Patent Publication Number: US-9406695-B2

Title: Circuit and method for improving ESD tolerance and switching speed

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from: 
     (1) U.S. Provisional Patent Application No. 61/906,843, filed Nov. 20, 2013, entitled “Floating Body Contact Circuit Method for Improving ESD Performance and Switching Speed”, the entire disclosure of which is hereby incorporated by reference; and 
     (2) U.S. patent application Ser. No. 14/521,331, filed Oct. 22, 2014, entitled “Floating Body Contact Circuit Method for Improving ESD Performance and Switching Speed”, the entire disclosure of which is hereby incorporated by reference, and which claims priority from U.S. Provisional Patent Application No. 61/906,843. 
    
    
     BACKGROUND 
     (1) Technical Field 
     This invention relates to electronic circuits, and more particularly to circuits and methods for improving electrostatic discharge (ESD) tolerance and switching speed in integrated circuits (ICs), such as ICs for radio frequency (RF) signal switching or processing. 
     (2) Background 
     The design of integrated circuits (ICs), particularly ICs for radio frequency (RF) signal switching or processing, often requires trading off or balancing different performance and functional parameters. For example, a conventional RF signal switch IC capable of switching at high speed may exhibit poor linearity, while a highly linear RF signal switch IC may be only capable of switching at much lower frequencies. 
     As another example, providing protection against ESD events (e.g., static electricity discharge that may be generated by a human body or other sources) is important in designing ICs, particularly ICs that include field effect transistors (FETs), which are notoriously susceptible to permanent damage when subjected to high voltages. For example, a human body induced ESD event may reach 15,000 volts or more and have a short rise time, on the order of nanoseconds. Such an ESD event may cause damage to a FET (e.g., gate-oxide punch-through), especially complementary metal-oxide-semiconductor (CMOS) FETs, metal-oxide-semiconductor FETs (MOSFETs), and other types of FETs, particularly those fabricated on semiconductor-on-insulator (“SOI”) and silicon-on-sapphire (“SOS”) substrates. However, providing adequate ESD guard circuitry or structures in an IC may affect other circuit parameters, such as switching time. 
     Accordingly, it may be desirable to improve both ESD tolerance and switching time for semiconductor devices while achieving high linearity, particularly in FETs fabricated on SOT and SOS substrates. The present invention provides system, apparatus, and methods for improvement of both ESD tolerance and switching time in such devices. 
     SUMMARY OF THE INVENTION 
     Various embodiments described herein relate generally to systems, methods, and apparatus for improving ESD tolerance and switching time for semiconductor devices, in particular floating body semiconductor devices including MOSFETs, and more particularly to MOSFETs fabricated on SOI and SOS substrates. 
     More specifically, embodiments of the invention provide an improved FET structure having accumulated charge sink (ACS) circuit, fast switching times, and improved ESD tolerance. Disclosed embodiments include circuit architectures that include a FET having a gate resistor and an ACS circuit, with the addition of respective (alternative) ACS resistors coupled to the ACS circuit and (directly or indirectly) a node on one side or the other of the corresponding gate resistor (i.e., parallel versus series connected to the gate resistor). 
     By inserting larger resistance value ACS resistors in series with the ACS circuit, during a positive polarity ESD strike, the total impedance seen at the base of the parasitic bipolar junction transistor (BJT) device inherent in a FET is large, compared to the low impedance that would otherwise be presented by just the ACS circuit. The addition of an ACS resistor essentially prevents significant impairment of the parasitic BJT protection mechanism. The ACS circuit function remains intact for linearity enhancement. In essence, in accordance with this description, the circuit parameters for determining switching speed of an ACS enabled FET are separated from the circuit parameters for setting an ESD tolerance level. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a circuit architecture with stacked semiconductor devices and accumulated charge sink modules according to various embodiments. 
         FIG. 2A  is a simplified block diagram of a first circuit architecture including a switching module and a first modified ACS circuit according to various embodiments. 
         FIG. 2B  is a simplified block diagram of a second circuit architecture including a switching module and a second modified ACS circuit according to various embodiments. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a simplified block diagram of a circuit architecture  100  with stacked semiconductor devices and accumulated charge sink modules according to various embodiments. Shown are a plurality of switching modules  102  in a stacked configuration. In the illustrated embodiment, each switching module  102  includes a FET  104  that includes conventional drain (D), source (S), and gate (G) connections. Connected to the gate of each FET  104  is a gate resistor  106 . In general, the stack height N of the circuit architecture  100  (i.e., the number of stacked switching modules  102 ) is determined by the amount of voltage that the circuit architecture  100  is required to handle. In some embodiments, only one switching module  102  may be needed. As shown in  FIG. 1 , the circuit architecture  100  also includes a first port  110  coupled to the drain D of a switching module  102 , second port  112  coupled to the source of a switching module  102 , and a control signal port  114  coupled to each gate resistor  106  of the switching modules  102 . The control signal port  114  would typically be coupled to a buffer or driver circuit (not shown). The first port  110  and the second port  112  may be dedicated (alternately) as either an input or an output port, or may function as both input and output ports. 
