Towards THz transistors

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Nov 2, 2013 (4 years and 2 months ago)

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Towards THz transistors
*

Luis Jauregui


* Yu
-
Ming et al. Nano letters 2009, vol. 9, No 1, 422
-
426

Outline


THz what and why?


RF CNT transistor model


RF CNT transistors


How do we improve the cutoff frequency?


Experimental setup


How can we improve them?.



THz what and why?


From 300 Ghz to 10+
Thz

(1mm to 1um).


Rich spectral signature information(molecular resonances).


Faster electronics, compared to RF electronics, wider bandwidths.


Applications:

1.
Security and defense, see through packages.

2.

Medicine, Imaging cancer tumors, identification of drugs

3.
Communications,


670
-
1000 GHz imaging systems

sub
-
mm
-
wave communications

RF CNT transistor model

P. Burke, Solid
-
state electronics 48 (2004) 1981
-
1986


L
M

<<
L
K

,
L
K
=16nH/µm


C
T

= C
Q

+ C
ES

, C
Q

= 100
aF
/
µ
m


C
T
= 50
aF
/
µ
m


RF CNT transistors


Relevant frequency scales
:


1)
RC time; CNT length = 100nm, then C =4aF, R can be = 6.25K
Ω



2)
Transconductance



P. Burke, Solid
-
state electronics 48 (2004) 1981
-
1986

2.6GHz transistor

How do we increase the
cuttoff

frequency?

Increasing g
m

and decreasing
C
gs

P. Burke, Solid
-
state electronics 48 (2004) 1981
-
1986

*
Guo

et al., Performance projections for ballistic CNT field effect transistors, APL 2002; 60(17) : 3192
-
4

Theoretical limit for g
m

= 60
μ
S

What now?

Is graphene a candidate for THz transistors?

Where:

Experimental setup



Graphene by mechanical Exfoliation on

300nm SiO
2
on highly doped P type silicon.



Raman used to verify single layer.



Source and drain 1nm/50nm Ti, Pd.



Top Gate electrode 10nm/50nm Pd, Au.



12nm Al
2
O
3
by ALD at 250 C

Important:

A
functionalization

layer of 50 cycles

of NO
2
-
TMA (
trimethylaluminium
)
deposited first. To be able to deposit
thin layer of alumina (12nm) without
producing pinholes that cause gate
leakage.


Lg

500nm

20um

DC Measurements
L
g
=360nm

Conductance before and after depositing the top gate

μ
=400cm2/(V.s)

Degradation probably due to the charged
impurity scattering associated with the
functionalization

layer.

They use the de
-
embedding procedure to
calibrate the network analyzer using as short and
open similar devices made on the same chip.



f
T

proportional to gm, meaning the graphene
device is working as a transistor.



Cg~72fF, geometrically
Cgs

= 80fF.

A
C Measurements
L
g
=360nm

A
C Measurements
L
g
=150nm

The measurement tool can not go to
the cutoff frequency but they point
out that extrapolating we reach a
cutoff frequency of 26GHz.

How can we improve them?

Increasing electron mobility or
Vds

and reducing gate length we can create faster transistors.

Approaching ballistic transport in suspended graphene, Nature Nanotechnology, August 2008

By graphene nanoribbons?.