U.S. patent application number 11/066961 was filed with the patent office on 2005-08-04 for insulated gate bipolar transistor and electrostatic discharge cell protection utilizing insulated gate bipolar tansistors.
This patent application is currently assigned to Micrel, Incorporated. Invention is credited to Imtiaz, Sohel, Mallikarjunaswamy, Shekar.
Application Number | 20050167753 11/066961 |
Document ID | / |
Family ID | 32710928 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050167753 |
Kind Code |
A1 |
Mallikarjunaswamy, Shekar ;
et al. |
August 4, 2005 |
Insulated gate bipolar transistor and electrostatic discharge cell
protection utilizing insulated gate bipolar tansistors
Abstract
IGBTs and circuits can be designed to improve the ability of
circuits and systems to withstand ESD events. In addition pads can
be designed to take advantage of the circuits and IGBTs to
withstand and dissipate ESD events.
Inventors: |
Mallikarjunaswamy, Shekar;
(San Jose, CA) ; Imtiaz, Sohel; (San Jose,
CA) |
Correspondence
Address: |
BEVER HOFFMAN & HARMS, LLP
TRI-VALLEY OFFICE
1432 CONCANNON BLVD., BLDG. G
LIVERMORE
CA
94550
US
|
Assignee: |
Micrel, Incorporated
San Jose
CA
|
Family ID: |
32710928 |
Appl. No.: |
11/066961 |
Filed: |
February 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11066961 |
Feb 25, 2005 |
|
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10336129 |
Jan 3, 2003 |
|
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6888710 |
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Current U.S.
Class: |
257/360 ;
257/E29.197 |
Current CPC
Class: |
H01L 27/0259 20130101;
H01L 29/7393 20130101; H01L 2924/0002 20130101; H01L 2924/0002
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/360 |
International
Class: |
H01L 023/62 |
Claims
We claim:
1. An insulated gate bipolar transistor comprising: a substrate; a
first well region within the substrate; a collector region within
the first well region; a second well region within the substrate;
an first emitter region within the second well region; a second
emitter region within the second well region; and a first electrode
connected with the first well region and the collector region.
2. The insulated gate bipolar transistor of claim 1 wherein the
insulated gate bipolar transistor comprises a lateral insulated
gate bipolar transistor.
3. The insulated gate bipolar transistor of claim 2 wherein the
insulated gate bipolar transistor comprises an isolation region
located between the substrate and the first well region.
4. The insulated gate bipolar transistor of claim 2 further
comprising a punch through reduction region located substantially
adjacent to the collector region.
5. The insulated gate bipolar transistor of claim 4 wherein the
punch through reduction region is approximately two microns in
width.
6. The insulated gate bipolar transistor of claim 2 further
comprising a buffer region within the first well, the buffer region
abutting the collector region.
7. The insulated gate bipolar transistor of claim 1 wherein the
insulated gate bipolar transistor comprises a vertical gate bipolar
transistor.
8. An insulated gate bipolar transistor comprising: a substrate; a
first well region within the substrate; a first collector region
within the first well region; a second collector region with the
first well region; a second well region within the substrate; an
first emitter region within the second well region; and a second
emitter region within the second well region.
9. The insulated gate bipolar transistor of claim 8 wherein the
insulated gate bipolar transistor comprises a lateral insulated
gate bipolar transistor.
10. The insulated gate bipolar transistor of claim 9 wherein the
lateral insulated gate bipolar transistor comprises an isolation
region located between the substrate and first well region.
11. The insulated gate bipolar transistor of claim 9 wherein the
lateral insulated gate bipolar transistor comprises a third well
region within the first well region.
12. The insulated gate bipolar transistor of claim 9 wherein the
lateral insulated gate bipolar transistor comprises an isolation
region located between the substrate and second well region.
13. The insulated gate bipolar transistor of claim 9 further
comprising a punch through reduction region located substantially
adjacent to the collector region.
14. The insulated gate bipolar transistor of claim 13 wherein the
punch through reduction region is approximately 2 microns in
width.
15. The insulated gate bipolar transistor of claim 9 further
comprising a buffer region within the first well, the buffer region
abutting the first collector region.
16. The insulated gate bipolar transistor of claim 8 wherein the
insulated gate bipolar transistor comprises a vertical insulated
gate bipolar transistor.
