as) United States
`a2) Patent Application Publication (10) Pub. No.: US 2003/0030488 Al
`
` Hueting et al. (43) Pub. Date: Feb. 13, 2003
`
`
`US 20030030488A1
`
`(54) TRENCH BIPOLAR TRANSISTOR
`
`(30)
`
`Foreign Application Priority Data
`
`(75)
`
`Inventors: Raymond J.E. Hueting, Helmond
`(NL); Jan W. Slotboom, Eersel (NL);
`Petrus H.C. Magnee, Leuven (BE)
`
`(GB) ces ecsecesscssseesseesennecsnneeere 0119215.2
`Aug. 7, 2001
`Publication Classification
`
`Correspondence Address:
`Corporate Patent Counsel
`U.S. Philips Corporation
`580 White Plains Road
`Tarrytown, NY 10591 (US)
`
`(73) Assignee: KONINKLIJKE PHILIPS
`TRONICSN.Y.
`
`ELEC-
`
`(21) Appl. No.:
`
`(22) Tiled:
`
`10/205,555
`
`Jul. 25, 2002
`
`the gate between connections 21,23.
`
`(ST) Ute C07 oc cccscsescentessnsteneec tne H03F 3/60
`(52) US. Che
`cesscssssssssssnstsstnsnseue sna 330/57; 257/565
`
`(57)
`
`ABSTRACT
`
`The invention relates to a trench bipolar transistor structure,
`having a base 7, emitter 9 and collector 4, the latter being
`divided into a higher doped region 3 and a lower doped drift
`region 5. Aninsulated gate 11 is provided to deplete the drift
`region 5 when the transistor is switched off. ‘lhe gate 11
`and/or doping levels in the drift region 5 are arranged to
`provide a substantially uniform electric field in the drift
`regionin this state, to minimise breakdown. In particular, the
`gate 11 maybe seminsulating and a voltage applied along
`
`
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`Patent Application Publication
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`US 2003/0030488 A1
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`Patent Application Publication
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`Feb. 13,2003 Sheet 9 of 10
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`Patent Application Publication
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`Feb. 13,2003 Sheet 10 of 10
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`US 2003/0030488 Al
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`Feb. 13, 2003
`
`TRENCH BIPOLAR TRANSISTOR
`
`[0001] The inventionrelates to a vertical bipolar transistor,
`and particularly to a vertical bipolar transistor suitable for
`use at high frequencies.
`
`[0002] Bipolar transistors are used in a numberof appli-
`cations, including in high-voltage radio frequency devices.
`The design of such structures is a trade-off between a
`number of factors. One factor is the breakdown voltage
`between the collector and the base or emitter, ic.
`the
`maximum voltage that may be applied between the collector
`and the base or emitter when applying a reverse potential to
`the collector without causing breakdown. Another factor is
`the cutoff frequency of operation. It would be desirable to
`increase the values is of both of these parameters.
`
`In alternative cmbodiments,a lateral structure may
`[0009]
`be provided, for example using an insulated buried layer as
`the gate.
`
`[0010] The gate maybe separated from the drift region by
`a gate insulating layer on the sidewalls of the trench.
`
`[0011] The structure is also typically easier to manufacture
`than structures involving multiple layers in the drift region.
`
`‘he collector region mayinclude a semiconductor
`[0012]
`substrate or body or a laycr or region formed on a substrate.
`The trench may extend through the emitter, base and drift
`regions.
`
`[0013] The invention is of particular application to high
`frequency devices. Such devices, may be for example het-
`erostructure bipolar transistors. ‘The invention can also be
`used for low frequency devices.
`
`Ilowever, it is well knownin the art that in general
`[0003]
`the product of the cutoff frequency f, and the breakdown
`voltage between the collector and the emitter has a maxi-
`‘lhe gate maybe of a semi-insulating material, and
`[0014]
`mum known as the Johnson limit. This product is accord-
`the structure may further compriseafirst gate connection at
`ingly an important parameter for bipolar transistors. Since
`the end of the gate adjacentto the boundary betweenthe drift
`the product has a maximum, it is not normally possible to
`region and the base region and a second gate connection at
`increase one of these parameters without reducing the other.
