Semiconductor-on-insulator electronic devices having trench isolated monocrystalline active regions

That which is claimed:

1. A semiconductor-on-insulator electronic device, comprising:

a monocrystalline semiconductor substrate;

a first monocrystalline semiconducting region of a first conductivity type in said monocrystalline semiconductor substrate;

a first trench in said monocrystalline semiconductor substrate at a face thereof, said first trench having a bottom and first and second opposing sidewalls which extend from the bottom of said first trench to the face of said monocrystalline semiconductor substrate;

a first trench insulating layer on the bottom of said first trench and on the second sidewall of said first trench so that the bottom and the second sidewall of said first trench define an interface between said monocrystalline semiconductor substrate and said first trench insulating layer;

a second epitaxial monocrystalline semiconducting region on said first trench insulating layer, opposite the first trench bottom, said second epitaxial monocrystalline semiconducting region forming a semiconductor junction, selected from the group consisting of rectifying and non-rectifying semiconductor junctions, with said first monocrystalline semiconductor region at the first sidewall of said first trench;

a first electronic device active region in said first monocrystalline semiconducting region; and

a second electronic device active region in said second epitaxial monocrystalline semiconducting region.

2. The semiconductor-on-insulator electronic device of claim 1, wherein said second epitaxial monocrystalline semiconducting region forms a P-N junction with said first monocrystalline semiconducting region.

3. The semiconductor-on-insulator electronic device of claim 1, further comprising a third electronic device active region adjacent said second electronic device active region and forming a P-N junction therewith.

4. The semiconductor-on-insulator electronic device of claim 3, wherein said first, second and third electronic device active regions comprise monocrystalline collector, base and emitter regions of a bipolar junction transistor, respectively.

5. The semiconductor-on-insulator electronic device of claim 4, wherein said monocrystalline base region comprises a monocrystalline extrinsic base region on said first trench insulating layer and a monocrystalline intrinsic base region on said monocrystalline collector regions, said monocrystalline extrinsic base region forming a non-rectifying semiconductor junction with said intrinsic base region at the first sidewall of said first trench.

6. The semiconductor-on-insulator electronic device of claim 5, further comprising a base contact on said monocrystalline extrinsic base region and in ohmic contact therewith.

7. The semiconductor-on-insulator electronic device of claim 1, further comprising a third electronic device active region adjacent said first electronic device active region and forming a P-N junction therewith.

8. The semiconductor-on-insulator electronic device of claim 7, wherein said first, second and third electronic device active regions comprise monocrystalline channel, source and drain regions of a field effect transistor, respectively.

9. The semiconductor-on-insulator electronic device of claim 1, further comprising:

a second trench in said monocrystalline semiconductor substrate at the face, said second trench having a bottom and first and second sidewalls which extend from the bottom of said second trench to the face of said monocrystalline semiconductor substrate;

a second trench insulating layer on the bottom of said second trench, so that the bottom of said second trench defines an interface between said monocrystalline semiconductor substrate and said second trench insulating layer;

a third epitaxial monocrystalline semiconducting region on said second trench insulating layer, said third epitaxial monocrystalline semiconducting region forming a semiconductor junction, selected from the group consisting of rectifying and non-rectifying semiconductor junctions, with said first monocrystalline semiconductor region at the first sidewall of said second trench; and

a third electronic device active region in said third epitaxial monocrystalline semiconducting region;

wherein said first, second and third electronic device active regions comprise monocrystalline channel, source and drain regions of a field effect transistor.

10. The semiconductor-on-insulator electronic device of claim 1, further comprising:

a second trench in said monocrystalline semiconductor substrate at the face, said second trench having a bottom and first and second sidewalls which extend from the bottom of said second trench to the face of said monocrystalline semiconductor substrate;

a second trench insulating layer on the bottom of said second trench, so that the bottom of said second trench defines an interface between said monocrystalline semiconductor substrate and said second trench insulating layer;

a third epitaxial monocrystalline semiconducting region of a first conductivity type on said second trench insulating layer, said third epitaxial monocrystalline semiconducting region forming a semiconductor junction, selected from the group consisting of rectifying and non-rectifying semiconductor junctions, with said first monocrystalline semiconductor region at the first sidewall of said second trench; and

a third electronic device active region in said third epitaxial monocrystalline semiconducting region;

wherein said first and second electronic device active regions comprise monocrystalline collector and base regions of a bipolar junction transistor; and

wherein said third electronic device active region comprises a monocrystalline source region of a field effect transistor.

