology, as will be seen below. 
     The operation of the MOSFET BCG diode shown in FIGS. 1-6 takes place in two current regions, which will be illustrated in greater detail in reference to FIG. 7. In the first current region, the BCG diode provides ideal diode characteristics. In the second current region the BCG diode provides ESD protection. In general, the first current region of the BCG diode corresponds to the functional voltage range from approximately 0 volts to approximately Vdd, wherein Vdd is the power supply voltage. The exponential portion of diode characteristics, though, is typically limited to zero to approximately +/- 0.7 volts because of external and parasitic series resistances. The second current region of the BCG diode corresponds to the range approximately below zero and above the power supply voltage, Vdd. 
     For the second current region, ESD protection is provided under two turn-on conditions of the MOSFET BCG diode. The first turn-on condition occurs when the voltage of the body 38 exceeds the voltage of the source 36. When this condition occurs, a forward-biased diode attribute allows a current flow from the body terminal to the source terminal. The first condition is given by the equation: 
     
         |V.sub.pad =V.sub.body |&gt;|V.sub.be +V*| 
    
     wherein, V pad  =applied voltage (voltage of a signal pad); 
     V body  =body voltage; 
     V be  =diode forward-biased voltage between body and source terminals; and 
     V*=Vdd when MOSFET BCG diode is an NFET, or Vss when MOSFET BCG diode is a PFET. 
     The second turn-on condition occurs when the voltage at the gate 32 exceeds the threshold voltage. That is, as the signal pad voltage increases, the body voltage and the gate voltage will also increase. When the body voltage increases, the threshold voltage decreases. Thus, when the gate voltage exceeds the threshold voltage, current will flow from the drain terminal to the source terminal. This condition is given by the equation: 
     
         |V.sub.pad =V.sub.gate |&gt;|Vt.sub.(Vbody=Vpad) +V*| 
    
     wherein, V pad  =applied voltage; 
     V gate  =gate voltage; 
     Vt.sub.(Vbody=Vpad) =the threshold voltage when the voltage of the body is equal to the voltage of the applied voltage; and 
     V*=Vdd when MOSFET BCG diode is an NFET, or Vss when MOSFET BCG diode is a PFET. 
     Thus, for the NFET BCG diode 10 (FIG. 1), when a positive pulse is applied to the terminal of the NFET (terminal A), the current is discharged through the P-N diode formed by the body 18 and drain 16 of the NFET structure. In parallel, as the body voltage increases, the threshold voltage of the MOSFET 30 decreases, creating a dynamic threshold and ideal diode characteristics. As the threshold voltage of the NFET decreases, the gate-coupling of the NFET turns on the NFET in parallel with the diode. A unique aspect of the BCG diode is the parallel operation of diodic characteristic of the diode and the body and gate coupled MOSFET interaction. The BCG diode uses body-coupling to lower the absolute value of the threshold voltage and gate-coupling to turn on the ESD MOSFET element prior to NFET snapback. 
     Likewise, for the PFET BCG diode 20 (FIG. 4), when a negative pulse is applied to the terminal of the PFET (terminal A), the current is discharged through the N-P diode formed by the body 28 and drain 26 of the PFET structure. In parallel, as the body voltage decreases, the magnitude of the threshold voltage of the PFET 20 decreases, again creating a dynamic threshold and ideal diode characteristics. As the threshold voltage of the PFET decreases, the gate-coupling of the PFET turns on the PFET in parallel with the diode. As aforementioned, a distinguishing aspect of the BCG diode as compared to other diodes is the parallel operation of diodic characteristic of the diode and the body and gate coupled MOSFET interaction. That is, the BCG diode uses body-coupling to lower the absolute value of the threshold voltage and gate-coupling to turn on the ESD MOSFET element prior to PFET snapback. 
     FIG. 7 illustrates the operation of the NMOSFET BCG diode 10 of FIG. 1 (lines 52 and 56) compared to the operation of a non-BCG diode (lines 54 and 58). On the left vertical axis of the graph is a log scale of the current at the first current region (pertaining to lines 52 and 54), indicating the subthreshold regions of MOSFET 30 (line 52) and the FET of the non-BCG diode (line 54). The other vertical axis of the graph illustrates the linear scale of the current at the second current region (pertaining to lines 56 and 58), indicating the ESD operational mode of MOSFET 30 (line 56) and the FET of the non-BCG diode (line 58). At the first current region, the BCG diode (line 52), unlike the non-BCG diode (line 54), attains an ideal subthreshold swing of 60 mV/decade. This ideality is possible through the coupling of the body to the gate of MOSFET 30. That is, the gate voltage is directly applied to the body instead of being capacitively coupled, as with the conventional non-BCG diode. The ideality factor n is given by the equation: 
     
         n-1+C.sub.D /C.sub.OX 
    
     wherein C D  /C OX  =the capacitive coupling. 
     Thus, the value of the ideality factor (n) is approximately 1 for the BCG diode, whereas n is greater than 1 for the non-BCG diode. The ideality factor is important to the resulting slope, which is found through the equation: 
     