`the boundary betweenthe drift region and the higher doped
`collector region. This allows a uniform field to be applied
`along the gate thereby providing a uniform field in the drift
`region to minimises the risk of breakdown at low voltages.
`The uniform field is achieved without complex doping
`profiles in the drift region being necessary.
`
`[0004] An exception is knownto the Johnson limit at very
`high frequencies. Non-local avalanche effects in the base-
`collector space charge region can allow the Johnson limit to
`be execeded at the very highest radio frequencies.
`
`[0005] However, it would be uscful to be able to design
`transistors for which the value of the threshold frequency
`and breakdown voltage product exceeds the Johnson limit
`over a broad radio frequencyrange.
`
`[0006] According to the invention there is provided a
`bipolartransistor structure, comprising: a collector including
`a higher doped collector region of semiconductor material of
`a first conductivity type doped to a first concentration; an
`emitter region of semiconductor material of the first con-
`ductivity type; a base region of semiconductor material of a
`second conductivity type opposite to the first conductivity
`type between the emitter region and the collector;
`the
`collector further
`including a
`lower doped drift
`region
`extending between the higher doped collector region and the
`base region, the drift region being ofthe first conductivity
`type and doped to a second concentration lower than the first
`concentration; a trench extending adjacent
`to the drift
`region; and a gate within the trench insulated from the drift
`region for controlling the drift region to be depleted of
`carriers in a voltage blocking made of operation.
`
`[0015] The contact at the end adjacentto the base may be
`electrically connected to the emitter (or base). ‘Thus, the first
`gate connection may be a gate contact in clectrical connce-
`tion with a base contact contacting the base or an emitter
`contact contacting the emitter. In alternative embodiments,
`there may be a separate connectionto the gate to allow the
`voltage across the gate to be controlled independently of the
`voltage on the emitter.
`
`[0016] The second gate connection may be a direct con-
`nection between the end of the drift control gate adjacent to
`the higher doped region of the collector and the higher doped
`region of the collector. Allernatively, the second gale con-
`nection may be a further contact so that the voltage applied
`along the gate can be controlled independently.
`
`[0017] Alternative embodiments replace the semi-insulat-
`ing gate with a p-i-n diode having voltage dropped across the
`intrinsic (i) region to very similar effect.
`
`[0018] Alternatively, the gate may be conducting and a
`uniform field in the drift region may be provided by a
`suitable graded doping profile in the drift region. In this
`particular embodiment the gate is isolated from the highly
`doped collector region.
`
`[0019] Other advantageous
`present
`invention are set out
`claims.
`
`the
`features of
`technical
`in the attached dependent
`
`‘The drift region in the collector is of lower doping
`[0007]
`concentration than the higher doped region of the,collector
`so that the drift region may be depleted of carriers. Using the
`gate in the trench the drift region can be depleted even with
`a higher doping in the drift region than would otherwise be
`possible. This allows the product of the threshold frequency
`and the breakdown voltage to be increased as compared with
`prior art structures. In embodiments of the invention, the
`Johnsonlimit may be exceeded.
`
`In another aspect the invention also relates to a
`[0020]
`method of manufacturing the bipolar transistor described
`above.
`[0008] Conveniently, the structure may beavertical struc-
`ture formed on a semiconductor body having opposedfirst
`and second faces. The emitter region may be connected to
`the first face and the collector region to the second face. The
`trench may extend substantially perpendicularly to the first
`face through the emitter and base regions to the drift region.
`
`the invention,
`a better understanding of
`For
`[0021]
`embodiments will now be described, purely by way of
`example, with reference to the accompanying drawings in
`which:
`
`
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`1 showsa first schematic embodiment of a
`[0022] FIG.
