Patent Document

FIELD OF THE INVENTION 
   The present invention relates generally to a semiconductor field effect transistor configured for operation as a source follower, and more particularly, to a method and apparatus for protecting the source follower transistor from breakdown under certain operating conditions. 
   BACKGROUND OF THE INVENTION 
   Integrated circuits (chips) comprise active transistors and passive components. The chips are designed and fabricated according to conventional design parameters and semiconductor process technologies that determine operating parameters and limits for the constituent transistors. Operating power is supplied to each transistor from one or more external power supplies such that each transistor is responsive to a specified power supply voltage that is below a maximum supply voltage limit for the transistor. To accommodate different transistor designs and their associated operating limits, the chip provides multiple power supply voltages, with each transistor connected to the appropriate supply voltage for safe and reliable operation. 
   As is known, a transistor comprises at least two pn junctions through which current flows to achieve transistor action. A MOSFET (metal-oxide semiconductor field effect transistor) comprises doped source and drain regions formed in a well of the opposite doping type. A region of the well between the source and drain is referred to as a channel. A conductive gate overlies the channel and is separated therefrom by a dielectric layer. Application of a voltage to the gate inverts the conductivity of the channel permitting current to flow between the source and the drain. 
   The equation for current flow for both an n-channel (NMOS) and a p-channel (PMOS) MOSFET is:
 
 I=K ( W/L )( Vgs−Vt )( Vgs−Vt ),
 
where: K is a constant for a given technology, Vgs is the voltage between the gate and the source, Vt is the threshold voltage (the Vgs at which current starts to flow through the channel) and W/L is a width/length ratio of the MOSFET structure. The quantity (Vgs−Vt) is commonly known as Veff.
 
