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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 constant and
breakdown voltage is VB  E m W (depletion width)
p   n Abrupt junction diode,
E(x) 
qN B
s
x
(W  x)   Em (1  )
W
where N B is the lightly doped bulk concentrat ion
 s Em2
1
VB  EmW 
2
2qN B
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 /( 2 wD )
Current in external circuit  v s . Charge is conserved 
I max
W
W
d
 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 IMPATT' s,  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%
vs  d 0.74vs
W
now  d 
f 

vs
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 the electrons travel at their
saturation velocity,  s , across the depletion region :
then I spchrg  υs ρ s A, where is the space charge density
A is the area. The disturbanc e E(x) in the electric
field due to the space charge is
W
E(x  W)   (ρ s /ε s )dx IW/(Aε s υs ). setting E(W)  E c
0
find the maximum current allowed to be I m  E c Aε s υs / W
Power Output -3
Therefore the upper limit on the power input is :
Pm  Vm I m  E c Aε s υs and the transit t ime
2
frequency, f  γ s / W, where is 1/2 for IMPATT
and 3/4 for BARITT and 1 for the TED operated
under the transit t ime domain. Rewrite as
2
2
γE c υs
Pm f  Vm I m 
, where
2π X c
2
X c is the device 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