Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device
Reexamination Certificate
2001-12-11
2004-12-07
Wilczewski, Mary (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
C257S288000, C257S342000, C257S404000, C257S487000
Reexamination Certificate
active
06828605
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a field-effect-controllable semiconductor component which is provided in a semiconductor body. The semiconductor component has at least one source zone and at least one drain zone of a first conductivity type. The semiconductor component has at least one body zone of a second conductivity type. The body zone is provided between the source zone and the drain zone. The semiconductor component has at least one gate electrode, which is insulated from the semiconductor body through the use of a dielectric and via which a channel zone can be formed in the body zone when a gate potential is applied to the gate electrode. The invention also relates to a method of fabricating the semiconductor component.
Such a field-effect-controllable semiconductor component is, for example, the MOS (Metal Oxide Semiconductor) transistor, or MOSFET (Metal Oxide Semiconductor Field Effect Transistor) for short. The construction and the method of operation of a MOSFET is known and requires no further explanation.
A MOSFET typically functions as a controllable switch and should therefore have the smallest possible ohmic resistance in the on state. In actual fact, however, MOSFETs in the on state have a non-negligible so-called “on resistance.” In accordance with J. P. Stengl, J. Tihanyi, Leistungs-MOS-FET-Praxis [Power MOS-FET technology], Pflaum-Verlag Munich, 1992, page 44, the on resistance R
ON
has the following composition:
R
ON
=R
K
+R
S
+R
CH
+R
AKJ
+R
EPI
+R
SUB
(1)
The individual resistance elements of the on resistance R
ON
in accordance with equation (1) are described briefly below: R
K
designates the contact resistance of the source electrode with respect to the semiconductor body at the front side of the wafer. The resistance R
S
is essentially determined by the doping concentration in the source region. R
AKJ
designates the so-called accumulation resistance, which is often also referred to as JFET (Junction Field Effect Transistor) resistance and results from the formation and mutual influencing of depletion regions of adjacent cells of a semiconductor component. R
EPI
designates the resistance caused by the doping concentration of the drift path—if present. In vertical semiconductor components, the drift path is usually realized by one or more epitaxial layers. In particular in high-blocking-capability semiconductor components having a blocking capability of several hundred volts, the resistance element R
EPI
contributes by far the greatest proportion of the on resistance R
ON
, whereas it is often negligibly low in the case of components in the low-voltage range. R
SUB
designates the resistance of the substrate or of the drain region. Since the source resistance R
S
and the substrate resistance R
SUB
typically have a very high doping concentration, they are negligibly low relative to the other resistance elements.
R
CH
designates the channel resistance. The channel resistance results in the event of application of a gate potential and of a voltage between drain and source electrodes from the inversion current induced beneath the gate electrode. In accordance with B. J. Baliga, Power Semiconductor Devices, PWS-Publishing Company, page 362, the channel resistance R
CH
is defined as follows:
R
C
H
=
L
W
·
μ
N
S
·
C
O
X
·
(
V
c
-
V
T
)
(
2
)
In equation (2), L and W designate the channel length and the channel width, respectively, &mgr;
NS
designates the mobility of the electrons in the channel region, V
G
and V
T
designate the gate potential and the thermal potential, respectively. C
OX
designates the oxide capacitance, which is essentially determined by the oxide thickness and also the doping concentration in the channel region. Given a predetermined transistor geometry—i.e. given a constant channel length L, channel width W and oxide capacitance—in a MOSFET the channel resistance is inversely proportional to the mobility &mgr;
NS
.
The mobility is characterized on the one hand by scattering of the electrons contributing to the channel current with dopants in the channel region, and on the other hand by scattering of the electrons at the interface between the semiconductor body and the gate oxide. These scattering mechanisms, which are also represented on pages 10-11 and 18-19 in the abovementioned book by B. J. Baliga, brake the electrons and thus reduce the effective current density J
eff
in accordance with
J
eff
=n
el
v
el
(3)
where n
el
and v
el
designate the number and velocity of the electrons. The effective mobility of the electrons decreases, as a result of which the channel resistance R
CH
increases undesirably in accordance with equation (2).
Furthermore, there are even further resistance elements that exist, for example mounting-governed resistances.
In order to minimize the power loss consumed by the semiconductor component itself, the on resistance R
ON
in accordance with equation (1) should be as small as possible. There are various measures for reducing the resistance elements, some of which measures are presented briefly below:
In order to reduce the source resistance R
S
and substrate resistance R
SUB
, the doping concentration in these regions is typically increased as far as possible. Furthermore, in the case of vertical MOSFETs, the substrate resistance R
SUB
can be reduced by reducing the thickness of the drain region by thinning the semiconductor body by grinding from the rear side of the wafer.
In vertically configured MOSFETs, the epitaxial resistance R
EPI
can be greatly reduced by forming the semiconductor component as a compensation structure. Semiconductor components according to the compensation principle are described for example in U.S. Pat. No. 5,216,275 and U.S. Pat. No. 4,754,310 and also in International Publication No. WO 97/29518 and in German Patent No. DE 43 09 764 C2.
When a semiconductor component is formed with trench structures or so-called trenches in which the gate electrodes are provided, the parasitic JFET effect can be suppressed to the greatest possible extent, as a result of which the accumulation resistance R
AK
is minimized.
In particular in the case of so-called “smart power MOSFETs”, which, as is known, are configured for very low voltages, the channel resistance R
CH
contributes by far the greatest proportion of the on resistance R
ON
. A relationship between breakdown voltage and channel resistance is presented in D. A. Grant, J. Gowar, Power MOSFETs—Theory and Applications, J. Wiley & Sons, 1989, on page 76. By way of example, in the case of a MOSFET rated for a voltage between 50 V and 100 V, the channel resistance R
CH
makes up about 35% of the total on resistance R
ON
. In the case of low-voltage MOSFETs having a much lower blocking capability of 20 V or 12 V, the channel resistance R
CH
even makes up a proportion of 60% or 80%, respectively, of the total on resistance R
ON
.
In order to reduce the influence of the channel resistance R
CH
, the transistor geometry—for example the channel length L, the channel width W and the oxide capacitance C
OX
—can be varied as much as possible, with the assistance of equation (2). However, the transistor geometry of a semiconductor component is to a very great extent predetermined, so that optimization to that effect is possible only to a limited extent. With the exception of the abovementioned optimization of the transistor geometry, however, further measures for reducing the channel resistance R
CH
are not known at the present time, so that low-voltage MOSFETs, in particular, have an on resistance that is greatly dependent on the channel resistance. This is a state which, understandably, should be avoided.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a field-effect-controllable semiconductor component which overcomes the above-mentioned disadvantages of the heretofore-known components of this gen
Eisele Ignaz
Fink Christoph
Hansch Walter
Werner Wolfgang
Greenber Laurence A.
Infineon - Technologies AG
Lewis Monica
Locher Ralph E.
Stemer Werner H.
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