17. An insulated gate bipolar transistor comprising: a substrate; a
first well region within the substrate; a collector region within
the first well region; a second well region within the substrate;
an first emitter region within the second well region; a second
emitter region within the second well region; and a third well
region within the first well region.
18. The insulated gate bipolar transistor of claim 17 wherein the
insulated gate bipolar transistor comprises a lateral insulated
gate bipolar transistor.
19. The insulated gate bipolar transistor of claim 18 wherein the
lateral insulated gate bipolar transistor comprises an isolation
region located between the substrate and first well region.
20. The insulated gate bipolar transistor of claim 18 wherein the
lateral insulated gate bipolar transistor comprises another
collector region within the first well region.
21. The insulated gate bipolar transistor of claim 18 wherein the
lateral insulated gate bipolar transistor comprises an isolation
region located between the substrate and second well region.
22. The insulated gate bipolar transistor of claim 18 further
comprising a punch through reduction region located substantially
adjacent to the collector region.
23. The insulated gate bipolar transistor of claim 22 wherein the
punch through reduction region is approximately 2 microns in
width.
24. The insulated gate bipolar transistor of claim 18 wherein the
insulated gate bipolar transistor comprises a vertical gate bipolar
transistor.
25. The insulated gate bipolar transistor of claim 18 further
comprising a buffer region within the first well, the buffer region
abutting the collector region.
Description
RELATED APPLICATION
[0001] The present application is a divisional of U.S. patent
application Ser. No. 10/336,129 filed by Shekar Mallikarjunaswamy
and Sohel Imtiaz on Jan. 3, 2003.
BACKGROUND
[0002] 1. Technical Field
[0003] The disclosure relates to electrostatic discharge protection
of integrated circuits and, in particular, to insulated gate
bipolar transistors for electrostatic discharge protection of
integrated circuits.
[0004] 2. The Related Art
[0005] A problem in designing integrated circuits is dealing with
electrostatic discharge (ESD), which is caused by static
electricity built up by the human body and machines that handle
integrated circuits. The static electricity is discharged into the
integrated circuit upon contact or close proximity with the
integrated circuit. Static electricity follows any discharge path
to alleviate the high electron build-up or deficiency. When an ESD
sensitive device, such as an integrated circuit, becomes part of
the discharge path, or is brought within the bounds of an
electrostatic field, the sensitive integrated circuit can be
permanently damaged.
[0006] ESD destruction of metal-oxide silicon field-effect
transistor (MOSFET) devices occurs when the gate-to-source or
gate-to-drain voltage is high enough to arc across the gate
dielectric of a transistor device. The arc burns a microscopic hole
in the gate oxide, which permanently destroys the MOSFET. Like any
capacitor, the gate of a MOSFET must be supplied with a finite
charge to reach a particular voltage. Larger MOSFETs have greater
capacitance and are therefore less susceptible to ESD than are
smaller MOSFETs. Typically, an immediate failure will not occur
until the gate-to-source or gate-to-drain voltage exceeds the
dielectric breakdown voltage by two to three times the rated
maximum voltage of the gate oxide. The voltages required to induce
ESD damage in some transistors can be as high as thousands of volts
or as low as 50 volts, depending upon the oxide thickness.
[0007] Electrostatic fields can also destroy power MOSFETs by
corona discharge. The failure mode is caused by ESD, but the effect
is caused by placing the unprotected gate of the MOSFET in a corona
discharge path. Corona discharge is caused by a positively or
negatively charged surface discharging into small ionic molecules
in the air.
[0008] When designing an integrated circuit, a voltage rating is
selected for the pad connecting a node in the circuit. The rating
is the maximum voltage that the integrated circuit or pad is
designed to withstand without causing damage. ESD protection
circuits are generally designed to protect integrated circuits or
pads from voltages above the rating for the integrated circuit or
housing.
[0009] Automotive applications, for example, demand robust
protection (typically 8 kV to 25 kV human body model on a system
level) against the threat of ESD or other transient pulses, such as
load dump. General applications typically require protection at a
minimum of 2,000 volts. Unfortunately, many power MOSFET device
designs are unable to meet this requirement.
