Review of Semiconductor Physics, PN Junction Diodes and Resistors Semiconductor fundamentals Doping Pn junction The Diode Equation Zener diode LED Resistors

What Is a Semiconductor?                                                                                                                                                                                                            Many materials, such as most metals, allow electrical current to flow through them These are known as conductors Materials that do not allow electrical current to flow through them are called insulators Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by the introduction of impurities

Semiconductors A material whose properties are such that it is not quite a conductor, not quite an insulator Some common semiconductors elemental Si - Silicon (most common) Ge - Germanium compound GaAs - Gallium arsenide GaP - Gallium phosphide AlAs - Aluminum arsenide AlP - Aluminum phosphide InP - Indium Phosphide

Crystalline Solids In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal Silicon crystal has a diamond lattice

Crystalline Nature of Silicon Silicon as utilized in integrated circuits is crystalline in nature As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron

What’s so special about Silicon? Cheap and abundant Amazing mechanical, chemical and electronic properties The material is very well-known to mankind SiO2: sand, glass Si is column IV of the periodic table Similar to the carbon (C) and the germanium (Ge) Has 3s² and 3p² valence electrons

Nature of Intrinsic Silicon Silicon that is free of doping impurities is called intrinsic Silicon has a valence of 4 and forms covalent bonds with four other neighboring silicon atoms

Semiconductor Crystalline Structure Semiconductors have a regular crystalline structure for monocrystal, extends through entire structure for polycrystal, structure is interrupted at irregular boundaries Monocrystal has uniform 3-dimensional structure Atoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium

Semiconductor Crystalline Structure Silicon atoms have 4 electrons in outer shell inner electrons are very closely bound to atom These electrons are shared with neighbor atoms on both sides to “fill” the shell resulting structure is very stable electrons are fairly tightly bound no “loose” electrons at room temperature, if battery applied, very little electric current flows

Conduction in Crystal Lattices Semiconductors (Si and Ge) have 4 electrons in their outer shell 2 in the s subshell 2 in the p subshell As the distance between atoms decreases the discrete subshells spread out into bands As the distance decreases further, the bands overlap and then separate the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom 4 possible electrons in the lower band (valence band) 4 possible electrons in the upper band (conduction band)

Energy Bands in Semiconductors The space between the bands is the energy gap, or forbidden band

Insulators, Semiconductors, and Metals This separation of the valence and conduction bands determines the electrical properties of the material Insulators have a large energy gap electrons can’t jump from valence to conduction bands no current flows Conductors (metals) have a very small (or nonexistent) energy gap electrons easily jump to conduction bands due to thermal excitation current flows easily Semiconductors have a moderate energy gap only a few electrons can jump to the conduction band leaving “holes” only a little current can flow

Insulators, Semiconductors, and Metals (continued) Conduction Band Valence Band Conductor Semiconductor Insulator

Hole - Electron Pairs Sometimes thermal energy is enough to cause an electron to jump from the valence band to the conduction band produces a hole - electron pair Electrons also “fall” back out of the conduction band into the valence band, combining with a hole pair elimination pair creation hole electron

Improving Conduction by Doping To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes elements with 5 outer electrons contribute an extra electron to the lattice (donor dopant) elements with 3 outer electrons accept an electron from the silicon (acceptor dopant)

Improving Conduction by Doping (cont.) Phosphorus and arsenic are donor dopants if phosphorus is introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current very loosely bound to atom and can easily jump to conduction band produces n type silicon sometimes use + symbol to indicate heavier doping, so n+ silicon phosphorus becomes positive ion after giving up electron

Improving Conduction by Doping (cont.) Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom boron is an acceptor dopant yields p type silicon boron becomes negative ion after accepting an electron

Epitaxial Growth of Silicon Epitaxy grows silicon on top of existing silicon uses chemical vapor deposition new silicon has same crystal structure as original Silicon is placed in chamber at high temperature 1200 o C (2150 o F) Appropriate gases are fed into the chamber other gases add impurities to the mix Can grow n type, then switch to p type very quickly

Diffusion of Dopants no new silicon is added It is also possible to introduce dopants into silicon by heating them so they diffuse into the silicon no new silicon is added high heat causes diffusion Can be done with constant concentration in atmosphere close to straight line concentration gradient Or with constant number of atoms per unit area predeposition bell-shaped gradient Diffusion causes spreading of doped areas top side

Diffusion of Dopants (continued) Concentration of dopant in surrounding atmosphere kept constant per unit volume Dopant deposited on surface - constant amount per unit area

Ion Implantation of Dopants One way to reduce the spreading found with diffusion is to use ion implantation also gives better uniformity of dopant yields faster devices lower temperature process Ions are accelerated from 5 Kev to 10 Mev and directed at silicon higher energy gives greater depth penetration total dose is measured by flux number of ions per cm2 typically 1012 per cm2 - 1016 per cm2 Flux is over entire surface of silicon use masks to cover areas where implantation is not wanted Heat afterward to work into crystal lattice

