Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
1999-12-21
2002-03-12
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S288000, C257S330000, C257S328000, C438S163000
Reexamination Certificate
active
06355944
ABSTRACT:
RELATED APPLICATIONS
Commonly-assigned, copending U.S. patent application Ser. No. 09/469,454, entitled “Self-Aligned Silicon Carbide LMOSFET”, filed Dec. 12, 1999.
Commonly-assigned, copending U.S. patent application Ser. No. 09/469,450, entitled “Silicon Carbide N-Channel Power LMOSFET”, FILED Dec. 12, 1999.
FIELD OF THE INVENTION
This invention relates to lateral metal-oxide-semiconductor field effect transistors (LMOSFETs) used in high-power applications such as UHF transmission which are especially suited for silicon carbide (SiC) technology. In particular, the invention relates to a SiC LMOSFET having a self-aligned gate structure with improved gate reach-through protection and methods of fabricating same.
BACKGROUND OF THE INVENTION
In recent years, the use of silicon lateral double-diffused metal-oxide-semiconductor field effect transistors (Si LDMOSFETs) in high-power and high-frequency applications has increased enormously. This is because Si LDMOSFETs offer simpler gate drive and faster response than bipolar devices.
Si LDMOSFETs are typically fabricated using self-aligned techniques, which minimize gate overlap of the source and drift/drain regions. Minimal overlap is critical for maintaining low gate-to-source and gate-to-drift/drain capacitances, which can adversely affect the high frequency performance of the device. It is also desirable to reduce the overlap to decrease the cell pitch and conserve the silicon area used by the device.
Silicon carbide (SiC) is an attractive semiconductor material for high frequency and high power applications. The properties which make SiC attractive for high power UHF applications are its large critical electric field (10 times that of Si) and its large electron saturation velocity (2 times that of Si). The large critical electric field helps increase the breakdown voltage of the device and the large saturation velocity helps increase the peak current.
FIG. 1
shows an LMOSFET
10
as disclosed in commonly-assigned copending U.S. patent application Ser. No. 09/469,454 entitled “Self-Aligned Silicon Carbide LMOSFET”. This SiC LMOSFET includes a self-aligned gate structure and offers protection against gate reach through. The LMOSFET
10
of
FIG. 1
includes highly n-doped source and drain regions
11
,
12
, a lightly n-doped drift region
13
formed by an N- epitaxial layer
14
, and an electrically insulated self-aligned gate structure
15
comprised of a gate oxide
16
and a gate metal
17
, formed on a lightly-doped p-type SiC epitaxial layer
18
(P-epilayer). The gate structure
15
has edges
19
which are substantially aligned with the edges
20
of the source and drift regions
11
,
13
. Accordingly, the gate-to-source and gate-to-drift region overlap can be advantageously controlled by the thickness of the gate metal
17
, which can be selected to be very small. A channel region
21
in the P- epilayer
18
. The channel region
21
changes from p-type to n-type due to inversion when a positive voltage greater than the threshold voltage of the LMOSFET
10
is applied to the gate
15
thereby providing a low resistance current path between the source region
11
and drift extension
13
of the drain region
12
.
The LMOSFET
10
of
FIG. 1
should provide many advantages in terms of better linearity, efficiency and power density at comparable frequencies, and higher frequency operation than Si LDMOSFETs. However, this LMOSFET may suffer from higher forward voltage drop, i.e., higher “on-resistance” due to the fact that the current at the source side has to flow around a corner
22
where the gate oxide
16
has a greater thickness. The greater oxide thickness results in a higher resistivity portion in inversion, which will likely result in higher forward voltage drop.
Therefore, a SiC LMOSFET is needed which overcomes the above problem.
SUMMARY OF THE INVENTION
Summarily described is an LMOSFET having a self-aligned gate with gate reach-through protection and method for making same. The LMOSFET comprises a first layer of SiC semiconductor material having a p-type conductivity and a second layer of SiC semiconductor material having an n-type conductivity formed on the first layer. Source and drain regions having n-type conductivities are formed in the second SiC semiconductor layer. An etched trench extends through the second SiC semiconductor layer and partially into the first SiC semiconductor layer so that the source and drain regions are substantially lateral thereto. The trench is coated with a layer of an electrically insulating oxide material and partially filled with a layer of metallic material thereby forming a gate structure. A channel region is defined in the first layer beneath the gate structure. The source comer of the gate structure is either rounded or surrounded by the source region to provide a current path in the channel region which avoids sharp corners. Electrical contacts associated with the source and drain regions, and the gate structure establish source, drain, and gate electrodes.
REFERENCES:
patent: 5726463 (1998-03-01), Brown et al.
patent: 5915180 (1999-06-01), Hara et al.
patent: 6011278 (2000-01-01), Alok et al.
patent: 6198127 (2001-03-01), Kocon
patent: 63287064 (1988-11-01), None
patent: 02110973 (1990-04-01), None
patent: 11103058 (1999-04-01), None
PHA 23,899, “Self-Aligned Silicon Carbide LMOSFET”, filed concurrently herewith.
PHA 23,900, “Silicon Carbide N-Channel Power LMOSFET”, filed concurrently herewith.
Biren Steven R.
Flynn Nathan
Forde Remmon R.
Philips Electronics North America Corporation
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