         I=I.sub.0 (e.sup.V/nkT/q -1) 
    
     wherein I=the resulting current; 
     I 0  =the reverse saturation current; 
     V=the voltage of the diode; 
     n=the ideality factor; 
     k=the Boltzman constant; 
     T=the absolute temperature (°K.); and 
     q=the electron charge. 
     The elements kT/q are approximately equal to 26 mV, and the current is proportional to e. The slope can then be approximated by multiplying 60 mV/decade (kT/q*ln(10)) by the ideality factor. The BCG diode, with an ideality factor of 1, produces a 60 mV/decade slope (line 52), and the non-BCG diode, with an ideality factor of approximately 1.45, produces a slope of around 87 mV/decade (line 54). As aforementioned, a 60 mV/decade slope is important in analog applications for functions such as voltage reference, phase-locked-loop, and voltage regulators. 
     At the second current region (lines 56 and 58), the ESD protection provided by the non-BCG diode (line 58) is minimal, quickly being dominated by series resistance 59, where the voltage begins to increment proportionally to the current. The ESD protection provided by the BCG diode is much greater (line 56). The BCG MOSFET at this point may be conceptually treated as a bipolar device with large (and sometimes infinite) current gain. The drain current can be modeled as the collector current, and the body (gate) current as the base current. Even though for ease of analysis this device may be treated as a bipolar device, it is indeed a MOSFET since the current conduction is through the surface channel and is controlled by the gate. The &#34;apparent&#34; gain of the &#34;bipolar&#34; device is large, because the threshold voltage (Vt) of the MOSFET is being modulated by the applied bias to the silicon film. This gives the appearance of large bipolar gain at low biases. 
     FIG. 8 demonstrates the modulation of the threshold voltage by the applied body bias. The modulation of two devices are depicted, one being of a shorter channel length than the other. Line 61 illustrates when L drawn , the drawn dimensions of the channel length, is equal to 0.25 μm, and line 62 illustrates when L drawn  is equal to 0.20 μm. Thus, the BCG diode uses body-coupling to lower the absolute value of the threshold voltage and create a dynamic threshold. Consequently, control of the threshold voltage allows for excellent control of the diode characteristics. Furthermore, by changing Vt-adjust implants (implants that are utilized to control the threshold voltage) of a MOSFET, the I 0  of the BCG diode (e.g., Ioff of the MOSFET) can easily be changed by several orders of magnitude. This change is depicted by moving line 52 to the left or right, which is not easily accomplished in regular diodes. 
     Carrying the bipolar analogy one step fuher, it becomes clear why connecting the collector and base together will form a diode. Here, the gate and the body may form the base terminal, the drain may be the collector terminal, and the source may be the emitter terminal. Even though the base (gate) current might have non-ideality, its sum with the collector current will remain ideal because the collector current is several orders of magnitude larger than the base current. As seen, this diode will have a much larger current than the conventional diode of the same size. 
     FIGS. 9-25 illustrate using the BCG diode as described above in a variety of ESD applications and circuits. 
     As shown in FIG. 9, a block diagram of a first stage ESD protection device 70 comprises diodes 64 and 66, at least one of them being a BCG diode. Diode 64 is coupled to a signal pad 68 and a predetermined high voltage V 1 . Diode 64 will sink an excessive amount of current from signal pad 68, thus clamping the voltage from the signal pad 68 to the predetermined high voltage V 1  when the signal voltage is greater than V 1 . Diode 66 is coupled to the signal pad 68 and a predetermined low voltage V 2 . Diode 66 clamps the signal voltage to the predetermined low voltage V 2  by the driving more current to the device when the signal voltage is below the low voltage V 2 . 
     FIGS. 10-13 illustrate using the NFET (N+/P) and PFET (P+/N) configurations of the BCG diode for diodes 64 and 66. As seen in FIG. 10, two N+/P diodes are used for diodes 64 and 66. In FIG. 11, diode 64 is an N+/P diode and diode 66 is an P+/N diode. In FIG. 12, diodes 64 and 66 are P+/N diodes. In FIG. 13, diode 64 is a P+/N diode and diode 66 is a N+/P diode. As aforementioned, and as can be seen through these four exemplary circuits, there is flexibility in creating the circuit when both N+/P and P+/N diodes may be used. 
     As seen in FIG. 14, a BCG diode may be used in combination with other non-BCG diodes in forming the ESD protection circuit of FIG. 9. One example is a BCG diode 64 used in conjunction with a diode having its source, drain and gate tied together 66. It is to be understood that the ESD protection device of the present invention is not limited by the combinations shown as examples. Other appropriate combinations, such as a gated diode and a BCG diode may also be used. 