     The FET  104  in each switching modules  102  may be floating body type semiconductor including an N-type MOSFET (NMOSFET) or P-type MOSFET (PMOSFET) fabricated on an insulator, such as silicon on insulator (SOI) or silicon on sapphire (SOS) substrates. In such embodiments, during an OFF state, when the source-to-gate voltage (Vsg) is not sufficient to turn the gate to the ON state (i.e., allow signal conduction between the source S and the drain D) but the Vsg is not insignificant, an accumulated charge may form in the semiconductor body  120  of the FET  104  below the gate oxide, causing the FET  104  to operate in an accumulated charge regime (ACR). 
     Semiconductor devices operating in an ACR may introduce parasitic capacitance non-linearities, affect the drain-to-source breakdown voltage, and introduce harmonic distortion and intermodulation distortion to a signal processed by the circuit architecture  100 . All of these ACR effects are especially problematic for RF applications, particularly when the circuit architecture  100  is employed in communication systems, where signal distortion may cause unacceptable interface with adjacent signals, violating system operating guidelines and government signal transmission rules. 
     The effects of ACR operation can be reduced or overcome in a FET, thereby improving the linearity of the FET, by providing a means for sweeping out accumulated charge. Such means includes the use of the “HaRP”™ accumulated charge sink (ACS) technology taught in U.S. Pat. Nos. 7,910,993 and 8,129,787, both entitled “Method and Apparatus for Use in Improving Linearity of MOSFET&#39;s Using an Accumulated Charge Sink”, and in pending U.S. patent application Ser. No. 13/277,108, filed on Oct. 19, 2011, and Ser. No. 13/412,529, filed on Mar. 5, 2012, all of which are assigned to the assignee of the present invention. The disclosures in each of U.S. Pat. Nos. 7,910,993 and 8,129,787 as well as pending U.S. patent application Ser. Nos. 13/277,108 and 13/412,529, are incorporated herein by reference in their entirety. 
     In particular, referring again to  FIG. 1 , ACR effects can be reduced or overcome by including at least an ACS circuit connecting the body  120  of a FET  104  to the gate G of the FET  104  in accordance with the referenced disclosures (noting that other specific connections are within the ambit of such disclosures). For purposes of this disclosure, the ACS circuit is shown in its simplest form in  FIG. 1  form as an ACS diode  122 , and referenced as such in this disclosure. However, it should be understood that the invention encompasses use of an ACS circuit that includes the equivalent of a diode function (i.e., a two-terminal electronic device having asymmetric conductance, with low (ideally zero) resistance to current in one direction and high (ideally infinite) resistance in the other direction). 
     While inclusion of an ACS diode  122  improves the linearity of a FET  104 , a circuit design tradeoff is that the ESD tolerance of the FET  104  is somewhat reduced. In an ACS enabled FET  104 , ESD is dissipated or redirected in part by the gate resistor  106  to a FET  104 , which limits the current generated by an ESD strike to the FET  104 . The larger the gate resistor  106  value, the greater the ESD tolerance. However, the FET  104  inherently includes parasitic capacitances between the gate and the source (C GS ), and between the gate and the drain (C GD ). A larger gate resistor  106  in combination with the C GS  and C GD  parasitic capacitances introduces a higher time delay (RC time constant) for the FET  104 , and thus an increased switching time, Tsw (other parameters, such as gate width and other parasitic capacitances, may affect switching time, but the value of the gate resistor  106  is a principal determinant). Increasing the switching time Tsw of a circuit architecture  100  may make the circuit unusable or unsuitable for some applications, particularly some RF switching applications. In some embodiments, a larger gate resistor  106  may introduce a frequency dependency with respect to signal insertion loss. Further, a higher value gate resistor  106  occupies a corresponding larger amount of IC die area. 
     In summary, desiring a small IC die size points to using small value gate resistors, specifying fast switching time points to using small value gate resistors, and having high ESD tolerance (i.e., better ESD performance) points to using large value gate resistors. 