11. The semiconductor-on-insulator electronic device of claim 10, further comprising:

a third trench in said monocrystalline semiconductor substrate at the face;

a third trench insulating layer at a bottom of said third trench;

a fourth epitaxial monocrystalline semiconducting region of a first conductivity type on said third trench insulating layer; and

a fourth electronic device active region in said fourth epitaxial monocrystalline semiconducting region;

wherein said fourth electronic device active region comprises a monocrystalline drain region of the field effect transistor.

12. The semiconductor-on-insulator electronic device of claim 11, wherein said third epitaxial monocrystalline semiconducting region and said fourth epitaxial monocrystalline semiconducting region each form a P-N junction with said monocrystalline semiconductor substrate.

13. The semiconductor-on-insulator electronic device of claim 11, wherein said third and fourth electronic device active regions comprise monocrystalline source and drain regions of a first conductivity type channel field effect transistor; and

wherein said fourth epitaxial monocrystalline semiconducting region also comprises monocrystalline source and drain regions of a second conductivity type channel field effect transistor on said third trench insulating layer.

14. A semiconductor-on-insulator electronic device, comprising:

a monocrystalline semiconductor substrate having a face thereon;

a buried layer of first conductivity type in said semiconductor substrate;

a first trench in said monocrystalline semiconductor substrate said first trench having first and second opposing sidewalls extending between said buried layer and the face;

a first trench insulating layer at a bottom of said first trench and adjacent the second sidewall thereof;

an epitaxial monocrystalline semiconducting region of a second conductivity type on said first trench insulating layer, said epitaxial monocrystalline semiconducting region forming a semiconductor junction, selected from the group consisting of rectifying and non-rectifying semiconductor junctions, with said monocrystalline semiconductor substrate at the first sidewall of said first trench;

a monocrystalline extrinsic base region of a bipolar junction transistor in said epitaxial monocrystalline semiconducting region;

a monocrystalline intrinsic base region of said bipolar junction transistor in said monocrystalline semiconductor substrate, adjacent said monocrystalline extrinsic base region; and

a monocrystalline intrinsic collector region of said bipolar junction transistor in said monocrystalline semiconductor substrate, said monocrystalline intrinsic collector region forming a P-N junction with said monocrystalline intrinsic base region; and

wherein said buried layer comprises an extrinsic collector region of said bipolar junction transistor and wherein said first trench insulating layer also extends adjacent a portion of the first sidewall of said first trench to thereby prevent the formation of a P-N junction between said monocrystalline extrinsic base region and said monocrystalline intrinsic collector region.

15. The semiconductor-on-insulator electronic device of claim 14, further comprising:

a monocrystalline emitter region of said bipolar junction transistor adjacent said monocrystalline intrinsic base region and forming a P-N junction therewith; and

wherein said monocrystalline intrinsic collector region has a first conductivity type doping concentration less than the doping concentration of said buried layer.

16. The semiconductor-on-insulator electronic device of claim 15, further comprising a base contact on said monocrystalline extrinsic base region and in ohmic contact therewith.

17. A semiconductor-on-insulator BiCMOS cell, comprising:

a monocrystalline semiconductor substrate;

a plurality of adjacent trenches in said monocrystalline semiconductor substrate at a face thereof, said plurality of adjacent trenches each having bottoms and first and second opposing sidewalls;

a plurality of trench insulating layers on respective bottoms and at least portions of the first and second sidewalls of said trenches to thereby form a plurality of insulated trenches;

a monocrystalline extrinsic collector region of a bipolar junction transistor in said monocrystalline semiconductor substrate, said monocrystalline extrinsic collector region extending adjacent the bottom and the second sidewall of first of said insulated trenches;

a monocrystalline intrinsic collector region of said bipolar junction transistor on said monocrystalline extrinsic collector region and extending adjacent the first sidewall of said first insulated trench;

a monocrystalline intrinsic base region of said bipolar junction transistor on said monocrystalline intrinsic collector region and extending adjacent the first sidewall of said first insulated trench;

a monocrystalline extrinsic base region of said bipolar junction transistor in said first insulated trench, said monocrystalline extrinsic base region forming a semiconductor junction with said monocrystalline intrinsic base region at the first sidewall of said first insulated trench, but being electrically insulated from said monocrystalline intrinsic and extrinsic collector regions by the trench insulating layer in said first insulated trench so that conduction of charge carriers from said monocrystalline extrinsic base region to said monocrystalline intrinsic and extrinsic collector regions is prevented but conduction of charge carriers from said monocrystalline intrinsic base region to said monocrystalline intrinsic and extrinsic collector regions is allowed;

a monocrystalline source region of a first enhancement-mode field effect transistor in a second of said insulated trenches; and

a monocrystalline drain region of said first enhancement-mode field effect transistor in a third of said insulated trenches.

18. The BiCMOS cell of claim 17, further comprising:

a monocrystalline source region of a second enhancement-mode field effect transistor in said third insulated trench, adjacent said monocrystalline drain region of said first enhancement-mode field effect transistor and forming a P-N junction therewith;

a monocrystalline channel region of said second enhancement-mode field effect transistor in said third insulated trench; and

a monocrystalline drain region of said second enhancement-mode field effect transistor in said third insulated trench.

19. The BiCMOS cell of claim 17, further comprising:

a monocrystalline source region of a second enhancement-mode field effect transistor in said third insulated trench; and

a monocrystalline drain region of said second enhancement-mode field effect transistor in said third insulated trench.

20. The BiCMOS cell of claim 19, further comprising a monocrystalline channel region of said first enhancement-mode field effect transistor between said second insulated trench and said third insulated trench.

21. The BiCMOS cell of claim 20, wherein said monocrystalline source and drain regions of said first enhancement-mode field effect transistor comprise lightly doped source and lightly doped drain regions adjacent said monocrystalline channel region of said first enhancement-mode field effect transistor, respectively.

22. The BiCMOS cell of claim 20,

wherein said bipolar junction transistor comprises an N-P-N bipolar junction transistor;

wherein said first enhancement-mode field effect transistor comprises an N-channel field effect transistor; and

wherein said second enhancement-mode field effect transistor comprises a P-channel field effect transistor.

23. The BiCMOS cell of claim 22, wherein said source region of said second enhancement-mode field effect transistor and said drain region of said first enhancement-mode field effect transistor form a P-N junction.

24. The BiCMOS cell of claim 19, wherein said monocrystalline source and drain regions of said second enhancement-mode field effect transistor comprise epitaxially overgrown silicon source and drain regions.

25. The BiCMOS cell of claim 19,

wherein said bipolar junction transistor is selected from the group consisting of N-P-N and P-N-P transistors;

wherein said first enhancement-mode field effect transistor comprises a first-channel MOSFET; and

wherein said second enhancement-mode field effect transistor comprises a second-channel MOSFET.

26. A semiconductor-on-insulator field effect transistor, comprising:

a monocrystalline semiconductor substrate of first conductivity type;

first and second adjacent trenches in said semiconductor substrate at a face thereof, said first and second trenches each having trench bottoms and first and second opposing sidewalls;

first and second trench insulating layers on the bottoms of said first and second trenches and on the second sidewalls of said first and second trenches, respectively;

a first epitaxial monocrystalline source region of second conductivity type on said first trench insulating layer, opposite said monocrystalline semiconductor substrate, said first epitaxial monocrystalline source region comprising a three dimensional in-situ doped semiconducting region therein having a uniform second conductivity type doping concentration in at least two orthogonal dimensions thereof;

a second epitaxial monocrystalline drain region of second conductivity type on said second trench insulating layer, opposite said monocrystalline semiconductor substrate, said second epitaxial monocrystalline source region comprising a three dimensional in-situ doped semiconducting region therein having a uniform second conductivity type doping concentration in at least two orthogonal dimensions thereof;

an insulated gate electrode on the face, between said first and second adjacent trenches: and

wherein said monocrystalline semiconductor substrate comprises a channel region of first conductivity type which extends between said source and drain regions and forms respective P-N junctions therewith at the first sidewalls of said first and second trenches.

27. The semiconductor-on-insulator field effect transistor of claim 26, wherein said monocrystalline source and drain regions comprise:

a relatively lightly doped region adjacent said channel region; and

a relatively highly doped region on said relatively lightly doped region, opposite said channel region.

Description Submit all comments and votes

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to Application Ser. No. 08/312,874, entitled METHOD OF FORMING SEMICONDUCTOR-ON-INSULATOR ELECTRONIC DEVICES BY GROWING MONOCRYSTALLINE SEMICONDUCTING REGIONS FROM TRENCH SIDEWALLS (Attorney Docket No. 5347-111), filed concurrently herewith.