`bipolar transistor structure according to the invention;
`
`[0023] FIG. 2 shows a second schematic embodiment of
`a bipolar transistor structure according to the invention;
`
`[0024] FIG. 3 shows a third embodiment of a bipolar
`transistor structure according to the invention;
`
`FIGS.4 to 6 illustrate steps used to manufacture the
`[0025]
`bipolar transistor structure of FIG. 3;
`
`[0026] FIG. 7 shows the equal potential
`structure of FIG.3;
`
`lines in the
`
`FIG.8 showsthevertical field in the drift region of
`[0027]
`the structure of FIG. 3;
`
`[0028] FIG. 9 shows the improved current density in a
`device according to the invention;
`
`[0029] FIG. 10 shows the improved performance of the
`device of FIG.3;
`
`[0030] FIG. 11 shows the improved high frequency per-
`formance of the device of FIG.3;
`
`[0031] FIG. 12 shows the electron concentration and
`current flow in the device of FIG.3;
`
`[0032] FIG. 13 shows the cutoff-frequency breakdown
`voltage product for the device of FIG. 3, an altcrnative
`device and a comparative example;
`
`[0033] FIG. 14 shows a fourth embodiment of the inven-
`tion;
`
`[0034] KIG. 15 shows a top view of a fifth, schematic,
`cmbodimentof the invention;
`
`[0035] FIG. 16 shows a lateral section through the
`embodiment of FIG. 15; and
`
`[0036] FIG. 17 shows a longitudinal section through the
`embodiment of FIG. 15.
`
`It should be noted that all of the Figures are
`[0037]
`diagrammatic and not to scalc: the relative dimensions of the
`parts have been exaggerated and reduced in size forclarity
`and convenience. Like reference numbersare generally used
`to refer to corresponding or similar features.
`
`[0038] Referring to FIG. 1, a vertical bipolar transistor
`according to a first embodiment of the invention has a
`collector 4 including a higher doped collector region 3 and
`a lower doped drift region 5, a base region 7 above the drift
`region 5 and an emitter region 9 above the base region. The
`regions are of semiconductor material; the base region 7 is
`of opposite conductivity type to the emitter 9 and collector
`3,4,5 regions. In the example, the bipolar transistor is an
`NPNtransistor with a p-type base 7 and an n-type emitter 9
`and collector 4, but the invention is equally applicable to
`PNPtransistors. The drift region 5 is more lightly doped than
`the higher doped collector 3, base 7 and emitter 9 regions.
`
`[0039] The various transistor regions may be formed in a
`known manner, for example in crystalline silicon, II-V
`semiconductor, Si—Gelayers or other alloys, II-V1 semi-
`conductors, Nitride containing layers, etc.
`
`In contrast to a conventional vertical bipolar tran-
`[0040]
`sistor structure a semi-insulating gate 11 is provided at either
`side of the drift region insulated from the drift region 5 by
`
`a thin insulating layer 13. The gate clectrode is of scmi-
`insulating material so that it can support a voltage alongit.
`For example, semi-insulating polysilicon may be used; As
`will be apparent, a variety of different semi-insulating mate-
`rials may be used to the same effect.
`
`[0041] The insulating layer may be formed of silicon
`dioxide, silicon nitride, or combinations of these or other
`layers as will be apparent to the skilled person.
`
`[0042] Collector 15, base 17 and emitter 19 contacts are
`provided connecting to the base 7, collector 3 and emitter 9
`respectively. A first gate connection 1s provided in the form
`of a first gate contact 21 is provided at the upper surface of
`the gate electrode 11. The lower end ofthe gate electrode 11
`is connected to a second gate contact 23.
`
`In use, the transistor may be,controlled to be in a
`[0043]
`blocking mode of operation in which voltage is dropped
`between base and collector with only a very small current
`passing through the base and collector. In this mode, the
`transistor can be said to be switched off. As well as voltage
`across base and collector, voltage is also supplied across the
`gale contacts 21, 23 and is dropped along the gate 11. This
`creates a uniform vertical electric field within the gate
`electrode, which applies a uniform electric field to the drift
`region 5. Conveniently, the upper gate contact 21 may be
`connected to the emitter contact 19 (or to the base contact
`17), and the lower gate contact 23 to the collector contact 15,
`though in alternative embodiments separate contacts may be
`provided. The uniform electric field is beneficial in keeping
`the collector-base breakdown voltage as high as possible.
`
`‘he structure shown allows high frequency opera-
`[0044]
`tion, and especially allows the increase of the breakdown
`voltage with less degrading effect on the cutoff frequency
`than otherwise possible. Thus, the maximum possible prod-
`uct of the threshold frequency and the collector base break-
`down voltage may exceed the Johnson limit.
`
`[0045] Although FIG. 1 showsonlya single emitter and
`collector surrounded by the gate, it will be appreciated that
`in practice an array of such structures may be provided,
`connected in parallel to increase the current handling capa-
`bility of the device. For example, in embodiments an array
`of mesas each containing a drift region 5, a base 7 and an
`emitter 9 and surrounded by gates 11 maybe provided.
`
`[0046] By ensuring depletion of the drift region when
`applying a reverse voltage to the collector and a substan-
`tially lmear voltage gradient in the drift region provided, in
`this state, by the gate, regions of local high electric field can
`be avoided thereby increasing the breakdown voltage.
`
`[0047] The depletion of the drift region has the benefit that
`the doping in the drift region can be higher than it would
`need to be without gates to deplete the drift region when the
`transistor is turned olf. This postpones the Kirk effect in the
`transistor and increases the maximumpossible current den-
`sity and conscqucntly increascs the switching frequency.
`Moreover, it reduces the resistance of the drift region when
`the device is switched on and increases the maximum
`possible current density.
`
`[0048] The inventors are not aware of prior examples of
`using an insulated gate to deplete a drift region in a bipolar
`transistor structure.
`
`
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`
`[0049] The transistor type illustrated is a conventional
`bipolar transistor but the invention is also of application in
`a number of other types of transistor. For example,
`the
`invention is of particular utility in transistors for use at high
`frequency. At radio frequencies, heterostructure bipolar tran-
`sistors (HBTs) may be used, thus the invention is of par-
`ticular application to HBTs. The ideas of the invention may
`also be of use in a number of other types of structure,
`including for example an insulated gate bipolartransistor, or
`other similar structures.
`
`1 is a vertical struc-
`[0050] The structure shown in FIG.
`ture. This term is used in the present specification to refer to
`a device in which the current flow is through the device,
`generally perpendicular to the faces of the substrate, rather
`than laterally parallel
`to the substrate. Often, a vertical
`device is one in which the collector and emitter contacts are
`on opposite faces. The invention is also applicable in a
`lateral structure, for which see below with reference to FIG,
`15.
`
`It will be appreciated that the arrangement of FIG.
`[0051]
`1 is not the only arrangement that allows depletion of the
`drift region when the device is turned off, together with a
`substantially uniform field in the drift region. An alternative
`approach, shown in FIG.2, uses a conductive gale 33 with
`a separate gate contact 35. In this arrangement however, the
`doping in the drift region 5 is selected to give a substantially
`uniform field with a constant voltage on the gate 33. Thisis
`achieved by a graded doping profile, with the doping con-
`centration higher near the collector electrode than near the
`base electrode.
`
`[0052] Referring to FIG. 3, in a specific embodiment of
`the inventionthe structure is formed on a n* semiconductor
`body,or substrate 3 doped with a doping concentration
`2x10 cm7>. The body 3 has opposed front 6 and rear 8
`faces. A rear contact 15 connects to the substrate 3, which
`acts as the higher doped callector region 3 of the bipolar
`transistor.
`
`[0053] The remaining layers forming the transistor proper
`are formed on the substrate as a plurality of mesas 25 each
`of width 0.3 wm each mesa being surrounded by trench 27
`separated from the mesa 25 by a thin silicon dioxide
`insulating laycr 13 of thickness 10 nm.
`
`[0054] A 3.5 um thick drift region 5 doped n-type with a
`doping concentration 10*’cm™?is provided abovethe higher
`doped collector substrate 3. A thin SiGe layer 29 having a
`20% germaniumpercentage is formed over the drift region
`5. The thin SiGe layer 29 acts as the base, and is doped
`p-type with a doping concentration of 3x107° cm7*. Thereis
`an emitter-base space charge region 31 having a doping of
`5x10"7 cm7? over the-base 29. The emitler region 9 is
`located over the emitter-base space charge region 31 and has
`a doping of 2x10°+ cm7>. An emitter contact 19 is provided
`on the emitter 9. It will be appreciated that these specific
`dimensions and doping concentrations may be varied.
