Field effect transistor of SiC for high temperature...

Details Field effect transistor of SiC for high temperature... Field effect transistor of SiC for high temperature...

1999-04-23

2001-08-21

Crane, Sara (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Specified wide band gap semiconductor material other than...

Diamond or silicon carbide

C257S253000

Reexamination Certificate

active

06278133

ABSTRACT:

The present invention relates to a field effect transistor of SiC for high temperature application having a source region layer, a drain region layer, a low doped channel region layer for conducting a current between the source region layer and the drain region layer, a gate electrode arranged to control the conduction properties of the channel region layer through varying the potential applied thereto as well as a front surface where the gate electrode is arranged, a use of such a transistor and a method for production thereof.
SiC has a number of properties that make it eminently suitable as a material in semiconductor devices, which have to function under extreme conditions. Its wide bandgap and high thermal stability, make it theoretically possible for semiconductor devices of SiC to function well at temperatures up to 1000 K. However, certain mechanisms in device structure can restrict the highest possible temperature of operation without failure to a much lower level.
Field effect transistors of SiC defined in the introduction and already known may fail at higher temperatures due to a charge injection mechanism. In the case of the presence of an insulating layer between the gate electrode and epitaxial layers of SiC, the energy barrier for electrons between the SiC conduction band edge and the insulating material (normally silicon dioxide) conduction band edge is small compared with for example silicon. This, coupled with the high electric fields encountered in SiC increases the chances of insulating layer breakdown caused by charge injection from the SiC into the insulating layer. This effect increases with temperature due to barrier lowering and can cause failure to occur at much lower temperatures than expected.
Another mechanism which has been previously noted to cause failure at higher temperatures is the reaction and consequent degradation of the contact metalization with gases in the surrounding atmosphere.
A possible use of a field effect transistor of SiC is as a gas sensor, for example in the flow of exhaust gases from cylinders of internal combustion engines of motor vehicles for sensing the composition of the exhaust gases passing. Such sensors already known may only withstand comparatively low temperatures without failure and have to be placed far from the cylinders at a point in the system where the exhaust gases have cooled significantly leading to long response times, and adjustment of individual cylinders is not possible as the sensors can only detect the joint output from the cylinders, SiC has as material a potential of enabling field effect transistors thereof to be placed near enough to monitor each cylinder separately with respect to the inherent thermal stability thereof. This would provide a much faster response time and the opportunity to adjust each cylinder individually in the event of it misfiring. This would lead to a reduction in petrol consumption and the production of cleaner exhaust gases, thus leading to a more environmentally-friendly system. It is pointed out that the invention is not at all restricted to this particular field of use, although it would be a very favourable application for a field effect transistor capable of withstanding very high temperatures.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a field effect transistor of SiC having a stable operation at considerably higher temperatures than such transistors already known and which may be constructed to function as an advantageous gas sensor but also finds other possible applications.
This object is according to the invention obtained by arranging the source region layer, the drain region layer and the channel region layer vertically separated from said front surface for reducing the electric field at said surface in operation of the transistor.
Separation of the active source, drain and channel regions from the front surface reduces the electric field in the vicinity of said front surface. This means that the charge injection discussed above into said insulating layer when present on front surface is reduced and the insulating layer may withstand considerably higher temperatures than before, actually up to 800° C.
Furthermore, the sensitivity of the transistor operation to surface effects will be reduced, since the active regions thereof are separated from the surface.
Separation of the active regions from the front surface also permits placement of the gate electrode over the entire active area, the source and drain being contacted at a distance, so that when used as a gas sensor, all electrodes excepting the catalytic gate electrode may be protected from the atmosphere by the encapsulation and thus prolong their lifetime.
Another advantage is obtained by this removal of active regions from the surface, namely a discontinuous gate electrode may be used and the device will still function. This is a very important feature, since the gate metal layer may with time become discontinuous, but here it will still function and not fail as for other types of device. This also constitutes a preferred embodiment of the present invention.
According to another preferred embodiment of the invention the transistor comprises a first layer of SiC separating the source region layer and the drain region layer from said front surface and being low doped according to the same, first conductivity type as the source region layer and the drain region layer. Such a low doped layer may be used to efficiently control the conduction properties of the channel region layer through the gate potential and it provides the necessary features for a normally off and a normally on device, i.e. a device operating in enhancement mode and depletion mode, respectively. The doping concentration of said first layer is lower than 10
16
cm
−3
, preferably lower than 2×10
15
cm
−3
.
According to another preferred embodiment of the invention both the source region layer and the drain region layer are buried in epitaxial layers of SiC and laterally separated for forming a lateral, i.e. horizontal, field effect transistor. Such a lateral transistor will have an advantageous function, and it will be possible to bury bars of source region layers and drain region layers alternating in the lateral direction in an interdigitated structure of such a transistor.
According to another preferred embodiment of the invention a second layer of SiC low-doped according to a second conductivity type opposite to the first one is arranged under the source region layer and the drain region layer for influencing the channel region layer arranged thereupon. The second layer and the gate electrode will in this way influence the channel region layer from opposite directions and thereby the appearance of a possible conducting channel between the source region layer and the drain region layer, so that a very sensitive transistor may be obtained.
According to another preferred embodiment of the invention one of the source region layer and the drain region layer is buried in epitaxial layers of SiC and the other one is arranged on a back side of the transistor opposite to said front surface for vertically separating the source region layer and the drain region layer for forming a vertical field effect transistor. Such a vertical transistor may in some applications be particularly advantageous.
According to another preferred embodiment of the invention said first conductivity type is n. This is preferred in cases in which the highest possible conductivity of the device is aimed at, since the mobility of electrons is much higher than that of holes in SiC. However, according to another preferred embodiment of the invention said first conductivity type is p. In some cases the conduction of holes may be preferred, and such a transistor may be more stable at high temperatures because of larger barrier heights, and when the transistor for example is used as a gas sensor it does riot matter that the total current will be lower, since the transistor only has to show variations in the current.
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