Bi-directional ESD diode structure

Details Bi-directional ESD diode structure Bi-directional ESD diode structure

1999-12-17

2002-12-31

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Load shunting by fault responsive means

C361S091100

Reexamination Certificate

active

06501630

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a device used for protecting circuits against electrostatic discharge (ESD), and more particularly, to an ESD diode having a reduced capacitance.
2. Discussion of the Prior Art
FIG. 1
shows conventional-circuit arrangement
100
which is protected against electrostatic discharge (ESD). The device being protected is a circuit
110
, which may be formed on a semiconductor substrate of an integrated circuit or chip. The input
115
and output
120
of the circuit
110
are connected to input and output pads
125
,
130
, respectively, which in turn are connected to pins of the integrated circuit or chip.
Typically, the input
115
and output
120
of the circuit
110
, are protected against an electrostatic discharge (ESD) using diodes D
1
, D
2
, D
3
and D
4
connected between the input/output pads
125
,
130
and power lines. The power lines include a ground bus
135
and a power supply bus
140
, which is connected to a voltage source for providing a positive voltage, referred to as Vcc.
As is well known in the art, each of the diodes D
1
, D
2
, D
3
and D
4
is formed by a P-N-junction, and may be integrated on the same chip or integrated circuit that includes the circuit
100
to be protected. For protection against positive ESD, the diodes D
1
, D
2
have their anodes (P-side) connected to the input
115
and output
120
of the circuit
110
. The cathodes (N-side) of the diodes D
1
, D
2
are connected to the power supply bus
140
having the positive voltage Vcc. For protection against negative ESD, the diodes D
3
, D
4
have their cathodes (N-side) connected to the input
115
and output
120
of the circuit
110
. The anodes (P-side) of the diodes D
3
, D
4
are connected to the ground bus
135
.
FIG. 1
shows the anode (P-side) of the diode D
1
as numeral
145
and the cathode (N-side) as numeral
150
. It is understood by those skilled in the art that the discussion of the diode D
1
is for illustration purposes, and is equally applicable to all the diodes D
1
-D
4
.
As is well known in the art, each diode (e.g., diode D
1
) blocks current when reverse biased, which is when the cathode (N-side)
150
is made positive with respect to the anode (P-side)
145
until the cathode voltage is high enough to cause breakdown. In the reverse bias mode of operation, the current from the cathode
150
to the anode
145
is very low and is called leakage current.
When the anode
145
or P-side is made positive with respect to the cathode
150
or N-side, the mode of operation is referred to as forward bias. Further, the voltage across the diode D
1
is referred to as the forward bias voltage, which is the voltage from the anode
145
to the cathode
150
. If the forward bias voltage is increased across the diode D
1
, the current from the anode
145
to the cathode
150
, referred to as the anode current, increases exponentially with the voltage as shown by the plot
410
shown in FIG.
4
. For a typical silicon diode, the effect of this rise in current switches the diode D
1
into an ON state at a threshold or TURN-ON voltage V
T
of approximately 0.7 volts. Above this TURN-ON voltage V
T
, i.e., in the ON state, the voltage increases gradually while the current increases significantly. Note that under high current conditions, e.g. during an ESD event, the voltage across the diode can rise to several volts, due to the internal resistance of the diode.
As shown in the plot
410
of
FIG. 4
, the diodes D
1
-D
4
provide an open circuit in the reverse direction or block current flow from the cathode
150
to the anode
145
. When the voltage on anode
145
is greater than the voltage on the cathode
150
by the TURN-ON voltage V
T
, the diode D
1
turns on in the forward direction, and provides a relatively low resistance path for current flow from the anode
145
or P-side to the cathode
150
or N-side.
ESD events can occur with either polarity between any pair of pins on an integrated circuit. ESD protection must therefore be provided from each input/output pin to both the power supply bus
140
and the ground bus
135
and to all other input/output pins. In addition, ESD protection is required for both positive and negative polarities between the power supply bus
140
and the ground bus
135
. For a positive ESD to the input/output pads, e.g., the input pad
125
, with respect to the ground bus
135
, the ESD current passes through diode D
1
to the power supply bus
140
. Next, this ESD current passes to the ground bus
135
though a clamp structure
155
which is located between the power supply bus
140
and the ground bus
135
. For a negative ESD to the input pad
125
with respect to the ground bus, the ESD current passes through diode D
3
to the ground bus
135
.
For positive ESD to the input/output pads
125
,
130
with respect to the power supply bus
140
, the ESD current passes through diodes D
1
, D
2
to the power supply bus
140
. For a negative ESD to the input/output pads
125
,
130
with respect to the power supply bus
140
, which is the same as a positive ESD from the power supply bus
140
with respect to the input/output pads
125
,
130
, the ESD current passes through the clamp structure
155
and through the diodes D
3
, D
4
to the pad.
For a positive ESD between input/output pads
125
and
130
, the ESD current passes through diode D
1
to the power supply bus
140
, through the power supply clamp
155
to the ground bus
135
and through the diode D
4
. For a negative ESD between input/output pads
125
and
130
, the ESD current passes through diode D
2
to the power supply bus
140
, through the power supply clamp
155
to the ground bus
135
and through the diode D
3
.
For positive ESD between the power supply bus
140
and the ground bus
135
, the ESD current passes through the power supply clamp
155
. For negative ESD between the power supply bus
140
and the ground bus
135
, the ESD current passes through one or more of the series diode strings D
1
and D
3
or D
2
and D
4
.
The conventional ESD protected circuit arrangement
100
provides an effective protection scheme for many situations. However, when reverse biased, each diode D
1
, D
2
, D
3
and D
4
provides a capacitive load on input/output signals which can significantly degrade the input and output signals and performance of the circuit
110
, particularly at high frequencies. Thus, the major disadvantage of the conventional diode D
1
occurs during normal (non-ESD) operation. In this case, the diodes D
1
, D
2
connected between the input or output pins or pads
125
,
135
and the power supply line
140
, as well as the diodes D
3
, D
4
connected between ground
135
and the pads
125
,
135
, are reverse biased where an analogue input signal is biased between the power supply and ground.
Each of the diodes D
1
-D
4
has a capacitance that is associated with the diode P-N junction where the capacitance depends on the area and the doping configuration. Part of the high frequency input signal is diverted through the diodes to circuitry other than the intended path. Reducing the size or area of the diodes reduces the capacitance, but it also reduces the level of ESD protection, since the level of ESD protection depends on the current density in the diode.
Accordingly, there is a need to reduce capacitive loads connected to input/output lines while maintaining a desired level of ESD protection.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an electrostatic discharge (ESD) protection device that significantly reduces the problems of conventional ESD protection devices.
The present invention accomplishes the above and other objects by providing an ESD protection device, referred to as a bi-directional ESD diode for example, which protects a circuit against electrostatic discharge and allows proper operation, particularly at high frequencies. The bi-directional ESD diode has first and second devices, referred to as ESD diodes, which are connected in an a

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