`
`[0055] The trenches 27 contain a semi-insulating polysili-
`con (SIPOS) gate 11. A contact 21 is formed on the top of
`the SIPOSgate 11 to form the first gate connection . ‘Ihe top
`of the SIPOS gate 11 is placed adjacent to the boundary
`between the base 7 and drift region 5 to avoid an inversion
`layer in the base adjacent to the gate 11 and to avoid field
`crowding, at the corner of the junction. The top contact 21 is
`
`connceted to the cmittcr contact 19. The basc contact to the
`base 29 is provided laterally of the mesa.
`
`It will be noted that the bottom of the SIPOSgate
`[0056]
`11 is connected directly to the collector to form the second
`gatc conncction at the opposite end of the gatc to the first
`gate connection.
`
`[0057] A method of manufacture of this structure is illus-
`trated with reference to FIGS. 4 to 6.
`
`[0058] The initial stages are exactly the same asthe stages
`used to form a heterostructure bipolar transistor having a
`drift region, and so will already be familiar to the skilled
`person in this field. An epilayer 5 to form the drift region is
`grown on n+ doped substrate 3 forming the collector region.
`The epilayer is doped n-type with a doping concentration of
`10’7em™.
`
`[0059] Next, a thin SiGe layer 29 is grown to form the
`basc, followed by a cmittcr-base space charge layer 31, and
`an emitter layer 9. It should be noted that as an alternative
`the emitter layer 9 may be grownlater.
`
`[0060] Next, a photoresist mask 41 is deposited and pat-
`terned. A plurality of trenches 27 are then ctched through the
`epilayer 5 as far as the collector substrate 3 using an
`anisotropic etch process. In the specific example, the process
`used is a dry etch process.It is not necessary to stop the etch
`exactly at the top of the collector substrate 3, so a slight
`overetch may be performed if required, resulting in the
`structure of FIG. 4.
`
`[0061] The photoresist 41 is stripped, and a gate insulation
`layer 13 formed to cover the structure, including the walls of
`the trench. In the specific embodiment the gate insulation
`layer 13 is a thermal oxidation layer formed by thermal
`oxidation in a suitable atmosphere. The thermal oxide layer
`13 is then etched using an anisotropic spacer etch process to
`remain only on the sidewalls of the trenches 27 (FIG.5).
`This etch process is not used to form alternative embodi-
`ments with a gate isolated from the collector.
`
`[0062] Chemical vapour depositionis then used to deposit
`scmi insulating polysilicon (SIPOS) 11 in accordance with
`known techniques,to fill the trenches. An etch back process
`is then used to clear any polysilicon from above the epilayer
`resulting in the structure and to etch the polysilicon11 to the
`level of the base 29 shown in FIG.6.
`
`Thefinal step is then to form contacts 15,19,21 by
`[0063]
`masking with photoresist, depositing contact metal, and
`removing the
`photoresist as is known to result
`in the
`structure of FIG. 3 of the application.
`
`[0064] As an alternative, especially for transistors that do
`not exceed 30 GHz,the base and collector may be implanted
`before or after
`trench formation.
`
`
`
`[0065] This method of manufacture is considerably easier
`to perform than manyaltcrnatives. In particular, there 1s no
`need for manylayers one on top of the other with different
`doping levels, which would be very hard to manufacture.
`
`[0066] The device according to FIG. 3 was modelled.
`FIG. 7 illustrates the potential contours in the device of
`FIG.3 with a voltage of 95 volts applicd between the base
`and collector—this being the breakdownlimit of this struc-
`ture. The left hand edge of the Figure corresponds to the
`centre of the mesa 25; the graph is symmetric aboutthis axis
`
`
`
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`
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`US 2003/0030488 Al
`
`Feb. 13, 2003
`
`and accordingly the left half of the mesa 25 is not shown. As
`can be seen, the contours are equally spaced in the gate
`region 11, whereas in the drift region 5 there is some
`horizontal field, and the contours are again equally spaced.
`This corresponds to a substantially constant vertical electric
`field in the drift region, except for near the base-collector
`junction. The vertical field (curve 43) as a function of
`distance is illustrated in FIG. 8. Except for near the base-
`collector junction, the vertical electric field is substantially
`constant in the drift region.
