Question

sketch, label & discuss the lattice structure showing the bonding & sketch band diagram. Show the Si chip connected to D.C. voltage, indicate charge carriers, and show donor/acceptor levels.

a. A Si doped with Ge( has 4 velectrons).

Answer 1

Solution:-

A p-n junction consists of two semiconductor regions with opposite doping type as shown in Figure The region on the left is p-type with an acceptor density N_a, while the region on the right is n-type with a donor density N_d. The dopants are assumed to be shallow, so that the electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density

We will assume, unless stated otherwise, that the doped regions are uniformly doped and that the transition between the two regions is abrupt. We will refer to this structure as an abrupt p-n junction.

Frequently we will deal with p-n junctions in which one side is distinctly higher-doped than the other. We will find that in such a case only the low-doped region needs to be considered, since it primarily determines the device characteristics. We will refer to such a structure as a one-sided abrupt p-n junction

The junction is biased with a voltage V_a as shown in Figure. We will call the junction forward-biased if a positive voltage is applied to the p-doped region and reversed-biased if a negative voltage is applied to the p-doped region. The contact to the p-type region is also called the anode, while the contact to the n-type region is called the cathode, in reference to the anions or positive carriers and cations or negative carriers in each of these regions

a)

• At 0^oK, no free carriers are available, Si behaves as an insulator.
• At room temperature, a few covalent bonds will be broken by the thermal energy, electron‐hole pair generation as free carriers.
• Both electrons and holes are free to move, can contribute to current conduction

The conductivity would not be affected much. This is because Si and Ge have an equal number of electrons (4) in their outermost bands. If the doping concentration is large enough, however, you’d actually find that the conductivity *drops* due to scattering mechanisms that result from lattice defects caused by the additive Ge.

You would also observe a larger effective lattice constant in your Ge-doped Silicon. If we place a Ge atom in a Si lattice site, the Ge atom (which is significantly larger) will exhibit a strong electrostatic force against the surrounding Si atoms, thus expanding the size of the lattice. However, such “expanded” lattice would only manifest itself in bulk material if there is so much Ge that you practically have a SiGe sample (i.e. a 1:1 ratio).