CA1181870A - Integrated circuit protection device - Google Patents

Abstract

INTEGRATED CIRCUIT PROTECTION DEVICE
ABSTRACT OF THE DISCLOSURE
Disclosed is a protection circuit which may be used, for example, in a television receiver to protect circuitry formed within an integrated circuit from damage due to excessively high voltage transients. The protection circuit comprises a PNPN structure forming a silicon controlled rectifier (SCR) and metal-oxide-semiconductor (MOS) transistor integral to the SCR structure. The SCR
and the MOS transistors are arranged to form a two terminal protection circuit which is rendered conductive when the potential difference across the two terminals is greater than a predetermined threshold. One terminal of the protection circuit is connected to an input or output signal terminal of the protected circuit, and the other terminal is connected to a reference terminal to which a reference potential such as ground potential is applied. Transient voltages appearing at the integrated circuit terminal greater than the predetermined threshold voltage causes the protection circuit to conduct current, thereby dissipating the energy of the high voltage transient and protecting the integrated circuit from damage. In one embodiment, the gate electrode of the MOS transistor is connected to the reference terminal, and in another to the signal terminal. The latter connection provides a much larger predetermined threshold voltage than the former, and typically considerably in excess of the supply voltage.

Description

-1 RCA 76,823A

INTEGRATED CIRCUIT P TECTION DEVICE

This invention relates to integrated pro-tection clrcults .

~any types of electîical equipment contain IC
(integrated circuit) devices which are vulnerable -to damage from high voltage transients.
In a television receiver, the anode of the image-producing kinescope is typically biased a-t a high potential, e.g., 25,000 volts. High-voltage transients may be produced when the high-voltage anode of the kinescope is rapidly discharged to points at lower potentials. Such high-voltage transients have positive and negative peaks often in excess of 100 volts and may last several microseconds. High-voltage transients may also be produced when electrostatic charges are discharged as a user contacts -the controls of the television receiver. ~igh-voltage transients may be coupled to the terminals of IC's employed in the television recei~er for video and audio signal processing. Accordingly, these IC~s may be damaged by high-voltage transients.

,~

1 -~_ RCA 76,fl23/A

In a televisi.on receiver, nar-ticular si.gnals applied to an IC may have positive voltac~e excursions 6which in normal operation exceed the positive supply ~otential.
For example, a typical television horizontal/vertical regulator IC re~uires a feedback connection fron the kinescope deflection coils ~o one of its input terminals.
While the power supply for the IC is typically ~ volts, 10 the peak feedback voltaqe Erom the deflection coils is typicall~ ~27 volts. Therefore, it is desirable to provide a positive transient protection circuit for such IC's that permits normal si~nal voltaaes to exceed the power supply potential without activatin~ such protection circuit 16and nevertheless protects the IC from excessively larae transients.

The present invention is embodied in an in-tegrated eireuit protection device eomprising a pair of complementary 20eonduetivity transistors and a metal oxide sPmiconductor (MOS) transistor formed intearal to the semiconductor strueture. The pair of eomplementary conductivity -transistors and the MOS transistors are arran~ed to form a two-terminal deviee eapable of conductinq a hi~h current when the 26potential difference across its two terminals exceeds a predetermined threshold. The protection device is connected at one terminal thereof to a circuit terminal of the circuit to be protected and at the other terminal thereof to a souree of reference potential. I~hen the 30potential at the circuit terminal of the protected circuit exceeds the predetermined threshold, which is preferably set above the maximum expected si~nal voltaae, the protection circuit is rendered conductive, thereby protecting the IC from damaqe.
3~ In one embodiment, the aate electrode of the .~OS
transistor is connected to the source of reference poten-tial, so that the predetermined threshold of the protection device is substantially equal to the threshold of the MOS transistor.
In a seeond embodiment, ~he ~ate electrode of the MOS
~0 37~
1 -3~ J~ 7(~ 3/A