     As shown in FIG. 15, other stages 76, 78 may be added onto the first stage ESD protection device 70 when mixed voltage interfacing is involved. The first stage ESD protection device 70, as described above with reference to FIG. 9, is connected to signal pad 68, V 2 , and subsequent stages. Each stage adds approximately an additional 0.7 volts onto the first stage, allowing for a delay in the turn on of the first stage protection device 70. Although specific examples and elements will be discussed in reference to the following figures, it is to be understood that the various stages of the ESD protection device are not limited thereby. Other appropriate elements and combination of elements, such as gated diodes, lubistors, any appropriate MOSFETs or dynamic threshold transistors, may also be used. 
     FIGS. 16-18 depict three exemplary elements used for the various stages 76, 78. As seen in FIG. 16, each stage 76, 78, etc. may comprise a diode having its source, drain and gate tied together. The body of diode 76 is connected to the first stage protection device 70 and the gate of the diode 76 is connected to Vdd and to the body of the next diode. The body of the Nth stage diode 78 is connected to the prior stage diode and the gate is connected to Vdd. In FIG. 17, each stage of diode 76, 78, etc. comprises a transistor with its body and drain tied together. The gate of diode 76 is coupled to the gates of the other stages of diodes 78 including the gates of the diodes in the 1st stage protection device 70 (not shown). The drain of diode 76 is connected to protection device 70 and its source is connected to the drain of the next diode. The source of the Nth stage diode 78 is connected to Vdd. 
     FIG. 18 shows a series of diodes 76, 78 wherein the gates and the drains are all tied together with the first stage protection device 70. The body of the second stage diode 76 is connected to the protection device 70 and the source of the second stage diode 76 is connected to the body of the third stage diode, etc. The source of the Nth stage diode 78 is connected to Vdd. 
     As shown in FIG. 19, a second power supply V 3  may also be used with the mixed voltage interfacing device of FIG. 15. Again, the first stage ESD protection device 70 is connected to signal pad 68, V 2 , and subsequent stages. FIGS. 20-22 show specific examples for FIG. 19. In FIG. 20, each stage of diode 76, 78, etc. comprises a transistor with its body and drain tied together. The gate of diode 76 is connected to the gates of the other stages of diodes 78 and Vcc. The drain of diode 76 is connected to Vcc and protection device 70 and its source is connected to the drain of the next diode. The source of the Nth stage diode 78 is connected to Vdd. 
     As seen in FIG. 21, each stage of diode 76, 78, etc. comprises a diode having its source, drain and gate tied together. The body of diode 76 is connected to Vcc and the first stage protection device 70 and the gate of diode 76 is connected to Vdd and to the body of the next diode. The body of the Nth stage diode 78 is connected to the prior stage diode and its gate is connected to Vdd. As seen in FIG. 22, each diode stage 76, 78, etc. comprises a BCG diode with its drain connected to the body and gate. The drain of diode 76 is connected to Vcc and protection device 70 and its source is connected to the drain of the next diode. The source of the Nth stage diode 78 is connected to Vdd. 
     FIG. 23 illustrates an ESD protection device 91 that may be used between two power supplies, V 1  and V 2 . The protection device 91 generally comprises a string of diodes, at least one being a BCG diode, wherein the string of diodes contains a set of diodes electrically coupled in a serial fashion, anode to cathode. The string of diodes may contain diodes such as lubistors, gated diodes, and other dynamic threshold (DT) MOSFETs, etc. FIGS. 24 and 25 illustrate specific implementations of FIG. 23. In FIG. 24, device 91 comprises a string of PFET BCG diodes 20. As seen in FIG. 25, device 91 comprises a string of NFET BCG diodes 10. 
     FIG. 26 shows the BCG diode used in conjunction with a typical analog application. Circuit 92 is a section of a band-gap reference circuit that is used in charge pumps and phase-locked loops. Circuit 92 comprises two PFETs 93 and 94 tied together at the gates, and two NFETs 95 and 96 tied together at the gates. The drain of PFET 93 is connected to the drain and gate of NFET 95. The drain and gate of PFET 94 is connected to the drain of NFET 96. The first terminal of BCG diode 97 is connected to the source of NFET 95, and the first terminal of BCG diode 98 is connected to the source of NFET 97. The second terminal of BCG diodes 97 and 98 are tied to ground. Typically, the width of BCG diode 98 is &#34;m&#34; times greater than the width of BCG diode 97. Hence, since both diodes are drawing the same current, the smaller diode (in this case, diode 97) will develop a larger voltage to drawn current ratio. The resulting voltage is given by the equation: 
     