   Application of an excessive voltage to a pn junction (such as the source/well of a MOSFET) can cause the junction to fail or break down, possibly resulting in transistor failure. Thus a maximum junction breakdown voltage is an important MOSFET operating limit. 
   Transistors designed to operate with different supply voltages may exhibit different breakdown limits, as there is a direct relationship between the transistor&#39;s nominal supply voltage and its junction breakdown characteristics. Transistors designed for operation at higher supply voltages generally exhibit higher breakdown voltages. Transistors can be operated near the maximum permitted supply voltage to increase operational speed in the case of a digital circuit and to provide maximum voltage headroom in an analog circuit. 
   Supplying a MOSFET with a voltage in excess of a breakdown voltage can shorten the transistor&#39;s life and cause performance limiting effects, all apparently related to an excessive electric field intensity within the transistor. The electric fields of interest include vertical and lateral fields within the transistor structure and fields across transistor junctions. Three known deleterious effects associated with high intensity electric fields are described below. 
   An excessive electric field across the gate dielectric causes current flow through the dielectric, possibly leading to dielectric breakdown. Also, in the case of a reverse biased pn junction, when the electric field across the junction is sufficient to cause either Zener or avalanche junction breakdown, excessive reverse current will flow, possibly leading to the generation of excess heat in the transistors or components with which it operates. 
   A MOSFET having a relatively high source-drain voltage creates a relatively large electric field intensity that accelerates the carriers through the channel from source to drain. If such a field is present while the MOSFET is operating in saturation (i.e., a high current flow from source to drain) the carriers may attain a sufficiently high energy such that upon collision with the channel silicon lattice atoms a fraction of the carriers are deflected into the gate dielectric (e.g., silicon dioxide). These high-energy carriers, referred to as “hot” carriers, degrade the quality of the gate dielectric, leading to premature failure of the transistor. 
   Continued operation of a transistor that is subject to one or more of these breakdown conditions will likely shorten the transistor&#39;s life and alter its performance over time. 
   Generally, there are three known approaches for overcoming the effects of an excessive electric field intensity within a MOSFET. One solution fabricates multiple MOSFET&#39;s on the chip, with each MOSFET having a different gate oxide thickness. Selection of the operative MOSFET for a specific circuit is based on the gate oxide thickness required to withstand the effects of the electric field generated by the circuit. While this technique provides a MOSFET capable of withstanding breakdown effects, fabrication process costs are increased by the extra masks and extra processing steps required, and an area penalty is incurred due to the increased chip area required to fabricate the multiple MOSFETS. 
   An extended-drain MOSFET includes an integrated resistor in series with the MOSFET drain. When a high current flows from the drain to the source, the voltage drop across the integrated resistor reduces the voltage at the drain and thus the magnitude of the source-drain electric field intensity. Generally, the simultaneous occurrence of a high source-drain voltage and a high drain current occurs only under transient operations. Thus, the only significant detrimental effect of the drain resistor is a slight reduction in response time, but the extended-drain MOSFET introduces an area penalty due to the area required by the integrated resistor. 
   A third solution uses a controllable protection circuit that prevents the application of excessive voltages to the MOSFET terminals. Within the protection circuit, voltages across transistor terminals are limited to values below breakdown. 
   A transistor&#39;s output voltage is related to its supply voltage, i.e., a first transistor operating at a first supply voltage provides a higher output voltage than a second transistor operating at a second supply voltage lower than the first supply voltage. In a circuit configuration where the first transistor output is supplied as an input to the second transistor, care must be exercised to ensure that the input does not exceed a breakdown voltage of the second transistor. For example, in a circuit where a signal is communicated between a first chip operating at 3.3 V and a second chip operating at 2.5 V, an input interface that can tolerate a 3.3 V input signal is disposed between the two chips to reduce the input voltage seen by the second chip, thereby limiting the breakdown effects on transistors and other components within the second chip. 
   One circuit configuration in which multiple transistors share a common output terminal is referred to as a “net.” In the net, multiple output drivers (i.e., transistors) drive (i.e., supply an input signal) to a network of components. At a given time, one of the output drivers drives the net while the others are in an off condition. Since all output drivers may not operate at the same power supply voltage, under certain operating conditions one or more of the off-state output drivers may be exposed to an excessive voltage, which can develop an excessive electric field in the transistor and possibly cause junction breakdown. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention comprises a breakdown protection circuit for a field effect transistor comprising a gate responsive to an input signal supplied to an input terminal, a first source/drain responsive to a power supply voltage, a second source/drain for connecting to an output terminal and a body. The protection circuit further comprises a first switching element for selectably connecting the gate to the input terminal or to the body; and a second switching element for selectably connecting the body to the power supply or to the output terminal. 