transistor is connec~ed to the circuit termil-al ol t~
c.ircuit to be protected so that the MO~ transistor is 6 conditioned for nonconduc~ion. The Predetermil-ed thl-eshold of th latter protection device is considerably ~reater than that of the form~r.
In the dxawin~:
FIGURE 1 is a plan view of an integra~ed circuit 10 pro~ection device in accordance with an e~bodi~ent of the present invention;
FIGURE 2 is a cross-sectional view of a semiconductor structure illustratina Further structural details of the protection device of FI~,~PE l;
16 FIG~RE 3 is a schematic dia~ra~ of the se~i-conductor protection device in FI~ ES 1 and 2;
FIGURE 4 is a plan view of an intearated circuit protection device in accordance with an alternate e~bodi~ent of the present invention;
FIGURE 5 is a cross-sectional view of a semiconductor structure illustratina further structural details of the protection device of FIGUR~ 4; and FI~,URE 6 is a schematic dia~ram of the semiconductor protection device of FI~URF~S 4 and 5.

2~
As shown in FI~URE~ 1 and 2, a semiconductor circuit is fabricated on a substrate 10 ~ade of P type silicon material. An epitaxial layer 12 of N- type conductivity is disposed on the substrate 10. A P region 30 1~ is disposed within N- epitaxial layer 12, formin~ a ~N
junction with layer 12. A P+ reaion 20 is further formed within P region 14u Another P region 16 is disPosed within N- epitaxial layer 12, forming a ~N junction with epitaxial layer 12. An N+ reaion 18 is disposed within P re~ion 16 formina a PN junction with P region 16. A buried N+
region 11 underlies re~ions 1~, 20, 16 and 18. The structure thus formed within N- epitaxial layer 12 is a protection circuit, the sche~atic for which is shown in FIGURE 3. A P+ region 32 surrounds N- epitaxial layer 12 ~0 4 RCA 76,823/A

and extends from the surface of epitaxial layer 12 -to the substrate 10 thereby isolatin~ the ~rotection circui-t formed 5within N- epitaxial layer 12 from other circuits on the substrate 10 in regions 21a and 21b. The P+ reqion 32 also overlaps P region 16 ~o provide a connection between the subs~rate 10 and P region 16.
An insulating layer 22, which may for exa~Ple be silicon dioxide, overlies the surface of N- epitaxial layer 12. Openings are formed in the insula~inq layer 22 over regions 20, 18 and 32 in order to make respec-tive electrical contact thereto. A conduc~ive layer 24, which may for example be aluminum, overlies the insulatin~ layer 22 and ~makes contact with P~ reqion 20. Another conductive layer 30 overlies the insula~ing layer 22 and makes contact with N+ region 18 and P~ reaion 32. A further conductive layer 26~ which is connected to conductive layer 30, overlies that portion of N- epitaxial layer 12 extendina between P reaion 2014 and P region 16 so as to form a P channel MO~ transistor.
A bond pad 28 is connected to P+ region 20 through conductive layer 24. The bond pad 28 is further connected to a si~nal terminal of utili.zation circuit 101 . . .. ... . . . .. ...... .
elsewhere on the IC chip, such as in regions 21a and 2621b. A terminal 3~ is further connected to P reaion 32 and N+ region 18 through conductive layer 30. Terminal 34 is connected to receive a source of reference potential, such as ground potential.
FIGURE 3 is a schematic circuit model of the 30 structure illustrated in FIGURES 1 and 2. The protection circuit comprises a PNP transistor Ql, an NPN transistor Q2,a P channel MOS transistor Pl, and a resistor Rl. The emitter electrode 114, base electro~e 112 and collector electrode 116 of transistor ~1 correspond to reoions 14,

3~ 12 and 16, respectively,in FIGURF.~ 1 and 2. P~ reqion 20 increases the injection efficiency of the emitter reqion 14 of transistor Ql which increases the com~on emitter forward current gain,commonl~ referred to as "beta" of that transistor. The emitter electrode 118, base electrode 116

