COURSE NAME: SEMICONDUCTORS Course Code: PHYS 473

Learning Outcomes  Define conductor, insulator and semiconductor, and state the resistance or conductance of each.  Name at least three semiconductor materials and state the most widely used.  Name the basic structure of material and explain how it is formed with atoms.  Define doping and name the two types of semiconductor material formed with doping.  Name the current carriers in N and P-type material.  Explain how current flows in semiconductor material.

Electronic Materials

Conductors Good conductors have low resistance so electrons flow through them with ease. Examples: 1. Metals- Copper, silver, gold, aluminum, & nickel. 2. Alloys- Brass & steel. 3. Liquids- Salt water

Conductor Atomic Structure The atomic structure of good conductors usually includes only one electron in their outer shell. 1. It is called a valence electron. 2. It is easily striped from the atom, producing current flow.

Insulators Insulators have a high resistance so current does not flow in them. Examples: Glass, ceramic, plastics, & wood. The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.

Semiconductors Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between. Examples: Carbon, Silicon, Germanium. Silicon is the best and most widely used semiconductor.

Silicon (Si) is in the IVA Column (4 valence electrons) Boron (B) is in the IIIA Column (3 valence electrons) Phosphorus (P) is in the VA Column (5 valence electrons)

Semiconductor Valence Orbit The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Crystal Lattice Structure The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice. The atoms link together with one another sharing their outer electrons. These links are called covalent bonds. 2D Crystal Lattice Structure

Si Covalent Bonds: Shared electrons to fill orbital 11

Si Poor conductor: No free electrons to carry current Need to engineer electrical properties (conduction) Intrinsic Silicon

Semiconductors can be Insulators? If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow. Good insulating semiconductor material is referred to as intrinsic. Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow. Silicon in this form is a great insulator. Semiconductor material is often used as an insulator.

Doping Doping is the incorporation of impurities into a semiconductor according to our requirements. “Impurities” are different elements. Examples: Boron, Arsenic

Semiconductors can be Conductors An impurity, or element like arsenic, has 5 valence electrons. Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms. The ONE electron left over for each arsenic atom becomes available to conduct current flow.

Si P P P Each N-type dopant brings an extra electron to the lattice N-type Doping

Resistance Effects of Doping If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely. If you use only a few boron atoms, there will be fewer free electrons so the resistance will be high and less current will flow. By controlling the doping amount, virtually any resistance can be achieved.

Position Energy Gap Conduction Band Valence Band N-Type Doping Doping the silicon lattice with atoms with 5 valence electrons (V) create sites in the band diagram that require little energy to break the bond to the dopant atom and become free to move in the lattice or in other words move into the conduction band. PPPPPPPPPPPPP

P-type Doping You can also dope a semiconductor material with an atom such as boron that has only 3 valence electrons. The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond. This place where a fourth electron should be is referred to as a hole. The hole assumes a positive charge so it can attract electrons from some other source. Holes become a type of current carrier like the electron to support current flow.

Si B B B Each P-type dopant is short an electron, creating a hole in the lattice P-type Doping

Position Energy Gap Conduction Band Valence Band P-Type Doping Doping the silicon lattice with atoms with 3 valence electrons (III) create sites in the band diagram that require little energy to trap an electron into the dopant atom. Holes are created in the valence band that are free to move. BBBBBBBBBBBBB

Types of Semiconductor Materials The silicon doped with extra electrons is called an “N type” semiconductor. “N” is for negative, which is the charge of an electron. Silicon doped with material missing electrons that produce locations called holes is called “P type” semiconductor. “P” is for positive, which is the charge of a hole.

Current Flow in Semiconductors 1. All electrons are associated with their covalent bonds. 2. The conduction band (above energy E C ) is empty of electrons. 3. The valence band (below energy E V ) is full. 4. Eg (energy-gap) = E C - E V. 5. Eg (energy-gap) is the forbidden gap. For a pure (intrinsic) semiconductor at T= 0° K;

Current Flow in Semiconductors (continued……) 1. Some electrons gain enough energy to break their bonds and jump the forbidden gap (only one shown). 2. The conduction band now contains free electrons, while the valence band now has free holes. For a pure (intrinsic) semiconductor at T > 0° K;

Current Flow in Semiconductors (continued……) 1. Electrons in the conduction band move to the right. 2. Electrons in the valence band also move to the right, but move by filling a hole. 3. This process is equivalent to holes moving to the left. 4. Number of holes in the valence band = Number of electrons in the conduction band. When we apply potential to a pure (intrinsic) semiconductor;

Current Flow in Semiconductors (continued……) 1. Electrons are the majority carrier. 2. There are more free electrons in the conduction band than holes in the valence band. When we apply potential to an n-type semiconductor;

Current Flow in Semiconductors (continued……) 1. Holes are the majority carrier. 2. There are more holes in the valence band than electrons in the conduction band. When we apply potential to a p-type semiconductor;

Forming a p-n Junction Doping one side of a piece of silicon with boron (a p-type dopant) and the other side with phosphorus (an n-type dopant) forms a p-n junction. Doped silicon.n-type and p-type materials brought together.

PN-junction model, schematic symbol, physical part. Diode: A semiconductor device, which conduct the current in one direction only.

Diffusion establishes “built-in” electric field. 1. In a process called "diffusion", the random motions of mobile electrons and holes over time cause them to attempt to distribute equally within the total volume. 2. As they cross the junction, the fixed ions they leave behind establish a "built-in" electric field at the junction.

Motion of mobile electrons and holes due to diffusion and the “built-in” electric field. A mobile electron or hole near the "built-in" electric field will be attracted and swept back into its original volume. At the junction there are two effects occurring: 1. Diffusion with electrons moving from n-type to p-type. 2. The "built in" electric field sweeping locally affected electrons back into the n-type volume. The holes are affected similarly but in opposite directions.

Depletion Region Within the depletion region, there are very few mobile electrons and holes. It is "depleted“ of mobile charges, leaving only the fixed charges associated with the dopant atoms. As a result, the depletion region is highly resistive and now behaves as if it were pure crystalline silicon: as a nearly perfect insulator. Depletion Region

Forward Bias of the p-n junction If a positive voltage is applied to the p-type side and a negative voltage to the n-type side, current can flow. This configuration is called "Forward Biased“. At the p-n junction, the "built-in" electric field and the applied electric field are in opposite directions.

Reverse bias of the p-n Junction If a negative voltage is applied to the p-type side and a positive voltage to the n-type side, no (or exceptionally small) current flows. This configuration is called "Reverse Biased". At the p-n junction, the "built-in" electric field and the applied electric field are in the same direction.

I-V curve of the silicon p-n junction diode

Applications of diodes 1. Temperature measuring 2. Ionizing Radiation detectors 3. Logic gates 4. Power conversion 5. Radio demodulation