MOS Transistor Properties Review 1
VLSI Chip Manufacturing Process Photolithography: transfer of mask patterns to the chip Diffusion or ion implantation: selective doping of Si substrate Oxidation: SiO 2 growth Deposition: Al and polysilicon thin films 2
NMOS Enhancement Device 3
Establishing a Channel v GS V t v DS =0 v GD =v GS v DS =v GS C ox = ox t ox F /cm 2 C gb =C ox WLF 4
Acts as a Voltage-Controlled Resistor v GD =v GS v DS v GS 5
NMOS Operation as v DS Increases 1. For small v DS : v GD =v GS v DS v GS 2. As v DS is increased for fixed v GS : v GD=v GS v DS -> decreases 3. When v DS =v DSsat =v GS V t v GD =V t channel pinches off at drain-end 4. As v DS is increased v DS v DSsat =v GS V t v GD V t channel L decreases. 6
Curve Flattens With Increased v DS 7
NMOS Enhancement Model Background If v GS V t,the drain to source current, i DS, increases until: v DS v GS V t The MOSFET saturates at this value of v DS and (except for an Early Effect ) i DS does not increase with further increases in v DS. This channel current is proportional to the channel width W, inversely proportional to its length L, and proportional to its transconductance parameter, k' n : ' k n = n C ox = cm2 V sec F cm2 C /V = 2 cm V sec cm = A 2 V 2 C ox is the capacitance per unit area of the gate-channel interface and n is the fabrication-dependent channel electron mobility. 8
Uniform Channel Width (Small v DS ) Model Drain-to-source current with fixed gate-to-source voltage is proportional to drain to source voltage: g DS =k n L v GS V t i D =i DS =g DS v DS =k n L v GS V t v DS r DS =1/ g DS = voltage-controlled resistance k n Intuitively, conductance g DS is proportional to the conductor cross-section - channel width, W, times channel depth (set by the gate to source voltage, v GS -V t ) and the conductivity of the channel material. Conductance is inversely proportional to L, the length of the current path. 1 L v GS V t 9
NMOS Enhancement Model Structure A satisfactory v DS - i DS model for the triode region is a quadratic expression that is linear for small v DS and reaches a maximum where the device saturates: Small v DS, where i D =i DS =k n L [ v GS V t v DS 1 2 v DS Saturation, where v DS =v GS V t : i D =i DS =k n 2 ] 1 2 v 2 v V v : DS i =i =k GS t DS D DS n L v GS V t v DS [ L v GS V t v GS V t 1 2] 2 v GS V t = k ' n 2 i D =i DS = k ' n 2 W L v GS V t 2 Triode region W L v GS V t 2 Saturation region 10
NMOS Enhancement Models For the NMOS enhancement mode (strong-inversion) transistor: For: For: v GS V t 0 i DS =0 v GS V t 0 (Channel non-existent) (Channel exists) v DS v GS V t 2 ] i D =i DS =k n L [ v GS V t v DS 1 2 v DS (Triode) v DS v GS V t i D =i DS = 1 2 k n L v V GS t 2 (Saturation) 11
Channel Length Modulation v DS v DSsat =v GS V t v GD V t L= Lv DS process parameter withunits V 1 Modify the saturation model to account for L: i D =i DS = 1 2 k n L v GS V t 2 1 2 k n L L v GS V t 2 = 1 2 k n L L L 1 L 1 L/ L v GS V t 2 i D =i DS 1 2 k n L 1 L L v GS V t 2 = 1 2 k n L v GS V t 2 1 n v DS 12
V A Early Voltage Relation to Lambda Saturation: i D =i DS 1 2 k n L v GS V t 2 1 n v DS small-signal output-resistance r o = V A I D = 1 I D where I D = 1 2 k n L V GS V t 2 slope=1/r o V A = 1 Triode: [ i D =i DS k n L v GS V t v DS 1 2 v ] 2 1 DS nv DS 13
NMOS Enhancement Circuit Symbols Sedra Symbols Multisim Symbols The body diode is indicated by the B terminal arrow. The substrate typically is connected to the most negative circuit voltage so the body diode is back biased. 14
NMOS Physical Representations 15
PMOS Enhancement Device PMOS Transistor on a p-type body (CMOS circuit). NOTE: p-type body is substrate for NMOS transistor n-well is local substrate for PMOS transistor 16
PMOS Enhancement Model PMOS device model is NMOS model with all polarities reversed: For: For: v SG V 0 t i D =i SD =0 v SG V 0 t v SD v SG V t (Channel non-existent) (Channel exists) [ i D =i SD =k p L v SG V t v SD 1 2 v ] 2 1 SD p v SD v SD v SG V t i D =i SD = 1 2 k p L v SG V 2 t 1 p v SD (Triode) (Saturation) 17
PMOS Enhancement Circuit Symbols Sedra Symbols Multisim Symbols another PMOS symbol 18
nmos Small Signal Model Low Frequency (approximate) v gs g m v gs r o g m= 2 nc ox W L I D r o = 1 n I D C gd C gs C High Frequency (approximate) V gs C gs g m V gs r o C gd C 19