MESFET models

1
MESFET models

• from Yngvesson, p. 313.

2
Simple limits

• simple results
• unity short-circuit current gain frequency fT
• unity power gain fmax

• from Sze, High Speed Devices

3
Velocities

• steady state
• ballistic for short channels

• from Yngvesson

4
Saturation condition

• from C. Liechti, Microwave Field-Effect
  Transistors 1976, IEEE Trans. Micro. Theory
  Tech., MTT-24, p. 279, 1976.

5
Channel resistance term Ri

• from Ladbrooke, MMIC Design GaAs FETs and HEMTs,
  p. 100,

6
Ri term

• from Ladbrooke, MMIC Design GaAs FETs and HEMTs.

7
breakdown limits

• from Sze, High Speed Devices

8
RF cmos trends

• from RF-CMOS Performance Trends, Pierre
  H.Woerlee, Mathijs J. Knitel, Ronald van
  Langevelde, Dirk B. M. Klaassen, Luuk F.
  Tiemeijer, Andries J. Scholten, and Adrie T. A.
  Zegers-van Duijnhoven, IEEE TRANSACTIONS ON
  ELECTRON DEVICES, VOL. 48, NO. 8, AUGUST 2001,
  pp. 1776-1782

Fig. 1. Measured f data of nominal gate length
NMOS devices from industrial CMOS technologies
versus minimum feature size. solid square
Philips, open triangle E. Morifuji, H. S.
Momose, T. Oghuro, T. Yoshitomi, H. Kimijima, F.
Matsuoka, M. Kinugawa, Y. Katsumata, and H. Iwai,
Dig. Tech. Papers VLSI Symp., 1999, p. 163.
closed triangle T. C. Holloway et al., Dig.
Tech. Papers VLSI Symp., 1997, p. 13. open
diamond J. N. Burghartz et al., IEDM Tech. Dig.,
1999, p. 853.).
9
MOSFET models

• from http//www.planetanalog.com/story/OEG20001024
  S0010

10
Silicon - germanium

• from Monte Carlo Investigation of Optimal Device
  Architectures for SiGe FETs, S. Roy, S. Kaya, S.
  Babiker, A. Asenov, J. Baker, http//www.elec.gla.
  ac.uk/groups/dev_mod/papers/iwce98/SiGe.pdf

11
Si-Ge fets

• from The Future of SiGe Beyond HBT Applications,
  Dr. Thomas Hackbarth, Dipl. Ing. Marco Zeuner,
  Dipl. Ing. and Dr. Ulf König, DaimlerChrysler
  Research Center, Ulm, Germany, http//www.semicond
  uctors.unaxis.com/en/download/The20Future20of20
  SiGe.pdf
• e Beyond HBT Applications, Dr. Thomas Hackbarth,
  Dipl. Ing. Marco Zeuner, Dipl. Ing. and Dr. Ulf
  König, DaimlerChrysler Research Center, Ulm,
  Germany, http//www.semiconductors.unaxis.com/en/d
  ownload/The20Future20of20SiGe.pdf

12
High Electron Mobility Transistor

• AKA
• heterojunction fet (HFET)
• modulation-doped FET, MODFET
• two-dimensional electron gas FET, 2DFET, TEGFET
• first device 1980
• Gate delays as low as 12 psec at room temperature
  (1984)
• At 77 K saturation velocities twice
  conventional MESFET.
• essentially achieve mobility of undoped GaAs in
  channel
• requires for fabrication
• MBE to grow both GaAs and AlxGa1-xAs
• sub-micron gate definition
• implantation for contacts

13
Basic idea

• GaAs and AlGaAs are latticed matched throughout
  their compositional range

from Sze, High Speed Semiconductor Devices, p.
285.
from Sze, High Speed Semiconductor Devices, p.
290.
14
Device structure

• critical parameters
• doping concentration in AlGaAs
• want fully depleted, as many carriers in channel
  as possible
• spacer layer thickness
• want to minimize scattering from ionised
  impurities
• still want max channel sheet carrier concentration

from Sze, High Speed Semiconductor Devices, p.
285, 300.
15
Results
from Sze, High Speed Semiconductor Devices, p.
297, 299, 298.
16
Other materials

• look for
• higher low field mobility
• higher velocities
• main issue
• can you grow the material?

from Sze, High Speed Semiconductor Devices, p.
301.
17
InGaAs hemts

• from 0.1 µm InGaAs/InAlAs/InP HEMT Production
  Process for High Performance and High Volume MMW
  Applications,
• R. Lai, M. Barsky, R. Grundbacher, L. Tran, T.
  Block, T.P. Chin, V. Medvedev, E. Sabin, H.
  Rogers, P. H. Liu, Y.C. Chen, R. Tsai and D.
  Streit, TRW Inc., Electronic Systems and
  Technology Division http//www.gaasmantech.org/dig
  est/1999/PDF/16.pdf

18
AlGaN/GaN

• higher breakdowns GaN Egap 3.4eV breakdown
  field 3MV/cm
• higher powers more than higher fmax
• AlGaN/GaN MODFET record output power density of
  3W/mm at 18GHz and 5.3W/mm at 10GHz 2
• T.S.Sheppard at al.,"High power microwave
  GaN/AlGaN HEMTs on Silicon Carbide", in Late News
  1998 56th Annual Device Research Conference,
  avail. at http//schof.colorado.edu/drc/drc1998/l
  ateprog.htm

from U.K. Mishra, P. Parikh, Y.F. Wu, AlGaN/GaN
HEMTs An overview of device operation and
applications, http//my.ece.ucsb.edu/mishra/cla
ssfiles/overview.pdf
19
SiGe hemts
from D.J. Paul, Silicon Germanium
Heterostructures in Electronics- The Present and
the Future, Thin Solid Films (Accepted for
publication)