[0010] Therefore, there exists a need to effectively protect
circuits from the effects of ESD both cost effectively and
efficiently.
SUMMARY OF THE DISCLOSURE
[0011] In accordance with the present invention, an electrostatic
discharge (ESD) protection circuit that includes an Insulated Gate
Bipolar Transistor (IGBT) is disclosed. In some embodiments, the
IGBT-based ESD circuit improves the ability to withstand ESD
events. In additional embodiments pad designs that take advantage
of the ESD circuits with IGBTs are disclosed.
[0012] In one embodiment, a circuit for protection from
electrostatic discharge events comprises an insulated gate bipolar
transistor comprising a gate, an emitter and a collector. The
collector is coupled with a pad, and the emitter is coupled with a
potential. A collector clamp is coupled with the pad, and the gate
of the insulated gate bipolar transistor, and a resistance is
coupled with the emitter and the gate of the insulated gate bipolar
transistor.
[0013] In another embodiment, a structure for electrostatic
discharge protection of pads housing integrated circuits comprises
a pad, an insulated gate bipolar transistor fabricated on the pad,
a collector clamp coupled with the pad and the insulated gate
bipolar transistor, and a resistor coupled with the insulated gate
bipolar transistor.
[0014] In a further embodiment, an insulated gate bipolar
transistor comprises a substrate, a first well region and a second
well region within the substrate, a collector region within the
first well region, first and second emitter regions within the
second well region, and a first electrode connected with the first
well region and the collector region.
[0015] In an additional embodiment, an insulated gate bipolar
transistor comprises a substrate, a first well region and a second
well region within the substrate, first and second collector
regions within the first well region, and first and second emitter
regions within the second well region.
[0016] In yet another embodiment, an insulated gate bipolar
transistor comprises a substrate, a first well region and a second
well region within the substrate, a first collector region within
the first well region, first and second emitter regions within the
second well region, and a third well region within the first well
region.
[0017] In yet a further embodiment, a method is provided for
protecting integrated circuits from electrostatic discharge events.
A circuit including an insulated gate bipolar transistor that is
coupled to a pad that is bonded to integrated circuits is provided.
The method comprises triggering the insulated gate bipolar
transistor, at a clamp trigger voltage, in response to an
electrostatic build up on the pad that is at least equal to the
clamp trigger voltage, and latching a parasitic thyristor of the
insulated gate bipolar transistor, at a latching voltage that is
greater than the clamp trigger voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic diagram of an embodiment of a circuit
for electrostatic discharge protection;
[0019] FIG. 2 is a simplified diagram of an embodiment of a lateral
insulated gate bipolar transistor for use in a circuit for
electrostatic discharge protection;
[0020] FIG. 3 illustrates another embodiment of a circuit for
electrostatic discharge protection;
[0021] FIG. 4 is a cross-sectional side view of an embodiment of a
lateral insulated gate bipolar transistor;
[0022] FIG. 5 is a graph of the current-voltage characteristics of
a collector of the lateral insulated gate bipolar transistor
illustrated in FIG. 4;
[0023] FIG. 6 is a cross-sectional side view of an another
embodiment of a lateral insulated gate bipolar transistor;
[0024] FIG. 7 is a cross-sectional side view of a further
embodiment of a lateral insulated gate bipolar transistor;
[0025] FIG. 8 is a graph of the current-voltage characteristics of
a collector of a lateral insulated gate bipolar transistor
illustrated in FIGS. 6 and 7;
[0026] FIG. 9 illustrates a well region of an embodiment of a
lateral insulated gate bipolar transistor;
[0027] FIG. 10 is a cross-sectional side view of an embodiment of a
lateral insulated gate bipolar transistor with leakage current
reduction;
[0028] FIG. 11 is layout of an embodiment of a pad with an
electrostatic discharge protection circuit fabricated upon it;
and
[0029] FIG. 12 is a graph of the direct current characteristics in
response to an ESD event of the electrostatic discharge protection
circuit fabricated on a pad as illustrated in FIG. 11, when a
parasitic thyristor latches.
[0030] In the present disclosure, like objects that appear in more
than one figure are provided with like reference numerals. Further,
objects in the figures and relationships in sizes between objects
in the figures are not to scale.
