Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-06-21
2001-06-26
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140LA
Reexamination Certificate
active
06252221
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to photo-electronic devices, and more particularly to a high performance photo-conductive switch that can be fabricated at a low cost.
BACKGROUND OF THE INVENTION
Microwave switches are being expected to perform at higher frequencies, with an improved ON to OFF ratio, a reduced insertion loss and increased isolation. A photo-conductive switch (PCS) uses light to control its electronic conductivity and therefore to modulate electronic signals passing through it. A photo-conductive switch has less stray electronic impedance than an equivalent electronically-controlled switch such as a transistor. Consequently, photo-conductive switches are potentially better suited for use as high-frequency and high-performance microwave switches.
U.S. Pat. No. 3,917,943 to Auston discloses a first type of PCS that is driven by an ultra-short optical pulse and is fabricated on a semiconductor substrate. Two gold micro-strip transmission lines separated by a narrow gap are located on the surface of a light-absorbing insulating semiconductor substrate. A first optical pulse directed to the substrate through the gap turns the PCS ON by generating copious electric charges on the substrate surface in the gap. A second optical pulse that begins during the first optical pulse and is directed to the gap generates copious electric charges in the bulk of the substrate extending down to the ground plane. This shorts the micro-strip transmission lines to ground, and switches the PCS OFF. The substrate is grown at a low temperature or is ion implanted to shorten the carrier lifetime to provide a very fast response. However, this also reduces the carrier mobility, which causes the PCS to have a high insertion loss.
U.S. Pat. No. 4,755,663 to Derkits, Jr. indicates that a disadvantage of the Auston PCS is that the electrical impulse created by the optical pulse is dominated by carrier recombination, rather than carrier transport. Derkits discloses a PCS in which the portion of the substrate constituting the gap includes a region composed of a textured-surface, graded-composition photosensitive semiconductor material. Illuminating the gap with a beam of light of sufficient intensity to generate charge carriers at the surface of the photosensitive semiconductor material causes the PCS to conduct.
FIG. 1
shows an embodiment of the PCS 1 disclosed by Derkits, Jr. In this, the semi-insulating semiconductor substrate 10, preferably of silicon, has the ground plane electrode 11 or an ohmic contact located on its major surface 21. On the opposite major surface 22 of the substrate is located the layer 18 of a wide band-gap energy semiconductor material. Overlaying the layer 18 is the layer 19 of a graded-composition alloy semiconductor material. Located on surface of the layer 19 and extending over part of the major surface 22 of the substrate are the electrodes 14 and 15 separated by the gap 13. The electrodes form the ohmic contact regions 17 with the layer 19.
The material of the layer 19 is an alloy of two semiconductor materials W and N. Semiconductor material W has a wide band-gap energy and semiconductor material N has a narrow band-gap energy. The fraction of the narrow band-gap energy semiconductor material N in the alloy increases monotonically with increasing distance from the layer 18 from a value of zero at the junction with the layer 18. The grooves 20 or other texturing are formed in the part of the layer 19 underlying the gap 13 to serve as charge separators.
Light falling on the layer 19 through the gap 13 creates charge carriers that provide electrical conduction between the electrodes 14 and 15. Extinguishing the light turns the PCS OFF by generating a quasi-electric field that sweeps the charge carriers into the region of the layer 19 where the narrow band-gap energy material is predominant. In this region, the grooves 20 separate the charge carriers and prevent further conduction between the electrode segments.
While the PCS disclosed by Derkits offers improved performance, PCSs with further performance improvements are required to meet the requirements of present-day technology.
SUMMARY OF THE INVENTION
The invention provides a photo-conductive switch (PCS) that has a lower insertion loss and provides greater isolation than a conventional PCS. The insertion loss is reduced as a result of the PCS according to the invention having a smaller ON resistance R
on
. The isolation is increased as a result of the PCS according to the invention having a smaller OFF capacitance C
off
. Thus, the PCS according to the invention has a smaller figure of merit F=R
on
×C
off
than a conventional PCS. A smaller figure of merit indicates a PCS with better performance.
The PCS according to the invention is based on a multi-layer semiconductor structure that includes multiple heterojunctions. Each of the heterojunctions is a junction between a layer of a semiconductor material having a wider band-gap energy (WB material) and a layer of a semiconductor material having a narrower band-gap energy (NB material). The semiconductor materials forming one of the heterojunctions are doped so that they have opposite conductivity types. Thus, this heterojunction is formed between a layer of p-type WB material and a layer of n-type NB material, or between a layer of n-type WB material and a layer of p-type NB material.
The PCS according to the invention comprises a three-layer semiconductor structure composed of a photo-conductive layer of NB material sandwiched between a top confinement layer and a bottom confinement layer. Both confinement layers are layers of WB material. The top confinement layer and the photo-conductive layer have opposite conductivity types. A first electrode and a second electrode, separated from each other by a gap, are located on the surface of the top confinement layer remote from the photo-conductive layer. The photo-conductive layer provides a conduction path between the electrodes when the photo-conductive layer is illuminated with incident light of an appropriate wavelength and intensity.
Preferably, the conductivity type of the photo-conductive layer is p-type and that of the top confinement layer is n-type since this combination of conductivity types provides a higher conductivity than the opposite combination.
The double heterojunction PCS structure just described prevents charge carriers from diffusing in a direction perpendicular to the plane of the layers and enhances the efficiency with which charge carriers are generated in response to the incident light. This results in a lower ON resistance R
on
, and, hence, a lower insertion loss.
The double heterojunction PCS structure just described also provides a lower OFF capacitance C
off
, which increases the isolation provided by the PCS according to the invention in its OFF state.
The PCS according to the invention may additionally comprise a graded-composition layer or a chirped superlattice located between the photo-conductive layer and the top confinement layer to reduce the ON resistance R
on
.
The PCS according to the invention may additionally comprise a mirror layer located to reflect incident light that passes through the photo-conductive layer back into the photo-conductive layer to reduce further the ON resistance R
on
.
The electrodes of the PCS according to the invention may additionally include a translucent conductive material to reduce further the ON resistance R
on
.
The PCS according to the invention may additionally comprise a substrate of a substrate material that is translucent in the range of wavelengths of the incident light that are absorbed by the photo-conductive layer. The substrate supports at least the photo-conductive layer and the top confinement layer. The incident light passes through the translucent substrate to illuminate the photo-conductive layer. Since the incident light is not partially obstructed by the electrodes, the ON resistance R
on
is reduced.
The PCS according to the invention may alternatively comprise an opaque substrate that su
Kaneko Yasuhisa
Low Thomas S.
Saito Mitsuchika
Agilent Technologie,s Inc.
Hardcastle Ian
Le Que T.
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