FIELD OF THE INVENTION

The present invention relates to semiconductor electronic devices, and more particularly semiconductor-on-insulator electronic devices.

BACKGROUND OF THE INVENTION

Semiconductor electronic devices may be formed on silicon-on-insulator (SOI) substrates with reduced parasitic and nodal capacitances, increased radiation hardness and reduced process complexity compared to bulk semiconductor devices. In particular, electronic devices such as SOI field effect transistors (FETs) typically have lower source/drain-to-substrate capacitance and higher latch-up immunity compared to their bulk counterparts. However, floating body effects caused by the presence of a continuous insulating layer may limit performance of conventional SOI FETs. Lower source-drain breakdown voltage caused by parasitic bipolar action also limits performance of conventional SOI FETs.

Electronic devices such as bipolar junction transistors (BJTs) may also benefit from being formed in monocrystalline SOI substrates, particularly when the extrinsic base and extrinsic collector contact regions are monocrystalline as well. Presently, the most popular commercial high-speed BJT structure is the Super Self-Aligned Transistor (SST) of FIG. 1. This BJT is typically characterized by a relatively low emitter-base capacitance (C.sub.eb), but a relatively high base-collector capacitance (C.sub.bc). Double Self-Aligned Transistors (DSTs) such as the Sidewall Contacted Structure (SICOS) of FIG. 2 typically have lower C.sub.bc, however these devices employ a polycrystalline extrinsic base contact region. To compensate for this, the edges of the base-collector junction are moved away from the polycrystalline extrinsic base contact region to prevent the depletion region in the intrinsic base from contacting the extrinsic base contact region during operation. For example, an extrinsic-to-intrinsic base link-up region is typically used adjacent the intrinsic collector region to provide the necessary separation, as illustrated by FIG. 2. This results in an increase in the base-collector junction area and prevents even smaller C.sub.bc from being achieved with DST devices.

Electronic devices which include a combination of Complimentary Metal-Oxide-Semiconductor (CMOS) FETs and BJTs (e.g., BiCMOS) can also be expected to achieve higher performance characteristics when formed on SOI substrates, particularly when all the active regions of the devices are monocrystalline and floating body effects have been eliminated. Attempts have been made to form BiCMOS devices on SOI substrates. For example, U.S. Pat. No. 5,049,513 to Eklund discloses a method of fabricating a BJT and a FET transistor on an SOI substrate. U.S. Pat. No. 5,212,397 to See et al. and U.S. Pat. No. 5,258,318 to Buti et al. also disclose BiCMOS devices and methods of forming the same on SOI substrates.

Notwithstanding these attempts to incorporate BJT and FET electronic devices on SOI substrates, there still exists a need for improved electronic devices such as FETs, BJTs and BiCMOS devices on SOI substrates, and methods of forming same, which have monocrystalline active regions, lower parasitic capacitances and reduced susceptibility to floating body effects and parasitic latch-up.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a semiconductor-on-insulator electronic device having monocrystalline active regions therein.

It is another object of the present invention to provide a semiconductor-on-insulator electronic device having trench isolated active regions.

It is a further object of the present invention to provide a semiconductor-on-insulator electronic device having reduced susceptibility to floating body effects, parasitic bipolar action and parasitic thyristor latch-up.

It is still a further object of the present invention to provide an improved BiCMOS cell.

These and other objects, features and advantages of the present invention are provided by an electronic device which has a plurality of trench isolated monocrystalline active regions. Trench isolation is preferably achieved by including electrically insulating layers at the bottoms of a plurality of trenches and including monocrystalline semiconducting regions on the insulating layers. The active regions of the electronic device are then formed in the semiconducting regions. The semiconducting regions are preferably grown from sidewalls of each trench using an epitaxial lateral overgrowth (ELO) technique. In particular, the semiconducting regions preferably comprise an epitaxially overgrown semiconductor, such as epitaxially overgrown silicon (EOS).

One embodiment of the present invention is a semiconductor-on-insulator (SOI) electronic device such as a bipolar junction transistor (BJT) or field effect transistor (FET) having at least one monocrystalline active region which is in a trench and is also electrically isolated from an adjacent semiconductor substrate by a trench insulating layer on a bottom of the trench. For example, a bipolar junction transistor according to the present invention includes an epitaxially overgrown extrinsic base region in a trench, and an intrinsic base region, an intrinsic collector region and an emitter region adjacent a sidewall of the trench. A field effect transistor according to the present invention includes an epitaxially overgrown source region and an epitaxially overgrown drain region in one or more adjacent trenches.