DETAILED DESCRIPTION OF THE DRAWINGS
[0031] Referring to FIG. 1, a circuit 2 for electrostatic discharge
protection is illustrated. A pad 15, along with any integrated
circuits coupled to pad 15, are protected by electrostatic
discharge (ESD) protection circuit 2. ESD protection circuit 2
includes an insulated gate bipolar transistor (IGBT) 5 that has a
collector 10 coupled to pad 15. IGBT 5 also has an emitter 20 that
is coupled to a potential 25 that can be, for example, a ground
potential. The ground potential can be a ground bus or a ground
pad, as is known in the art. Gate 30 of IGBT 5 is coupled through a
collector clamp 35, which comprises one or more diodes, to pad 15.
A resistor 45 is coupled between emitter 20 and gate 30. Further,
an emitter clamp 40, which also comprises one or more diodes, may
be coupled between emitter 20 and gate 30. Emitter clamp 40 is
designed to protect gate 30 of IGBT 5 from current surges through
collector clamp 35 that could burn through the gate dielectric of
gate 30.
[0032] Collector 35 and emitter clamp 40 can, for example, be zener
diodes, diodes, or active clamps, e.g., gate shorted MOSFETs.
[0033] When the voltage at pad 15 is below the trigger voltage of
collector clamp 35, collector clamp 35 is in a blocking state. As
long as collector clamp 35 is not triggered, i.e., does not
conduct, emitter 20 and gate 30 are both at potential 25, thus
preventing IGBT 5 from conducting. At the onset of an ESD event,
when a voltage greater than the trigger voltage of collector clamp
35 appears at pad 15, collector clamp 35 will begin conduction.
Conduction by collector clamp 35 causes a current to flow along
path 55 through resistor 45.
[0034] Once a positive voltage with respect to emitter 20 appears
at gate 30, due to the current flow through resistor 45, IGBT 5
will enter its forward conduction state, resulting in an increasing
collector to emitter voltage 50. As the collector to emitter
voltage 50 increases, it will reach a level at which the current
through IGBT 5 latches a parasitic thyristor that exists in the
structure of IGBT 5. Latching of the parasitic thyristor causes a
substantial decrease in the collector to emitter voltage 50. The
substantial decrease in collector to emitter voltage 50 results in
dissipation of charge at the pad 15, almost instantaneously. The
parasitic thyristor structure of IGBT 5 will continue to conduct
until all of the charge at the pad 15 is dissipated.
[0035] In some embodiments, ESD protection circuit 2 is fabricated
upon pad 15, an embodiment of which is depicted in FIG. 11.
[0036] Referring to FIG. 2, a simplified diagram of an embodiment
of a lateral insulated gate bipolar transistor for use in a circuit
2 for electrostatic discharge protection is illustrated. In FIG. 2,
n-well 84 forms a junction with p-well 86. A collector region 90,
which is a p.sup.+-type material, is formed in n-well 84. A first
emitter region 88, which is a n.sup.+-type material, and a second
emitter region 96, which is a p.sup.+-type material, are formed in
p-well 86.
[0037] When a voltage is applied at gate 30 that exceeds the
threshold voltage for IGBT 5, an inversion layer 82 forms on the
surface of n-well 84. A voltage at gate 30 will arise when a
current flows along path 55 (FIG. 1), thus pulling up the gate
voltage due to the current flowing through resistor 45.
[0038] Formation of inversion layer 82 causes electrons to flow
through inversion layer 82 from first emitter region 88 on the
surface of p-well 86 to n-well 84. Holes flow in the reverse
direction of the electrons, when collector 90 is forward biased
with respect to n-well 84, e.g., when the collector 90 is at a
potential of 0.7 volts greater than n-well 84. The flow of holes
into n-well 84 turns on bipolar transistor 92, as n-well 84 forms
the base of a pnp bipolar transistor 92. Bipolar transistor 92,
which is formed from collector region 90 (transistor collector),
n-well 84 (transistor base), and p-well 86 (transistor emitter), is
inherent in the structure of IGBT 5.
