Semiconductors and the Computer Chip Jim Felton The Johns Hopkins University April 30, 2002
Outline • Semiconductors • Doping • p-n Junctions • Transistors • The Integrated Circuit • Integrated Circuit Manufacturing
Semiconductors • Intermediate between conductors and insulators • Defined as crystal with a band gap < 2 eV (at 300K) • Does not take much to excite electrons from valence to conduction band • At T = 0 valence band is full and conduction band is empty...behaves as insulator • For T > 0, electrons are thermally excited into the conduction band and become mobile • Holes are left behind in the valence band • Common semiconductors: • Si (IV, Egap = 1.11 eV) • Ge, (IV, Egap = .66 eV) • GaAs (III-V, Egap = 1.43 eV)
Doping • Semiconductors become useful when impurities are added • There are two types of impurities: donors and acceptors • Result is either an excess of electrons (n-type) or holes (p-type) • Typical impurity concentration is 1015 cm-3
p-n Junctions • Junction of p-type and n-type semiconductors is in dynamic equilibrium • Electrons diffuse from n to p to fill holes, establish an electric field...recombination current • Thermally generated electrons move from p to n due to the electric field...generation current balances recombination current • Now apply potential across junction in reverse direction: • The increased potential prevents electrons from moving from n to p, so recombination current is eliminated • Generation current is unaffected (but very small)
p-n Junctions • For applied potential in forward direction: • Decreased potential increases recombination current • Generation current is again unaffected • Result: p-n junction acts as a rectifier • Current only flows for forward biasing
The First Transistor • Invented in 1947 by Shockley, Bardeen, and Brattain at Bell Labs (Nobel Prize 1956) • Called the point-contact transistor
The Junction Transistor • Composed of two p-n junctions, one forward-biased, one reverse-biased. • Current flows in the emitter circuit, won't in the collector circuit • Holes from the emitter enter collector circuit and create current • Base current controls the collector current • Currents in both circuits are the same regardless of load, so a signal can be amplified • Thus transistors are useful for two things: • Amplification • Switching
Modern Transistors • Field Effect transistors (MOSFET, JFET, etc) • MOSFET = Metal-oxide semiconductor field effect transistor • Gate made of conductor (polysilicon) surrounded by insulator (silicon dioxide) • Gate voltage regulates flow from source to drain • Advantage: smallest, fastest transistor
The Transistor Revolution • Transistors superior to old technology (vacuum tube) in every way • Faster • Smaller • Less expensive • More durable • New devices possible • New era in electronics, but problems still exist • Transistors must be connected with other components • Complicated devices require huge numbers of components • Problem called the Tyranny of Numbers • Transistor usefulness is limited, until...
The Integrated Circuit • Integrated circuit invented by Jack Kilby at Texas Instruments in 1958 • The Monolithic Idea: • "The following circuit elements could be made on a single slice: resistors, capacitor, distributed capacitor, transistor" • The first integrated circuit (1958):
Competition Begins • Bob Noyce independently invents the integrated circuit six months later, and patents the idea first • Kilby files appeal, but loses in ten year legal battle (consolation is Nobel Prize in 2000) • Patent stays with Noyce, who cofounds Intel in 1968 with Hopkins APL alumnus Gordon Moore • Integrated circuits are developed to perform specific tasks • Intel's breakthrough: a single, general perpose integrated circuit that can be programmed to perform any task...the CPU • First microprocessor, the Intel 4004, is sold in 1971 • Moore's "Law": number of transistors on a chip doubles every year
Integrated Circuit Manufacturing • Wafer manufacturing • Oxidation / Deposition
IC Manufacturing, continued • Photolithography • UV • EUV • Etching • Doping • DiffusionIon • Implantation • Interconnections
Finished IC's • Completed chips...with up to 42,000,000 transistors each!!