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P n-junc-diode1


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Pn junction

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P n-junc-diode1

  1. 1. THE p-n JUNCTION Structure of p-n Junction Diode: p-n Junction Diode Fabrication: 1. Ion Implantation Process 2. Diffusion process
  2. 2. Figure 1 (c)Planar p-n junction diode fabrication on epitaxial substrate by diffusion, (b) Planar p-n junction diode fabrication on epitaxial substrate by ion implantation. Ion Implantation System
  3. 3. p-n Junction Diode Physics Space Charge Region: The net positively and negatively charge region shown in Figure 7.2 is called space charge region.
  4. 4. Built-in Voltage (Potential) Barrier (Vbi): • Assume that no voltage bias is applied across the p-n junction barrier. • So the p-n junction is in the thermal equilibrium- the Fermi level is constant throughout the system. • The conduction band and valency band energies must bend in the space charge region to accommodate the carriers. • The energy separation between the conduction bands in p- and n-type semiconductor is the built-in voltage Vbi. The built-in voltage can be found from the energy separation of intrinsic Fermi level in p- and n-regions. We can define the potentials ΦFn and ΦFp.
  5. 5. Electric Field: An electric field is created in the space charge region by the separation of positive and negative charges. We will assume that the space charge region abruptly ends in the n-region at x = +xn and abruptly ends in the p-region at x = - xp (xp is a positive quantity).
  6. 6. E Figure 7.5 is a plot of the electric field in the space charge region. The electric field direction is from the n to the p-region or in the negative x direction for this geometry.
  7. 7. Where C1 = constant of integration, The potential is equal zero at x = -xp The constant of integration is found as:
  8. 8. Space Charge Width: The space charge region extends into the p- and n- regions from the metallurgical junction. The space charge region can be defined using Equation (7.17) as:
  9. 9. Comparative Study of Space Charge for Various Bias Condition F. Biasing R. Biasing Barrier potential Height BPH: Vbi - Vf BPH: Vbi+VR (BPH): Vbi (Vo) Vf = F. Biasing VR = R. Biasing
  10. 10. Space Charge Width and Electric Field: Figure 7.8 shows a p-n junction with an applied reverse-bias voltage VR. The electric field in the space charge region and the applied electric field Eapp are induced by the applied electric field. E • At the edge of depletion region (space charge region) at xn and xp, the electric field is almost zero. • Therefore, the electric field at the neutral zone is zero. • The direction of the electric field from n-regions (+ve charge) to p-region (-ve) charge.
  11. 11. The total space charge width can be expressed as:
  12. 12. Home Work Problem:
  13. 13. JUNCTION CAPACITANCE: Since a separation of positive and negative charges is existing, a capacitance is associated in the p-n junction. The charge densities are changing with variation of R. biasing VR to dVR. The junction capacitance can be expressed as: The space charge width under R. Biasing
  15. 15. Home Work: