Ch 1: Semiconductors PDF

# Ch #1: Semiconductors ## Introduction Electronics - Is the science and technology of the motion of charges in gas, vacuum, or semiconductors. ## Materials The materials can be classified into: 1. **Conductors**: Have low resistance which allows electrical current flow. 2. **Insulators**:...

# Ch #1: Semiconductors ## Introduction Electronics - Is the science and technology of the motion of charges in gas, vacuum, or semiconductors. ## Materials The materials can be classified into: 1. **Conductors**: Have low resistance which allows electrical current flow. 2. **Insulators**: Have high resistance which suppresses electrical current flow. 3. **Semiconductors**: Can allow or suppress electrical current flow. The conductivity (and at the same time the resistivity) of semiconductors lie between that of conductors and insulators. **Conductivity:** - (Ω-m)-1 **Insulators:** - Quartz - Diamond - Glass **Semi-conductors:** - Silicon - Germanium **Metals:** - Copper - Iron - Silver ## THE ATOM ### The Bohr Model - The Bohr model of an atom showing electrons in orbits around the nucleus, which consists of protons and neutrons. - The "tails" on the electrons indicate motion. ## Electrons and Shells - **Energy Levels** Electrons orbit the nucleus of an atom at certain distances from the nucleus. Electrons near the nucleus have less energy than those in more distant orbits. - **Each discrete distance (orbit)** from the nucleus corresponds to a certain energy level. In an atom, the orbits are grouped into energy levels known as **shells**. - **Each shell** has a fixed maximum number of electrons. ## Valence Electrons - Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus. This is because the force of attraction between the positively charged nucleus and the negatively charged electron decreases with increasing distance from the nucleus. - Electrons with the highest energy exist in the outermost shell of an atom and are relatively loosely bound to the atom. - The outermost shell is known as the **valence shell** and electrons in this shell are called **valence electrons**. ## Ionization - When an atom absorbs energy from a heat source or from light, for example, the energies of the electrons are raised. The valence electrons possess more energy and are more loosely bound to the atom than inner electrons, so they can easily jump to higher energy shells when external energy is absorbed by the atom. - The process of losing a valence electron is known as **ionization**, and the resulting positively charged atom is called a **positive ion**. - The atom that has acquired the extra electron is called a **negative ion**. ## Materials ### Insulators - Insulators have tightly bound electrons in their outer shell. - The electrons require a very large amount of energy to free them for conduction. - The force on each electron is not enough to free it from its orbit and the insulator does not conduct. - Insulators are said to have a high resistivity / resistance. - They are materials that offer a very low level of conductivity when voltage is applied. - **Examples:** Glass, Plastic and Ceramic ### Conductors - **Examples**: Copper, Aluminum, Silver and Gold - Conductors have loosely bound electrons in their outer shell. - The electrons require a small amount of energy to free them for conduction. - The force on each electron is enough to free it from its orbit and it can jump from atom to atom – the conductor conducts. - Conductors are said to have a low resistivity / resistance. ### Current in Conductors - **I = Q/T = Nq/T** - **va = με** - **va = L/T** - **I = Nqva/L** - **J = I/A = Nqva/LA** - **J = nqva = nque = σε** - **σ = nqμ = 1/ρ** - **I** = Current - **T** = Time in Seconds - **q** = 1.6x10-19 C - **va** = Drift Velocity - **μ** = Electron Mobility - **V/L** = Electric Field - **J** = Current Density - **n** = Electron Concentration - **σ** = Material Conductivity ### Current in Conductors - **I = JA = σεΑ = σΑL/L = σΑV/L = V/R** ## Semiconductors - A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. - The most common single-element semiconductors are silicon 'Si', germanium 'Ge', and carbon 'C'. Compound semiconductors such as gallium arsenide are also commonly used. - The single-element semiconductors are characterized by atoms with four valence electrons. ## Semiconductors - Four valence electrons in the outer (valence) shell. - **Diagram of a carbon atom** - **Silicon Atom** - **Germanium Atom** ## Semiconductors - The center silicon atom shares an electron with each of the four surrounding silicon atoms, creating a covalent bond with each. The surrounding atoms are in turn bonded to other atoms, and so on. - **Bonding diagram.** - **Covalent bonds in a silicon crystal.