IEEE Spectrum October, 2007 - 11

ENERGY

ENERGY

20	 IEEE	Spectrum	|	October	2007	|	NA		

technical	officer	Jeffrey	Gilbert	
says	his	company's	rF	chip	
looks	for	the	receiver,	and	if	it's	
not	directly	in	sight,	the	receiver	
figures	out	the	best	path	to	ricochet	the	signal-off	a	wall	or	the	
floor-to	get	to	the	target.
	
not eVeryone is	convinced	
that	Phiar's	technology	will	
make	it	into	upcoming	consumer	
devices.	"I	would	personally	be	
very	skeptical	of	anyone	saying	
they	will	put	semiconductors	
out	of	business	anytime	soon,"	
says	John	cressler,	a	professor	of	electrical	engineering	at	
Georgia	Institute	of	technology,	
in	Atlanta,	who	studies	the	
60-Ghz	band.	though	Phiar's	
	tunneling-based	approach	will	
produce	fast	circuits,	he	says,	
silicon-germanium	and	cmOS	
chips	have	the	advantage	at	
60	Ghz	if	for	no	other	reason	
than	that	they	are	already	nearing	production.
Ibm's	brian	Gaucher	agrees.	
his	company	is	developing	silicongermanium-based	60-Ghz	
chips.	"I	don't	doubt	the	device	
physics,"	says	the	yorktown	
heights,	N.y.-based	research	
staff	member,	"but	I	think	that	
traditional	silicon,	due	to	its	
maturity,	is	the	technology	that	
will	likely	be	leveraged	to	enable	
the	hd-multimedia	revolution."	
behzad	razavi,	an	electrical	engineering	professor	at	
University	of	california,	los	
Angeles,	adds	that	while	Phiar	
has	demonstrated	an	ability	
to	use	quantum	tunneling	to	
make	fast	devices,	it	is	not	clear	
whether	those	fast	devices	
can	be	integrated	into	actual	
applications.	"Everything	they	
have	in	their	products	is	single	
devices,"	he	says.	"diodes,	
transistors-these	are	single	
components."	In	an	integrated	
system,	countless	components	
running	at	a	60-Ghz	frequency	
must	work	together	flawlessly.	
but	the	metal	interconnects	
between	them	are	extremely	
difficult	to	manage	at	such	high	
frequencies.	"you	have	these	
little	components,	each	good	
for	60	Ghz,	but	the	wires	have	
introduced	their	own	problems,"	
razavi	says.	
-sarah aDee
www.spectrum.ieee.org

bryAN	chrIStIE	dESIGN

NEWS

where	in	this	issue].	but	
inexpensive	technology.	In	fact,	
work-you	can	beam	your	home	
Phiar	took	advantage	of	the	 rentschler	maintains	that	Phiar	
movie	from	your	dVd	player	to	
phenomenon	by	finding	a	
devices	can	be	fabricated	on	
your	tV	without	worrying	about	
way	to	make	electrons	tunalmost	any	substrate.	"tree	bark	 your	neighbors	watching,	too.	
nel	easily	in	one	direction	
is	probably	a	really	bad	choice,"	
but	it	also	may	mean	that	people	
and	not	at	all	in	the	other.	the	
he	says,	"but	we	can	pretty	
walking	through	the	beam	can	
result	is	a	diode	made	up	of	two	 much	deal	with	anything."
disrupt	the	link.
very	thin	insulators	sandwiched	
tree	bark	aside,	the	company	
Phiar's	rentschler	says	that	
between	two	metal	layers.
is	hoping	to	build	its	circuits	
instead	of	being	confined	to	a	
From	the	electron's	perspecon	a	variety	of	substrates	in	an	
single	chip	in	a	single	location	
tive,	Phiar's	proprietary	blend	of	
metals	and	insulators	makes	the	
QUANTUM
FORWARD BIAS
energy	barrier	between	the	metTUNNELING: Phiar
als	thinner	in	one	direction	than	
Corp.'s gigahertzQUANTUM
frequency diodes
the	other.	that	happens	because	
WELL
rely on the phenomwhen	the	electron	travels	in	the	
enon of quantum
"easier"	direction,	the	interface	
tunneling. In one
between	the	two	insulators	
direction, a quancreates	a	quantum	well,	a	structum well forms
ELECTRON
between two insuture	that	confines	electrons	in	
lators. An electron
two	dimensions.	the	quantum	
that tunnels the
well	sits	at	the	halfway	point	
short distance to
between	the	two	metals,	and	
the well can easily
tunnel the rest of
once	the	electron	has	tunneled	
the way.
that	far,	the	quantum	well	boosts	
the	chances	that	it	will	make	it	
INSULATOR
through	the	second	insulator.	
2
METAL 1
METAL 2
1
V+
but	the	quantum	well	appears	
only	when	the	electron	is	being	
DISTANCE
pushed	in	one	direction,	called	
the	forward	bias	[see	illustration].	When	the	electron	is	being	
pushed	in	the	opposite	direction,	
In the other direcreverse	bias,	there	is	no	well,	
REVERSE BIAS
tion, no quantum
and	without	it,	the	insulation	is	
well forms, and
too	thick	to	tunnel	through.	
the distance is
the	metal-insulator-insulatortoo great for an
electron to tunnel
metal	(mIIm)	structure	makes	
through, so no curelectrons	"10	billion	times	
rent flows.
more	likely	to	tunnel,"	says	
rentschler.	Quantum	tunneling	lets	an	electron	traverse	a	
device	in	just	1	femtosecond,	
thousands	of	times	as	fast	as	
an	electron	traveling	through	a	
typical	semiconductor	transistor.	
INSULATOR
"Semiconductors	are	called	semi2
METAL 1
METAL 2
1
V+
conductors	because	an	electron	
doesn't	move	through	them	very	
DISTANCE
well,"	he	says.	"It	spends	a	lot	of	
time	bumping	around	through	a	
slow,	molasses-like	atmosphere."	 effort	to	get	around	one	of	the	
in	a	computer	or	tV,	his	firm's	
rentschler	says	the	devices	are	
peculiarities	of	the	60-Ghz	band.	 antennas	and	transceivers	can	
so	fast	that	the	60-Ghz	band	
radiation	at	that	frequency	is	
be	distributed	in	tiny	strips	all	
is	the	lowest	frequency	the	comabsorbed	by	oxygen	in	the	atmoover	a	consumer	device,	allowpany	is	interested	in	and	that	
sphere.	to	get	a	strong	enough	
ing	it	to	pick	up	a	signal	from	
its	circuits	have	been	clocked	
connection	between,	say	a	
any	direction.
at	110	Ghz.
video	player	and	an	hdtV,	the	
Of	course,	other	radio	
Phiar	contends	that	its	
electronics	must	communicate	
developers,	such	as	the	leading	
devices	can	be	manufactured	
in	a	tightly	focused,	highly	direc60-Ghz	silicon	firm	Sibeam,	in	
directly	on	top	of	a	cmOS	
tional	beam.	In	some	ways	this	
Sunnyvale,	calif.,	have	soluchip,	potentially	making	them	
attribute	makes	60	Ghz	perfect	
tions	for	the	band's	directionala	simple	addition	to	an	already	
for	a	wireless	personal	area	netity	problem,	too.	Sibeam	chief
http://www.spectrum.ieee.org
Table of Contents for the Digital Edition of IEEE Spectrum October, 2007