4~

-5- RCA 76,823/A

and collector electrode 112 of -transis-tor ~2 correspond to regions 18, 16 and 12, respectively,in FI~,UR~ I and 2.
6Source electrode 114 and drain electrode 116 oE transistor Pl correspond to reg.ions 14 and 16, re.spec~ively, in FI~URE~ 1 and 2. The gate electrode 126 of trarlsistor P1 corresPonds to conductor 26 in F`IGURE~ 1 and 2. Resistor R1 corresponds to the extended portion of P region 16 between N-~ re~ion 18 and P+ region 32 plus the pinch resistor formed by that portion of P region 16 that underlies N~ reaion lfl.
The value of resistor Rl is determined by the resistivity of the P region 16, and the ~eometry of N~
region 18 relative to P region 16 (see FI~U~ 2). For example, the resistance of resistor Rl may be increased by further extendinq P region 16 further away from N+ reaion 18 or making the extension narrower. Also, as is known -to those skilled in the art, the value of resistor Rl attributable to the pinch resistor beneath N+ region 18 20may be increased by diffusin~ N+ reaion 18 deeper into P
region 16. The buried N~ region 11 provides increased conductivity across the lower reqion of epitaxial layer 12 which increases the ability of transistors Ql and ~2 to conduct current when a hiqh-voltaae transient has triaoered 2~the protection device.
As shown in FIGURE 3, transis~ors Ql and Q2 are connected to form a silicon control rectifier (~CR).
Specifically, the base electrode of Ql is connected to the collector electrode of Q2,and the base electrode of Q2 is 30connected to the collector electrode of ~1. Resistor Rl is connected between the base and emitter electrodes of transistor Q2. The source electrode of transistor Pl is connected to the emitter electrode of transistor Ql and the drain electrode of transistor P1 is connected to the 3~collector electrode of transis~or ~1 so that the conduction channel of Pl ls connected in parallel with the main conduction path of transistor Ql. r~ate electrode 126 of transistor Pl is connected to the emitter electrode of transistor Q2. The resultino protection device is connected ~0 ~.~h~ 7~.~

-6- ~CA 76,~2yA

between bond pad 28, which is a signal ter~inal (either for input or output signals) of a TV utiliza-tion circuit 101 to 6be protected,and terminal 34, which is connected to ground potential.
The present structure differs from a conventional SCR device in that the integral ~OS -transistor and its connections to transistors Ql and Q2 converts the three-terminal SCR device into a two-terminal device tha-t is rendered conductive when the vol-tage across its terminals exceeds a predetermined threshold. Since ~he ~ate and source electrodes are connected between sianal ter~inal 2~ and ground terminal ~4, the predetermined threshold of the $~protection device is .substantially eaual to the gate-to-source threshold voltage of transistor Pl, i.e., the gate voltage at which transistor Pl conducts.
In operation, assume that transistors Ql and Q2 are initially non-conductive. Resistor Rl prevents ~ electrical and thermal noise from inadvertently causina transistors Ql and ~2 to conduct. So long as the signal applied to bond pad 28 has a potential below the qate-to-source threshold vol~age of transistor Pl, -transistor~ Ql and Q2 remain non-conductive.
~6 A high-voltaqe transient appearing a-t bond pad 28 having a potential greater than the aate-to-source threshold voltage of Pl causes the gate-to-source voltage of transistor Pl to exceed the threshold voltage of Pl, which causes channel current to flow in transistor Pl. Conduction by 30 transistor Pl provides base c~rrent to transistor Q2. The resulting collector current of transistor 02 provides base current to transistor Ql, causing that transistor to conduct.
The conduction between collector and eritter electrodes of transistors Ql and Q2 is regenerative, thereby drivina 36 transistors Ql and Q2 into high conduction. The eneray of the high-voltage transient is diverted by virtue of the conduction of transistors Ql and Q2 to around, thereby protecting the TV signal processing utilization circuit 101 from damage.
4~