Hole and Electron Concentrations To produce reasonable levels of conduction doesn’t require much doping silicon has about 5 x 1022 atoms/cm3 typical dopant levels are about 1015 atoms/cm3 In undoped (intrinsic) silicon, the number of holes and number of free electrons is equal, and their product equals a constant actually, ni increases with increasing temperature This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the number of holes np = ni2

INTRINSIC (PURE) SILICON At 0 Kelvin Silicon density is 5*10²³ particles/cm³ Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice Higher temperatures create free charge carriers. A “hole” is created in the absence of an electron. At 23C there are 10¹º particles/cm³ of free carriers

There are two types of doping N-type and P-type. The N in N-type stands for negative. A column V ion is inserted. The extra valence electron is free to move about the lattice The P in P-type stands for positive. A column III ion is inserted. Electrons from the surrounding Silicon move to fill the “hole.”

Energy-band Diagram A very important concept in the study of semiconductors is the energy-band diagram It is used to represent the range of energy a valence electron can have For semiconductors the electrons can have any one value of a continuous range of energy levels while they occupy the valence shell of the atom That band of energy levels is called the valence band Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy levels This is the conduction band

Band Gap Between the valence and the conduction band is a range of energy levels where there are no allowed states for an electron This is the band gap In silicon at room temperature [in electron volts]: Electron volt is an atomic measurement unit, 1 eV energy is necessary to decrease of the potential of the electron with 1 V.

Impurities Silicon crystal in pure form is good insulator - all electrons are bonded to silicon atom Replacement of Si atoms can alter electrical properties of semiconductor Group number - indicates number of electrons in valence level (Si - Group IV)

Impurities Replace Si atom in crystal with Group V atom substitution of 5 electrons for 4 electrons in outer shell extra electron not needed for crystal bonding structure can move to other areas of semiconductor current flows more easily - resistivity decreases many extra electrons --> “donor” or n-type material Replace Si atom with Group III atom substitution of 3 electrons for 4 electrons extra electron now needed for crystal bonding structure “hole” created (missing electron) hole can move to other areas of semiconductor if electrons continually fill holes again, current flows more easily - resistivity decreases electrons needed --> “acceptor” or p-type material

COUNTER DOPING Insert more than one type of Ion The extra electron and the extra hole cancel out

A LITTLE MATH pi-number of holes in intrinsic silicon= 10¹º/cm³ n= number of free electrons p=number of holes ni=number of electrons in intrinsic silicon=10¹º/cm³ pi-number of holes in intrinsic silicon= 10¹º/cm³ Mobile negative charge = -1.6*10-19 Coulombs Mobile positive charge = 1.6*10-19 Coulombs At thermal equilibrium (no applied voltage) n*p=(ni)2 (room temperature approximation) The substrate is called n-type when it has more than 10¹º free electrons (similar for p-type)

P-N Junction Also known as a diode One of the basics of semiconductor technology - Created by placing n-type and p-type material in close contact Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type

P-N Junction Region of charges left behind (dopants fixed in crystal lattice) Group III in p-type (one less proton than Si- negative charge) Group IV in n-type (one more proton than Si - positive charge) Region is totally depleted of mobile charges - “depletion region” Electric field forms due to fixed charges in the depletion region Depletion region has high resistance due to lack of mobile charges

THE P-N JUNCTION

The Junction  The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region

Biasing the P-N Diode THINK OF THE DIODE AS A SWITCH Forward Bias Applies - voltage to the n region and + voltage to the p region CURRENT! Reverse Bias Applies + voltage to n region and – voltage to p region NO CURRENT

P-N Junction – Reverse Bias positive voltage placed on n-type material electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal width of depletion region increases allowed current is essentially zero (small “drift” current)

P-N Junction – Forward Bias positive voltage placed on p-type material holes in p-type move away from positive terminal, electrons in n-type move further from negative terminal depletion region becomes smaller - resistance of device decreases voltage increased until critical voltage is reached, depletion region disappears, current can flow freely

P-N Junction - V-I characteristics Voltage-Current relationship for a p-n junction (diode)

Current-Voltage Characteristics THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE

The Ideal Diode Equation

Semiconductor diode - opened region The p-side is the cathode, the n-side is the anode The dropped voltage, VD is measured from the cathode to the anode Opened: VD  VF: VD = VF ID = circuit limited, in our model the VD cannot exceed VF

Semiconductor diode - cut-off region Cut-off: 0 < VD < VF: ID  0 mA

Semiconductor diode - closed region Closed: VF < VD  0: VD is determined by the circuit, ID = 0 mA Typical values of VF: 0.5 ¸ 0.7 V

Zener Effect Zener break down: VD <= VZ: VD = VZ, ID is determined by the circuit. In case of standard diode the typical values of the break down voltage VZ of the Zener effect -20 ... -100 V Zener diode Utilization of the Zener effect Typical break down values of VZ : -4.5 ... -15 V

LED Light emitting diode, made from GaAs VF=1.6 V IF >= 6 mA

Resistor in an Integrated Circuit