   The present invention further comprises a method for isolating a source follower output terminal from a voltage impressed on the output terminal by a circuit external to the source follower, wherein the source follower comprises a gate responsive to an input signal, a first source/drain responsive to a power supply voltage, a second source/drain for connecting to an output terminal and a body. The method comprises connecting the gate to the input signal and the body to the output terminal for operating in a source follower mode, and in response to a relation between the power supply voltage and the voltage impressed on the output terminal, isolating the body from the output terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIGS. 1 and 2  are schematic illustrations of an NMOSFET and a PMOSFET, respectively, connected in a source follower configuration. 
       FIG. 3  illustrates prior art source followers for driving a net. 
       FIG. 4  is a schematic illustration of a source follower breakdown protection circuit according to the teachings of the present invention. 
       FIGS. 5 ,  6  and  7  are schematic illustrations of operational modes of the source follower breakdown protection circuit of  FIG. 4 . 
       FIG. 8  is a schematic illustration of a second embodiment of a source follower breakdown protection circuit according to the teachings of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Before describing in detail the particular method and apparatus for providing breakdown protection for a source follower circuit, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe other elements and steps pertinent to understanding the invention in greater detail. 
     FIG. 1  illustrates a source follower  10  comprising an NFET (NMOSFET)  14  further comprising a body or well  14 B shorted to a source  14 S. It is known common practice to connect an NFET body (a p-well region) to the source. A drain  14 D is connected to a power supply VH. 
   The source follower  10  receives an input signal at an input terminal  20  connected to a gate  14 G of the NFET  14 . An output terminal  22  is connected to ground through a resistor  23 . The source follower circuit  10  reproduces the input signal at the output terminal  22  with a voltage offset of about (Vth+Veff) (typically about 0.8 volts) and a gain of slightly less than unity. The source follower configuration is known to exhibit a relatively high input impedance and a relatively low output impedance. 
   The source follower circuit is placed in an off state (i.e., turned off) by grounding the input terminal  20 , which grounds the gate  14 G. 
   When a first and a second source follower circuit are connected to drive a net from their respective output terminals, an output terminal of the first source follower (such as the output terminal  22 ) is connected to an output terminal of the second source follower through the net circuit. See  FIG. 2 , for example, where source followers  25  and  27  (comprising MOSFETS  29  and  30 ) drive a net  31  from respective output terminals V out1 , and V out2 . Within the net  31 , the output terminals V out1  and V out2  are connected together so that one or the other of the source followers  25  and  27  can provide drive signals for the net  31 . 
   The MOSFETS  29  and  30  are responsive to power supply voltages VH 1  and VH 2 , respectively, where VH 1 &lt;VH 2 . When the source follower  27  is in an on state, the voltage at an output terminal  22 B is V out2 =VH 2 . If the source follower  25  is concurrently in an off state (by grounding the gate terminal G of the MOSFET  29 ), the source follower  27  pulls an output terminal  22 A of the MOSFET  29  to a voltage V out1 =VH 2  (due to the common connection within the net  31 ), which is higher than the voltage VH 1 . 
   When the output terminal  22 A rises above the power supply voltage VH 1 , the normally reverse biased pn junction between the body B (a p-type region) of the MOSFET  29  and the drain D (an n-type region) can become forward biased, causing current flow from the output terminal  22 B (at the voltage VH 2 ), through the forward biased body/drain junction of the MOSFET  29  into the power supply VH 1 . 
   Since the MOSFET  29  is designed to operate with a power supply voltage VH 1 , the MOSFET  29  can be damaged by driving the output terminal  22 A above the power supply voltage VH 1 . As the output terminal  22 A is pulled above VH 1  to VH 2 , the voltage between the gate G (which is grounded since the MOSFET  29  is off) and the source S/output terminal  22 A may exceed the breakdown voltage of the gate dielectric. 
   The same potentially damaging situation arises if the source follower  25  and a tristate driver are connected to drive the net  31 , i.e., the output terminal  22 A is connected to an output terminal of a tristate push-pull driver (not shown in  FIG. 1 ), with the tristate driver operating from a power supply voltage VH 3  greater than VH 1 . 
     FIG. 3  illustrates a PFET  33  configured as a source follower  32 . A source  33 S is connected to the power supply via the resistor  23  and to a body  33 B. It is common practice to connect the PFET body (an n-well region) to the highest available voltage. When driving a net such as the net  31 , the PFET source follower  32  is subject to the same potentially damaging conditions as the NFET source followers  25  and  27  described above. 
   As is known in the art, a source follower can also be constructed using a junction field effect transistor (JFET) or a bipolar junction transistor, and thus the teachings of the present invention can be extended to such source follower implementations. 
     FIG. 4  illustrates a source follower breakdown protection circuit  40  according to the teachings of the present invention for overcoming certain potentially damaging operating conditions, including those described above for the source followers  25  and  27  in  FIG. 2 . The breakdown protection circuit  40  operates in one of three modes to control a source follower  42  by controlling connections to a gate  43 G, a well  43 B, a source  43 S and drain  43 D of an NFET  43  operating as the source follower  42 . The three operational modes comprise: an on state for the source follower, a first off state for the source follower when the voltage at the output terminal  22  is less than the power supply voltage VH, and a second off state when the voltage at the output terminal  22  is greater than the power supply voltage VH. 
   Table 1 below sets forth a state of the NFETS and PFETS of the source follower breakdown protection circuit  40  for each of the three operational modes. 
   
     
       
             
             
           
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               STATE 
             
           
        
         
             
                 
               NFET46 
               NFET47 
               PFET48 
               PFET68 
               PFET56 
               NFET70 
               NFET88 
               PFET90 
             
             
                 
                 
             
           
        
         
             
               Source follower 
               ON 
               ON 
               OFF 
               ON 
               OFF 
               OFF 
               ON 
               OFF 
             
             
               40 is on (FIG. 5) 
             
             
               Source follower 
               OFF 
               OFF 
               ON 
               OFF 
               ON 
               ON 
               ON 
               OFF 
             
             
               40 is off and 
             
             
               output terminal 
             
             
               22 &lt; VH (FIG. 6) 
             
             
               Source follower 
               OFF 
               OFF 
               ON 
               OFF 
               ON 
               ON 
               OFF 
               ON 
             
             
               40 is off and 
             
             
               output terminal 
             
             
               22 &gt; VH (FIG. 7) 
             
             
                 
             
           
        
       
     
   
   Operation of the source follower breakdown protection circuit  40  in the source follower on state will be described first. For proper on-mode operation, the breakdown protection circuit  40  configures the drain  43 D, the gate  43 G, the p-well  43 B and the source  43 S of the NFET  43  to function as a conventional source follower, such as the source follower  10  of  FIG. 1 . Configuration of these regions is controlled by the state of NFETS  46 ,  47 ,  70 ,  88  and  90  and PFETS  48 ,  56  and  68  as described below. 
   To turn on the source follower  42  according to the first operational mode, an enable terminal  44  is set to a high state by an externally supplied signal, turning on the NFETS  46  and  47  in response to the high voltage on the gates thereof, and turning off the PFETS  48  and  56  in response to a high voltage on the gates thereof 
   A node  64  is grounded through the source/drain path of the on NFET  46 , turning on the PFET  68  by grounding a gate thereof The grounded node  64  also turns off the NFET  70  by grounding a gate thereof 
   With the PFET  56  and the NFET  70  both off (i.e., exhibiting an open source/drain current path) the source follower  42  is on, as set forth in Table 1 above, and the connection between the p-well  43 B and the gate  43 G is open, as required for normal source follower operation. 
   Since the PFET  68  and the NFET  47  are on, an input terminal  80  drives the gate  43 G, as required for normal on-mode operation of the source follower  42 . The source follower  42  responds to input signals applied at the gate  43 G as does the conventional source follower  10  illustrated in  FIG. 1 . As with a conventional source follower, if the input signal drives the gate  43 G to a voltage that is more than Vt above VH, the source follower output will not be able to follow the input signal. 
   A gate G of an NFET  88  is connected to the power supply voltage VH, turning on the NFET  88 , shorting the p-well  43 B to the output terminal  22  via the conducting source/drain (S/D) circuit of the NFET  88 . Referring to  FIG. 1 , for an operational source follower the body  14 B of the MOSFET  14  is connected to the output terminal  22 . Thus the p-well  43 B is properly configured for source follower operation. 
   Because a gate G of an NFET  90  is tied to the output terminal  22 , the NFET  90  is off, isolating the p-well  43 B from the power supply VH, as required for proper operation of the source follower  42 . 
     FIG. 5  is a schematic illustration of certain components of the breakdown protection circuit  40  when the source follower  42  is in the on mode, i.e., the source follower operates conventionally. In  FIG. 5  the on FETs  47 ,  68  and  88  are represented as short circuits and the off FETs  56 ,  70  and  90  are represented as open circuits. The configuration of  FIG. 5  is operationally identical to the conventional source follower  10  illustrated in  FIG. 1 , i.e., the p-well  43 B is connected to V out  and the input signal is supplied to the gate  43 G. 
   To turn the source follower  42  off, the enable input  44  is driven low by an externally-supplied signal turning off the NFETs  46  and  47  and turning on the PFETs  48  and  56 . Since the NFET  46  is off and the PFET  48  is on, the node  64  is high, which turns off the PFET  68  and turns on the NFET  70 . See Table 1 above. 
