In general, diffusion systems are similar to oxidation furnaces. The dopant can be provided in the form of gas, liquid, or solid.

The basic approach is to deposit a glassy layer containing the dopant on the surface of the silicon. A reaction between the glass and the silicon transfers dopant into the silicon.

Figure below shows a solid source diffusion system where a platinum boat is used to hold a source of dopant oxide upstream from the carrier with the semiconductor wafer.

Source shut off is done by moving the dopant source to colder region of the furnace.

In operation the carrier gas transports vapors from this source and deposits them on the semiconductor slices.

The success of this technique depends critically on the vapor pressure of the source.

Examples of sources:

1. Diffusion of p-type impurity –

   a. Boron is an almost exclusive choice as an acceptor impurity in silicon.

   b. Source wafers are made of nitride (BN).

   An oxidizing reaction transfers $B_2O_3$ to silicon surface.

2. Diffusion of n-type impurity –

   a. $P_2O_5$ is a solid source for phosphorus impurity and can be used.

   b. $P_2O_5$ transfers directly to the surface of the silicon when the source wafers are heated.

   c. Ammonium dihydrogen phosphate $(NH_4HPO_4)$ can also be used.