[0039] When the holes collected by well 86 flow under first emitter
region 88 into second emitter region 96, they forward bias the
junction between first emitter region 88 and second emitter region
96, which causes a parasitic thyristor 94 to latch up. Parasitic
thyristor 94 is formed from collector region 90, n-well 84, p-well
86, and first emitter region 88. At latch up, parasitic thyristor
94 will not respond to changes in the current or voltage at gate 30
of IGBT 5. Current will flow through parasitic thyristor 94 until
all of the charge at pad 15 is dissipated.
[0040] Triggering a parasitic thyristor in the structure of an IGBT
to dissipate ESD-induced voltages provides several advantages over
MOSFET-based ESD protection schemes. One advantage is improved
power dissipation by ESD protection circuit 2. The improved power
dissipation increases the useful life of ESD protection circuit
2.
[0041] Referring again to FIG. 1, when collector clamp 35 consists
of diodes, the trigger voltage would be the sum of the reverse
breakdown voltages of the one or more diodes that comprise
collector clamp 35. By changing the trigger voltage of collector
clamp 35, the voltage at which IGBT 5 begins conduction is altered,
allowing a system designer to change the rating of pad 15 without
having to redesign or change out IGBT 5. This results in a
substantial cost saving and design flexibility, since IGBT 5 can be
used regardless of the rating of the pad. In some embodiments, the
breakdown voltage is altered by changing the number of diodes that
make up collector clamp 35, without having to change the rating of
the diodes.
[0042] Referring to FIG. 3, another circuit for electrostatic
discharge protection is illustrated. In FIG. 3, another IGBT 60 is
added to ESD protection circuit 2. An emitter 65 of IGBT 60 is
coupled to emitter 20 of IGBT 5. The gate 70 of IGBT 60 is coupled
to its emitter 65 through emitter clamp 41 and resistor 46.
Collector 80 of IGBT 60 is coupled to pad 81. Another collector
clamp 75 couples gate 70 of IGBT 60 to pad 81. IGBT 5 is coupled
essentially the same way as illustrated in FIG. 1.
[0043] The circuit in FIG. 3 is especially advantageous in handling
bi-directional ESD events, where a voltage at pad 81 is greater
than a potential at pad 15 as well the reverse. This is because
IGBT 5 responds to positive ESD events, while IGBT 60 responds to
negative ESD events. Further, both collector clamps 35 and 75 can
be optimized either together or separately to allow flexibility in
the ESD rating of pad 15.
[0044] It should be noted that the ESD protection circuits of FIGS.
1 and 3 can be integrated circuits for ease of use and manufacture
onto pad 15.
[0045] The circuits described in FIGS. 1 and 3 can be utilized
regardless of the desired voltage rating of pad 15 without changing
IGBT 5 or IGBT 60. IGBT 5 can, for example, withstand 5,000 volts
during an ESD event, or any other amount. However, the circuit can
operate for a pad 15 rated to almost any value, simply by changing
the trigger voltage of collector clamp 35 or collector clamp 75 to
the desired rating. In the case where either collector clamp 35 or
collector clamp 75 comprises diodes, the trigger voltage can be
changed by adding or removing diodes. This greatly increases the
utility and cost effectiveness of the ESD protection circuits 2
illustrated in FIGS. 1 and 3 over conventional ESD protection
designs.
[0046] Referring to FIG. 4, a cross-sectional side view of a
lateral insulated gate bipolar transistor 5 for electrostatic
discharge protection circuit 2 is illustrated. In FIG. 4, IGBT 5
comprises a substrate 100, which may be p-type. An epitaxial region
105, which may be n-type, is diffused into substrate 100. An
isolation region 110, which may also be an up-diffused p-type
region, is also formed in substrate 100, along with a p-well 115. A
first emitter region 120, which may be p.sup.+-type, and a second
emitter region 125, which may be n.sup.+-type, are formed within
p-well 115. A collector region 130, which also may be p.sup.+-type,
is formed in a n-well 135 that is formed in epitaxial region 105.
Portions of n-well 135 and p-well 115 are separated by epitaxial
region 105, while other portions form a junction.
[0047] A gate electrode 140, which in one embodiment is comprised
of a polycrystalline silicon material, is located above an extended
portion of p-well 115, a portion of second emitter region 125, and
a portion of n-well 135. A field oxide 145 is interposed between
gate electrode 140 and another portion of n-well 135. An emitter
electrode 150 is in common contact with both first emitter region
120 and second emitter region 125. A collector electrode 155 is in
contact with collector region 130. An insulation film 165, formed
of a thermally oxidized film, a boron phosphorous silicate glass
(BPSG) or other insulation, is disposed over IGBT 5 for surface
protection and for surface stabilization.
