High Electron Mobility Transistors

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High Electron Mobility Transistors for Low-Noise
Operation
Day 3B, May 29, 2008, Pisa
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High electron-mobility Transistor

• Note the Schottky barrier

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Schottky barrier band diagram
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Schottky barrier under bias

• Negative potential on n-type semiconductor

• discontinuity in EF

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Forward bias in SB- and PN-diodes
2. What is the driving force here?
1. What is the bottleneck here?
FB
-qVa
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Two heterojunctions in a HEMT
Metal/AlGaAs HJ
AlGaAs/GaAs HJ
2-DEG in the potential "well"
y
Note the doping
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Simplifying the quantum well

• Triangular to finite square

• Finite to infinite square

• SWE becomes

• wavenumber is

Ey
a

• bcs

? E is quantized

• solution

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Energy is quantized
?E?1/m ? 10X GaAs vs. Si
Wavefunction
Energy
Probability density
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For a finite well

• Wavefunction not completely confined

• Use undoped spacer

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Employment of a spacer layer
Provision of electrons from remote donors is
called MODULATION DOPING
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Formation of sub-bands and 2DEG
2m
2m
Empty
Partially filled 2nd sub-band

• ns0 ?1013 cm-2

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2DEG concentration ns
Independent of E !!
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Controlling ns by VGS
Thick barrier layer
Thick-enough barrier layer
qVGS
qVGS
Threshold condition
Depleting the channel

• How would you make an enhancement HEMT?

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• Often modeled by SPICE LEVEL 1
  IDsatWg?Cg(VGS-VT)2 /2Lg

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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 lt ?e
• i.e., about 15nm for GaAs at 300K.
• Size-quantization of energy levels
• - standing waves
• - only 2-D scattering
• ? ? ? ? ?
• and gm ?

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Start with a high ? and preserve it!
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High performance HEMT
Why the funny gate?
fT 270 GHz, fmax490 GHz
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NOISE
Noisy DC signal
dB
?use RMS values
What is a signal of -30dBm ?
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Thermal noise
So, an equivalent circuit representation of
thermal noise is
Brownian motion
vR gt vd, so present without current
From Nyquist

• What "colour" is this noise?

This P can be transferred from a real resistor R
to a noiseless resistor R.

• How much thermal noise in 50 Ohm R? -gt 1nV over
  1Hz

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Shot noise
Forward-biased junction
microscopically -gt
EC
Transition over barrier is random event
(probability of state occupancy)
Important in HBTs, but not in FETs, except in
sub-threshold operation.
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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.
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Induced gate noise

• The induced gate noise is correlated with the
  channel (drain-current) noise.
• Coupling is via capacitance
• Impedance decreases with frequency
• Important at high frequencies

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Non-Quasi-Static operation
q(x, y, z, t' ) f( VTerminals, t') q(x, y, z,
t' ) ? f( VTerminals, t lt t')
Recall QSA
At high frequencies, this breaks down. Consider
example of charging a capacitor
R
C
q(t')f(v(tltt'))
v(t')
v(t')
q(t')f(v(t'))
Model capacitor by
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Non-Quasi-Static Equivalent Circuit including
noise sources
vnRg
RG
Cgd
D
G
vns
Rgd


Cgs
Zps

RL
Rgs
vsig
ind

vng
S
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Noise Figure
Important to have high fmax
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• What is Associated Gain?
• What is the "black" gain?