         V.sub.A -V.sub.B =kT/q*In (m) 
    
     wherein V A  =the voltage of the smaller diode; 
     V B  =the voltage of the larger diode; 
     kT/q=the terminal voltage (described in reference to FIG. 7, approximately 26 mV); and 
     m=the ratio of the width of the larger diode to the width of the smaller diode. 
     For example, if m is equal to 10, then the resulting fixed voltage that is supplied to the circuit (V A  -V B ) would be approximately 60 mV. Although m is equal to 10 in this example, it is to be understood that other appropriate values may be used. The BCG diodes 97 and 98 provide the ideality necessary for a fixed voltage, which allows the band-gap reference circuit to function correctly. 
     FIG. 27 illustrates a second embodiment of the BCG diode 40. As with the first embodiment, the gate 32 and the body 38 are tied together. The source 36 forms terminal B, and the drain 34 forms terminal A. The diode shown is in NFET configuration, but may also be made in PFET configuration. The drain 34 is indirectly connected to the body 32 and gate 38 through delay device 110 as shown in subsequent figures. Thus, the operation of diode 40 is similar to the operation as described in reference to FIGS. 1-6. 
     As seen in FIG. 28, the drain 34 of the BCG diode is tied to the body 38 and gate 32 through delay device 110 comprising level shifting device 120. The level shifting device 120 is used to delay the turn on of diode 40. Terminal A is normally connected to a signal pad (not shown), and terminal B is normally connected to the substrate or to ground. FIGS. 29 and 30 illustrate exemplary circuits used for level shifting device 120 between nodes C and D. FIG. 29 shows a string of BCG diodes constructed from SOI PFETs. As in all diode strings made reference to in this application, each string may consist of only one diode, or several diodes. The diodes turn each other on in sequential manner, thus allowing for a delay in the turn on of diode 40. In the FIG. 29 example, Node C connects to the source and drain of diode 122. The source and drain of each diode 122, 124 is tied together and the body connection of the first diode 122 is connected to the drain of the next diode 124. The body of the last diode connects to node D. 
     FIG. 30 shows a string of BCG diodes constructed from SOI NFETs. Node C connects to the drains of each of the diodes 126, 128 and the body connection of the first diode 126. The source of the first diode 126 is connected to the body/gate connection of the next diode 128. The source of the last diode connects to node D. Although BCG diodes are specifically shown for exemplary circuits of level shifting device 120, one of ordinary skill in the art will understand that other devices, such as lubistors, gated diodes, and other dynamic threshold (DT) MOSFETs, may be used in the diode string. 
     As shown in FIG. 31, delay device 110 comprises rise-time device 130 and level shifting device 120, rise time device 130 being connected to level shifting device 120 at node C. Rise-time device 130 also connects to terminals A and B and is generally a resistor/capacitor (RC)-coupled device. The drain of BCG diode 40 is now connected to the body/gate connection of BCG diode 40 through rise-time device 130 and level shifting device 120. The purpose of rise-time device 130 is to cause the body and gate of BCG diode 40 to rise, thus providing an earlier turn-on. 
     FIG. 32 depicts a specific exemplary RC circuit used for rise time device 130. The drain of a PFET BCG diode 134 is connected to the gate of an NFET 133. The body of transistor 133 is connected to the body/gate connection of the BCG diode and to terminal A, which in this case is Vss. A modulated ESD clamp, PFET 132 and NFET 131, uses rise-time device 130 to clamp voltage Vdd. 
     Referring to FIGS. 33-43, an NFET configuration of the body- and dual gated-coupled (BDG) diode in accordance with a preferred embodiment of the present invention is shown. Although an NFET configuration is shown, a PFET configuration may also be used as depicted in FIG. 41. The BDG diode 210 is similar to the BCD diode 10 of FIG. 1, except for an addition of a second gate, which, as will be discussed below, provides additional ESD protection. As depicted in FIG. 33, an NFET BDG diode 210 is formed from an SOI MOSFET structure 230 having an isolation region 224, and buried oxide 212. Three N+ regions are formed, a source region 216 and a drain region 217 on P-type body region 218, and an N+ region 215 on P-type region 213 below the buried oxide 212. A diode (shown in FIGS. 37 and 40) may be used to tie the N+ region 215 to the substrate. Gate electrode 222 overlies gate insulator 221 and defines a first gate of the MOSFET 230. The second gate is defined by N+ region 215 below buried oxide 212. The source 236, drain 234, body 238 and gate 232 and 233 terminals are affixed to the source region 216, drain region 217, body node region 218 and gate electrodes 222, 215 respectively. Terminal A, coupled to the body 238, drain 234, and gate 232, 233 terminals, and terminal B, coupled to the source terminal 236, form the input and output of an N+/P type BDG diode 210. Although not shown, the connections of the terminals can be easily done at any of the metal layers of MOSFET structure 230. FIG. 34 is similar to FIG. 33 except that the second gate 211 is disposed within the buried oxide region 212. 