** ## Semiconductors Semiconductors classified into: * Pure Semiconductors (Intrinsic Semiconductors). * Impure Semiconductors (Extrinsic Semiconductors). ## Intrinsic Semiconductors - At a very low temperature (0 K) the crystal of semiconductor behaves as an insulator. - **Energy band diagram for an unexcited atom** ## Semiconductors - An intrinsic (pure) silicon crystal at room temperature derives heat (thermal) energy from the surrounding air. - This causes some valence electrons to gain sufficient energy to jump the gap from the valence band into the conduction band, - Electrons in the conduction band are free electrons (conduction electrons) - The absence of the electron in the covalent bond is represented by a small circle and is called a hole. - **Energy band diagram for an excited atom** ## Electron Movement in Silicon - **Free Electron** - **Hole** ## Heating Silicon - This creates electron-hole pairs which are then available for conduction. - **Diagram of a silicon lattice with electron-hole pairs** ## Intrinsic Conduction - Take a piece of silicon... - Apply a potential difference across it... - This sets up an electric field throughout the silicon - seen here as dashed lines - When heat is applied an electron is released and... - This releases a free electron. - **Diagram of a silicon lattice with electric field** ## Intrinsic Conduction - The electron feels a force and moves in the electric field. - It is attracted to the positive electrode and re-emitted by the negative electrode. - This causes the current to flow. - **Diagram of a silicon lattice with electric field and an electron moving through it.** ## Intrinsic Conduction - Now, let's apply some more heat... - Another electron breaks free... - And moves in the electric field. - We now have a greater current than before... - And the silicon has less resistance... - This causes additional electrons to be released. - **Diagram of a silicon lattice with electric field and two electrons moving through it.** ## Intrinsic Conduction - If more heat is applies the process continues... - More heat... - More current... - Less resistance... - This causes even more electrons to be released. - **Diagram of a silicon lattice with electric field and three electrons moving through it.** ## Semiconductors - Valence electron moves into 4th hole and leaves a 5th hole. - Valence electron moves into 5th hole and leaves a 6th hole. - Valence electron moves into 2nd hole and leaves a 3rd hole. - Valence electron moves into 3rd hole and leaves a 4th hole. - Valence electron moves into 1st hole and leaves a 2nd hole. - Free electron leaves hole in valence shell. - **Diagram of valence electrons moving between energy shells** ## Energy diagrams for the three types of materials. - **(a) Insulator** - **(b) Semiconductor** - **(c) Conductor** ## Intrinsic Semiconductors - For intrinsic semiconductor: **n = p = n₁** - Where: - P = hole concentration - n = electron concentration - n₁ = intrinsic concentration ## Intrinsic Semiconductors - The current density J that results from an electric field ε is obtained by: **J = q (nu + pup) ε = σε** - The conductivity is given by: **σ = q (ημ₁ + pup) (Ω-m)-1** - For intrinsic semiconductors: **p = n = n₁** - The intrinsic conductivity is given by: **σ₁ = q n₁(μ₁ + μp) (Ω-m)-1** ## The Thermistor - The thermistor is a heat sensitive resistor. - When cold it behaves as an insulator i.e. it has a very high resistance. - When heated, electron hole pairs are released and are then available for conduction as has been described – thus its resistance is reduced. - **Diagram of a thermistor** ## The Thermistor - Thermistors are used to measure temperature. - They are used to turn devices on, or off, as temperature changes. - They are also used in fire-warning or frost-warning circuits. - **diagram of a thermistor** ## The Light Dependent Resistor (LDR) - The LDR is very similar to the thermistor – but uses light energy instead of heat energy. - When dark its resistance is high. - As light falls on it, the energy releases electron-hole pairs. - They are then free for conduction. - Thus, its resistance is reduced. - **Diagram of an LDR** ## The Light Dependent Resistor (LDR) - LDR's are used as light meters. - LDR's are also used to control automatic lighting. - LDR's are used where light is needed to control a circuit - e.g. Light operated burglar alarm. - **Diagram of an LDR** ## Extrinsic Semiconductors ### N-TYPE AND P-TYPE SEMICONDUCTORS ## Doping ## Extrinsic Semiconductors - Since semiconductors are generally poor conductors, their conductivity can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductive material. This process, called doping, increases the number of current carriers (electrons or holes). The two categories of impurities are n-type and p-type. - The goal of electronic materials is to generate and control the flow of an electrical current. ## Extrinsic Semiconductors ### N-Type Semiconductor - To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb). - Because the pentavalent atom gives up an electron, it is often called a donor atom. - The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. - **Diagram of a silicon lattice with a pentavalent impurity atom** ## Extrinsic Semiconductors ### N-Type Semiconductor - The Phosphorus Atom - It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell. - **Diagram of a phosphorus atom** ## N-Type Semiconductor - **Diagram of a 2-D crystal lattice of silicon doped with phosphorous** ## Doping – Making N-type Silicon - Relying on heat or light for conduction does not make for reliable electronics. - Suppose we remove a silicon atom from the crystal lattice... - ...and replace it with a phosphorus atom. - We now have an electron that is not bonded – it is thus free for