1. Grace Xing---EE30357 (Semiconductors II: Devices) 1 EE 30357: Semiconductors II: Devices Lecture Note #19 (02/27/09) MOS Field Effect Transistors Grace Xing Outline: 1.Last class: Small signal equivalent circuit model & Short channel effects 2.Compound semiconductor based devices 1.MESFET 2.JFET 3.HEMT 3.Quick revisit of MOS capacitors and FETs (read on your own how DRAM, CCD, flash memory etc. work, they are all based on MOS structures!) – next class 1.Flat band voltage 2.Effects of oxide charges (interface charges, fixed charges)

2. Grace Xing---EE30357 (Semiconductors II: Devices) 2 Schematic showing the structure of a GaAs MESFET. Figure 8.26

3. Grace Xing---EE30357 (Semiconductors II: Devices) 3 (a) Cross section of a MESFET at equilibrium indicating the depletion regions; (b) the energy band diagram perpendicular to the gate. The channel thickness is t. Figure 8.27 metal SiO 2 Si Similar to the SOI devices (ultra-scaled MOSFET), electrons are confined in the channel by large bandgap materials

4. Grace Xing---EE30357 (Semiconductors II: Devices) 4 MESFET energy band diagram and depletion region (a) at equilibrium and (b) for an applied gate voltage that depletes the channel. In the second case, the channel is still n type but it is empty of carriers because of the increased barrier height. Figure 8.28 Depletion region

5. Grace Xing---EE30357 (Semiconductors II: Devices) 5 Cross section of a MESFET under small V DS bias and the corresponding energy band diagrams at the source end and drain end of the gate. Figure 8.29 1 V 1 eV Mistakes in textbook (There are quite some mistakes on MESFET and HEMTs in Anderson. Use my slides instead.)

6. Grace Xing---EE30357 (Semiconductors II: Devices) 6 The JFET. The shaded areas represent the depletion regions. (a) Cross-sectional diagram; (b) the energy band diagram, (E C ), for V DS = 0; (c) when V DS  0 (but not yet in saturation) the depletion region at the drain end increases, narrowing the channel and increasing the channel resistance. Figure 8.32 Substrate gate

7. Grace Xing---EE30357 (Semiconductors II: Devices) 7

8. 8

9. 9 Gradual channel approximation Channel is long such that electric field  L <<  T Gate depletion w is independent of  L (y)

10. Grace Xing---EE30357 (Semiconductors II: Devices) 10 Gradual channel approximation n(y) ~ N D ’ long channel and constant low- field mobility  lf No need to remember these equations

11. Grace Xing---EE30357 (Semiconductors II: Devices) 11 High electron mobility transistor (HEMT) (other names are Modulation Doped FET – MODFET and Heterojunction FET - HFET) Mobile electrons are separated from the ionized donors  less scattering  high mobility! Error in A&A, Fig.8.24 (b) and description of how gate works are wrong. The other parts are reasonably correct.

12. Grace Xing---EE30357 (Semiconductors II: Devices) 12

13. Grace Xing---EE30357 (Semiconductors II: Devices) 13 High speed IC – NTT 80 Gbits/s multiplexer HEMTs can operate much faster than MOSFETs  high speed ICs