Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – Having only two terminals and no control electrode – e.g.,...
Reexamination Certificate
2001-02-16
2004-03-02
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
Having only two terminals and no control electrode , e.g.,...
C257S132000
Reexamination Certificate
active
06700140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a communications switch and more particularly to a thyristor switch for microwave signals.
2. Description of Related Art
(a) Thyristors
The name “thyristor” applies to a general family of semiconductor devices that exhibit bistable characteristics and that can be switched between a high-impedance, low-current OFF state and a low-impedance high-current ON state. Thyristors are well-known in the art. (See, for example, “Physics of Semiconductor Devices”, S. M. Sze, Wiley (1981); “Semiconductor Power Devices”, S. Ghandhi, Wiley (1977).) Operationally, thyristors are analogous to bipolar transistors, in which both electrons and holes are involved in the transport process. The thyristor is a solid state semiconductor device usually made up of four layers with dopant sequence p-n-p-n, or to be more specific, p
+
-n
−
-p-n
+
, where the semiconductor material can be either Si (silicon) or GaAs (gallium arsenide) although most commercially made thyristors are constructed out of Si.
FIG. 1
shows a schematic of a representative two-terminal thyristor that is sometimes called a “Shockley Diode.” For silicon devices, the typical doping of the four layers between an anode
2
and a cathode
4
is as follows: p
+
(10
19
cm
−3
), n
−
(10
14
cm
−3
), p (10
16
cm
−3
) and n
+
(10
19
cm
−3
). This doping profile can be made by diffusion or by using epitaxial layers of the desired doping.
Another two terminal thyristor design used in the industry is a p
+
-p-n
−
-p-n
+
structure as shown by the thyristor
5
in
FIG. 2
where the doping profile
9
is also illustrated. This thyristor
5
consists of deep p type diffusions made simultaneously into either side of a slice of high resistivity n− type silicon, with an alloyed or diffused n
+
type region on one end to form the cathode
8
. An aluminum layer is usually alloyed to the other end of the device to form a p
+
type anode
6
. Typically, thyristors are made from silicon and can be used for large power devices (e.g., 10 cm×10 cm). However, it is also possible to fabricate a thyristor out of GaAs using epitaxial layers as shown in FIG.
3
.
In
FIG. 3
, the p
+
, n
−
, p and n
+
semiconductor layers of the thyristor
10
are shown in a mesa-like structure with sloped walls disposed on a substrate
10
. A metallic ohmic contact
12
to the p
+
region serves as the anode. A metal air bridge
14
forms an ohmic contact to the n
+
region and to a metallic ohmic contact
16
that serves as the cathode. The metal air bridge
14
can be fabricated by depositing photoresist, opening a via in the photoresist atop the n
+
region, depositing metal through a mask, and dissolving the photoresist to leave the air bridge
14
as shown in FIG.
3
. Alternatively, an air bridge design may include a dielectric material used for structural support.
A thyristor (e.g.,
FIGS. 1-3
) has hysteresis or memory and is characterized by a high-resistance OFF state and a low-resistance ON state.
FIG. 4
shows Va
18
as an operating voltage in the OFF state and Vc
22
as an operating voltage of the ON state. Transitions between the ON state and the OFF state are characterized by a break over voltage Vb
20
and a holding voltage Vh
26
as described in the following sequence.
The OFF state resistance is relatively high, and so the operating voltage Va
18
is essentially the applied voltage across the thyristor; that is, the resistance of the load has little effect. In the OFF state the current (I) is minimal.
When the thyristor is in the OFF state, a Turn-ON pulse voltage greater than the break over voltage Vb
20
causes the thyristor to transition to the ON state at the operating voltage Vc
22
.
The operating voltage Vc
22
in the low-resistance ON state is less than the operating voltage Va
18
in the high-resistance OFF state, as characterized by a load line
24
that connects these operating points. The slope of the load line
24
is determined by the resistance of the load.
When the thyristor is in the ON state, A Turn-OFF pulse voltage less than the holding voltage Vh
26
causes the thyristor to transition to the OFF state at the operating voltage Va
18
.
Repeat, etc
When the thyristor is in the OFF state, there is no transition when a pulse causes the voltage to decrease (e.g., below the holding voltage Vh); instead, the current continues to decrease along the continuous curve shown in FIG.
4
. Similarly, when the thyristor is in the ON state, there is no transition when a pulse causes the voltage to increase; instead the current continues to increase along the continuous curve shown in FIG.
4
.
Pulse circuits are typically used for operating the thyristor. Examples of a Turn-ON pulse
30
and a Turn-OFF pulse
32
are presented in
FIG. 5
with reference to the thyristor I-V curve shown in FIG.
4
. In the initial OFF state, the operating voltage is Va before the ON pulse
30
is applied. Because the amplitude Vg of the ON pulse
30
is greater than the break over voltage Vb, the thyristor switches from OFF to ON and the operating voltage drops to Vc. Similarly, in the initial ON state, the operating voltage is Vc before the OFF pulse
32
is applied. Because the amplitude (zero volts) of the OFF pulse
32
is less than the holding voltage Vh, the ON state collapses and the OFF state is obtained with the operating voltage Va.
The lightly doped n
−
region shown in
FIGS. 1-3
is critical to the operation of the thyristor. The thickness (sometimes called width) and the doping level of this n
−
region both affect the voltage required to obtain reach through of the n
−
region and therefore the magnitude of the break over voltage Vb.
Typically the application of thyristors has been mostly limited to applications such as power systems with relatively low frequencies (e.g., 60 Hz power control). Thyristors generally have not been used in applications involving higher frequencies including the range of microwaves (e.g., roughly 300 MHz-300 GHz).
(b) Telecommunications Switch Arrays
FIG. 6
illustrates a permutation switch element for use in the telecommunications industry. At each node there is the possibility of a connection between the input rows and the ouput columns. For example, Input r
2
is connected to output s
3
as shown in the diagram. There are N! different configurations possible in a permutation switch of dimension N (e.g., N=6 in FIG.
6
). The important case where there are N inputs and N outputs is called an N×N switch or an N×N switch array, where an array may be made from a combination of switch elements.
A typical wavelength switch element used in the telecommunications industry is called an optical crossconnect switch (OXC). The OXC uses mirrors that can move a light spot from one location to another. The OXC is a permutation switch; that is, any one input is connected to only one output and vice versa. The net result is that the light intensity is retained during its passage through the switch and not diluted by a multiplicity of connecting paths.
A major disadvantage of the OXC is that it is not possible to vary the wavelength between input and output. That is, the wavelength of input r
2
and output s
3
must be the same. Many optical networks require the additional flexibility of assigning to the output s
3
a wavelength different from that of the input r
2
. This can be done in the network by adding much more complex and costly extra equipment that effectively adds considerable cost to the OXC.
In
FIG. 6
, the array size is drawn for N=6. However, the array size for a crossconnect application should be appreciably larger, perhaps large enough to accommodate ~50 fibers in each cable and ~20 wavelengths in each fiber. A typical crossconnect switch can therefore have N ~1,000 to best optimize the performance of the communication network.
It is possible to use tiling t
Freske Stanley
Holden Thomas
LaRue Ross
Levine Jules D.
Katten Muchin Zavis & Rosenman
Loke Steven
Teraburst Networks Inc.
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