[0048] Operation of the IGBT 5 of FIG. 4 will now be described.
Once a voltage, higher than the threshold voltage and positive with
respect to a potential of emitter electrode 150, is applied to gate
electrode 140, an inversion layer 170 on the surface of p-well 115
is created between second emitter region 125 and n-well 135.
Electrons then flow from second emitter region 125 through p-well
115 into n-well 135 through inversion layer 170. The electron flow
into n-well 135 functions as a base current of a pnp transistor
175, which is formed by collector region 130 (transistor emitter),
n-well 135 (transistor base), and p-well 115 (transistor
collector).
[0049] Once collector region 130 reaches a voltage greater than 0.7
volts above that of n-well 135, collector region 130 begins to
inject holes into p-well 115 (transistor collector), thereby
causing conduction by PNP transistor 175. The difference of 0.7
volts, between collector region 130 and n-well 135, can be altered
by changing the doping of the collector region 130 and the n-well
135, which effects the potential at which pnp transistor 175
conducts.
[0050] Parasitic thyristor 180 latches up when the holes collected
in p-well 115 flow under second emitter region 125 and forward bias
first emitter region 120. Parasitic thyristor 180 then will conduct
all of the current flowing through the IGBT 5. Parasitic thyristor
180 consists of second emitter region 125 (thyristor cathode),
p-well 115 (npn transistor base), n-well 135 (pnp transistor base),
and collector region 130 (thyristor anode). Further, parasitic
thyristor 180 will not cease conduction until all of the charge at
collector electrode 155 is dissipated. Latching of parasitic
thyristor 180 varies based upon the pinched resistance of p-well
115. The pinched resistance of p-well 115 is a function of the
dimensions of second emitter region 125 and p-well 115, as
discussed with respect to FIG. 9.
[0051] Triggering the parasitic thyristor 180 in IGBT 5 runs
counter to the accepted and desired use of IGBTs. This is because,
as described above, parasitic thyristor 180 will not cease
conduction until the charge at collector electrode 155 is
dissipated. The result is that, once the parasitic thyristor 180 is
latched up, the IGBT 5 cannot be controlled by its bias circuitry
and cannot operate in its linear amplification or switching
region.
[0052] It should be noted that isolation region 110 is used to
reduce the surface electric fields (RESURF) between p-well 115 and
n-well 135. Further, by varying the depth of isolation region 110
the collector to emitter breakdown voltage, which is the forward
blocking voltage of IGBT 5, can be varied.
[0053] Referring to FIG. 5, a graph of the current-voltage
characteristics of a collector of the lateral insulated gate
bipolar transistor illustrated in FIG. 4 is illustrated. In FIG. 5,
as collector to emitter voltage 200 increases, it will snap-back
210 when parasitic thyristor 180 latches up. Also, as the voltage
at gate 30 is increased the snap-back voltage of the parasitic
thyristor 180 decreases, as shown by gate voltage levels 215, 220,
225, and 230.
[0054] Additionally, FIG. 5 illustrates the advantage of the use of
an IGBT for ESD protection by showing operation of parasitic
thyristor 180. Specifically, parasitic thyristor 180 latches up at
a voltage which is a sum of the clamp trigger voltage 235 and the
gate voltage of the IGBT. This can be altered by changing the
pinched resistance of p-well 115. Upon latching up, the parasitic
thyristor 180 begins conducting, thereby reducing the charge at pad
15 (FIG. 1) until all of the charge at pad 15 is dissipated. The
operation of parasitic thyristor 180 is shown by curve 240.
[0055] The collector to emitter breakdown voltage 245 is the
voltage at which IGBT 5 is not able function in a forward blocking
state. In ESD protection circuit 2, the breakdown voltage of
collector clamp 35 must be set to a level below the collector to
emitter breakdown voltage 245 minus the gate voltage of IGBT 5
required to trigger parasitic thyristor 280.