conduction. - **Diagram of a silicon lattice with a phosphorus atom** ## Doping – Making n-type Silicon - Let's remove another silicon atom... - ...and replace it with a phosphorus atom. - As more electrons are available for conduction we have increased the conductivity of the material. - Phosphorus is called the dopant (pentavalent impurity). - If we now apply a potential difference across the silicon... - **Diagram of a silicon lattice with two phosphorus atoms** ## Extrinsic Conduction – n-type Silicon - A current will flow. - Note: - The negative electrons move towards the positive terminal. - **Diagram of a silicon lattice doped with phosphorus with a positive and negative terminal** ## N-type Silicon - From now on n-type will be shown like this. - This type of silicon is called n-type. - This is because the majority charge carriers are negative electrons. - A small number of minority charge carriers – holes – will exist due to electrons-hole pairs being created in the silicon atoms due to heat. - The silicon is still electrically neutral as the number of protons is equal to the number of electrons. - **Diagram of a silicon lattice doped with phosphorus** ## Extrinsic Semiconductors ### P-Type Semiconductor - To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (В), indium (In), and gallium (Ga). - Because the trivalent atom can take an electron, it is often referred to as an acceptor atom. - The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. - **Diagram of a silicon lattice with a trivalent impurity atom** ## Extrinsic Semiconductors ### P-Type Semiconductor - The Boron Atom - It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell. - **Diagram of a boron atom** ## P-Type Semiconductor - **Diagram of a 2-D crystal lattice of silicon doped with Boron** ## Doping – Making p-type Silicon - As before, we remove a silicon atom from the crystal lattice... - ...this time we replace it with a boron atom. - Notice we have a hole in a bond - this hole is thus free for conduction. - **Diagram of a silicon lattice with a boron atom** ## Doping – Making p-type Silicon - Let's remove another silicon atom... - ...and replace it with another boron atom. - As more holes are available for conduction we have increased the conductivity of the material. - Boron is the dopant in this case. - If we now apply a potential difference across the silicon... - **Diagram of a silicon lattice with two boron atoms** ## Extrinsic Conduction – p-type silicon - A current will flow – this time carried by positive holes. - Note: - The positive holes move towards the negative terminal. - **Diagram of a silicon lattice doped with boron with a positive and negative terminal** ## P-type Silicon - From now on p-type will be shown like this. - This type of silicon is called p-type. - This is because the majority charge carriers are positive holes. - A small number of minority charge carriers – electrons – will exist due to electrons-hole pairs being created in the silicon atoms due to heat. - The silicon is still electrically neutral as the number of protons is equal to the number of electrons. - **Diagram of a silicon lattice doped with boron** ## Extrinsic Semiconductors ### The Mass-Action Law - Under thermal equilibrium, the product of the free negative and positive concentrations is a constant independent of the amount of donor and acceptor impurity doping. - **n p = n₁²** ## Extrinsic Semiconductors ### Charge Densities in a Semiconductor - Since the semiconductor is electrically neutral, the magnitude of the total positive charge density (ND +p) must equal that of the negative concentration (NA +n), - Hence; - For n-type: **NA=0, n>>p** - For p-type: **Np=0, p>>n** - **(ND +p)= (NA +n)** - **n≈ND, p=n₁²/n ≈n₁²/ND** - **-p≈NA, n=n₁²/p≈n₁²/NA** ## Extrinsic Semiconductors | N-Type Semiconductor | P-type Semiconductor | |---------------------------------|------------------------------------| | Donor Atoms = ND | Acceptor Atoms = NA | | Majority Carriers : Electrons | Majority Carriers: Holes | | Minority Carriers : Holes | Minority Carriers : Electrons | | n >>>>>p | P >>>>> n | | n ≈ ND | P ≈ NA | | **n = p + ND** | **p = n + NA** | | For mass action law | For mass action law | | ND * p = ni² | n * NA = ni² | | **P = ni²/ND** | **n = ni²/NA** | ## Conductivity in Semiconductors - **Metal's Current:** The current comes from the movement of free electrons. - **Semiconductor's Current:** The current comes from both Electrons and Holes. - **Currents in semiconductors**: - Drift Current due to applied electric field. - Diffusion current due to non-uniform concentration. ## Drift Current - Drift current density for Conductors: **J = I/A = n q με** - Drift current density for Semiconductors: **J = I/A = n q un E +p q μp E** - **J = q( η μη + p μp )E = σε** - **σ = q( η μη + p up ) = 1/ρ** - **μη**: Mobility of Electrons - **μp**: Mobility of holes ## Example # 1 - Show that the resistivity of intrinsic germanium at 300K is 452.cm, and also find the resistivity of intrinsic silicon at 300 K. - **Table of Silicon and Germanium Properties at 300K** ## Solution - For intrinsic germanium: - **ρ₁ = 1 / qn₁(μη + μp)** - **ρ₁ =1 / (1.6 × 10-19 × 2.5x1013 × (3800 + 1800))** - **ρ₁ = 45Ω.cm** ## Thank You # ??? ## Have a Wonderful Semester