Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – Device protection
Reexamination Certificate
2002-09-05
2004-02-24
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
Device protection
C257S163000, C257S167000, C257S174000, C257S177000, C257S182000
Reexamination Certificate
active
06696709
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to thyristors and other four-layered devices, and more particularly to the fabrication of thyristor devices having low breakover voltages.
BACKGROUND OF THE INVENTION
Thyristors, SIDACtor® overvoltage devices and other four-layer devices are commonly used to provide overvoltage protection to circuits requiring the same. The SIDACtor® overvoltage devices are two-terminal thyristors that have bidirectional current carrying capability. Such devices are obtainable from Teccor Electronics at many different breakover voltage values. When utilized in conjunction with telephone lines, for example, of the type in which 220 volt ringing signals are carried, a 250 volt breakover voltage SIDACtor® overvoltage device can be utilized to allow normal operation of the telephone line, but operate at 250 volts, or greater, in response to lightning strikes or power line crosses to thereby safely clamp the line to a very low voltage. This type of a device provides high surge current capabilities for protecting equipment from damage due to the extraneous voltages that may be coupled to the telephone line. Five-pin telephone line protection modules utilizing these high voltage devices have typically been commercially available.
Many telephone circuits and equipment operate on a −48 volt supply voltage. To that end, SIDACtor® overvoltage devices that operate at a nominal 64 volts are often utilized to protect such type of circuits. A nominally operating 30 volt SIDACtor® device can be advantageously utilized to protect many 24 volt circuits, such as fire alarm and other systems that are susceptible to extraneous voltages. It can be appreciated that the lines that generally require protection from damage due to extraneous voltages are often in environments where energy from lightning strikes can be induced into the lines, where high voltage AC circuits are in close proximity thereto, and for a host of other reasons.
While low-voltage digital lines, such as those driven by 5-volt TTL drivers are extensively employed in computerized and other equipment, such lines have not yet found a large application in outside installations. However, in view of the fact that computer networks and communications are increasing at a substantial rate, such low-voltage lines are being used in environments where overvoltage protection is required. Such overvoltage protection need not be due solely to lightning and power line crosses, but can be due to other standard voltages that are commonly found in indoor equipment.
It is well known in the thyristor and SIDACtor® overvoltage device field that the impurity level of a semiconductor wafer can be adjusted to thereby achieve a desired breakover voltage. It is commonly known that lightly-doped silicon substrates are characterized by high breakover voltages. As the doping or impurity level of the substrate is increased, the breakover voltage is reduced. It is also well known that the impurity level of a semiconductor material is inversely proportional to the resistivity thereof.
It has also been found that the use of buried regions in the semiconductor substrate facilitates the operational characteristics of a SIDACtor® overvoltage device. See, for example, U.S. Pat. No. 5,479,031 by Webb, et al. Referring to
FIG. 1
, if the SIDACtor® device is constructed so as to have a P-type emitter
18
, an N-type base
16
and a P-type substrate
12
or mid-region, a heavily doped P-type buried region
14
can be implanted between the base region
16
and the silicon substrate
12
to thereby reduce the breakover voltage. Important advantages are achieved when the buried region
14
is directly beneath the emitter region
18
, with the base region
16
material therebetween. Without significantly changing the impurity levels of the emitter
18
, base
16
and substrate
12
, the breakover voltage can be changed by simply changing the impurity level of the buried region
14
. Moreover, in achieving breakover voltages from 250 volts down to 64 volts, the buried region need only be more heavily doped. In like manner, to achieve 30-volt breakover voltage devices, the buried region is required to be even more heavily doped.
As the impurity level of the buried region
14
increases, the junctions
20
-
26
formed between the buried region
14
and the base region
16
are displaced upwardly toward the emitter region
18
. Indeed, as the doping level of the buried region
14
increases, the distance between the buried region-base junction
20
and the base-emitter junction becomes smaller and smaller. The reason for this is that the junction
20
is formed at a location in the semiconductor material where the donor states of one impurity are cancelled by the acceptor states of the opposite impurity. Stated another way, the junction of two semiconductor materials exists where the impurity concentration of one region is equal to the impurity concentration of the other region. The formation of a low breakover voltage SIDACtor® overvoltage device is not an elementary task.
It has been found that to fabricate nominal 10-volt breakover voltage SIDACtor® devices, the impurity level of the buried region must be so high that the buried region can often be effectively short circuited to the emitter region. In any event, even after fine tuning the processes so as to prevent short circuiting between the buried region and the emitter, the yield of workable devices is low, and thus such devices become costly.
Another problem attendant with upward migration of the junction of the buried region is that the base region under the emitter becomes thinner. The distance in the base region between the emitter junction and the buried region junction defines, in part, a holding current (I
h
) parameter. The holding current is that current required to maintain an on-state of the device. A thinner base region adversely affects the ability to control a desired holding current.
Various other attempts have been made to make low breakover voltage thyristors. One endeavor involves a semiconductor design in which the breakover voltage occurs at the surface of the device. In other words, the concentration of the impurities at the surface of the device is controlled to achieve a low breakdown voltage.
Standard twisted pair telephone lines are protected by various circuits from hazardous voltages and currents that may be imposed on the lines. It is a standard practice to provide primary protection by the use of five-pin protection modules in the central offices and other high density conductor applications. Such modules have a standard pin configuration so that the modules of many different suppliers can be inserted into the same type of socket.
The basic protection to telephone lines includes primary protection modules and secondary protection modules. The primary protection module provides overvoltage protection against lightning strikes and power line crosses to the telephone lines. Such primary protectors may include gas discharge tubes and other semiconductor devices that can withstand high voltages. Secondary protection circuits often include semiconductor devices, resistors, positive temperature coefficient devices and other components to provide lower voltage protection to the customer side equipment. A family of overvoltage protection SIDACtor® devices providing the secondary protection is available from Teccor Electronics, Irving, Tex. The primary protection module is effective to limit the hazardous line voltages to approximately 300 volts. The secondary protection circuits, for example in line cards, provide additional protection to the telephone lines at levels below 300 volts.
While numerous five-pin primary protection modules are commercially available to provide primary protection, there is a limited selection of five-pin secondary protection modules providing secondary protection.
Recent changes in regulatory requirements suggest the use of DC isolation as well as overvoltage protection in secondary protection circuits of certain
Casey Kelly C.
Pelegris Dimitris Jim
Turner, Jr. Elmer L.
Chauza & Handley LLP
Chauza, Esq. Roger N.
Ngo Ngan V.
Teccor Electronics, LP
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