Semiconductor Devices

Semiconductor Devices • Atoms and electricity • Semiconductor structure • Conduction in semiconductors • Doping • epitaxy • diffusion • ion implantation • Transistors • MOS • CMOS • Implementing logic functions

Electricity • Electricity is the flow of electrons • Good conductors (copper) have easily released electrons that drift within the metal • Under influence of electric field, electrons flow in a current • magnitude of current depends on magnitude of voltage applied to circuit, and the resistance in the path of the circuit • Current flow governed by Ohm’s Law V = IR + electron flow direction -

Electron Bands • Electrons circle nucleus in defined shells • K 2 electrons • L 8 electrons • M 18 electrons • N 32 electrons • Within each shell, electrons are further grouped into subshells • s 2 electrons • p 6 electrons • d 10 electrons • f 14 electrons • electrons are assigned to shells and subshells from inside out • Si has 14 electrons: 2 K, 8 L, 4 M L K M shell d p s 10 6 2

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 • 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

Metal-Oxide-Semiconductor Transistors • Most modern digital devices use MOS transistors, which have two advantages over other types • greater density • simpler geometry, hence easier to make • MOS transistors switch on/off more slowly • MOS transistors consist of source and drain diffusions, with a gate that controls whether the transistor is on Gate S D metal n+ n+ silicon dioxide p monosilicon

+ S D + n+ n+ p - MOS Transistors (continued) • Making gate positive (for n channel device) causes current to flow from source to drain • attracts electrons to gate area, creates conductive path • For given gate voltage, increasing voltage difference between source and drain increases current from source to drain

Complementary MOS Transistors • A variant of MOS transistor uses both n-channel and p-channel devices to make the fundamental building block (an inverter, or not gate) • lower power consumption • symmetry of design • If in = +, n-channel device is on, p-channel is off, out is connected to - • If in = -, n-channel is off, p-channel is on, out is connected to + • No current flows through battery in either case!! P out in N

CMOS (continued) • CMOS geometry (and manufacturing process) is more complicated • Lower power consumption offsets that • Bi-CMOS combines CMOS and bipolar (another transistor type) on one chip • CMOS for logic circuits • Bi-polar to drive larger electrical circuits off the chip S D S D n+ n+ p+ p+ n p

Logic Functions Using CMOS p A p B out two input NAND - if both inputs 1, both p-channel are off, both n-channel are on, out is negative; otherwise at least one p-channel is on and one n-channel off, and out is positive n n input 0 input 1

Semiconductor Devices