Protection circuit for integrated circuit devices

The protection circuit is a four layer PNPN device which includes a PMOS IGFET. The device is designed to pass current to ground when large transients are imposed across its two external terminals, thereby protecting the integrated circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to a protection circuit for integrated 
circuit devices. 
Integrated circuits are often damaged by voltage transients which overload 
one or more individual devices contained within the integrated circuit 
thereby melting or otherwise destroying the device. Heretofore, various 
devices and circuits have been employed for protective purposes on 
integrated circuit structures in order to prevent their destruction by 
such transients. In the past, diodes and transistor circuits have been 
used for internal transient protection. While such devices provided some 
measure of protection to the integrated circuits in which they were 
included, additional protection has been desired. 
SUMMARY OF THE INVENTION 
The present invention relates to a protection circuit which provides 
transient protection for an integrated circuit. The protection circuit 
comprises a silicon controlled rectifier (SCR) which is constructed as a 
two terminal device, preferably as a part of the integrated circuit which 
is to be protected. The protection circuit comprises a PNPN structure in 
which an insulating layer overlies the N type region which is intermediate 
to two P type regions. A conductive layer overlies the insulating layer 
and makes contact to the N type region at the end of the PNPN structure, 
thereby acting as the gate of the P channel MOS (PMOS) transistor while 
simultaneously acting as one of the two terminals of the protection 
circuit. Thus, if there is a transient which is negative with respect to 
the P type region at the end of the PNPN structure, the PMOS transistor 
will be turned on and the protection circuit will act like a diode through 
which the current can flow without harm to the protected circuit.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
Referring to FIG. 1, a cross-sectional view of the protection circuit 10, 
in accordance with the preferred embodiment of the present invention, is 
shown. The protection circuit 10 is comprised of a substrate 12, which is 
P type silicon material in the preferred embodiment of the invention. An 
N- epitaxial layer 14 forms a PN junction 16 with the P type substrate 12. 
A P type region 18 is formed within the N type epitaxial layer 14, forming 
a PN junction 20 with the layer 14. An N+ region 22 is formed within the P 
type region 18, and it forms a PN junction 24 with the P type region 18. 
A P+ region 32 extends from the surface of the device 10 to make ohmic 
contact to substrate 12. The P+ region 32 preferably surrounds the device 
10. A conductor 34 contacts the P+ region 32 adjacent layer 28, as shown 
in FIG. 1. 
An insulating layer 26 overlies the surface of the device 10. In the 
preferred embodiment of the invention, the insulating layer 26 is 
comprised of silicon dioxide. A conductive layer 28 overlies the 
insulating layer 26, overlying the area where the N- type region 14 is 
adjacent the surface of the device 10, and at least partially overlying 
the P+ region 32 and the P type region 18. The conductive layer also 
extends through an aperture 30 in the insulating layer 26 to make contact 
to the N+ region 22. The conductive layer 28 and the conductor 34 are 
typically comprised of aluminum, but they may be comprised of any other 
suitable material, such as a trimetal system. 
Referring now to FIG. 2, a schematic representation 100 of the protection 
circuit 10 of FIG. 1, is shown. In the schematic representation 100, the 
protection circuit comprises a PNP transistor Q1, an NPN transistor Q2, a 
P channel insulated gate field effect transistor (IGFET) Q3, and a pair of 
capacitors C1, C2. Transistor Q1 models the P, N-, P regions 32, 14, 18 of 
FIG. 1. Accordingly, the emitter, base, and collector of transistor Q1 are 
referred to using reference numerals 132, 114 and 118, respectively, in 
the schematic representation 100. Similarly, the transistor Q2 represents 
the N-, P, and N+ layers 14, 18, 22, respectively, of FIG. 1. Accordingly, 
the collector, base, and emitter of transistor Q2 are represented by the 
reference numerals 114 (which is also the base of transistor Q1), 118 
(which is also the collector of transistor Q1), and 122, respectively. 
Similarly, the IGFET Q3 includes a drain 118, a source 132, and a gate 128 
which is also a terminal of the protection circuit 100. The capacitors C1 
and C2 model the junction capacitance of the PN junctions 20 and 24 of the 
structure shown in FIG. 1. The two terminals 128, 134 of the schematic 
representation 100 correspond to the two metal interconnects 28, 34, 
respectively. 
The protection circuit is similar in operation to a silicon controlled 
rectifier (SCR) except that it is constructed as a two terminal device 
which includes a P channel IGFET. Also, the protection circuit is designed 
to be triggered by either a high voltage across the two terminals 128, 134 
or by a high rate of change of voltage (dv/dt) across the two terminals 
128, 134. Accordingly, the protection circuit differs from a conventional 
SCR in that a conventional SCR is a three terminal device which is 
designed to avoid triggering based upon either the voltage between its 
anode and cathode or upon the rate of change of voltage between its anode 
and cathode. 
In practice, the conductor 34 (terminal 134) is connected to ground 
potential, whereas the conductor 28 (terminal 128) is connected across the 
circuitry which is designed to be protected. Accordingly, if terminal 128 
goes negative with respect to ground at a high rate, the protection 
circuit will be turned on (terminals 128 and 134 will be electrically 
connected together via a low resistance path as provided when terminals 
128 and 134 are in close proximity to each other, as shown in FIG. 1) 
insuring excess current is passed to ground. Unlike the present protection 
device, a conventional SCR would have a low value resistor across 
capacitor C2 which would prevent such firing. In the event that there is a 
slow change of the voltage on terminal 128, a very small current, on the 
order of nanoamps, will flow through transistor Q2 without causing the 
circuit to latch, because the total loop again is selected to be less than 
1. When the voltage on terminal 128 is negative enough, IGFET Q3 will turn 
on causing transistor Q2 to turn on thereby providing sufficient loop gain 
to insure that the total loop gain is greater than 1. Again, the 
protection circuit will pass excess current to ground. 
In order to manufacture the device of the present invention, one starts 
with a semiconductor substrate, preferably of P type (100) silicon having 
a resistivity of about 10 to 30 ohm-cm. An N type epitaxial layer having a 
resistivity of about 1000 ohms/square is then grown to a thickness of 
between about 10 and 12 microns. Next, a layer of photoresist is applied 
over the surface of the device. 
The photoresist is defined using a photomask and developed to form openings 
through which a suitable P type dopant, such as boron nitride, is 
deposited and diffused to form the P+ isolation regions 32. The P+ 
isolation regions 32 have a surface conductivity of about 5 ohms/square, 
and they contact the substrate 12 after diffusion. Next, a new photoresist 
layer is applied and defined using a second photomask to form an opening 
where the P type region 18 will be formed. A suitable acceptor impurity is 
deposited (either directly or by ion implantation), and it is diffused to 
form the P type region 18 to a depth of approximately 2.1 to 2.2 
micrometers. The P type region 18 will preferably have a surface 
resistivity of about 200 ohms/square. 
In a similar manner the N+ region 22 is formed using a third photomask and 
photolithographic step. Donor impurities are deposited and diffused to 
form the region 22 with a surface resistivity of approximately 2-5 
ohms/square. 
Next, the oxide layer 26 is grown and openings are defined and formed 
therein using another photolithographic step. 
Finally, a conductive layer 28 such as an aluminum layer, is applied to the 
surface of the device. The conductive layer 28 is defined using a fourth 
photolithographic step, thereby completing the formation of the device 10.