A semiconductor electronic device according to another embodiment of the present invention includes a BiCMOS cell, wherein epitaxially overgrown monocrystalline active regions of the cell are formed in adjacent trenches having electrically insulating layers along the bottoms thereof. In particular, the bipolar junction transistor (BJT) of the BiCMOS cell includes a completely monocrystalline extrinsic base region. This region eliminates the need for complex intrinsic-to-extrinsic link-up regions adjacent the base-collector junction, and this helps to minimize the base-collector junction area and capacitance. In addition, the PMOS transistor of the cell is an SOI device having an insulating layer extending between the active regions of the device and the semiconductor substrate. However, the NMOS transistor of the cell is preferably a partial-SOI device which combines the lowered device parasitic advantage of SOI with the ability to electrically connect a neutral NMOS substrate (channel region) to a fixed potential. This eliminates the hot-electron induced parasitic bipolar action associated with conventional SOI devices, but preserves the latch-up immunity of these devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art Super Self-Aligned Transistor (SST).

FIG. 2 illustrates a prior art Double Self-Aligned Transistor (DST).

FIGS. 3A-3L are schematic cross-sectional views of intermediate structures illustrating a method of forming an electronic device fabricated according to a first embodiment of the present invention.

FIGS. 4A-4L are schematic cross-sectional views of intermediate structures illustrating a method of forming an electronic device fabricated according to a second embodiment of the present invention.

FIGS. 5A-5M are schematic cross-sectional views of intermediate structures illustrating a method of forming an electronic device fabricated according to a third embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of a method of forming semiconductor-on-insulator (SOI) electronic devices and electronic devices formed therefrom are illustrated. This invention may, however, be embodied in different forms and should not be construed as limited to only the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For greater clarity, the thickness of the illustrated layers and regions have been exaggerated.

Referring now to FIGS. 3A-3L, a method of forming an electronic device according to a first embodiment of the present invention is illustrated. In particular, FIGS. 3A-3L illustrate the steps of forming a preferred SOI NMOS transistor having SOI monocrystalline source and drain regions and a substrate-connected channel region. This transistor has the benefits of conventional SOI FETs such as low source/drain-to-substrate capacitance, latch-up immunity and dielectric isolation which allows for higher integration densities. However, this embodiment does not suffer from floating body effects or low source-drain breakdown voltage caused by parasitic bipolar action, which are commonly associated with conventional SOI devices.

Referring specifically to FIG. 3A, an exemplary <100> oriented monocrystalline silicon substrate 10 (shown as P-) is illustrated having a plurality of layers 12'-18' thereon. The method according to this embodiment begins with the step of forming a first insulating layer 12' on a face 10a of the silicon substrate 10. The first insulating layer 12' may be silicon dioxide (SiO.sub.2) or another dielectric having similar characteristics. The thickness of the first insulating layer 12' is preferably in the range of about 15 to 50 .ANG. and can be formed using conventional deposition or dry oxidation techniques. Thereafter, an electrically conductive layer 14', such as a relatively thick polycrystalline silicon layer (poly-Si) (or another conductive layer such as a silicide or salacide), is formed on the first insulating layer 12'. As understood by those skilled in the art, a poly-Si layer having a thickness of about 3500 .ANG. may be deposited using Low Pressure Chemical Vapor Deposition (LPCVD).

Next, a second insulating layer 16' is formed on the conductive layer 14'. The second insulating layer 16' is also preferably SiO.sub.2 or a similar dielectric. For example, the second insulating layer 16' can be formed by growing a relatively thick wet SiO.sub.2 of about 2000 .ANG. from the conductive layer 14'. The second insulating layer forming step is then followed by the step of forming a reactive-ion-etch resistant (RIE-resistant) layer 18', such as silicon nitride (Si.sub.3 N.sub.4) having a thickness of about 1000 .ANG., on the second insulating layer 16'. This layer 18' can be deposited by LPCVD as well. A first masking layer such as photoresist, having a thickness in the range of about 1,000 to 10,000 .ANG., may also be formed and patterned, as illustrated, using techniques well known to those skilled in the art.

