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Figure 8.1 (a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET.

1. 8.1 MOSFET PRINCIPLES. 8.1.1 MOSFET Structure. Figure 8.1 (a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET. Figure 8.2 Types of MOSFETs. 8.1.2 MOSFET as a Voltage – Controlled Switch. Figure 8.3 Cross-sectional illustrations of a MOSFET in (a) off

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Figure 8.1 (a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET.

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  2. 8.1 MOSFET PRINCIPLES 8.1.1 MOSFET Structure Figure 8.1(a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET. Principles of Semiconductor Devices Dimitrijev

  3. Figure 8.2Types of MOSFETs. Principles of Semiconductor Devices Dimitrijev

  4. 8.1.2 MOSFET as a Voltage – Controlled Switch Figure 8.3Cross-sectional illustrations of a MOSFET in (a) off (b) on modes, along with (c) the corresponding current-voltagecharacteristics. (8.1) Principles of Semiconductor Devices Dimitrijev

  5. Figure 8.4Two-dimensional energy-band diagram for the semiconductor part of an N-channel MOSFET in the flat-band condition. Thetwo different colors in the conduction band indicate the two different types of doping: N type in the source and drain regions, andP type in the body. Darker colors indicate higher carrier concentration, and the nearly white areas indicate the depletion region. (8.2) (8.3) Principles of Semiconductor Devices Dimitrijev

  6. Figure 8.5Two-dimensional energy-band diagrams for an N-channel MOSFET acting as a switch in (a) off mode and (b) on mode. The two colors in the conduction band indicate the concentrations of electrons and holes: darker colors correspond to higher carrier concentrations, whereas the nearly white areas indicate depleted regions. Note that the depleted regions correspond to the areas with sloped energy bands and therefore with existence of built-in and externally applied electric field. Principles of Semiconductor Devices Dimitrijev

  7. Figure 8.6(a) The circuit of CMOS inverter. (b) Typical input/output signals. Principles of Semiconductor Devices Dimitrijev

  8. 8.1.3 The Threshold Voltage and the Body Effect Figure 8.7Illustration of the body effect. (a) VSBvoltage increases the barrier between the electrons in the source and the drain. (b) The surface potential of 2fFdoes not reduce the barrier sufficiently for the electrons to be able to move into the channel. (c) The surface potential needed to form the channel is 2fF+VSB. Principles of Semiconductor Devices Dimitrijev

  9. (8.4) (8.5) effective gate-to-body voltage voltage across the gate oxide (8.6) (8.7) Principles of Semiconductor Devices Dimitrijev

  10. (8.8) (8.9) (8.10) (8.11) Principles of Semiconductor Devices Dimitrijev

  11. 8.1.4 MOSFET as a Voltage – Controlled Current Source : Mechanisms of Current Saturation Figure 8.8(a) Cross section of a MOSFET in the saturation region. (b) The corresponding ID-VDScharacteristics. Principles of Semiconductor Devices Dimitrijev

  12. Figure 8.9Two-dimensional energy-band diagrams for an N-channel MOSFET in saturation due to channel pinch off. A comparison of the smaller VDSbias in (a) to the larger VDSvalue in (b) shows that the channel is shortened by the increased drain-to-source bias, but the concentration of electrons in the channel is not changed. As in the waterfall analogy, the drain current is limited by the concentration of the electrons in the channel and not by the height of the fall. Principles of Semiconductor Devices Dimitrijev

  13. 8.2 PRINCIPAL CURRENT – VOLTAGE CHARACTERISTICS AND EQUATIONS (8.12) Figure 8.10(a) Output and (b) transfer characteristics of a MOSFET. Principles of Semiconductor Devices Dimitrijev

  14. 8.2.1 SPICE LEVEL 1 Model (8.13) (8.14) (8.15) (8.16) Principles of Semiconductor Devices Dimitrijev

  15. (8.17) (8.18) (8.19) (8.20) (8.21) Principles of Semiconductor Devices Dimitrijev

  16. Figure 8.11Output characteristics corresponding to SPICE LEVEL 1 model. Solid lines, Eq. (8.20); dashed lines, saturation current. (8.22) (8.23) (8.24) (8.25) (8.26) Principles of Semiconductor Devices Dimitrijev

  17. 8.2.2 SPICE LEVEL 2 Model Figure 8.12N-channel MOSFET diagram, indicating the surface potential at the source and the drain ends of the channel. (8.27) (8.28) Principles of Semiconductor Devices Dimitrijev

  18. (8.29) (8.30) (8.31) Principles of Semiconductor Devices Dimitrijev

  19. (8.32) (8.33) Principles of Semiconductor Devices Dimitrijev

  20. 8.2.3 SPICE LEVEL 3 Model : Principal Effects (8.34) (8.35) Principles of Semiconductor Devices Dimitrijev

  21. (8.36) (8.37) Principles of Semiconductor Devices Dimitrijev

  22. (8.38) (8.39) (8.40) (8.41) Principles of Semiconductor Devices Dimitrijev

  23. Figure 8.13Comparison of LEVEL 1, LEVEL 2, and LEVEL 3 models. Principles of Semiconductor Devices Dimitrijev

  24. 8.3 SECOND – ORDER EFFECTS 8.3.1 Mobility Reduction with Gate Voltage Figure 8.14Influence of mobility reduction with gate voltage on (a) transfer and (b) output characteristics. (8.42) Principles of Semiconductor Devices Dimitrijev