     FIG. 35 illustrates the circuit schematic of NFET BDG diode 210. As in FIGS. 33 and 34, FIG. 35 shows BDG diode 210 comprising a source 236, drain 234, body 238, and gate 232 and 233 terminals. BDG diode 210 comprises two terminals, A and B. As aforementioned, terminal B is coupled to the source terminal 236, and terminal A is coupled to the body 238, gate 232 and drain 234 terminals being tied together. Although in this specific example, only two terminals A and B are shown, the second gate may be tied to a third potential instead of terminal A. BDG diode 210 of FIG. 35 is represented by the diode symbol 210 as shown in FIG. 36. 
     FIG. 37 illustrates a second circuit schematic of NFET BDG diode that includes a gate diode. As in FIG. 35, FIG. 37 shows BDG diode 225 comprising a source 236, drain 234, body 238, and gate 232 and 233 terminals. BDG diode 225 comprises two terminals, A and B. As aforementioned, terminal B is coupled to the source terminal 236, and terminal A is coupled to the body 238, gate 232 and drain 234 terminals being tied together. Although in this specific example, only two terminals A and B are shown, the second gate may be tied to a third potential instead of terminal A. A gate diode 280 is coupled from the substrate to the gate 233, providing voltage from the substrate to compensate for a negative voltage swing as will be discussed below. BDG diode 225 of FIG. 37 is represented by the diode symbol 225 as shown in FIG. 38. 
     FIGS. 39 and 40 illustrate exemplary circuit diagrams of an ESD protection device 260 using a BDG diode 210 of FIG. 35 (for FIG. 39) or a BDG diode 225 of FIG. 37 (for FIG. 40). FIG. 39 operates similarly to the BCD diode as described in FIGS. 1-8. That is, the BDG diode uses body-coupling to lower the absolute value of the threshold voltage and gate-coupling to turn on the ESD MOSFET element prior to NFET snapback. FIG. 40 operates similar to FIG. 9, except only one BDG diode is required to achieve substantially similar results in terms of ESD protection. That is, for positive pulses, as the body voltage of the BDG diode 225 increases, the threshold voltage decreases. As the first gate 232 and/or second gate 233 voltage increases, the first and/or second gate surface starts to conduct after the respective threshold voltage is reached. Also, as the body voltage exceeds the supply voltage V 1 , the gate diode 280 formed between the body 213 and the N+ implant 215 or 211 will conduct. For negative pulses, the second gate conducts to the bulk substrate. The area of the ESD network as seen in this example is thus reduced in half. That is, the circuit of FIG. 40 uses half the amount of space as shown in FIG. 9. 
     The BDG diode of FIG. 35 may also be used in the same configuration as FIG. 9, as shown in FIG. 41. FIG. 41 illustrates an exemplary circuit diagram of an ESD protection circuit using two BDG diodes, an N-channel diode 210 and a P-channel diode 260. Again, the operation is similar to that of FIG. 9. 
     FIG. 42 illustrates an exemplary circuit diagram of an ESD protection circuit using BDG diodes 210 in a series configuration between any two power rails V1 and V2. Although BDG diodes 210 are shown, other diodes such as lubistors, gated diodes, and other dynamic threshold (DT) MOSFETs, etc. may also be included in the series configuration. 
     FIG. 43 illustrates a third embodiment of the BDG diode 210 of the present invention. The source 236 forms terminal B, and the drain 234 forms terminal A. The diode shown is in NFET configuration, but may also be made in PFET configuration. The drain 234 is indirectly connected to gate 232 through control element 270, and the drain 234 is indirectly connected to gate 233 through control element 272. Control elements 270 and 272 are used to modify the gate coupling of the top or bottom gate channels. 
     Thus, a diode formed from an SOI FET structure is disclosed in the invention that provides electrostatic discharge (ESD) protection and conserves space. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.