1 ~7- RCA 76,823 /A

When the current supplied by the hiah-voltaae transient from bond p~d 28 to power suPply terminal 34 falls 5below a minimum sustaining current, transistor ~2 is provided with insufficient base current to remain conductive, and therefore Q2 ~urns off. In response, the base current to transistor Ql is removed, causing ~1 to turn off. Accordingly, the protection circuit becomes nonconductive. ~e~istor Rl, lOin addition to stabilizing the protection device against inadvert~nt firing, also determines the minimum holding current below which Ql and Q2 beco~e nonconductive. As the value of resistor Rl is increased, the minimum holding current is decreased, and vice versa.
16 The predetermined threshold voltage of the protection device is substantially equal to the threshold voltage of transistor Pl. As is known in the art, the threshold voltage of an r10S transistor is related to the oxide thickness beneath the gate electrode thereof and the 20 conductivity of the channel material. Typical values for the threshold voltage of MOS transistors such as Pl are in the range between 20 and 30 volts. Accordingly, by appropriate desi~n of ~IOS transistor Pl, the predetermined threshold of the protection circuit can be set at a 2~ relatively high value, e.g. 30 volts,which is typically much higher than the most positive power supply voltaae, e.g. 10 volts.
An alternate embodiment of the present inventlon wherein the predetermined threshold voltaae of the ~ protection device is substantially increased,compared with that of the protection device shown in FI~URE5 1, 2 and 3, is shown in FIGU~ES 4, S and 6. The structure of the protection circuit of FIGURES 4,5 and 6 is the same as that shown in FIGURES 1, 2 and 3 except that the gate electrode of transistor Pl is not connected to ground potential as in the first embodiment, but rather to the bond pad 28 via a connection between conductive layer 26 and conductive - layer 24. Such connection between the qate and source electrodes of transistor Pl conditions that transistor to 1 -8- RCA 76,823 /A

be nonconductive for all positive voltages at boncl pad 2~.
In this embodiment, the predetermined ~hreshold voltage of 6the protection device depends on -the reverse bias breakdown voltage between collec-tor and hase electrodes of transistors ~1 and Q~ rather than on the threshold voltage of transistor Pl. The collector-to~base reverse bias breakdown voltage is that voltaye applied to the collector which causes base current to be applied from the collector. As lonq as -the collector to base current is not excessive, the transistor conducts, but no damage occurs to the transistor. ~he purpose of transistor Pl is to increase the reverse bias breakdown voltage of transistors ~1 and ~2. Towards this ~5 end, it is believed that the electric field induced beneath the gate electrode of transistor Pl tends to inhibit the collertor to base breakdown of transistors ~1 and Q2 fro~
occurring near the surface of the integrated circuit. As a result, the collector to base breakdown phenomena tends 20 to occur at a greater depth into the semiconductor wafer, which has the effect o~ increasing the collector-to-base breakdown voltage. Thus, connectinq the gate electrode of transistor Pl to bond pad 28,where it receives the positive transientlincreases the predetermined threshold voltage of 26 the SCR formed by transistors ~1 and Q2.
The reverse bias breakdown occurs at the junction of regions 12 and 16. Therefore, the predetermined threshold voltage of the prot-ection device is substantially e~ual to the reverse bias collector-to-base breakdown voltage of 30 transistor Q2-The breakdown voltage of transistor Q2 is, tosome extent, determined by the value of resistor Rl. In particular, as the value of resistor Rl is decreased, the reverse bias collector-~o-base breakdown voltaae of ~6 transistor Q2 is increased, and vice versa. ~lso, the reverse bias breakdown voltage of transistor Q2 is affected by particular parameters of transistor Pl.
For example, the thinner the oxide insulator beneath the gate electrode 26 o~ transistor P1, the deeper the ~h~-r~