     It may be noted that the ESD tolerance of one or more switching modules  102  is also dependent on other parameters and factors, such as: the stack height, N, which determines the gate voltage bounce which helps divide applied voltage from drain-to-gate-to-source; the actual total width and other geometries of a FET device which relate to current handling; and the number of metal contacts and ohmic losses and connections to the FET device. If some of these factors and parameters are not properly designed or accounted for with respect to a particular circuit, then they may be a—or even the—primary contributor to an ESD failure. 
     It was experimentally determined by the present inventors that the reduction in ESD tolerance on a version of an ACS enabled FET  104  with a lower resistance value gate resistor was primarily observed with a positive ESD strike potential relative to ground and not with a negative ESD strike potential. This asymmetry in the circuit performance was determined by the present inventors to be attributable to the ACS diode  122 , which is a unidirectional device and thus presents different characteristics when exposed to different polarities. 
     For fuller understanding, as is well known, a FET formed on an insulator effectively forms a parasitic bipolar junction transistor (BJT) device within its body. The BJT device may be able to direct some or all ESD energy into regions of the body below the more sensitive FET gate-body junction and thereby prevent or limit damage in the operative transistor body area near the gate G, source S, and drain D. For example, during a negative ESD event (e.g., −3 kV at the drain D of a FET  104 ), an impact ionization current (a known phenomenon) may be formed during a strike to the drain region of the FET, which may cause a body current I B  to flow, activating the base of the parasitic BJT device. The activated BJT device may direct the energy from its collector (C) to its emitter (E) via respective parasitic currents, I C  and I E , to regions deeper within the FET body, away from the sensitive operative regions of the FET structure. However, during a positive ESD event (e.g., +3 kV at the drain D of a FET  104 ), impact ionization current may be transmitted to the FET gate G through the ACS diode  122 . In such an event, the base current I B  may not be sufficient to activate the parasitic BJT device, and thus may not direct sufficient energy from the ESD strike away from the gate junction to prevent damage. 
     Accordingly, based on the mechanics of ESD tolerance, which relies upon the parasitic BJT inherent in the FET structure for ESD discharge, the present inventors observed that a positive polarity ESD strike turned the ACS diode  122  ON (i.e., low impedance), thus essentially significantly impairment of the parasitic BJT protection mechanism. A negative polarity ESD strike did not affect the parasitic BJT protection mechanism because the ACS diode  122  remained reversed biased during the negative polarity strike and the impedance at the body remained high. In short, the ACS diode  122  may cause an increased level of damage to the FET  104  during a positive ESD strike by preventing the operation of the parasitic BJT device inherent in the FET  104 . 
     Based on this new understanding of the interaction of the ACS diode  122  with ESD events and the tradeoffs of increased ESD tolerance versus decreased switching speed by increasing the value of the gate resistor  106 , embodiments of the invention provide an improved FET structure having an ACS circuit, fast switching times, and improved ESD tolerance. 
       FIG. 2A  is a simplified block diagram of a first circuit architecture  200  including a switching module  102  and a first modified ACS circuit according to various embodiments.  FIG. 2B  is a simplified block diagram of a second circuit architecture  202  including a switching module  102  and a second modified ACS circuit according to various embodiments. Both circuit architectures  200 ,  202  are similar to the circuit architecture  100  shown in  FIG. 1 , but with the addition of respective (alternative) ACS resistors  210 ,  220  coupled to the ACS diode  122  and (directly or indirectly) either of a first node  212  or a second node  222  on, respectively, the FET  104  gate side or the control signal port  114  side of the corresponding gate resistor  106  (i.e., parallel versus series connected to the gate resistor  106 ). 
     As should be apparent, the ACS resistors  210 ,  220  can be series connected to either side of the ACS diode  122  (i.e., the series order changed, since the resulting series equivalent circuit would remain unchanged), and each ACS resistor  210 ,  220  may be configured as one or more resistor structures. Thus, for example, some resistance may be placed on both sides of the ACS diode  122  to form an equivalent series connection for the ACS resistors  210 ,  220 . Further, the switching modules  102  shown in both figures may be stacked in the manner shown in  FIG. 1 . In implementation in ICs, both the gate resistor  106  and the ACS resistors  210 ,  220  may be fabricated using the same or different technologies and structures. 