[0056] Referring to FIG. 6, a cross-sectional view of another
embodiment of a lateral insulated gate bipolar transistor 5 for
electrostatic discharge protection circuit 2 is illustrated. In
FIG. 6, a second collector region 300, which may be n.sup.+-type,
is added, so as to form a junction with the collector region 130.
The second collector region 300 acts as a short between collector
electrode 155 and n-well 135 during conduction by IGBT 5.
[0057] Further, another p-type well region 305 is used to improve
the injection of holes from the collector region 130 into the
second portion 310 of the n-well 135, which is separated from the
first portion 315 of the n-well 135. The first portion 315 and
second portion 310 are connected by a buried region 317, which is
n-type. The collector region 130 is p.sup.+-type.
[0058] Referring to FIG. 7, a cross-sectional view of a further
embodiment of a lateral insulated gate bipolar transistor for
electrostatic discharge protection is illustrated. In FIG. 7, a
metal contact 320 is then added to collector electrode 155. The
metal contact 320, which is a Schottky contact, acts as a short
between collector electrode 155 and n-well 135 during conduction by
IGBT 5. The short between collector electrode 155 and n-well 135
improves bi-directional ESD event dissipation and allows for
conduction by IGBT 5 at a lower voltage. Further, metal contact 320
improves the homogenous turn on of parasitic thyristor 180.
[0059] Alternatively, collector electrode 155 can itself be
completely or partially formed of a metallic material to form
either an ohmic or a Schottky contact to collector region 135.
[0060] An advantage of the lateral IGBTs in FIGS. 6 and 7 over that
of FIG. 4 is the reaction of the IGBT to negative ESD events, where
the charge at the pad 15 (FIG. 1) is negative with respect to
potential 25. Substrate 100 and n-well 135 form a diode between
substrate 100 and collector electrode 155 due to the short between
n-well 135 and collector electrode 155. The diode conducts current
induced by negative ESD events from pad 15 to substrate 100, and
allows IGBT 5 to dissipate voltages induced by negative ESD events.
The use of the structures of FIGS. 6 and 7 improves the response to
negative ESD events versus that of FIG. 4.
[0061] Referring to FIG. 8, a graph of the current-voltage
characteristics of a collector of a lateral insulated gate bipolar
transistor 5 of FIGS. 6 and 7 is illustrated. As the collector to
emitter voltage 400 increases, it will snap-back at 410 as
parasitic thyristor 180 latches-up, as described with respect to
FIG. 5. Also, as the voltage on gate 30 is increased, the latch-up
voltage of the parasitic thyristor 180 decreases, as shown by gate
voltage levels 415, 420 and 430 as described with respect to FIG.
5.
[0062] A feature of the IGBT structures of FIGS. 6 and 7 is
collector conduction prior to the turn on of pnp transistor 175, as
shown by early current flows 435.
[0063] Although exemplary doping characteristics are discussed with
respect to FIGS. 4, 6 and 7, other doping including those that
result in complimentary structures to those disclosed are possible
and can be used in the circuits of FIGS. 1 and 3. For example,
doping concentrations of 3.times.10.sup.16 for p-well 115 and
7.times.10.sup.16 for n-well 135 can be used.
[0064] Further, additional variations may be made to the IGBT
structures discussed with respect to FIGS. 4, 6, and 7. For
example, a p-well or p-body region, or complementary doping in a
complementary IGBT, that is self-alligned with gate electrode 140
may be included. Also, for low voltage applications, field oxide
145 can be replaced with thin oxide.
[0065] Referring to FIG. 9, an enlarged cross-sectional view of a
region in a well at the emitter of a lateral insulated gate bipolar
transistor from any of FIGS. 4, 6, and 7 is illustrated. In FIG. 9,
a region 500 and a well region 505 are illustrated. Region 500 has
a width 510 and length 515, while well region 505 has a depth 520.
The pinch resistance of well region 505 is then equal to: 1 R well
= well L WZ Equation 1
[0066] where .rho..sub.well is the resistivity of the material that
comprises well region 505, L is width 510, W is depth 520, and Z is
length 515. By increasing width 510 or depth 520, the resistance of
the well region 505 decreases, thereby increasing the latching
current of the parasitic thyristor 180. Also, if the length of
region 500 is decreased, then the resistance of well region 505
will also increase the latching current parasitic thyristor
180.
