ECE 342 Electronic Circuits Lecture 6 MOS Transistors Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jesa@illinois.edu 1
NMOS Transistor Typically L = 0.1 to 3 m, W = 0.2 to 100 m, and the thickness of the oxide layer (t ox ) is in the range of 2 to 50 nm. 2
NMOS Transistor NMOS Transistor N-Channel MOSFET Built on p-type substrate MOS devices are smaller than BJTs MOS devices consume less power than BJTs 3
NMOS Transistor - Layout Top iew Cross Section 4
MOS Regions of Operation Resistive Triode DS T small Nonlinear T < ( ) DS T Active Saturation T DS T 5
MOS Transistor Operation As G increases from zero Holes in the p substrate are repelled from the gate area leaving negative ions behind A depletion region is created No current flows since no carriers are available As G increases The width of the depletion region and the potential at the oxide-silicon interface also increase When the interface potential reaches a sufficiently positive value, electrons flow in the channel. The transistor is turned on As G rises further The charge in the depletion region remains relatively constant The channel current continues to increase 6
MOS Triode Region - 1 W ID Cox T DS L DS T C ox t ox ox 3.9 o t ox C ox : gate oxide capacitance : electron mobility L: channel length W: channel width T : threshold voltage 7
MOS Triode Region - 1 FET is like a linear resistor with r ds 1 C W L n ox T 8
MOS Triode Region - 2 T DS T Charge distribution is nonuniform across channel Less charge induced in proximity of drain W 1 I C L 2 2 D n ox T DS DS 9
MOS Active (Saturation) Region Saturation occurs at pinch off when DS T DSP T DS T (saturation) W I C 2L 2 D n ox T 10
NMOS Circuit Symbols 11
NMOS Regions of Operation Cut off T I D 0 Triode T DS T W 1 I C L 2 2 D n ox T DS DS Saturation T DS T W I C 2L 2 D n ox T 12
NMOS Drain Current 13
NMOS I Characteristics characteristics for a device with k n (W/L) = 1.0 ma/ 2. 14
MOS Threshold oltage The value of G for which the channel is inverted is called the threshold voltage T (or t ). Characteristics of the threshold voltage Depends on equilibrium potential Controlled by inversion in channel Adjusted by implantation of dopants into the channel Can be positive or negative Influenced by the body effect 15
nmos Device Types Enhancement Mode Normally off & requires positive potential on gate Good at passing low voltages Cannot pass full DD (pinch off) Depletion Mode Normally on (negative threshold voltage) Channel is implanted with positive ions ( T ) Provides inverter with full output swings 16
Types of MOSFETS 17
PMOS Transistor 0 PMOS -100-200 =-1.0-300 -400-500 -600 =-1.0 =-1.5 =-2.0 =-2.5-700 -2.5-2 -1.5-1 -0.5 0 - All polarities are reversed from nmos - v, v DS and t are negative - Current i D enters source and leaves through drain - Hole mobility is lower low transconductance - nmos favored over pmos ds 18
PMOS Regions of Operation Cut off I D : drain current flowing from drain to source TP I D 0 Triode TP DS TP W 1 I C L 2 2 D p ox TP DS DS Saturation TP DS TP W I C 2L 2 D p ox TP 19
Cut off Triode SG TP PMOS Alternative Equations In positive quantities ( SG, DS, TP ) I D 0 SG TP SD SG TP Saturation SG TP SD SG TP W 1 I C L 2 2 D p ox SG TP SD SD W I C 2L 2 D p ox SG TP 20