Semiconductor devices with graded dopant regions
Most semiconductor devices manufactured today, have uniform dopant concentration, either in the lateral or vertical device active (and isolation) regions. By grading the dopant concentration, the performance in various semiconductor devices can be significantly improved. Performance improvements can be obtained in application specific areas like increase in frequency of operation for digital logic, various power MOSFET and IGBT ICS, improvement in refresh time for DRAM's, decrease in programming time for nonvolatile memory, better visual quality including pixel resolution and color sensitivity for imaging ICs, better sensitivity for varactors in tunable filters, higher drive capabilities for JFET's, and a host of other applications.
CROSS-REFERENCE TO RELATED APPLICATION
This application is not related to any pending application.
FIELD OF INVENTION
This present invention relates to all semiconductor devices and systems. Particularly it applies to diffused diodes, avalanche diodes, Schottky devices, power MOS transistors, JFET's, RF bipolar transistors, IGBTs (Insulated Gate Bipolar Transistors), varactors, digital VLSI, mixed signal circuits and sensor devices including camera ICs employing CCD (Charge Coupled Device) as well as CMOS technologies.
BACKGROUND OF INVENTION
Bipolar Junction transistors (BJT) are minority carrier devices as the principle device conduction mechanism. However, majority carriers also a small yet finite role in modulating the conductivity in BJTs. Consequently, both carriers (electrons and holes) play a role in the switching performance of BJTs. The maximum frequency of operation in BJTs is limited by the base transit time as well as the quick recombination of the majority carriers when the device is switched off (prior to beginning the next cycle). The dominant carrier mechanism in BJTs is carrier diffusion. Carrier drift current component is fairly small, especially in uniformly doped base BJTs. Efforts have been made in graded base transistors to create an ‘aiding drift field’, to enhance the diffusing minority carrier's speed from emitter to collector. However, most semiconductor devices, including various power MOSFETs (traditional, DMOS, lateral, vertical and a host of other configurations), IGBT's (Insulated Gated Base Transistors), still use a uniformly doped ‘drift epitaxial’ region in the base.
‘Retrograde’ wells have been attempted, with little success, to help improve soft error immunity in SRAM's and visual quality in imaging circuits.
BRIEF DESCRIPTION OF DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 3(a), 3(b), 3(c), 3(d) illustrate cross sections commonly used CMOS silicon substrate with two wells (one n-well in which p-channel transistors are subsequently fabricated, and, one p-well in which n-channel transistors are subsequently fabricated)—typical IC, EEPROM using tunnel insulator, DRAM and NAND flash;
FIGS. 5(a), 5(b), 5(c) illustrate the cross sections of a MOS silicon substrate with two wells, and, an underlying layer using embodiments of the invention to improve performance in each application—VLSI logic, DRAM/image IC, nonvolatile memory IC.
DETAILED DESCRIPTION OF THE INVENTION
The relative doping concentrations of emitter and collector regions varies from 1018 to 1020/cm3, where as the base region is 1014 to 1016/cm3 depending on the desired characteristics of the BJT. In graded base p-n-p transistors, the donor dopant concentration may be 10 to 100× at the emitter-base junction, relative to the base-collector junction (1×). The gradient can be linear, quasi linear, exponential or complimentary error function. The relative slope of the donor concentration throughout the base, creates a suitable aiding drift electric field, to help the holes (p-n-p transistor) transverse from emitter to collector. Since the aiding drift field helps hole conduction, the current gain at a given frequency is enhanced, relative to a uniformly-doped-(base) BJT. The improvement in cut-off frequency (or, frequency at unity gain, fT) can be as large as 2×-5×. Similar performance improvements are also applicable to n-p-n transistors.
As illustrated in
As illustrated in FIGS. 5(a), 5(b), 5(c), donor gradient is also of benefit to very large scale integrated circuits (VLSI)—VLSI logic, DRAM, nonvolatile memory like NAND flash. Spurious minority carriers can be generated by clock switching in digital VLSI logic and memory IC'S. These unwanted carriers can discharge dynamically-held ‘actively held high’ nodes. Statically held nodes (with Vcc) can not be affected, in most cases. Degradation of refresh time in DRAM's is one of the results, because the capacitor holds charge dynamically. Similarly, degradation of CMOS digital images, in digital imaging IC's is another result of the havoc caused by minority carriers. Pixel and color resolution can be significantly enhanced in imaging IC's with the embodiments described here. Creating ‘Sub Terrain’ recombination centers underneath the wells (gold doping, platinum doping) as is done in some high-voltage diodes is not practical for VLSI circuits. Hence, a novel technique has been described here by creating a drift field to sweep these unwanted minority carriers into the substrate as quickly as possible, from the active circuitry at the surface. In a preferred embodiment, the subterrain n-layer has a graded donor concentration to sweep the minority carriers deep into the substrate. One or more of such layers can also be implemented through wafer to wafer bonding or similar “transfer” mechanisms. This n-layer can be a deeply-implanted layer. It can also be an epitaxial layer. The n-well and p-well also can be graded or retrograded in dopants, as desired, to sweep those carriers away from the surface as well. The graded dopant can also be implemented in surface channel MOS devices to accelerate majority carriers towards the drain. In nonvolatile memory devices, to decrease programming time, carriers should be accelerated towards the surface when programming of memory cells is executed. The graded dopant can also be used to fabricate superior Junction field-effect transistors where the “channel pinchoff” is controlled by a graded channel instead of a uniformly doped channel (as practiced in prior art).
One of ordinary skill and familiarity in the art will recognize that the concepts taught herein can be customized and tailored to a particular application in many advantageous ways. For instance, minority carriers can be channeled to the surface, to aid programming in nonvolatile memory devices (NOR, NAND, multivalued-cell). Moreover, single well, as well triple-well CMOS fabrication techniques can also be optimized to incorporate these embodiments, individually and collectively. Any modifications of such embodiments (described here) fall within the spirit and scope of the invention. Hence, they fall within the scope of the claims described below
Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.
1. A semiconductor device with graded dopant concentration in the active region, to aid carrier movement from emitter to collector.
2. A semiconductor device with graded dopant concentration in the active region, to aid carrier movement from source to drain.
3. A semiconductor device with graded dopant concentration in the well regions, to aid carrier movement away from the active surface regions, towards the substrate.
4. A semiconductor device with graded dopant concentration in the substrate region to aid carrier movement away from the active surface regions, deeper towards the substrate.
5. A semiconductor device with at least one graded dopant concentration of donor or acceptor, to aid or impede carrier movement in selected regions in the monolithic die.
6. A semiconductor device with at least one each of dopant concentration of both donor and acceptor, to optimize the operating performance of the device.
7. A semiconductor device with at least one graded dopant concentration fabricated with ion implantation, to provide an aiding or retarding electric field locally in a monolithic integrated circuit.
8. A semiconductor device with at least one graded dopant concentration in an epitaxial layer.
9. A semiconductor device where one layer of dopant from one wafer, is transferred to another wafer having either same polarity or different polarity dopant through wafer bonding or similar processes.
Filed: Sep 3, 2004
Publication Date: Mar 9, 2006
Inventor: G.R. Mohan Rao (McKinney, TX)
Application Number: 10/934,915
International Classification: H01L 29/74 (20060101);