[0067] Referring to FIG. 10, a cross-sectional view of a lateral
insulated gate bipolar transistor with leakage current reduction is
illustrated. The IGBTs shown in FIGS. 4, 6, and 7 each have a small
leakage current on the surface of the pnp transistor 175. This
leakage current, which is inherent to the structure of an IGBT, can
cause erroneous latching of the parasitic thyristor 180 due to the
leakage current that occurs prior to triggering of collector clamp
35. Inserting a punch through reduction region 550 that forms a
butting junction with collector region 130 on the side of field
oxide 145 can substantially reduce or eliminate the leakage
current. This punch through reduction region 550 should be of a
complementary doping to collector region 130, which for example
would be an n-type punch through reduction region for a
p.sup.+-type collector region. The punch through reduction region
550 can be relatively small in width; in one embodiment, the width
is no more than two (2) microns. The use of a punch through
reduction region 505 reduces inaccurate and premature latching of
the parasitic thyristor 180.
[0068] Alternatively, to reduce the leakage current, a buffer
region can be added to the n-well 135. The buffer region can be
formed by heavily doping the portion of the n-well 135 abutting the
collector region 130.
[0069] Referring to FIG. 11, of a pad 15 with an electrostatic
discharge protection circuit fabricated upon it is illustrated. Pad
15 has a trace 700 that forms resistor 45 and diffusions 705 that
form collector clamp 35, which is made up of a number of diodes.
Diffusions 705 have a number of contacts 710 to pad 15.
[0070] Gate 140, which has rounded corners as depicted, overlies
n-well 135, p-well 115, and second emitter region 125. Each of
n-well 135, p-well 115, and second emitter region 125 have a
similar shape to gate 140. A number of contacts 720 provide bonding
to pad 15.
[0071] Collector region 130 is diffused along a periphery of pad
115 and also has a number or contacts 720.
[0072] By fabricating IGBT 5 on a pad, the ruggedness of IGBT 5 is
increased due to the charge distribution on the device. Further,
the response time of IGBT 5 improves by fabricating it on pad 15,
thereby reducing the potential for damage to integrated circuits
bonded to pad 15. In another embodiment, IGBT 5 is fabricated on at
least two sides of one of the surfaces of pad 15.
[0073] While FIG. 11 depicts IGBT 5 in a substantially oval
configuration, other configurations of IGBT can be used. For
example, race-track, or configurations having multiple fingers can
be used. Further, IGBT 5 can be fabricated on two or three sides of
a surface of pad 15.
[0074] Referring to FIG. 12 is a graph of the direct current
characteristics in response to an ESD event of the electrostatic
discharge protection circuit 2 fabricated on pad 15 (FIG. 11), when
the parasitic thyristor 180 latches. As can be seen, the amount of
time required to dissipate the large voltage at pad 15 is minimal.
Specifically, the triggering of parasitic thyristor 180 almost
instantaneously reduces the voltage to a low holding voltage.
Further, parasitic thyristor 180 snaps-back to the low holding
voltage without damage to electrostatic discharge protection
circuit 2 and IGBT 5.
[0075] It should be noted that while IGBT 5 is illustrated as a
lateral IGBT in FIGS. 4, 6, and 7, a vertical IGBT can also be
utilized based upon the principles and utilizing the same region
constituents as described herein. Further, it would be advantageous
to use a vertical IGBT in an integrated ESD protection circuit.
[0076] Embodiments of ESD protection circuits and transistors
capable of being used with the ESD circuits described herein are
also depicted and described in a copending U.S. patent application
entitled "Thick Gate Oxide Transistor And Electrostatic Discharge
Protection Utilizing Thick Gate Oxide Transistors," Ser. No.
10/336,202, which is assigned to the Assignee of the current
application, and was filed on the same date (i.e., Jan. 3, 2003) as
the present application.
[0077] The detailed description provided above is merely
illustrative, and is not intended to be limiting. While the
embodiments, applications and advantages of the present inventions
have been depicted and described, there are many more embodiments,
applications and advantages possible without deviating from the
spirit of the inventive concepts described and depicted herein. The
invention should only be restricted in accordance with the spirit
of the claims appended hereto and is not restricted by the
embodiments, specification or drawings.
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