THE p-n JUNCTION
Structure of p-n Junction Diode:
p-n Junction Diode Fabrication:
1. Ion Implantation Process
2. Diffusion process
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
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.
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.
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).
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
Where C1 = constant of integration,
The potential is equal zero at x = -xp
The constant of integration is found as:
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:
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
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.
• 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
The total space charge width can be expressed as:
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