~9- RCA 76,823/A

respective collector-to-base breakdown occurs,resulting in potentially h~her hreakdown voltages~ Breakdown voltages in 6the ranae of 40 to 60 volts are obtainable.
In operation, a signal is applied at bond pad 28, and transistors Ql and Q2 are initially nonconductive. A
high-voltage transient appearing at bond pad 28 will cause the potential at bond pad 28 to rlse sharply. Such positive potential substantially appears across the collec-tor-to-base junction of transistors nl and Q2. ~hen the potential applied exceeds the reverse bias breakdown of transis-tor ~2, base current is provided to transistor ~1 which in turn supplies base current to transistor 02,regeneratively driving ~both transistors into high conduction.
When the current supplied by the high-voltage transient from bond pad 28 to power supply terminal 34 falls below the minimum holding current, transistors O1 and Q2 will turn off and the protection circuit becomes 20nonconductive. In such manner, the energy of hiah-voltaae transients producing a positive voltage at bond pad 28 in excess of the threshold voltage of the pxotection circuit is dissipated by the conduction of transistors ~1 and 02 to power supply terminal 34. Furthermore, since the ~6predetermined threshold voltage of the protec-tion device is 40 volts or more, the input signal variation may considerably exceed the positive power supply potential, typically in the order of ~10 volts,without tri~ering the protection device.
While the present invention has been described wi h reference to a specific structure,it will be understood that modification within the scope of the invention as defined in the following claims are contemplated. For example, P and N ~ype semiconductor 36regions may be interchanged to provide a protection device that is rendered conductive for negative voltage transients.
Also,i~ is to be understood that conductive layer 26,which forms the gate elec~rode of ~OS ~ransis~or Pl,may be a conductor other than aluminum,and the insulatinq material 4~

-lO- RCA 76,823/A

beneath the gate may be an insulator other than silicon dioxide .

This is a divisional application of Canadian Serial No.
399,038 :Eiled March 22, 1982 2~1 ~6 3~

Claims (8)

CLAIMS:

1. A semiconductor protection circuit comprising:
a semiconductor substrate of a first conductivity type;
a semiconductor layer of a second conductivity type disposed on said substrate, said semiconductor layer having a surface;
first and second semiconductor regions of said first conductivity type, each disposed in PN junction forming relation with said semiconductor layer;
a third semiconductor region of said second conductivity type disposed in PN junction forming relation with said second semiconductor region;
a layer of insulating material disposed on the surface of said semiconductor layer lying between said first and second semiconductor regions;
a layer of conductive material disposed on said insulating layer; and conductive means for connecting said conductive layer to one of said first and third semiconductor regions.

2. A semiconductor protection circuit in accordance with claim 1 wherein said conductive means connects said conductive layer to said first semiconductor region.

3. A semiconductor protection circuit in accordance with claim 1 wherein said conductive means connects said conductive layer to said third semiconductor region.

4. A semiconductor protection circuit in accordance with claim 1 wherein:
said second semiconductor region extends in a direction along the surface of said semiconductor layer so as to form a resistor comprising that portion of said second semiconductor region disposed between said third semiconductor region and the end of said extension of said second semiconductor region.

5. A semiconductor protection circuit in accordance with claim 4 further comprising means for connecting said extended end of said second semiconductor region to said semiconductor substrate.

6. A semiconductor protection circuit according to claim 5 wherein said means for connecting said extended end of said second semiconductor region to said semi-conductor substrate comprises a fourth semiconductor region of said first conductivity type extending from said surface of said semiconductor layer to said substrate, said fourth semiconductor region intersecting with said second semi-conductor region at said extended end thereof, said fourth semiconductor region surrounding said semiconductor layer.

7. A semiconductor protection circuit in accordance with claim 5 further comprising:
a reference terminal for receiving a reference supply potential;
means for connecting said reference terminal to said substrate and to said third semiconductor region;
a utilization circuit including a signal terminal for receiving a signal; and means for connecting said signal terminal to said first semiconductor region.

8. A semiconductor protection circuit in accordance with claim 4 further comprising a buried semi-conductor region of said second conductivity type, said buried semiconductor region disposed between said semiconductor layer and said substrate, and extending beneath said first semiconductor region, said third semiconductor region, and that portion of said semiconductor layer between said first and said third semiconductor regions, said buried semi-conductor region having a lower resistivity than that of said semiconductor layer.