Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-10-10
2003-09-16
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S173000, C257S162000
Reexamination Certificate
active
06621126
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to the field of electrostatic discharge (ESD) protection circuitry and, more specifically, improvements for silicon controlled rectifier (SCR) structures in the protection circuitry of an integrated circuit (IC).
BACKGROUND OF THE INVENTION
Integrated circuits (IC's) and other semiconductor devices are extremely sensitive to the high voltages that may be generated by contact with an electrostatic discharge (ESD) event. As such, ESD protection circuitry is essential for integrated circuits. An ESD event commonly results from the discharge of a high voltage potential (typically, several kilovolts) and leads to pulses of high current (a few amperes) of a short duration (typically, 100 nanoseconds). An ESD event is generated within an IC, illustratively, by human contact with the leads of the IC or by electrically charged machinery being discharged in other leads of an IC. During installation of integrated circuits into products, these electrostatic discharges may destroy the IC's and thus require expensive repairs on the products, which could have been avoided by providing a mechanism for dissipation of the electrostatic discharge to which the IC may have been subjected.
FIG. 1
depicts a schematic diagram of a prior art multi-fingered SCR ESD protection device
101
, which serves as protection circuitry for an integrated circuit (IC)
100
. As the distances between the pads (i.e., pad pitches) become smaller the ESD protection circuitry has been provided with multiple SCR fingers. An illustrative prior art integrated circuit
100
includes a SCR protection circuit
101
having multiple SCR fingers, and is illustratively depicted in
FIG. 1
having two SCR “fingers”
102
1
and
102
2
. Generally, prior to an ESD event, the SCR fingers are in a nonconductive state. Once the high voltage of an ESD event is encountered, the SCR fingers then change to a conductive state to shunt the current to ground. Each of the SCR fingers maintains this conductive state until the voltage is discharged to a safe level.
In particular, the SCR protection circuit
101
is connected from a pad
132
to ground
124
. The pad
132
is also connected to the protected circuitry of the IC, optionally through a current limiting resistor R
L
(not shown). The SCR protection circuit
101
comprises a trigger device
105
(discussed further below), a first SCR
102
1
(i.e., “first finger”), and a second SCR
102
2
(i.e., “second finger”). The first SCR
102
1
further comprises a NPN transistor QN
1
131
1
and a PNP transistor QP
1
132
1
. In particular, the SCR protection device
101
includes an anode
122
, which is connected to the pad
132
and to one side of a resistor R
n1
142
. The resistor R
n1
142
represents the resistance of the N-well, which is seen at the base of the PNP transistor QP
1
132
1
of the SCR
102
1
, which is discussed in further detail below. Additionally, the anode
122
is coupled to an emitter
108
1
of the PNP transistor QP
1
132
1
, which is parallel to the N-well resistance R
n1
142
1
.
A first node
134
1
includes the base of the PNP transistor QP
1
132
1
, the other side of the resistor R
n1
142
1
, and the collector
104
1
of the NPN transistor QN
1
131
1
. A second node
136
1
includes the collector
106
1
of the PNP transistor QP
1
132
1
, the base of the NPN transistor QN
1
131
1
, and connects to one side of a resistor R
p1
141
1
. The resistor R
p1
141
represents the resistance of the P-well, which is seen at the base of the NPN transistor QN
1
of the SCR
102
1
and is discussed in further detail below. The other side of resistor R
p1
141
1
is connected to a third node
124
, which is grounded and serves as the cathode of the SCR
102
1
. Furthermore, the emitter
112
1
of the NPN transistor QN
1
131
1
is also connected to the grounded third node
124
.
A second SCR
102
2
is formed exactly in the same manner as described with regard to the first SCR
102
1
. In particular, an emitter
108
2
of a second PNP transistor QP
2
132
2
is coupled to the anode
122
, which is common to all of the multi-finger SCR's
102
and the pad
132
. Furthermore, an emitter
112
2
of a second NPN transistor QN
2
131
2
is coupled to the cathode
124
, which is common to all of the multi-finger SCR's
102
and ground. In addition, the two fingers
102
1
and
102
2
of the multi-finger SCR protection circuit
101
are coupled together by a common P-substrate and shared N-well regions therein. That is, the bases of the first and second NPN transistors QN
1
131
1
and QN
2
131
2
are coupled via a P-well coupling resistance R
pc
103
p
. Similarly, the bases of the first and second PNP transistors QP
1
132
1
and QP
2
132
2
are coupled via a N-well coupling resistance R
nc
103
n
. The coupling resistances R
pc
and R
nc
typically have high resistance values in the range of 100 to 2000 Ohms.
A single triggering device providing a positive trigger current to the trigger gate
105
has been used to turn on all of the SCR fingers
102
. Alternatively, a single trigger device providing a negative trigger current to the trigger gate
107
may be used. It has been observed however, that providing the trigger current to the trigger gate
105
(or
107
) has not been sufficient to trigger all of the SCR fingers
102
as is discussed below.
In operation, each protective multi-finger SCR circuit
102
, which illustratively comprise the NPN and PNP transistors QN
1
131
1
and QP
1
132
1
, will not conduct current between the anode
122
and the grounded cathode
124
. That is, the SCR fingers
102
are turned off, since there is no high voltage (e.g., ESD voltage) applied to the SCR
102
, but only the regular signal voltage of the functional parts of the IC. Once an ESD event occurs at the pad
132
, a high voltage potential appears on the anode
122
. A triggering device senses the high voltage potential and provides a trigger current to the trigger gate
105
and causes the base potential of the NPN transistor QN
1
131
1
to rise, which subsequently turns on the NPN transistor QN
1
131
1
. Furthermore, the collector of the NPN transistor QN
1
131
1
is coupled to the base of the PNP transistor QP
1
132
1
, which turns on the PNP transistor QP
1
132
1
.
As such, once the NPN transistor QN
1
131
1
is turned on, the collector
104
1
provides the base current to the PNP transistor QP
1
132
1
. Therefore, the base current of the PNP transistor QN
2
132
1
is greater than the base current of the NPN transistor QN
1
131
1
. Moreover, the current gain of the PNP transistor QP
1
132
1
is realized as the QP
1
132
1
collector current, which is then fed back to the base of the NPN transistor QN
1
131
1
, thereby amplifying the base current of the NPN transistor QN
1
131
1
. Amplification of the base currents in the SCR
102
progressively continues to increase in a feedback loop between both transistors QN
1
131
1
and QP
1
132
1
. Therefore, the conduction in a turned on SCR is also called a “regenerative process”.
The SCR
102
1
becomes highly conductive and sustains (i.e., holds) the current flow with a very small voltage drop (i.e., holding voltage) between the anode and cathode (typically, 1-2 V). Accordingly, once the SCR
102
1
is turned on, the current from the ESD event passes from anode
122
to the grounded cathode
124
. Once the ESD event has been discharged from the anode
122
to the cathode
124
, the SCR
102
turns off because it cannot sustain its regenerative conduction mode.
There is usually a large voltage difference between the triggering point and holding point. One problem that has been observed is that the multiple SCR fingers
102
do not always trigger. That is, even though the first SCR finger
102
1
may trigger, the other SCR fingers (e.g., SCR
102
2
) may not trigger because almost the entire triggering voltage quickly collapses, which fails to enable the other SCR fingers (e.g., SCR
102
2
) to reach their trigger voltages. Also the coupling through the relatively high-
Burke William J.
Jackson Jerome
Sarnoff Corporation
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