`[0067] The collector current of this structure has been
`simulated as a function of base-emitter voltage (V,.) and
`compared with that of a conventionalstructure (FIG. 9). The
`conventional hetero-structure bipolar transistor structure for
`comparison has a similar collector 3, basc 29, cmittcr-basc
`
`
`space charge region 31 and emitter 9. However, the con-
`
`
`ventional structure differs in two regards. Firstly, there are
`no trenches or gates, and secondly the doping in the drift
`region is only 10°°cm7? not 10*7cm™>. The device used for
`comparison also had a drift length of 3.8 wm as the device
`according to the embodiment of the invention has a drift
`length of 3.5 wm. The resulling comparison device has a
`breakdown voltage of around 95 V, similar to that of the
`embodiment of FIG.3.
`
`[0068] As can be seen from FIG. 9, the collector current
`density of the conventional hetero-structure bipolar transis-
`tor (curve 45) starts lo behave non-linearly in base-emitter
`voltage at 500 A/em? which roughly corresponds with the
`Kirk limit. This can be raised somewhat by supplying 10
`volts to the collector contact (curve 47).
`[0069] However,for the device according to the invention
`(curve 49) the current density can be much higher whichis
`due to the increase in doping possible in the drift region 5.
`The reason that higher doping is possible is that the device
`can be turned off by depletion from the trench regions 11.
`Another effect which increases the maximum collector cur-
`rent density is the horizontal field present in the drift region
`
`5 which forecs the holes out of the drift region. This lattcr
`
`
`
`effect is howeverless importantin this specific embodiment.
`[0070] FIG. 10 illustrates the collector current density as
`a function of collector-emitter voltage. Here again,
`the
`greatly improved performance of the device shown in FIG.
`3 according to the invention (curves 51,52) compared with
`a conventional device (curves 53,54) may be seen. The
`lower graphs (52, 54) are the resulis of a base-emitter
`voltage of 0.7V, the upper graphs (51, 53) of a base-emitter
`voltage of 0.75V.
`[0071] Calculations of the cutoff frequency have been
`
`performed with the upper contact 21 of the SIPOS layer
`connected to the emitter (FIG. 11). The maximumcutoff
`frequencyfor a conventional transistor (curve 55) is 2 GHz,
`while that of the device according to the invention (curve 57
`is 10 GHz. These calculations have been performed for
`collector-base voltages of 0 volts. The maximumcut o
`frequencyfor the new structure is reached at higher curren
`densities because the Kirk effect only kicks in at high
`densities. Hence,
`the invention allows the conventional
`maximum product of threshold frequencyand collector base
`breakdown voltage to be exceeded since the breakdown
`voltages of both the conventional and the newstructure are
`substantially the same (95 volts).
`[0072] FIG. 12 shows contourplots of the electron con-
`centration 59 and electron current flow 61 for the device
`
`—
`
`according to the invention. As in FIG.7, the left hand cdge
`of the graph represents the centre of the drift region 5. The
`current flows inhomogeneously through the drift region, as
`illustrated.
`
`the maximum available product of
`In FIG. 13,
`[0073]
`cut-off frequency with collector-base breakdown voltage is
`plotted as a function of collector-base breakdown voltage for
`a conventional HBT (curve 69), a device with the same low
`drift concentration as the conventional HBTbutincluding a
`trench gale according to the invention (curve 71) and an
`optimised HBT formed withthe structure of the embodiment
`of FIG. 3 (curve 73). As can be seen, the use of the trench
`field gates to control the drift region gives much better
`results. Indeed, these values are record valuesforsilicon and
`compete with the highest values available in III-V devices.
`A conventional $i(Ge) HBTbipolar device may have a
`maximum cut-off frequency of about 85 GHz for a break-
`down voltage of 2V. These values maybe exceeded by the
`invention, which may provide, for example, a 10V break-
`downvoltage with a 50 GHz cut-off.
`[0074] A further embodimentofthe invention is shown in
`FIG. 14,
`in which the semi
`insulating gate 11 of the
`embodiment of FIG. 3 is replaced with a p-i-n diode. The
`p-i-n diode has an p-layer 63, an intrinsic layer 67 and an
`n-type layer formed by the collector 3 and can carry a
`uniform field across the intrinsic layer with similar results to
`the uniform electric field produced in the embodiment of
`FIG. 3. A contact 65 connects to the p-layer 63. The
`structure of the FIG. 14 embodimentis particularly com-
`patible with existing BiCMOSprocesses.