Referring now to FIG. 3B, the conductive layer 14', second insulating layer 16' and the RIE-resistant layer 18' of FIG. 3A are then chemically etched in the exposed areas 20a. This step defines a gate electrode layer 14 having first and second ends 141-b, a second gate insulating layer 16 and a first RIE-resistant mask layer 18. As illustrated by FIG. 3C, the masking layer 20 is removed and then electrically insulating spacers 16a-b are formed on the first and second ends 14a-b of the gate electrode layer 14. Preferably, the insulating spacers 16a-b are SiO.sub.2 having a thickness of about 2000 .ANG.. As understood by those skilled in the art, the insulating spacers can be formed by oxidizing the first and second ends 14a-b of a poly-Si gate electrode layer 14. This oxidizing step also defines the actual length of the gate electrode since part of the gate electrode layer is consumed during oxidation. Accordingly, the actual length of the gate electrode 14 is defined by new ends 14c-d. A second RIE-resistant mask layer 22 such as Si.sub.3 N.sub.4 is then lithographically patterned on the first insulating layer 12', as illustrated.

A blanket reactive ion etching step using Freon 115 is then preferably performed to selectively etch SiO.sub.2 and Si anisotropically, using the first and second RIE-resistant layers 18 and 22 as etch masks. This etching step forms an insulated gate electrode 26 on a mesa 25 which has exposed vertical sidewalls 25a-b. The mesa 25 is preferably of stripe or similar geometry. The RIE etching step also defines adjacent trenches 24, each of which has opposing vertical sidewalls 24a-b in the silicon substrate 10. The SiO.sub.2 insulating spacers 16a-b are also partially etched, but the gate electrode 14 still remains insulated by thinner insulating spacers 16c-d.

Referring now to FIGS. 3E-3F, additional spacers 28a-b are also formed adjacent the sidewalls 26a-b of the insulated gate electrode 26 and adjacent the sidewalls 25a-b of the mesa 25. This spacer forming step is performed so that relatively thick trench insulating layers can be selectively formed adjacent the trench bottoms 24c and adjacent the outside sidewalls 24b, but not on the mesa sidewalls 25a-b. These spacers 28a-b are preferably Si.sub.3 N.sub.4 spacers which have a sufficient thickness of about 1000 .ANG. to inhibit SiO.sub.2 growth adjacent the mesa sidewalls 25a-b. Trench insulating layers 30 are then formed on the trench bottoms 24c and sidewalls 24b by performing a wet oxidation of the structure of FIG. 3E and then removing the nitride spacers 28a-b and the first and second RIE-resistant layers 18 and 22, preferably with phosphoric acid. As illustrated, the trench insulating layers 30 ultimately extend along the outside trench sidewalls 24b, on the trench bottoms 24c and on lowermost portions of the mesa sidewalls 25a-b (i.e., inner trench sidewalls 24a), however, the insulating layers 30 do not extend adjacent uppermost portions of the mesa sidewalls 25a-b (24a).

Referring now to FIGS. 3G-3H, monocrystalline semiconducting regions 32 are then formed from the uppermost (i.e., exposed) portions of the mesa sidewalls 25a-b onto the trench insulating layers 30. Preferably, this step includes the step of growing monocrystalline silicon from the vertical seed windows 32a-b using vertically-seeded epitaxial lateral overgrowth (VELO). This can be done by placing the structure of FIG. 3F in a LPCVD epi growth reactor (not shown) and then growing monocrystalline epitaxial silicon from the windows 32a-b. The monocrystalline silicon semiconducting regions 32 fill the trenches 24 as illustrated, however, they are then chemically/mechanically planarized using the top of the second gate insulating layer 16 as a "planarizing/etch stop", as illustrated by FIG. 3H. As described more fully hereinbelow, one or more active regions of an electronic device such as an FET can be formed in the semiconducting regions 32. These active regions will be electrically isolated from the substrate 10 by the trench insulating layers 30.

Referring again to FIG. 3G, in-situ doping using phosphine as an N-type dopant gas in the epi reactor can also be performed simultaneously with the growing step to form N-type semiconducting regions 32. The concentration of the dopant gas can also be increased during the growing step so that relatively lightly doped N-type regions extend adjacent the mesa 25 and more highly doped N-type regions extend adjacent an upper surface of the semiconducting regions 32.