   With the NFET  47  and the PFET  68  both off, the connection between the input terminal  80  and the gate  43 G is open. Also, since the PFET  56  and the NFET  70  are on, the gate  43 G is shorted to the p-well  43 B via the source/drain current path of the PFET  56  and the NFET  70 , turning off the MOSFET  43  and thus the source follower  42 . 
   As described above, the relationship among VH, Vt and V out  controls the two off modes of the source follower. In a first off mode the source follower is in an off state in response to the voltage at the output terminal  22  less than (VH−Vt). In a second off mode the source follower is off in response to the voltage at the output terminal  22  greater than the (VH+Vt). In an intermediate mode the output voltage V out  is between (VH−Vt) and (VH+Vt). 
   In the first off mode, (V out &lt;VH−Vt) a threshold condition for the NFET  88  is satisfied and the NFET  88  is turned on, shorting the p-well  43 B to the output terminal  22 . Since the gate  43 G is shorted to the p-well  43 B as described above, the gate  43 G, the p-well  43 B and the source  43 S are shorted to the output terminal  22 . 
   With the gate G of the NFET  90  tied to the output terminal  22  and the output voltage V out &lt;VH−Vt, the NFET  90  is turned off, isolating the p-well  33 B from the power supply VH. 
   As described above with respect to the prior art, forward biasing of the pn junction between the body and the drain of a source follower is to be avoided to prevent current flow from the output terminal into the source follower power supply. With regard to the present invention, in the first off state condition (V out &lt;VH+Vt or V out −Vt&lt;VH), the pn junction between the drain  43 D (the n-type region connected to the power supply VH) and the p-well  43 B (the p-type region connected to the output terminal  22  with a voltage V out −Vt) cannot be forward biased since the p-type region is at a lower voltage (V out −Vt) than the n-type region (VH). 
     FIG. 6  is a schematic illustration of certain components of the breakdown protection circuit  40 , when the source follower is in the first off mode.  FIG. 6  represents the on NFETS  70  and  88  and the on PFET  56  with short circuits. The gate  43 G is connected to the body or p-well  43 B through the source/drain path of the PFET/NFET  56 / 70 . The p-well  43 B is connected to the source  43 S and to the output terminal  22  through the source/drain of the NFET  88 . The NFET/PFET  47 / 68  depicted as open circuits, disconnect the gate  43 G from the input terminal  80 . 
   The second off state condition occurs when V out  is greater than (VH+Vt). Under these conditions the MOSFET  90  turns on and the MOSFET  88  turns off, shorting the p-well  43 B to the power supply voltage VH. The off state of the NFET  88  isolates the p-well  43 B from the output terminal  22 . Since the gate  43 G remains connected to the p-well  43 B through the on transistors, PFET/NFET  56 / 70 , the MOSFET  43  (and the source follower  42 ) remains off. An input signal applied to the input terminal  80  cannot reach the gate  43 G due to the open NFET/PFET  47 / 68 . 
     FIG. 7  is a schematic representation of this state of the breakdown protection circuit  40 . Noting that the source and drain of a MOSFET are interchangeable, a terminal of the NFET  43  connected to the power supply VH is designated the source  43 S and a terminal connected to the output terminal  22  is designated the drain  43 D, which is a reversal of the terminal designations from  FIGS. 5 and 6 . 
   According to the prior art source follower, when the voltage at V out  is greater than the voltage at the power supply VH (or greater than the VH+Vt), current can flow into the power supply via the forward biased body/drain pn junction of the source follower MOSFET, wherein the body (p-type) is connected to V out  and the drain (n-type) is connected to the power supply. Also, with the source connected to V out  and the gate connected to ground (since the MOSFET is off) the gate dielectric can breakdown due to excessive voltage (i.e., electric field intensity) between the source and ground through the gate dielectric. 
   According to the teachings of the present invention, these difficulties are avoided. Current cannot flow from V out  to the power supply VH through the body/drain junction of the MOSFET  43  since the body or p-well  43 B is not connected to the output terminal  22 , but instead is connected to the power supply VH. 
   With regard to possible breakdown of the gate dielectric, as can be seen in  FIG. 7 , the gate  43 G is tied to VH. Thus the voltage V out  can increase to the sum of VH and the breakdown voltage of the gate dielectric, before dielectric breakdown. According to the prior art, V out  is limited to the dielectric breakdown voltage because the gate is at ground potential. Thus the present invention provides additional margin with respect to limits on the voltage V out  when the source follower is off. 