`[0075]
`Ina fifth embodiment of the invention, illustrated
`schematically in FIGS. 15, 16, and 17, a lateral structure is
`provided. FIG. 15 shows a top view of the structure, and
`FIGS. 16 and 17 sections along A-A and B-B respectively.
`In this embodiment, an insulating substrate 91, for example
`a glass substrate or a semiconductor substrate with an
`insulating layer deposited over it is provided and the tran-
`sistor structure is formed in semiconductor layers over the
`substrate.
`
`the structure
`[0076] Reterring to FIGS. 16 and 17,
`includes, arranged along longitudinal axis B-B,a collector4,
`a base region 7 and an emitter region 9. The collector region
`4 is divided into a higher doped region 3 and a lower doped
`drift region 5 extending longitudinally between the higher
`doped region 3 and the base 7. It will be noted that the
`direction B-B is referred to as the longitudinal axis and
`direction A-A as the lateral axis since the transistor current
`flow is along B-B.
`[0077] A seminsulating gate electrode 95 insulated from
`the drift region 5 bygate insulation 97 is provided above the
`drift region 5. Contacts 81,83 are provided on the gate
`electrode 95 at longitudinal ends of the gate electrode 95.
`These contacts 81,83 are used for applying a voltage-along
`the gale electrode 95 in like mannerto the vertical embodi-
`ments.
`
`[0078] Referring to FIGS. 15 and 17, the semiinsulating
`gate electrode 95 extends laterally around the drift region 5
`in trenches 93 arranged oneitherside of the drift region. The
`gate oxide layer 97 extends on the sidewall and base of the
`trench to insulate the gate electrode from the drift region 5.
`[0079]
`In use, in a voltage blocking mode of operation
`voltage is supplied along the gate contacts 81, 83 to deplete
`
`
`
`Dell Ex. 1035
`
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`

`US 2003/0030488 Al
`
`Feb. 13, 2003
`
`the drift region 5 and also to maintain a substantially
`uniform longitudinal electric field in the drift region to
`improve the collector-base breakdown voltage,
`in a like
`manner to the vertical embodiments described above.
`
`Thelatcral structure is relatively casy to manufac-
`[0080]
`ture. The gates are in insulated trenches.
`
`It should be noted that the invention is not limited
`[0081]
`to the example described above and in particular may be
`uscd for any hetcrostructure bipolar transistor or indced
`other (bipolar) transistor structure.
`
`[0082] The specific sizes and materials of the specific
`embodiments described above may be varied, as will be
`appreciated by the skilled person.
`
`[0083] Moreover, the invention is not limited to n-type
`transistors and can be used also for PNPtransistors. Also, the
`device is not limited to silicon structures and can also be
`uscd in germanium, germanium silicon, ITI-V, TI-N and SiC
`bipolar devices.
`
`In particular, the use of a SiGe HBT structure is
`[0084]
`useful for devices operating at voltages lower than 30V,
`especially below 10V. For higher voltage devices, a more
`conventional Si bipolar junction transistor design may be
`used. Accordingly, for a preferred arrangement having a
`breakdown voltage of 95V the Si—Ge-base layer of the
`embodimentillustrated in FIG. 3 may be replaced with a Si
`layer having a lower doping concentration of typically about
`107° cm".
`
`[0085] From reading the present disclosure, other varia-
`tions and modifications will be apparent to persons skilled in
`the art. Such variations and modifications may involve
`equivalent and other features which are already known in the
`design, manufacture and use of semiconductor devices and
`which may be used in addition to or instead of features
`described herein. Although claims have been formulated in
`this application to particular combinations of features,
`it
`should be understood that
`the scope of disclosure also
`includes any novel feature or any novel combination of
`features disclosed herein either explicitly or implicitly or
`any generalisation thereof, whetheror notit mitigates any or
`all of the same technical problems as does the present
`invention. The applicants hereby give notice thal new claims
`may be formulated to any such features and/or combinations
`of such features during the prosecution of the present
`application or of any further applications derived therefrom.
`
`L. A bipolar transistor structure, comprising:
`
`a collector including a higher doped co