Protection circuit for semiconductor device

Details Protection circuit for semiconductor device Protection circuit for semiconductor device

1999-11-23

2001-01-09

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Load shunting by fault responsive means

Reexamination Certificate

active

06172861

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a protection circuit for a semiconductor device such as a semiconductor integrated circuit (IC, LSI etc.), more particularly to a protection circuit provided in such a semiconductor device for protecting an internal circuit from high-surge voltages, such as those generated by static electricity, accidentally applied to a pad serving as a terminal for connection with an external circuit.
2. Description of the Related Art
Variously configured protection circuits are used to protect the internal circuitry of semiconductor devices from high-surge voltages, such as those generated by static electricity. One example is shown in FIG.
24
.
FIG. 24
is a circuit diagram showing an example of the input circuit of a semiconductor device equipped with an ordinary protection circuit
9
and an internal circuit
3
.
The protection circuit
9
is constituted of diodes
91
,
92
and a resistor
4
, while the internal circuit
3
is constituted of a P-channel metal-insulator-semiconductor (MIS) field effect transistor
1
and an N-channel MIS field effect transistor
2
.
“MIS field effect transistor” is a general term encompassing field effect transistors of the metalinsulator-semiconductor structure, including MOS field effect transistors. It is abbreviated as MISFET in this specification.
In the input circuit of this semiconductor device, a pad
10
is connected to the anode of the diode
91
and one terminal of the resistor
4
constituting the protection circuit
9
. The other terminal of the resistor
4
is connected to the cathode of the diode
92
and the gates of the P-channel MISFET
1
and the N-channel MISFET
2
constituting the internal circuit
3
.
A first power supply terminal
11
is connected to one terminal of the P-channel MISFET
1
and the cathode of the diode
91
, and a second power supply terminal
12
is connected to the other terminal of the N-channel MISFET
2
and the anode of the diode
92
.
The first power supply terminal
11
is supplied with a base voltage (VDD) and the second power supply terminal
12
is supplied with a negative supply voltage (VSS).
The other terminal of the P-channel MISFET
1
and the other terminal of the N-channel MISFET
2
are both connected to an output terminal
13
.
The protection circuit
9
is required to protect the internal circuit
3
from several KV to ten-plus KV of static electricity of either polarity that may be accidentally applied to the pad.
10
.
When positive static electricity applied to the pad
10
reaches the connection point between the anode of the diode
91
and the resistor
4
, the diode
91
turns on in the forward direction to pass current to the first power supply terminal
11
. The voltage at which the diode
91
begins to pass this current is called the threshold voltage. Since the positive voltage applied to the pad
10
is clamped at the forward threshold voltage of the diode
91
, no voltage higher than this forward threshold voltage is applied to the internal circuit
3
.
When negative static electricity applied to the pad
10
reaches the cathode of the diode
92
through the resistor
4
, the diode
92
turns on in the forward direction to pass current to the pad
10
through the resistor
4
. Since the negative voltage applied to the pad
10
is therefore clamped at the forward threshold voltage of the diode
92
, no voltage of an absolute value higher than this forward threshold voltage is applied to the internal circuit
3
.
Since the resistor
4
is connected in series between the pad
10
and the internal circuit
3
, it also serves to smooth sharply rising noise components produced by static electricity.
The shrinking dimensions of MISFETs in recent years has led to increasingly thin MISFET gate insulating films. Since a thinner MISFET gate insulating film exhibits lower breakdown strength, the importance of the protection circuit is greater than in the past.
The protection capability of the conventional protection circuit using two diodes as described in the foregoing is dependent on the area of the PN junctions of the diodes. In the semiconductor device shown in
FIG. 24
, for instance, the protection of the internal circuit
3
by the protection circuit
9
can be enhanced in terms of the breakdown strength of the MISFETs
1
and
2
constituting the internal circuit
3
by increasing the area of the PN junctions of the diodes
91
,
92
constituting the protection circuit
9
.
This is because increasing the area of the PN junctions of the diodes
91
,
92
enables the-diodes
91
,
92
to pass a greater amount of current per unit time and reduces the current passage per unit area of the PN junctions constituting the diodes
91
,
92
. As a result, the protection capability of the protection circuit increases.
Decreasing the amount of current passed per unit area of the PN junctions of the diodes
91
,
92
also suppresses generation of heat by the current passing through the PN junctions. Since this prevents thermal breakdown of the diodes
91
,
92
, it prevents breakdown of the protection circuit
9
itself.
However, an attempt to secure these advantages by increasing the area of the PN junctions of the diodes
91
,
92
leads to a major problem, namely, that it increases the area of the semiconductor device accounted for by the protection circuit
9
.
In order to protect the internal circuit
3
by clamping high voltages applied to the pad
10
owing to static electricity, the protection circuit
9
of
FIG. 24
requires a separate clamping element for voltage of each of the positive and negative polarities generated by static electricity and thus requires the two diodes
91
,
92
.
An attempt to improve the protection capability of the protection circuit by increasing the area of the PN junctions of the diodes serving as the clamping devices therefore greatly increases the area occupied by the protection circuit. Moreover, additional space is taken up by the power lines of opposite polarity required for enabling the two diodes to pass surge currents produced when high voltages occur.
This reduces the amount of space available in the vicinity of the pad
10
for provision of circuitry other than the protection circuit
9
and, in turn, increases the area of the semiconductor device as a whole. Since this way of increasing protection capability therefore runs counter to the desire to reduce semiconductor device area and lower cost, it is best avoided.
On the other hand, an attempt to prevent increase in the overall area of the semiconductor device by reducing the layout area of the protection circuit
9
thwarts securement of adequate area for the PN junctions of the diodes
91
,
92
constituting the protection circuit
9
and therefore degrades the protection of the internal circuit
3
from static electricity applied to the pad
10
. Since it also reduces the width of the power lines of the protection circuit
9
and thus lowers the current capacity thereof, it also increases the risk of the protection circuit
9
itself breaking down.
This has led to the use of the protection circuit
9
′ shown in
FIG. 25
, which is configured using the diode
91
as the only clamping element. When a high negative voltage is applied to the pad
10
, the voltage is clamped at the breakdown voltage of the diode
91
.
Since this configuration requires only a single clamping element and a power line of only one polarity for each pad, it enables the area of the PN junction of the diode serving as the clamping element to be increased and the durability of the power line to be enhanced by increasing its width.
However, in the case of the protection circuit
9
′, the clamp voltage at the time of application of positive voltage to the pad
10
is equal to the forward threshold voltage of the diode
91
, but the clamp voltage at the time of application of negative voltage to the pad
10
is equal to the breakdown voltage of the diode
91
(about 50 V) and is therefore fairly large. Another disadvantage of this arran

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