  25. 8.3.2 Velocity Saturation ( Mobility Reduction with Drain Voltage) (8.43) (8.44) Principles of Semiconductor Devices Dimitrijev

  26. 8.3.3 Finite Output Resistance (8.45) (8.46) Figure 8.15Energy-band diagrams along the channel of a MOSFET illustrating the drain-induced barrier lowering (DIBL). Principles of Semiconductor Devices Dimitrijev

  27. Figure 8.16Output characteristics with (solid lines) and without (dashed lines) the influence of VDSon VT. Principles of Semiconductor Devices Dimitrijev

  28. 8.3.4 Threshold – Voltage – Related Short – Channel Effects Figure 8.17Illustration of the threshold voltage related short-channel effect. (8.47) Figure 8.18Threshold voltage dependence on the MOSFET channel length. Principles of Semiconductor Devices Dimitrijev

  29. 8.3.5 Threshold – Voltage – Related Narrow – Channel Effects Figure 8.19Illustration of the narrow channel effect. Fringing electric field wastes the gate voltage, causing a threshold voltage increasein narrow-channel MOSFETs. (8.48) Principles of Semiconductor Devices Dimitrijev

  30. 8.3.6 Subthreshold Current Figure 8.20 The use of a logarithmic drain axis emphasizes the subthreshold region of a transfer characteristic (the subthreshold currentcannot be seen with a linear axis). The subthreshold swing is 60 mV/decade, corresponding to ns=1 in Eq. (8.49). (8.49) Principles of Semiconductor Devices Dimitrijev

  31. 8.4 NANOSCALE MOSFETs 8.4.1 Down – Scaling Benefits and Rules Table 8.1General Down-Scaling Rules and Their Effects Figure 8.21Deep-submicron MOSFET structure. Principles of Semiconductor Devices Dimitrijev

  32. 8.4.2 Leakage Currents Figure 8.22Transfer characteristics of 100-nm MOSFETs (the channel width is 1 mm). (a) Devices with similar characteristics were experimentally demonstrated as early as 1987, but they exhibit pronounced short-channel effects and very high off current. (b) As knownfrom the down-scaling rules, an increase in the substrate doping eliminates the short-channel effects, but also increases the threshold voltage, in this case above the 1.0-V supply voltage. (c) Down-scaling rules require a 1.1-nm gate oxide that leaks a very high input current. Principles of Semiconductor Devices Dimitrijev

  33. Figure 8.23(a) Energy-band diagram and (b) equipotential contours for the MOSFET with the transfer characteristics shown in Fig. 8.22a (VGS=0 V, VDS=1.0 V). Principles of Semiconductor Devices Dimitrijev

  34. Figure 8.24Energy-band diagram (a) and equipotential contours (b) for the MOSFET with the transfer characteristics shown in Fig. 8.22c(VGS=0 V, VDS=1.0 V). Principles of Semiconductor Devices Dimitrijev

  35. 8.4.3 Advanced MOSFETs (8.50) gate-dielectric capacitance per unit area : Figure 8.25MOSFET with engineered doping profile in the substrate. Figure 8.26SOI MOSFET with ultra-thin body. Principles of Semiconductor Devices Dimitrijev

  36. Figure 8.27FinFET: the most promising structure for nanoscale MOSFETs. Principles of Semiconductor Devices Dimitrijev

  37. 8.4.4 Reliability Issues Figure 8.28 Failure causes and mechanisms leading to changes of oxide charge, and border and interface trap densities. Figure 8.29The threshold voltage “turn-around” effect is a manifestation of the link between hole trapping and the creation ofinterface traps. Principles of Semiconductor Devices Dimitrijev

  38. High Oxide Field Figure 8.30Mechanisms of positive gate-oxide charge creation by a high gate-oxide field. (a) Negative gate bias. (b) Positive gate bias. (8.51) Principles of Semiconductor Devices Dimitrijev

  39. Hot Carriers Figure 8.31Mechanism of channel hot-electron injection into the gate oxide. (a) Illustration at the device cross section. (b) Energy bandsat the point of hot electron injection. Principles of Semiconductor Devices Dimitrijev

  40. Figure 8.32 Mechanism of avalanche hot-carrier injection into the gate oxide. (8.52) (8.53) Principles of Semiconductor Devices Dimitrijev

  41. Example 8.5 Table 8.2Results of Hot-Carrier Accelerated Testing Principles of Semiconductor Devices Dimitrijev

  42. 8.5.1 1C1T DRAM Cell Figure 8.33The architecture of DRAMs with 1C1T memory cells. Figure 8.34The cross section of a 1C1T memory cell. Principles of Semiconductor Devices Dimitrijev

  43. 8.5.2 Flash – Memory Cell Figure 8.35Flash-memory MOSFET. (a) The cross section. (b) The transfer characteristics. (c) The energy bands in erased state. (d) The energy bands in programmed state. Principles of Semiconductor Devices Dimitrijev

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