Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
2003-02-04
2004-03-23
Baumeister, Bradley (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S190000
Reexamination Certificate
active
06710379
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to High Electron Mobility Transistor (HEMT) devices and more particularly, to a HEMT device having a partially relaxed channel.
Referring to
FIG. 1
, a typical prior art HEMT device
10
comprises a buffer layer
12
which is grown on a substrate
14
. A pseudomorphic channel
16
is grown on the buffer layer
12
, and, a barrier layer
18
is grown on the channel
16
. The barrier layer
18
typically includes a doping layer
20
disposed near the channel
16
. A cap layer
22
is disposed on the barrier layer
18
. The buffer layer
12
and the barrier layer
18
provide confinement of the carriers in the channel
16
. The buffer layer
12
additionally isolates the channel
16
from the substrate
14
. To complete the HEMT device
10
, a gate
23
is positioned on the barrier layer
18
through a recess
24
formed in the cap layer
22
. A source
24
and a drain
24
are positioned on the cap layer
22
on either side of the gate
23
.
A thick channel
16
is desirable because a thicker channel
16
provides improved containment of the carriers which improves the electrical properties of the HEMT device
10
. Therefore, it is desirable to fabricate the channel
16
as thick as possible. However, the thickness of the channel
16
is limited to a thickness which maintains the material in the channel
16
in a pseudomorphic state. In a pseudomorphic state, the material of the channel
16
is under strain with no defects in the material resulting from strain relaxation. Defects in the material of the channel
16
caused by strain relaxation begin to appear once the channel
16
is fabricated to a thickness greater than the critical thickness. When defects are present in the channel
16
, the channel
16
is referred to as being either partially-relaxed or fully-relaxed. Partially-relaxed and fully-relaxed are terms used in the industry to distinguish between levels of defects in the material. A partially-relaxed material has some level of defects but not enough to cause the material to lose all beneficial material properties, whereas a fully-relaxed material has enough defects to cause the material to lose most of its beneficial properties.
Any level of defects in the material of the channel
16
are considered by those skilled in the art to be undesirable because defects are thought to reduce the DC and RF performance of the HEMT device
10
. Therefore, the channel
16
of the prior art HEMT device
10
is only grown below the critical thickness, so that defects in the channel
16
can be avoided. This means that the typical HEMT device
10
has a relatively thin channel
16
. A thin channel
16
poorly confines the carriers in the channel
16
which limits the gain and frequency response of the HEMT device
10
.
What is desired therefore is a HEMT device which provides a higher gain and frequency response than provided by the prior art HEMT device
10
.
SUMMARY OF THE INVENTION
The proceeding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a HEMT device comprising a buffer layer disposed on a substrate. A partially-relaxed channel is disposed on the buffer layer and a barrier layer is disposed on the channel. A cap layer is disposed on the barrier layer and a gate is positioned on the barrier layer. A source and a drain are positioned on the barrier layer on opposite sides of the gate.
In a second aspect the channel is fabricated of a material which, when disposed to a first thickness is pseudomorphic; and, when disposed to a second thickness, is fully-relaxed. The channel is fabricated to a thickness intermediate the first and second thicknesses.
In a third aspect, the present invention provides a method for fabricating a HEMT device having a gate, a source and a drain. A buffer layer is deposited on a substrate. A partially-relaxed channel is deposited on the channel and a barrier layer is deposited on the channel. A cap layer is deposited on the barrier layer and the gate is positioned on the cap layer through a recess formed in the cap layer. The source and the drain are positioned on the cap layer on opposite sides of the gate.
REFERENCES:
patent: 5221367 (1993-06-01), Chisholm
patent: 5367182 (1994-11-01), Matsugatani
patent: 5668387 (1997-09-01), Streit et al.
patent: 0495452 (1992-07-01), None
patent: 2304998 (1997-03-01), None
Barsky Michael E.
Chin Tsung-Pei
Grundbacher Ronald W.
Wojtowicz Michael
Baumeister Bradley
Northrop Grumman Corporation
Thousand Connie M.
LandOfFree
Fully relaxed channel HEMT device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fully relaxed channel HEMT device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fully relaxed channel HEMT device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3279378