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Rectifying and Ohmic Contacts

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Silicon Analog Components

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Abstract

A contact between metal and silicon can be rectifying or ohmic. The most commonly used rectifying contact is the Schottky barrier diode (SBD). Because of its fast response to signals, the SBD has found several applications in analog circuits where switching speed is important. A contact is said to be ohmic, i.e., non-rectifying, if it exhibits negligible resistance to current in both voltage polarities. Most semiconductor devices are interconnected on the chip and brought to the “outside world” by means of ohmic contacts and metal wires. Understanding the physical nature of contacts and methods to reduce their resistance is becoming increasingly important as contact dimensions are reduced. The first part of this chapter discusses SBD properties, characterization, and applications. The second part describes the formation and characterization of ohmic contacts.

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Notes

  1. 1.

    Unless otherwise stated, E, EF, Ei, EC, EV, Eg represent energies in eV; ϕ, ψ, χ represent potentials in V. Energies and potentials will then have the same numerical values but different units. Example: At 25 °C, kT ≅ 0.026 eV, kT/q ≅ 0.026 V.

  2. 2.

    The combined potential energy due to depleted charge in silicon and approaching single electron reaches its maximum (saddle point) at \( \Delta x=\sqrt{q/\left(16{\pi \varepsilon}_0{\varepsilon}_{\mathrm{Si}}{\mathbf{E}}_{\mathrm{peak}}\right)} \).

  3. 3.

    Latch-up is discussed in Chap. 11.

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Author information

Authors and Affiliations

  1. PIYE, Cedar Park, TX, USA

    Badih El-Kareh

  2. Lou Hutter Consulting, Dallas, TX, USA

    Lou N. Hutter

Authors
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Problems

Problems

  1. 1.

    A CoSi2 Schottky barrier diode of effective barrier height ϕB = 0.60 V and area 50 × 50 μm2 is formed on a uniformly N-type silicon of concentration ND = 2 × 1016 cm−3. Assume thermal equilibrium at 300 K and find

    1. (a)

      The depletion width in silicon

    2. (b)

      The peak field

    3. (c)

      The diode capacitance

    4. (d)

      The barrier seen by bulk electrons at the bottom of the conduction band

  2. 2.

    The forward characteristic in Fig. P2 was obtained on a SBD of area 2.5 × 10−5 cm2 at 300 K. Find the ideality factor, the barrier height, and the series resistance of the diode.

    Fig. P2
    figure 33

    The ideality factor, the barrier height, and the series resistance of the diode

    Full size image
  3. 3.

    Consider the structure in Fig. P3. The silicide contacting the P-region extends into the N-region to form a SBD of barrier height 0.6 V and ideality factor n = 1.04. A forward-biased voltage of 0.5 V is applied to both the P-region and SBD. The temperature is 300 K, the PN junction area is 10 μm2, the N-region is uniformly doped at ND = 5 × 1016 cm−3, and the injected minority carriers from the P-region immediately recombine when they reach the buried N+-layer.

    Fig. P3
    figure 34

    The silicide contacting the P-region extends into the N-region to form a Schottky barrier diode

    Full size image
    1. (a)

      Find the SBD area that is necessary to ensure that only 1% of the forward current consists of minority-carrier injection from the P-region.

    2. (b)

      Estimate the leakage current for a reverse voltage of 2.5 V applied to both diodes. Neglect surface effects.

  4. 4.

    The room temperature capacitance of a reverse-biased SBD of area 100 × 100 μm2 is 5.09 pF at VR = 1 V and 2.73 pF at VR = 5 V. Knowing that silicon is N-type and uniformly doped, find the dopant concentration ND and the SBD barrier height ϕB.

  5. 5.

    A contact chain of the type described in Fig. 4.32 is designed on a P-well having a concentration NA = 1017 cm−3. The chain contains of 1000 links, each link consisting of one N+-diffusion of width W = 2 μm and two contacts at a space s = 1 μm between contacts. The sheet resistance of the N+-diffusion is 36 Ω/□. To extract the average contact resistance, 1 mA is forced between the chain end terminals and the voltage measured between the terminals. Why would this test give erroneous results?

  6. 6.

    A SBD of area 4 × 10−6 cm2 is formed on N-type silicon of concentration ND = 1016 cm−3. The barrier height at zero applied voltage is 0.45 V. Assume that barrier lowering follows the relation in (4.13) and estimate the room temperature leakage current at a reverse voltage VR = 5 V.

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El-Kareh, B., Hutter, L.N. (2020). Rectifying and Ohmic Contacts. In: Silicon Analog Components. Springer, Cham. https://doi.org/10.1007/978-3-030-15085-3_4

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  • DOI: https://doi.org/10.1007/978-3-030-15085-3_4

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