Digital VLSI: Understanding Layers, Semiconductors, and Parasitic Capacitance, Study notes of Electrical and Electronics Engineering

Download Digital VLSI: Understanding Layers, Semiconductors, and Parasitic Capacitance and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! Introduction to Digital VLSI Layers Semiconductors Conducting Layers Diffusion N-type P-type Polysilicon small broken-up crystalline regions of silicon Metal 1,2 aluminum Metal 3 aluminum, about twice as thick as metal 1,2 Sheet Resistance Materia l Example: Sheet resistance of 4 Squares of N-diffusion R = 4 squares * 28 /square = 112 Metal Layers Used for interconnecting transistors routing power and clocks bonding pads Metal 3 Top interconnect layer Metal 3 is about twice as thick as Metal 1,2 More current carrying capability Metal 3 is preferred for global power routing Maximum Current Density Metal migration failures result from carrying too much current (metal erosion) JAl is maximum current density of aluminum = 1 to 2 mA/um of width Problem for power buses. Too much current causes electromigration of aluminum Wire Delay: Distributed RC Effects r c r c r c r c r c r c r c r c Inpu t Outpu t Wire Length Design Guide And we can use lumped C of wire and ignore R . . . Otherwise, we must treat the wires as transmission lines: metal1 > 5000 poly > 200 diffusion > 60 RC Line Example r = 20/m c = 4E-4 pF/m t = 2.8 E-15 x l2 (delay of this wire) This is even a problem with metal lines. 1 mm 1 mm 2 mm With buffer: t = 2.8E-15(10002) + tbuf + 2.8E-15(10002) = 5.6ns + tbuf Without buffer: t = 2.8E-15(20002) = 11.2ns Line width factored in! Energy Bands Metals have an unfilled Valence band Electrons move freely without extra energy Insulators have full Valence bands It take tremendous amounts of energy to jump to a conduction band. Semiconductors Energy can excite the electrons to the conduction band Once in the conduction band the current can flow. Shockley-Hall-Read at Bell Labs took this and found the semiconductor industry. Carriers Electrons Presence of electrons Overall negative charge N type semiconductor Holes Lack of electrons Overall positive charge P type semiconductor - + - - - -- - + + + + + + + ++ Semi Variations III-V semiconductors InP GaAs GaN Typical p dopants Boron Gallium N_A number of acceptors Typical n dopants Arsenic Phosphorous N_D number of donors carrier concentration [log scale] The Junction space . charge . neutral region region neutral region electrons i << | E-field ' “Diffusion force" on holes _ <_— “Diffusion force" on electrons ' i) E-field force on holes <—— __ E-field force on electrons ' ' Junction Where two dopants meet p-n junction Excess carriers from the silicon cross the junction and recombine on other side Forming Space Charge