Question

A. Germanium is a semiconductor. If small amounts of the elements In, P, Sb, or Ga are placed into the Ge as impurities (not at the same time but in four separate instances) what types of semiconductors are manufactured in the four cases?

B. Normally, in conducting materials, we think of current as being carried by electrons as they move through a solid. In semiconductors, it is also common to talk about the current being carried by the “holes” in the valence band.

a. Explain how “holes” move through a solid material

b. If, in a p-type semiconductor device, electric current is moving from left to right, in which direction will the holes be moving?

Answer 1

A.

For Germanium,

Atomic number: 32

Valence electrons: 4

Thus, germanium is a tetravalent atom.

For indium (In),

Atomic number: 49

Valence electrons: 3

Thus, indium is a trivalent atom.

When a tetravalent atom is doped with trivalent atom, a p-type extrinsic semiconductor is formed.

Therefore, a p-type extrinsic semiconductor is formed when In impurities is doped with Ge.

For Phosphorus (P),

Atomic number: 15

Valence electrons: 5

Thus, P is a pentavalent atom.

When a tetravalent atom is doped with pentavalent atom, a n-type extrinsic semiconductor is formed.

Therefore, a n-type extrinsic semiconductor is formed when P impurities is doped with Ge.

For Antimony (Sb),

Atomic number: 51

Valence electrons: 5

Thus, indium is a pentavalent atom.

When a tetravalent atom is doped with pentavalent atom, a n-type extrinsic semiconductor is formed.

Therefore, a n-type extrinsic semiconductor is formed when Sb impurities is doped with Ge.

B.

a.

There is a vacany or hole when a Si is doped with impurities having 3 electrons in its valence shell. This makes three of the Si atom to form covalent bond with three electrons of the impurities atoms. But there is a fourth electron of the Si which is not formed covalent bond with any electron. There is a tendency for the fourth electron of Si to formed covalent bond with an electron. An electron from the outermost orbit of the neighbourhood Si will jump to bond with the fourth electron which leaves a hole or vacant on its own site. Thus, a hole is created in the neighbouring Si atom. In this way, many holes are created inside the lattice. The movement of these electrons in one way can be seen as a hole is moving inside the lattice. This makes the direction of the movement of hole just opposite to the electrons. Therefore, the movement of holes is due to the movement of electrons in the solid material.

b.

It is given that an electric current is moving from left to right in a p-type semiconductor device.

Holes are the majority carrier in p- type semiconductor device. Thus, the current is due to the flow of holes.

Therefore, the direction of the holes will be moving from left to right.

For Gallium (Ga),

Atomic number: 31

Valence electrons: 3

Thus, Ga is a trivalent atom.

When a tetravalent atom is doped with trivalent atom, a p-type extrinsic semiconductor is formed.

Therefore, a p-type extrinsic semiconductor is formed when Ga impurities is doped with Ge.