THE p-n JUNCTION

Structure of p-n Junction Diode:

p-n Junction Diode Fabrication:

1. Ion Implantation Process2. 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).

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.

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.

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.

The total space charge width can be expressed as:

Home Work Problem:

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

ONE SIDED ABRUPT JUNCTION:

Home Work: