Germanium Wafer | The Doping Technique Involved in Germanium Wafer Fabrication

Doping plays a crucial role in Germanium wafer fabrication. It is the process of adding impurities into a semiconductor crystal to alter its conductivity. In this article, we will discuss the technique used to dope wafers.

How are Germanium Wafers Doped?

Diffusion

Diffusion is a net transport of molecules from an area of higher concentration to another area of lower concentration through random molecular motion. This movement results in a continuous mixing of materials. In a Ge crystal, there's a solid lattice of the atom through which the dopant has to migrate. This can be done in three different ways:

• Empty Space Diffusion- This refers to the process where impurity atoms fill empty spaces in the crystal lattice.
• Inter Lattice Diffusion- This refers to the process where impurity atoms migrate in-between the silicon atoms in the crystal lattice.
• Changing of Places- This refers to the process of exchanging Ge atoms with the impurity atoms found in the crystal lattice.

Different Types of Diffusion

Diffusion with an Exhaustible Source- This means that the dopant is available in a definite supply only. The longer the diffusion process goes on, the lower the concentration is at the surface, which increases the depth of penetration into the wafer.
Diffusion with an Inexhaustible Source- This means that there's an unlimited supply of dopants during the diffusion process. This helps keep the concentration at the surface constant throughout the diffusion process. The particles that have pierced into the wafer are also constantly replenished.

Methods of Diffusion

• Diffusion from the Gas Phase- This process utilizes a carrier gas which is enhanced with the desired dopant that is also in gaseous form. It is then transported towards the Germanium wafer, where the concentration balance occurs.
• Diffusion with Solid Source- Wafer slices that contain the dopants are situated in-between the wafers. Once the temperature inside the quartz tube is heightened, the dopant from the source discs disseminates into the atmosphere.
• Diffusion with Liquid Source- A carrier gas is brought through the liquids, moving the dopant in a gaseous state.

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