ShOC Rectifier: A New Metal-Semiconductor Device with Excellent Forward and Reverse Characteristics

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  • 8/14/2019 ShOC Rectifier: A New Metal-Semiconductor Device with Excellent Forward and Reverse Characteristics

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    Shielded Ohmic Contact (ShOC) Rectifier: A New

    Metal-Semiconductor Device with Excellent Forward

    and Reverse Characteristics

    M. Jagadesh Kumar

    Department of Electrical Engineering,

    Indian Institute of Technology, Delhi,

    Hauz Khas, New Delhi 110 016, INDIA.

    Email: mamidala@ieee.org Fax: 91-11-2658 1264

    Abstract

    We report a new structure, called the Shielded Ohmic Contact (ShOC) rectifier which utilizes

    trenches filled with a high barrier metal to shield an Ohmic contact during the reverse bias.

    When the device is forward biased, the Ohmic contact conducts with a low forward drop.

    However, when reverse biased, the Ohmic contact is completely shielded by the high barrier

    Schottky contact resulting in a low reverse leakage current. Two dimensional numerical

    simulation is used to evaluate and explain the superior performance of the proposed ShOC

    rectifier.

    Index Terms: Schottky barrier, Ohmic Contact, forward voltage drop, reverse leakage

    current, Rectifier, Diode, breakdown, Simulation

    Final Manuscript submitted to

    IEEE Trans on Electron Devices17 November 2004

    M. Jagadesh Kumar, "ShOC Rectifier: A New Metal-Semiconductor Device with Excellent Forward and ReverseCharacteristics," IEEE Trans. on Electron Devices, Vol.52, pp.130-132, January 2005.

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    IntroductionApplications requiring low power dissipation and fast switching speed widely use low barrier

    Schottky rectifiers. One has to always make a trade-off between the forward voltage drop and

    the reverse leakage current, since a low barrier metal gives small forward voltage drop but at

    the cost of high leakage current and the high barrier metal gives a lower leakage current but

    with a high forward voltage drop. To overcome the above problem, several novel lateral

    Schottky rectifiers have been reported in the recent past [1-5]. Among them, in the lateral

    merged double Schottky (LMDS) rectifier [1,2], two Schottky barriers are used - the low

    barrier Schottky contact conducts during forward bias and the high barrier Schottky contact

    effectively shields the low barrier Schottky contact during the reverse bias. One drawback of

    the LMDS rectifier is that it requires two Schottky metals. In this paper, we report a new

    device called the Shielded Ohmic Contact (ShOC) rectifier in which the low barrier Schottky

    contact in the LMDS rectifier is replaced by an Ohmic contact so that during the forward

    bias, the Ohmic contact conducts and is effectively shielded by the high barrier Schottky

    contact during the reverse bias. Using numerical simulations we demonstrate that using the

    ShOC rectifier, which uses only one high barrier Schottky contact, we can still realize low

    forward drop, low reverse leakage current and high breakdown voltage similar to that of the

    LMDS rectifier which uses two Schottky contacts.

    Simulation results and discussion

    A cross-sectional view of the ShOC rectifier is shown in Fig.1 which is implemented in

    MEDICI[6], a 2D device simulator. The tunneling Ohmic contact formed by the high barrier

    metal on the diffused N+

    region between the two high-barrier metal trenches forms the anode

    contact of the device. This N+ region can be formed before creating the trenches in the oxide

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    window region. The cathode contact is placed symmetrically on both sides of the anode. To

    reduce the electric field crowding at the trench edges, metal field termination is used[7]. We

    have compared the ShOC rectifier with LMDS and the conventional low barrier Schottky

    (LBS) and high barrier Schottky (HBS) structures. The parameters used for the simulation of

    these rectifiers are given in Table 1.

    The simulated current voltage characteristics of the ShOC rectifiers are compared

    with that of the LMDS, conventional low barrier and high barrier Schottky structures in Fig.

    2. We can see that the forward characteristics (as shown in Fig. 2(a)) and the reverse

    characteristics (as shown in Fig. 2(b)) are approximately identical for the ShOC and LMDS

    rectifiers. Just as in the case of the LMDS rectifier, the ShOC rectifier also exhibits a higher

    breakdown voltage when compared to the LBS and HBS rectifiers. The improved

    performance of the ShOC rectifier can be understood from the current flow lines shown in

    Fig.3. When the ShOC rectifier is forward biased, the Ohmic contact conducts all the forward

    current giving rise to a low forward voltage drop. However, under reverse bias conditions, we

    notice that the Ohmic region is effectively shielded by the high barrier trench and the reverse

    leakage current corresponds to that of the high-barrier trench. In the above simulations, we

    have taken an optimum value for the Ohmic contact width (m=0.2 m) at the given trench

    depth (d=1.5 m). Although we have chosen a trench aspect ratio(d/w) equal to 7.5, our

    simulations show that the trench width w can be increased to reduce the trench aspect ratio

    without affecting the device characteristics.

    The performance of the ShOC rectifier, i.e. the how efficiently the Ohmic region can

    be shielded by the high-barrier trench is, however, dependent on the d/m ratio. The forward

    voltage drop (at a forward current density of 100 A/cm2) and the reverse current density (at a

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    reverse voltage of 25V) versus the d/m ratio are shown in Fig. 4 ( with d fixed at 1.5 m and

    m as a variable) and in Fig. 5 ( with m fixed at 0.2 m and das a variable) We can easily

    observe from Fig. 4 that as m increases for a fixed d, the shielding provided by the high

    barrier trench becomes inefficient and the Ohmic contact region plays a greater role in current

    conduction. This will result in a reduced forward voltage drop but will also increase the

    reverse leakage current. Therefore, the d/m ratio should be chosen such that the reverse

    leakage current is at its minimum. Both the forward voltage drop and reverse leakage current

    will approximately match with that of the LMDS for a d/m ratio of 7 or above. On the other

    hand, we observe from Fig. 5 that for a fixed m, ifdis decreased, the Ohmic contact region is

    again not efficiently shielded for a d/m ratio less than 7. The important observation that needs

    to be made here is that the Ohmic contact region is not efficiently shielded if m is long in

    relation to d. Therefore, for the given trench depth in our simulations, we have chosen the

    Ohmic contact width m to be 0.2 m so that d/m ratio is greater than 7. Thus for a given

    trench depth, by choosing an appropriate width for the Ohmic region, we can easily realize the

    same current voltage performance for the ShOC rectifier as compared to the LMDS rectifier.

    Conclusion

    A novel shielded ohmic contact rectifier (ShOC) having a high barrier metal filled in a trench

    surrounding an Ohmic contact is presented. The results show that this structure is as good as

    the previously reported LMDS structure with the advantage of using only one high barrier

    Schottky metal contact. The ShOC rectifier also exhibits significantly enhanced breakdown

    voltage compared to the conventional high barrier or low barrier Schottky rectifiers. The

    combined low forward voltage drop and excellent reverse blocking capability make the

    proposed ShOC rectifier attractive for use in low-loss high speed smart power IC applications.

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    References

    [1] Y. Singh and M.J. Kumar, A new 4H-SiC lateral merged double Schottky (LMDS) rectifier

    with excellent forward and reverse characteristics, IEEE Trans. on Electron Devices, Vol.48,pp. 2695-2700, Dec 2001.

    [2] Y. Singh and M.J. Kumar, Novel lateral merged double Schottky (LMDS) rectifier: proposaland design,IEE Proc. Circuits, Devices and Systems, Vol. 148 pp. 165-170, June, 2001.

    [3] Y. Singh and M.J. Kumar, Lateral thin-film Schottky (LTFS) rectifier on SOI: a device with

    higher than plane parallel breakdown voltage, IEEE Trans. on Electron Devices, Vol.49, pp.181-184, January 2002.

    [4] M.J. Kumar and Y. Singh, A new low-loss lateral trench sidewall Schottky (LTSS) rectifieron SOI with high and sharp breakdown voltage, IEEE Trans. on Elec. Devices, Vol. 49 pp.

    1316-1319, July 2002.

    [5] M. J. Kumar and C.L. Reddy, "A New High Voltage 4H-SiC Lateral Dual SidewallSchottky (LDSS) Rectifier: Theoretical Investigation and Analysis", IEEE Trans. on

    Electron Devices, Vol.50, pp.1690-1693, July 2003.

    [6] MEDICI 4.0, a 2D device simulator. TMA, Palo Alto, CA.

    [7] V. Saxena, J. N. Su, and A. J. Steckl, High-voltage Ni- and Pt-SiC Schottky diodes utilizing

    metal field plate termination, IEEE Trans. on Electron Devices, Vol. 46, pp. 456-464, Mar.

    1999.

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    Table 1. MEDICI parameters for the LBS, HBS, ShOC and LMDS structures

    Parameter Value

    N+

    doping for Ohmic contact 1019

    cm-3

    Drift region doping, ND 1016cm-3

    Drift region length, L 12 m

    Drift region thickness, tsi 2 m

    Buried oxide thickness, tbox 3 m

    Field oxide thickness, tox 0.5 m

    Trench depth, d 1.5 m

    Trench width, w 0.2 m

    Mesa width, m 0.2 m

    Low Schottky barrier height(Ni), L 0.57eV

    High Schottky barrier height(W), H 0.67eV

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    Figure captions

    Fig. 1 Cross-section of the Shielded Ohmic Contact (ShOC) rectifier.

    Fig. 2 (a) Forward characteristics and (b) Reverse characteristics of the ShOC

    rectifier compared with that of the LMDS, conventional low-barrierSchottky (LBS) and high-barrier Schottky (HBS) rectifiers

    Fig. 3 (a) Forward current flow lines of ShOC rectifier at a forward voltage of 0.3

    V and (b) Reverse current flow lines of ShOC rectifier at a reverse voltage of120 V.

    Fig. 4 (a) Forward voltage drop versus d/m ratio at a forward current density of

    100A/cm2

    and (b) Reverse leakage current density versus d/m ratio at areverse bias of 25 V for the ShOC and the LMDS rectifiers. Here m is varied

    for a fixed trench depth w.

    Fig. 5 (a) Forward voltage drop versus d/m ratio at a forward current density of 100A/cm2 and (b) Reverse leakage current density versus d/m ratio at a reverse

    bias of 25 V for the ShOC and the LMDS rectifiers. Here the trench width wis varied for a fixed m.

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    Cathode Cathode

    m

    W

    Buried Oxide

    N+ Substrate

    N+ N+

    tSid

    N Drift Region

    L

    High Barrier Schottky and Anode

    N+ Ohmic contact

    Field Oxide

    Fig.1

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    0.0 0.1 0.2 0.3 0.4 0.5 0.6

    10-4

    10-5

    10-6

    10-7

    10-8

    10-9

    10-10

    HBSLBSLMDSShOC

    Forward

    currentIF,A/m

    Forward voltage drop VF

    , [V]

    Fig. 2(a)

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    0 20 40 60 80 100 120 140

    10-8

    10-9

    10-10

    10-11

    10-12

    10-13

    HBSLBSLMDS

    ShOC

    Revers

    ecurrentIR,A/m

    Reverse voltage VR, [V]

    Fig. 2(b)

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    Fig. 2(a)

    Fig. 3(a)

    Fig. 3(b)

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    2 4 6 8 10 12 14 160.15

    0.20

    0.25

    0.30

    0.35

    0.40

    0.45

    d=1.5m

    (m variation)

    LMDS

    ShOC

    Forwa

    rdvoltagedropVF,[

    V]

    d/m

    Fig.4(a)

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    2 4 6 8 10 12 14 16

    d=1.5m

    LMDSShOC

    (m variation)

    -2

    2

    0

    4

    6

    8

    10

    12

    14

    16

    18

    Reversec

    urrentIR,A/m(

    1X10-6)

    d/m

    Fig.4 (b)

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    0 2 4 6 8 10

    0.05

    0.10

    0.15

    0.20

    0.25

    0.30

    0.35

    0.40

    0.45

    0.50

    m=0.2m

    ShOC

    (d variation)

    LMDS

    ForwardvoltagedropVF,

    [V]

    d/m

    Fig.5 (a)

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    0 2 4 6 8 10

    m=0.2m

    LMDSShOC

    (d variation)

    0

    1

    2

    4

    3

    5

    Reverse

    currentIR,A/m(

    1X1

    0-4)

    d/m

    Fig. 5 (b)

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