Referring now to FIGS. 3I-3J, the semiconducting regions 32 can alternatively be doped after the growing step has been completed. For example, the semiconducting regions 32 can be etched back using a blanket RIE etching step and then sidewall spacers 34, such as Si.sub.3 N.sub.4, can be formed adjacent the second gate insulating layer 16. At this stage, a heavy dose blanket arsenic or phosphorus N+ implant 35 can be used to highly dope the semiconducting regions 32 (shown as N+) away from the windows 32a-b, as illustrated by the dotted lines of FIG. 3J. The sidewall spacers 34 can then be removed and a corresponding light dose blanket N-type implant 37 can be performed to lightly dope the semiconducting regions 32 (shown as N-) adjacent the windows 32a-b, as illustrated by FIG. 3K.

Finally, dopant anneal, oxidation and contact metallization steps are performed to form an enhancement-mode NMOS transistor, as illustrated by FIG. 3L, wherein first, second and third monocrystalline semiconducting regions extend in the substrate 0 and in the adjacent trenches 24, respectively. These three semiconducting regions include the channel region (C) (shown as P-), and the source (S) and drain (D) regions (shown as N-/N+) of the NMOS transistor. As illustrated, vertically displaced source and drain ohmic contacts 36 are provided on the source and drain regions, adjacent the highly doped portions thereof (shown as N+). As understood by those skilled in the art, the application of a sufficiently positive bias to gate electrode 4 will cause the formation of an N-type inversion-layer channel 38 in the channel region (C), which electrically connects the source (S) to the drain (D). However, the channel region (C) is not electrically isolated from the bulk portion of the substrate 10 like conventional SOI FETs. Accordingly, the NMOS FET of FIG. 3L has reduced susceptibility to floating body effects and parasitic latch-up.

Referring now to FIGS. 4A-4L, a method of forming an electronic device according to a second embodiment of the present invention is illustrated. In particular, FIGS. 4A-4L illustrate the steps of forming a bipolar junction transistor (BJT) which is generally compatible with the steps illustrated and described by FIGS. 3A-3L and the accompanying description. The BJT of the present invention includes, among other things, monocrystalline intrinsic and extrinsic base regions. The present invention eliminates the need for the complex intrinsic-to-extrinsic base link-up regions of the prior art as illustrated by the SICOS DST of FIG. 2. This helps to minimize the base-collector junction area and capacitance and improves the electrical performance of the BJT.

Referring specifically to FIG. 4A, an exemplary <100> oriented monocrystalline silicon substrate 110 (shown as P-) is illustrated having a lightly doped epitaxial layer 12 thereon (shown as N-). A relatively highly doped buried layer 114 (shown as N+) is also provided in the substrate 10, as illustrated. A stress relief layer 16 is then formed on the substrate 110, at a face 110a thereof. The stress relief layer 116 is preferably a 900 .ANG. thick layer of grown SiO.sub.2. A nitride layer 118 such as Si.sub.3 N.sub.4 is also preferably formed on the stress relief layer 116 by LPCVD. This is followed by the formation and patterning of a masking layer 119 such as photoresist on the nitride layer 118 using techniques well-known to those skilled in the art. As described more fully hereinbelow, the masking layer 119 is patterned to define an intrinsic collector mesa and an extrinsic collector contact mesa of the aforementioned BJT.

Referring now to FIG. 4B, the nitride layer 118, the stress relief layer 116 and the epitaxial layer 112 are then anisotropically etched in the exposed ares 119a. In particular, dry etching with SF.sub.6 is preferably performed to remove the nitride layer 118 in the exposed areas 119a and then Freon 115 is used to remove the stress relief layer 116 and the epitaxial layer 112. These steps define a plurality of adjacent trenches in the substrate and particularly a first trench 120 which extends between the N+ buried layer 114 and the face 1108. The adjacent trenches also define the intrinsic collector mesa 121 and the collector contact mesa 123 which extend in a third dimension orthogonal to the illustrated cross-section. As understood by those skilled in the art, the intrinsic collector mesa 121 may be of cylindrical, rectangular, stripe or similar geometry. For example, the illustrated trenches on either side of the intrinsic collector mesa 121 may extend parallel to each other or be part of a single surrounding trench.

Referring now to FIG. 4C, the masking layer 119 is removed and then trench insulating layers 122 such as SiO.sub.2 are formed at the bottoms and along the sidewalls of the adjacent trenches. As illustrated, relatively thick SiO.sub.2 is formed adjacent the bottoms of the trenches and relatively thin SiO.sub.2 is formed adjacent the sidewalls of the trenches. This can be achieved by taking advantage of the enhanced oxidation rate of the