     The first circuit architecture  200  and second circuit architecture  202  both allow the value of the gate resistor  106  to be low to achieve a fast switching time for the FET  104 . By inserting larger resistance value ACS resistors  210 ,  220  in series with the ACS diode  122 , during a positive polarity ESD strike, the total impedance seen at the parasitic BJT base is large (compared to the low impedance that would otherwise be presented by just the ACS diode  122 ), thus allowing the BJT protection mechanism to properly function. Thus, the addition of an ACS resistor  210 ,  220  essentially prevents significant impairment of the parasitic BJT protection mechanism. The ACS diode  122  function remains intact for linearity enhancement. 
     The value of the gate resistor  106  and an ACS resistor  210 ,  220  can be selected as follows: The gate resistor  106  can be chosen to meet the switching time requirement for an application of the circuit architecture  200 ,  202 . For example, a typical value for a switching time of less than about 70 nsec would be about 10 kilo-ohms. The ACS resistors  210 ,  220  should be as large as possible to improve ESD tolerance, yet be reasonable with consideration for IC die area impact, and not so large as to negate or substantially impair the function of the ACS diode  122  (i.e., if the ACS resistor  210 ,  220  is very large, it effectively behaves as an open circuit and the ACS diode  122  essentially would be disconnected and thus no longer provide any switch linearity improvement by allowing the discharge of accumulated charge). ESD tolerance provided by the ACS resistors  210 ,  220  may be computed or measured in terms of discharge rate and is usually on the order of milliseconds, which, for example, would allow for an ACS resistor  210 ,  220  resistance value from about 100 kilo-ohms to about a few mega-ohms (noting that, in the case of the second circuit architecture  202 , the resistance coupled to the ACS diode  122  includes both the ACS resistor  220  and the series connected gate resistor  106 ). In general, the resistance value of the ACS resistors  210 ,  220  should be much greater than the resistance of the gate resistor  106 , typically at least 10 times greater. 
     In essence, in accordance with this description, the circuit parameters for determining switching speed of an ACS enabled FET are separated from the circuit parameters for setting an ESD tolerance level. The invention encompasses improved FET structures having an ACS circuit, fast switching times, and improved ESD tolerance. 
     Another aspect of the invention includes a method for improving ESD tolerance and switching speed in an integrated circuit that includes an ACS circuit, including adding a resistance in series with the ACS circuit. 
     Yet another aspect of the invention includes a method for improving ESD tolerance and switching speed in an integrated circuit, including: 
     STEP 1: fabricating an integrated circuit including a field effect transistor (FET) having a gate, a drain, a source, and a body; 
     STEP 2: connecting a gate resistor in series with the gate of the FET; 
     STEP 3: connecting an accumulated charge sink (ACS) circuit to the body of the FET; 
     STEP 4: connecting an ACS resistance in series with the ACS circuit; 
     STEP 5: connecting the series-connected ACS resistance and the ACS circuit to the gate resistor. 
     Further aspects of the above method include: sizing the resistance of the ACS resistance to provide substantial ESD tolerance without substantially impairing the function of the ACS circuit; setting the ACS resistance to be at least 10 times the resistance of the gate resistor; connecting the series-connected ACS resistance and the ACS circuit to the gate resistor of the FET between the gate resistor and the gate; connecting the series-connected ACS resistance and the ACS circuit to the gate resistor of the FET at a node opposite to the connection of the gate resistor to the gate; using a diode for the ACS circuit; and fabricating the integrated circuit on one of a semiconductor-on-insulator substrate or a silicon-on-sapphire substrate. 
     Applications 
     In some applications, the circuit architectures  200 ,  202  in  FIGS. 2A and 2B  may be used as a binary (pass or block) switch, including a radio frequency (RF) switch. In other applications, the circuit architectures  200 ,  202  may modulate a signal transmitted between the first port  110  and the second port  112  by means of the control signal port  114 . Examples of portable devices in which the circuit architectures  200 ,  202  may be used include mobile phones, personal data assistants (PDAs), tablets, laptops, digital cameras, digital audio and video players, and other devices including ESD sensitive elements or modules. Examples of non-portable devices in which the circuit architectures  200 ,  202  may be used include automotive, communication, test equipment, medical, RADAR, and satellite devices. 
     Other applications that may include the novel apparatus and systems of various embodiments include electronic circuitry used in high-speed computers, communication and signal processing circuitry, modems, single or multi-processor modules, single or multiple embedded processors, data switches, and application-specific modules, including multilayer, multi-chip modules. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., MP3 players), vehicles, medical devices (e.g., heart monitor, blood pressure monitor, etc.) and others. Some embodiments may include a number of methods. 
     A number of embodiments of the invention have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. Various activities described with respect to the methods identified above can be executed in repetitive, serial, or parallel fashion. It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims.