N Channel MOSFET Characteristics - Lecture Slides | EE 534, Study notes of Electrical and Electronics Engineering

Download N Channel MOSFET Characteristics - Lecture Slides | EE 534 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! EE 534 fall 2003 University of South Alabama Summary of the last lecture EE 534 fall 2003 University of South Alabama p-Channel MOSFET In p-channel enhancement device. A negative gate-to- source voltage must be applied to create the inversion layer, or channel region, of holes that, “connect” the source and drain regions. The threshold voltage VTP for p-channel enhancement load device is always negative and positive for depletion- mode PMOS. Cross-section of p-channel enhancement mode MOSFET The operation of the p-channel is same as the n-channel device , except that the hole is the charge carrier, rather than the electron, and the conventional current direction and voltage polarities are reversed. EE 534 fall 2003 University of South Alabama EE 534 fall 2003 University of South Alabama N-channel Depletion mode MOSFET The term depletion mode means that a channel exists even at zero gate voltage. A negative gate voltage must be applied to the n- channel depletion mode MOSFET to turn the device off. Lecture #6 Lecture #6 EE 534 fall 2003 University of South Alabama EE 534 fall 2003 University of South Alabama Summary of the MOSFET Current-Voltage relationship Table 5.1 EE 534 fall 2003 University of South Alabama Example Given that for n-channel enhancement mode MOSFET, VTN=1.2V and VGS=2V Determine the region of the operation when: i) VDS=0.4V ii) VDS= 1V iii) VDS=5V Sol: We know at the saturation, VDS(sat)=VGS-VTN= 2-1.2=0.8V Case1: when VDS=0.4V ⇒ VDS < VDS(sat) ⇒ transistor is in nonsaturation region CaseII: when VDS=1V ⇒VDS > VDS(sat)⇒ transistor is in saturation mode CaseIII: when VDS=5V ⇒ VDS > VDS (sat) ⇒ transistor is in saturation mode EE 534 fall 2003 University of South Alabama Secondary effects Short-channel effects: Short channel device has channel length comparable to depth of drain and source junctions and depletion width Causes threshold voltage and I/V curve variations Narrow-channel effects: Narrow channel device has small channel width Subthreshold conduction (leakage current) EE 534 fall 2003 University of South Alabama Threshold voltage variation Until now, threshold voltage assumed constant VT changed only by substrate bias VSB In threshold voltage equations, channel depletion region assumed to be created by gate voltage only Depletion regions around source and drain neglected: valid if channel length is much larger than depletion region depths In short-channel devices, depletion regions from drain and source extend into channel EE 534 fall 2003 University of South Alabama Threshold voltage variation Short-channel effects cause threshold voltage variation: VT roll off As channel length L decreases, threshold voltage decreases Drain-induced barrier lowering As drain voltage VDS increases, threshold voltage decreases Hot-carrier effect Threshold voltages drift over time EE 534 fall 2003 University of South Alabama Source depletion region Drain depletion region Gate-induced depletion region Threshold voltage variation Even with VGS=0, part of channel is already depleted Bulk depletion charge is smaller in short-channel device → VT is smaller N+ source N+ drain EE 534 fall 2003 University of South Alabama Threshold voltage variation Change in VT0: xdS, xdD: depth of depletion regions at S, D xj: junction depth                 −++        −+•=∆ 121121 2 2210 j dD j dSj FASi ox T x x x x L x Nq C V φε • ∆VT0 is proportional to (xj/L) – For short channel lengths, ∆VT0 is large – For large channel lengths, term approaches 0 EE 534 fall 2003 University of South Alabama Threshold voltage variations Graphically: VT0 versus channel length L VT Low VDS threshold VDS VT Roll-off: VT decreases rapidly with channel length VT0 L Long-channel VT Lnom Threshold as a function of As a function of length (for low VDS) Drain-induced barrier lowering (for low L) EE 534 fall 2003 University of South Alabama Drain-induced barrier lowering (DIBL) Drain-induced barrier lowering (DIBL) Drain voltage VDS causes change in threshold voltage As VDS is increased, threshold voltage decreases Cause: depletion region around drain Depletion region depth around drain depends on drain voltage As VDS is increased, drain depletion region gets deeper and extends further into channel For very large VDS, source and drain depletion regions can meet → punch-through! Issue: results in uncertainty in circuit design EE 534 fall 2003 University of South Alabama Threshold voltage variation Hot-carrier effect increased electric fields causes increased electron velocity high-energy electrons can tunnel into gate oxide This changes the threshold voltage (increases VT for NMOS) Can lead to long-term reliability problems EE 534 fall 2003 University of South Alabama Threshold voltage variation Hot electrons High-velocity electrons can also impact the drain, dislodging holes Holes are swept towards negatively- charged substrate → cause substrate current Called impact ionization This is another factor which limits the process scaling → voltage must scale down as length scales EE 534 fall 2003 University of South Alabama Threshold voltage variations Summary of threshold variations in short- channel devices VT rolloff: threshold voltage reduces as channel length L reduces DIBL: threshold voltage reduces as VDS increases Hot-carrier effect: threshold voltage drifts over time as electrons tunnel into oxide EE 534 fall 2003 University of South Alabama Narrow-channel effects Narrow-channel device: Channel width W is comparable to maximum depletion region thickness xdm Narrow-channel effect: Threshold voltage of narrow-channel device is larger than threshold of conventional device EE 534 fall 2003 University of South Alabama Narrow-channel effect Cause of narrow-channel effect Edges of gate metal are over field oxide (FOX) This field oxide causes a small depletion region Gate voltage must support this additional depletion region charge EE 534 fall 2003 University of South Alabama Narrow-channel effect Change in threshold voltage: W xNq C V VVV dm FASi ox T TTT κφε •=∆ ∆+= 221 channel) narrow( 0 000 κ is empirical parameter: depends on shape of the fringe depletion region Change in VT0 proportional to (xdm/W)