   Thus when the source follower  42  is off (whether the voltage at the output terminal  22  is less than the power supply voltage (VH−Vt), greater than (VH+Vt) or between (VH−Vt) and (VH+Vt), the gate  43 G and the p-well  43 B are shorted by PFET/NFET  56 / 70 . The NFET  88  and the NFET  90  cooperate to short the gate  43 G and the p-well  43 B to whichever of the drain/source terminals  43 D/ 43 S is at a lower voltage. By definition, the terminal to which the p-well  43 B and the gate  43 G are shorted is the source of the NFET  43 . Use of the source follower protection circuit  40  allows the net  31  to drive the voltage at the output terminal  22  above or below the voltage of the power supply VH without damaging the source follower  43 , allowing use of source followers driven from different power supply voltages to drive the net without fear of source follower damage. 
   In the intermediate mode when the voltage V out  is between (VH−Vt) and (VH+Vt) both of the MOSFETS  88  and  90  are off and the p-well  43 B floats. If the p-well  43 B floats to a voltage below (VH−Vt), the MOSFET  88  turns on, connecting the p-well  43 B to the output terminal  22 . The MOSFET  90  remains off. If the p-well  43 B floats to a voltage above (VH+Vt), the MOSFET  90  turns on, connecting the p-well  43 B to the power supply VH. The MOSFET  88  remains off. 
   The use of two parallel MOSFETS (such as the NFET/PFET  47 / 68  pair and the NFET/PFET  70 / 56  pair) are known in the art and commonly referred to as a pass gate or transfer gate. It is known that if only an NFET is employed to provide a closed path between its source and drain terminals, the NFET passes a signal having a voltage range from ground to (VH−Vt). If only a PFET is used, the PFET passes a signal having a voltage range from (ground+Vt) to the power supply voltage VH. Using both a PFET and an NFET allows a signal from ground potential to VH to pass through the parallel MOSFETS. Further with respect to the pass gates, according to the prior art it is known to connect the body of the PFET to the power supply and the body of the NFET to ground. However, this configuration is not necessarily required for the pass gates of the present invention; in other embodiments the bodies can be tied to other voltages. 
   Although the present invention is described as comprising enhancement mode MOSFETS for the pass gates, in another embodiment depletion mode MOSFETS can be utilized, with corresponding circuit modifications as understood by those skilled in the art. In yet another embodiment, the pass gates are replaced by a low threshold voltage MOSFET (e.g., having a threshold voltage of about 0.1 volts) permitting either a PFET or an NFET to pass a signal having a voltage extending nearly the entire range from ground to VH. In still another embodiment other types of electronic (e.g., junction field effect transistors), mechanical and nanotechnology devices can be used in place of the pass gate MOSFETs. 
   One application for the breakdown protection circuit  40  of the present invention comprises a hard disk drive of a computer or other data storage device. In certain disk drives a source follower drives a load operative with the read/write head for improving the data reading and data writing process. The protection circuit of the present invention is used advantageously with the source follower to protect against the conditions as described above, that are known to occur according to prior art source followers. 
   The breakdown protection circuit  40  can also be used in applications where the source follower power supply voltage is not fixed, but is variable as desired. Changing the power supply voltage changes the relationship between VH and Vout, possible leading to the damaging conditions described above. The source follower protection circuit  40  can protect the source follower against such conditions. 
   In yet another embodiment, a source follower breakdown protection circuit  120  illustrated in  FIG. 8  comprises a power supply voltage VH′ connected to the drain D of the MOSFET  90  and to the gate G of the MOSFET  88 . According to the embodiment of  FIG. 4 , the state of the MOSFETS  88  and  90  is determined by whether V out , is less than (VH−Vt) or greater than (VH+Vt). According to the embodiment of  FIG. 8 , VH is replaced by VH′, which can be less than or greater than VH. Thus the V out  value at which the MOSFETS  88  and  90  change state is shifted by the difference between VH and VH′. 
   An architecture and process have been described as useful for a source follower breakdown protection circuit. Specific applications and exemplary embodiments of the invention have been illustrated and discussed, and provide a basis for practicing the invention in a variety of ways and with a variety of circuit structures. Numerous variations are possible within the scope of the invention. Features and elements associated with one or more of the described embodiments are not to be construed as required elements for all embodiments. The invention is limited only by the claims that follow.

Technology Category: 5