4. MOS: Device Operation and Large Signal Model Lecture notes: Sec. 4 Sedra & Smith (6 th Ed): Sec. 5.1-5.3 Sedra & Smith (5 th Ed): Sec. 4.1-4.3 ECE 65, Winter2013, F. Najmabadi
Operational Basis of a Field-Effect Transistor (1) Consider the hypothetical semiconductor below: (constructed similar to a parallel plate capacitor) Electrical contact Metal Insulator Dopent ions P-type semiconductor Holes (majority carries) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (2/29)
Operational Basis of a Field-Effect Transistor (2) If we apply a voltage v 1 between electrodes, a Depletion Region (no majority carrier) charge Q = C v 1 will appear on each capacitor plate. o The electric field is strongest at the interface with the insulator and charge likes to accumulate there. Holes are pushed away from the insulator interface forming a “depletion region”. Depth of depletion region increases with v 1. Inversion layer If we increase v 1 above a threshold value ( V t ), (“channel”) the electric field is strong enough to “pull” free electrons to the insulator interface. As the holes are repelled in this region, a “channel” is Depletion Region formed which contains electrons in the (no majority carrier) conduction band (“inversion layer”). Inversion layer is a “virtual” n-type material. F. Najmabadi, ECE65, Winter 2013, Intro to MOS (3/29)
Operational Basis of a Field-Effect Transistor (3) We apply a voltage across the p-type semiconductor: (Assume current flows only in the n-type material, ignore current flowing in the p-type semiconductor) No inversion layer ( v 1 < V t ): With inversion layer ( v 1 > V t ): No current will flow A current will flow in the channel Current will be proportional to electron charge in the channel or ( v 1 − V t ) Magnitude of Current i 2 is controlled by voltage v 1 (a Transistor!) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (4/29)
Operational Basis of a Field-Effect Transistor (4) We need to eliminate currents flowing in the p-type, i.e., current flows only in the “channel” which is a virtual n-type. Make n-type material terminals (set up diodes between terminals & p-type “body”) Heavy doping of the n-type terminals provides a source of free electrons for the channel. Make insulator layer as thin as possible to increase the electric field. Body-source and body-drain junctions should always be in reverse bias for FET to work! F. Najmabadi, ECE65, Winter 2013, Intro to MOS (5/29)
Channel width (L) is the smallest feature on the chip surface MOSFET “cartoons” for deriving MOSFET (or MOS): Metal-oxide field-effect transistor MOSFET characteristics NMOS: n-channel enhancement MOS MOSFET implementation on a chip F. Najmabadi, ECE65, Winter 2013, Intro to MOS (6/29)
NMOS i-v Characteristics (1) To ensure that body-source and body-drain junctions are reversed bias, we assume that Body and Source are connected to each other and v DS ≥ 0 . o We will re-examine this assumption later Without a channel, no current flows (“Cut-off”). For v GS > V tn , a channel is formed. The total charge in the channel is = = |Q| CV C WL ( v -V ) ox GS tn = C C W L ox ε = ox C : Capacitanc e per unit area ox t ox t : Thickness of insulator ox ε : permitivit y of insulator ox ε = ε = × − 11 3 . 9 3 . 45 10 F/m (for SiO ) ox 0 2 Overdrive Voltage: V OV = v GS –V tn F. Najmabadi, ECE65, Winter 2013, Intro to MOS (7/29)
NMOS i-v Characteristics (2) v GS > V tn A channel is formed Apply a small v DS between drain & source. A current flows due to the “drift” of electrons in the n-channel: W = µ − i C ( v V ) v D n ox GS tn DS L W = µ i C V v D n ox OV DS L For small v DS , MOS acts as a resistance with its conductivity controlled by V OV (or v GS ). W = = µ i g v with g C V D DS DS DS n ox OV L F. Najmabadi, ECE65, Winter 2013, Intro to MOS (8/29)
NMOS i-v Characteristics (4) When v DS is increased the channel becomes narrower near the drain (local depth of the channel depends on the difference between V G and local voltage). Triode Mode [ ] W = µ − 2 i C V v 0 . 5 v D n ox OV DS DS L When v DS is increased further such that v DS = V OV , the channel depth becomes zero at the drain (Channel “pinched off”). When v DS is increased further, v DS > V OV , the location of channel pinch-off remains close to the Saturation Mode drain and i D remains approximately constant. W = µ 2 i 0 . 5 C V D n ox OV L F. Najmabadi, ECE65, Winter 2013, Intro to MOS (9/29)
NMOS i-v Characteristics (5) For a given v GS (or V OV ) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (10/29)
NMOS i-v Characteristics Plot (1) NMOS i-v characteristics is a surface i = f ( v , v ) D GS DS * Plot for V t,n = 1 V and µ n C ox ( W/L ) = 2.0 m A/V 2 F. Najmabadi, ECE65, Winter 2013, Intro to MOS (11/29)
NMOS i-v Characteristics Plot (2) Looking at surface with v GS axis pointing out of the paper* *Note: surface was truncated (i.e., v GS < 5 V) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (12/29)
NMOS i-v Characteristics Plot (3) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (13/29)
Channel-Width Modulation The expression we derived for saturation region assumed that the pinch-off point remains at the drain and thus i D remains constant. In reality, the pinch-off point moves “slightly” away from the drain: Channel-width Modulation W ( ) = µ + λ 2 i 0 . 5 C V 1 v D n ox OV DS L λ = 1 / V A F. Najmabadi, ECE65, Winter 2013, Intro to MOS (14/29)
Body Effect Recall that Drain-Body and Source-Body diodes should be reversed biased. o We assumed that Source is connected to the body ( v SB = 0) and v DS = v DB > 0 In a chip (same body for all NMOS), it is impossible to connect all sources to the body (all NMOS sources are connected together. Thus, the body (for NMOS) is connected to the largest negative voltage (negative terminal of the power supply). Doing so, changes the threshold voltage (called “Body Effect”) ( ) = + γ φ + − φ V V | 2 V | | 2 | tn tn , 0 F SB F In this course we will ignore body effect as well as other second- order effects such as velocity saturation. F. Najmabadi, ECE65, Winter 2013, Intro to MOS (15/29)
P-channel Enhancement MOS (PMOS) A PMOS can be constructed analogous to an NMOS: (n-type body), heavily doped p-type source and drain. A virtual “p-type” channel is formed in a P-MOS (holes are carriers in the channel) by applying a negative v GS . i-v characteristic equations of a PMOS is similar to the NMOS with the exception: o Voltages are negative (we switch the terminals to have positive voltages: use v SG instead of v GS ). o Use mobility of holes, µ p , instead of µ n in the expression for i D F. Najmabadi, ECE65, Winter 2013, Intro to MOS (16/29)
MOS Circuit symbols and conventions NMOS PMOS F. Najmabadi, ECE65, Winter 2013, Intro to MOS (17/29)
MOS i-v Characteristics Equations NMOS ( V OV = v GS – V tn ) ≤ = Cut - Off : V 0 i 0 OV D [ ] W ≥ ≤ = µ − 2 Triode : V 0 and v V i 0 . 5 C 2 V v v OV DS OV D n ox OV DS DS L W [ ] ≥ ≥ = µ + λ 2 Saturation : V 0 and v V i 0 . 5 C V 1 v OV DS OV D n ox OV DS L PMOS ( V OV = v SG – | V tp | )* ≤ = Cut - Off : V 0 i 0 OV D [ ] W ≥ ≤ = µ − 2 Triode : V 0 and v V i 0 . 5 C 2 V v v OV SD OV D p ox OV SD SD L W [ ] ≥ ≥ = µ + λ 2 Saturation : V 0 and v V i 0 . 5 C V 1 v OV SD OV D p ox OV SD L *Note: S&S defines | V OV |= v SG – | V tp | and uses | V OV | in the PMOS formulas. F. Najmabadi, ECE65, Winter 2013, Intro to MOS (18/29)
MOS operation is “Conceptually” similar to a BJT -- i D & v DS are controlled by v GS Controlled part: i D & v DS are set by transistor state (& outside circuit) Controller part: Circuit connected to GS sets v GS (or V OV ) A similar solution method to BJT: o Write down GS-KVL and DS-KVL, assume MOS is in a particular state, solve with the corresponding MOS equation and validate the assumption. MOS circuits are simpler to solve because i G = 0 ! o However, we get a quadratic equation to solve if MOS in triode (check MOS in saturation first!) F. Najmabadi, ECE65, Winter 2013, Intro to MOS (19/29)
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