Download HEMTs

High Electron Mobility Transistors
for
Low-Noise Operation
D.L. Pulfrey
Department of Electrical and Computer Engineering
University of British Columbia
Vancouver, B.C. V6T1Z4, Canada
[email protected]
http://nano.ece.ubc.ca
Day 3B, May 29, 2008, Pisa
High electron-mobility Transistor
• Note the Schottky barrier
Schottky barrier band diagram
Schottky barrier under bias
• Negative potential on
n-type semiconductor
• discontinuity in EF
Forward bias in SB- and PN-diodes
2. What is the driving force here?
ΦB
-qVa

1. What is the bottleneck here?
Two heterojunctions in a HEMT
Metal/AlGaAs HJ
AlGaAs/GaAs HJ
2-DEG
in the
potential
"well"
y
Note the doping
Simplifying the quantum well
• Triangular to finite square
• Finite to infinite square
• SWE becomes:
• wavenumber is
Ey
a
• bc’s:
• solution:
 E is quantized
Energy is quantized
E1/m*
 10X
GaAs vs. Si
Energy
Wavefunction
Probability density
For a finite well
• Wavefunction
not completely
confined
• Use undoped spacer
Employment of a
spacer layer
Provision of electrons from
remote donors is called
MODULATION DOPING
Formation of sub-bands and 2DEG
2m
Empty
2m
Partially filled 2nd sub-band
• ns0 1013 cm-2
2DEG concentration ns
Independent
of E !!
Controlling ns by VGS
Thick barrier layer
Thick-enough barrier layer
qVGS
qVGS
Depleting the channel
Threshold condition
• How would you make an enhancement HEMT?
• Often modeled by SPICE LEVEL 1: IDsat=WgCg(VGS-VT)2 /2Lg
HEMT attributes
• Excellent lattice match
 no surface scattering ( ).
• Electrons and donors separated
 no I I scattering, i.e.,   
• Undoped spacer also helps mobility.
• Electrons confined to a well of width < e
i.e., about 15nm for GaAs at 300K.
• Size-quantization of energy levels
- standing waves
- only 2-D scattering

• and gm 
Start with a high  and preserve it!
High performance HEMT
Why the funny gate?
fT= 270 GHz, fmax=490 GHz
NOISE
Noisy DC signal
dB
use RMS values
What is a signal of -30dBm ?
Thermal noise
Brownian motion
So, an equivalent circuit representation of thermal
noise is
vR > vd, so present without current
From Nyquist:
This P can be transferred from a real resistor R to a
noiseless resistor R.
• What "colour" is this noise?
• How much thermal noise in 50 Ohm
R? -> 1nV over 1Hz
Shot noise
Forward-biased junction
microscopically ->
EC
Transition over barrier is random event
(probability of state occupancy)
Important in HBTs, but not in FETs, except in sub-threshold operation.
Flicker noise
Defects cause ''traps"
Escape time: tends to be long
Empirical expression:
Colour of this noise?
Prevalent in MOSFET channel.
Keep L
short.
Use a HEMT.
Induced gate noise
Gate
• The induced gate noise is correlated with the channel (drain-current) noise.
• Coupling is via capacitance
• Impedance decreases with frequency
• Important at high frequencies
Non-Quasi-Static operation
Recall: QSA
q(x, y, z, t' ) = f( VTerminals, t')
q(x, y, z, t' )  f( VTerminals, t < t')
At high frequencies, this breaks down.
Consider example of charging a capacitor:
R
v(t')
C
q(t')=f(v(t'))
v(t')
q(t')=f(v(t<=t'))
Model capacitor by C  
C
1  jCR
Non-Quasi-Static Equivalent Circuit
including noise sources
vns
G
RG
+
Rgd
Cgd
D
+
Cgs
Zps
+
vnRg
Rgs
vsig
gm
1  j
+
S
vng
RL
ind
Noise Figure
Important to
have high fmax
• What is Associated Gain?
• What is the "black" gain?