Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-11-12
2004-08-10
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S152000, C257S153000, C257S154000, C257S242000, C257S302000, C257S335000
Reexamination Certificate
active
06774434
ABSTRACT:
This invention relates to a field effect transistor semiconductor device and is particularly, though not exclusively, concerned with the trade off relationship between the on-resistance and the breakdown voltage of the device.
It is well known that the voltage blocking capability of a field effect transistor device can be increased by reducing the dopant concentration and increasing the size of the drain drift region. However, this also increases the resistivity and length of the majority charge carrier path through the device when the device is conducting. This means that the series resistivity of the current path for majority charge carriers through the device, and thus the on-resistance of the field effect device, increases in proportion to approximately the square of the desired breakdown voltage.
U.S. Pat. No. 4,754,310, the contents of which are hereby incorporated herein by reference, (our reference PHB32740) addresses this problem by providing the drain drift region as a zone formed of first regions of one conductivity type interposed with second regions of the opposite conductivity type with the dopant concentrations and dimensions of the first and second regions being such that, when the device is operated in voltage blocking mode and the zone is depleted of free charge carriers, the space charge per unit area in the first and second regions balances at least to the extent that the electric field resulting from the space charge is less than the critical field strength at which avalanche breakdown would occur. This enables the required breakdown voltage characteristics to be obtained using interposed semiconductor regions which individually have a higher dopant concentration, and thus lower resistivity, than would otherwise be required so that the series resistivity of the first and second regions and thus the on-resistance of the device can be lower than for conventional devices.
For best results using the invention of U.S. Pat. No. 4,654,310 the charge balance between each pole in the drain drift region needs to be precise. That is to say the integral of the doping concentration perpendicular to the junction of the two interposed regions of one conductivity type and opposite conductivity type needs to have the same value of about 2×10
12
cm
−2
. Doping concentration to this level of precision in integrated circuit processing techniques is difficult, and a small fluctuation in doping concentration in either of the two regions results in a correspondingly large deviation from the desired charge balance along the drain drift region and a corresponding large reduction in the breakdown voltage of the device.
International patent application published as W001/59847, the contents of which are hereby incorporated herein by reference, (our reference PHNL 000066) provides another way of improving the trade off between breakdown voltage and on resistance in the case of vertical high voltage insulated gate field effect devices. Field shaping regions extend through the drain drift region from the body regions of the device to the drain region. These field shaping regions are semi-insulative or resistive regions which provide current leakage paths from the source regions when the device is non-conducting and a voltage is applied between the main electrodes of the device so as to cause an extension of a depletion region in the drain drift region towards the drain region to increase the breakdown voltage of the device. The small leakage current along the resistive paths causes a linear electrical potential drop along these paths. Hence a substantially constant vertical electric field is generated along these paths and accordingly in the adjacent drain drift region, and this results in the breakdown voltage being greater than for a non-uniform electric field which would occur in the absence of the field shaping region. Thus, as for the invention of U.S. Pat. No. 4,754,310, for a given required breakdown voltage of the device, it is possible to increase the doping concentration of the drain drift region and hence reduce the on-resistance of the device compared with a conventional device.
An object of the present invention is to provide a field effect transistor semiconductor device which also has a field shaping region adjacent the drift region but in which the substantially constant electric field is generated in the field shaping region in a different manner and by a different structure.
According to the present invention there is provided a field effect transistor semiconductor device comprising a source region, a drain region and a drain drift region, the device having a field shaping region adjacent the drift region and arranged such that, in use, when a voltage is applied between the source and drain regions and the device is non-conducting, a substantially constant electric field is generated in the field shaping region and accordingly in the adjacent drift region, characterised in that the field shaping region is arranged to function as a capacitor dielectric region between a first capacitor electrode region and a second capacitor electrode region, the first and second capacitor electrode regions being adjacent respective ends of the dielectric region and having different electron energy barriers.
By substantially constant electric field it is meant herein that the maximum electric field in the field shaping region and hence in the adjacent drift region at a given voltage is reduced in comparison with the absence of the field shaping region with the consequence that the breakdown voltage of the device is comparatively greater. Associated with the reduced maximum electric field is an increased integral of the electric field along the length of the field shaping region and the drift region and hence the greater breakdown voltage. It is possible to have a perfectly uniform electric field along both the field shaping region and the adjacent drift region but that depends on a number of factors including the device geometry, for example the extent of the field shaping region along the length of the drift region and the extent of influence of the field shaping region across the width of the drift region.
In a device according to the present invention, it is the different electron energy barriers of the first and second capacitor electrode regions which ensure that in use, when a voltage is applied between the source and drain regions and the device is non-conducting, the field shaping region functions as a capacitor dielectric region rather than a resistive region, there is substantially no space charge in the field shaping region, and within the drift region there is a charge balance between the space charge in the first capacitor electrode region, together with the drain drift region, and the second capacitor electrode region. That is to say, the charge in the drain drift region plus the charge in the first capacitor electrode region compensates the charge of, the second capacitor electrode region. It is an applied voltage which capacitively generates the substantially constant electric field in the field shaping region in the present invention rather than the leakage current applied through the field shaping region which is provided in the arrangement disclosed in W001/59847. Also, the problem with the arrangement of U.S. Pat. No. 4,754,310 of providing a precise charge balance between the two opposite conductivity type regions along the length of the drift region does not arise in the arrangement of the present invention.
In a device according to the present invention, the capacitor dielectric region may be intrinsic semiconductor material, or it may be extrinsic semiconductor material which is lower doped than the drift region, or it may be semi-insulating material, for example comprising one of oxygen doped polycrystalline silicon and nitrogen doped polycrystalline silicon.
In a device according to the invention, the capacitor dielectric region may be separated from the drain drift region by an insulating region. This insulating region tends to inhibit the possib
Hueting Raymond J. E.
Magnee Petrus H. C.
Slotboom Jan W.
Koninklijke Philips Electronics , N.V.
Waxler Aaron
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