Transcript ECE 662 - George Mason University

ECE 662
Microwave Devices
Transit-Time Diodes
February 17, 2005
Two-Terminal Negative
Resistance Devices
Avalanche Transit-Time Devices
Avalanche Transit-Time Devices
Avalanche Transit-Time Devices
Measured Ionization
rates for electrons and
holes vs reciprocal field
for Si and GaAs
Ref: Sze
Diode Configurations
p  i  n Diode, field, E m is constantand
breakdown voltageis VB  E m W (depletionwidth)
p   n Abrupt junctiondiode,
qN B
x
E(x) 
(W  x)   Em (1  )
s
W
where N B is thelight lydoped bulk concent ration
 s Em2
1
VB  EmW 
2
2qNB
IMPATT Mode Diodes
IMPATT Mode Diodes
Injected carriers therefore traverse the length wD of the drift region
During the negative half-cycle if we choose the transit time to be
½ oscillation period.
  d  wD / vs  0.5(1 / f ) or f  v /(2wD )
Current in ext ernalcircuit  vs . Charge is conserved
W
W
d
I max  I inj
I dc  I inj
 I max
d
2
2
1
Pdc  VB I dc and Prf 
2
2
I
0
ind
(t )Vrf sin(t )d (t )
Prf  I dcVrf
 cos m  cos( m   D )  sin(W / 2)
;


d

 W / 2
Vrf sin(W / 2)  cos m  cos( m   d ) 




Pdc VB
W / 2 
d

Prf
for IMP AT Ts,'  m  
Vrf sin(W / 2)  cos d  1


;
VB
W / 2   d 
for best efficiency, W is small
Vrf  cos d  1
sin(W / 2)
so
 1, and  


W / 2
VB   d

Max max occurs when  d  0.74 so   0.72
typically,  20 to 30%
now d 
W
vs
vs  d 0.74vs
f 

W 2
2W
Vrf
VB
,
IMPATT Mode Diodes
Double-Drift Region IMPATTs
TUNNETT Mode
BARrier Injection Transit Time Devices
(BARITTs)
BARrier Injection Transit Time Devices
(BARITTs)
The injected carrier density increases with the ac voltage.
Then the carriers will traverse the drift region.
o
The injected hole pulse at 90 and the corresponding
induced current which travels 3/4s of a cycle to reach the
negative terminal. Or w/vs = ¾ (1/f)
Note that for /2 t, both the ac voltage and external
current are positive therefore ac power is dissipated in the
device.
Consequently, the BARITT diodes have low power
capabilities and low efficiencies but they also have low
noise (avoiding the avalanche phenomena).
TRApped Plasma Avalanche Triggered
Transit Time Devices (TRAPATTs)
TRApped Plasma Avalanche Triggered
Transit Time Devices (TRAPATTs)
Comparison of Microwave
Devices
• An important figure of merit for microwave
devices is power output as a function of oscillation
frequency.
• Due to limitations of semiconductor materials, the
maximum power of a single device at a given
frequency is limited.
• Two basic limitations:
– Critical field, at which avalanche breakdown occurs
– Saturation velocity which is the maximum attainable
velocity in semiconductors
Power Output -1
• The maximum voltage that can be applied
across a semiconductor sample is limited by
the break down voltage.
– For a uniform avalanche this is Vm = EcW
where W is the depletion layer width
• The maximum current that can be carried by
the semiconductor is also limited by the
avalanche breakdown process, because the
current in the space charge region causes an
in crease in the electric field.
Power Output -2
Assume that theelectronstravelat their
saturat ionvelocity, s , across thedeplet ionregion :
then Ispchrg  υs ρ s A, where is thespace chargedensity
A is thearea. T hedisturbance E(x)in theelectric
field due to thespace chargeis
W
E(x  W)   (ρ s /ε s )dx IW/(Aε s υs ). settingE(W) E c
0
find themaximumcurrentallowed to be I m  E c Aε s υs / W
Power Output -3
T hereforetheupper limit on t hepower input is :
Pm  Vm I m  E c Aε s υs and the transit ime
t
2
frequency,f  γ s / W, where is 1/2 for IMP AT T
and 3/4 for BARIT Tand1 for theT EDoperated
under thetransit ime
t domain. Rewrite as
2
2
γE c υs
Pm f  Vm I m 
, where
2π X c
2
X c is thedevice reactance(2f s A / W ) 1
Noise - Microwave Devices
Devices
IMPATT
BARITT
TED
GaAs
TED InP
TUNNEL
MESFET
Bipolar
Noise
Figure
in dB
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
Solid-State
Device
Power
Output vs
Frequency
ref: Sze
and
modified
by Tian