Transit time device

Transit-Time Device
Course coordinator: Arpan Deyasi
2/7/2021 1
Arpan Deyasi, RCCIIT

Q: What is transit time?
time between the injection and the collection of
carriers in a semiconductor device
the movement is considered between
two electrodes
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Q: What is transit time effect?
any effect caused due to transit time
is called transit-time effect
In microwave vacuum devices as well as
semiconductor devices, several effects are
observed due to transit time
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Q: What is transit time device?
transit time causes a change of phase between
voltage and current in a semiconductor device
Q: How the effect becomes significant?
If the effect causes 180° phase shift between voltage
and current, then it exhibits negative resistance
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Avalanche Transit Time Device
If both avalanche and transit time effect together
cause the negative resistance, then the device is
called Avalanche Transit Time Device
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Types of ATT
IMPATT: IMPact ionization Avalanche Transit Time
TRAPATT: TRApped Plasma Avalanche Triggered Transit
BARITT: BARrier Injection Transit Time
QWITT: Quantum Well Injection Transit Time
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IMPATT
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Features of IMPATT
Operating region: 16-300 GHz ---- specifically operates
at window Frequencies (16, 34, 94, 140, 220, 301 GHz)
Generates high level of phase noise due to avalanche
Process (~40 dB)
High power capability
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Classification of IMPATT
SDR: Single Drift Region
DDR: Double Drift Region
DAR: Double Avalanche Region
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SDR IMPATT: Read type
n+ p i p+
E
z
z
Neff
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SDR IMPATT: Read type
n+ p i p+
E
z
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SDR IMPATT
n+ p p+
E
z
z
Neff
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SDR IMPATT
n+ p p+
E
z
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DDR IMPATT
E
z
z
Neff
n+ n p p+
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DDR IMPATT
E
z
n+ n p p+
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DAR IMPATT
E
z
z
Neff
n+ p i p+
n
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DAR IMPATT
E
z
n+ p i p+
n
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carriers are drifted in respective sides after generation due
to applied reverse bias
Carrier flow in IMPATT
Avalanche multiplication always takes place at the junction
of highly doped and moderately doped region
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Materials for IMPATT fabrication
 GaAs
 Si
 InP
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Microwave
generation
in
IMPATT
n+ p i p+
E
z
Vac
z
z
I Iava
Iext
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Microwave generation in IMPATT
external current due to moving carriers is delayed by 90°
relative to pulsed current
pulsed current is delayed by 90° relative to ac voltage
phase difference becomes 180° between external
current and applied ac voltage
negative conductance occurs which leads to
microwave oscillation
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Conversion Efficiency
dc to RF conversion efficiency is the ratio of output
ac power to input dc power
ac
dc
P
P
η =
( )
2
m
0 0
0
I sin( )( sin( )
m
t V t dt
I V
π
ω ω
η = ∫
m
0 0
I
m
V
V I
η =
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Power output of IMPATT
at low ‘f’, power output is inversely proportional
to frequency
at high ‘f’, power output is inversely proportional
to square of frequency
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2/7/2021 Arpan Deyasi, RCCIIT 24
Advantages of IMPATT diode
Operates from 3 - 100 GHz frequency range
high power capabilities compare to other microwave
diodes
output is more reliable compare to other microwave
diodes
acts as a narrow band device when used as amplifier
can be used as excellent microwave generators
can produce carrier signal for microwave transmission
system

Disadvantages of IMPATT diode
has high noise figure due to avalanche process & higher
operating current
shot noise is generated in the device due to high
operating current
noise figure of IMPATT is about 35 dB
produces spurious noise (AM and FM) with higher levels
compare to klystron and Gunn diodes
tuning range of IMPATT diode is not as good as Gunn
diode
offers lower efficiency compare to TRAPATT diode

Application of IMPATT
Negative resistance parametric amplifier
Microwave source
CW Doppler RADAR transmitter
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BARITT
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Features of BARITT
generates low noise microwave power at lower
microwave frequency (up to X-band)
large transit time
uses thermionic emission rather avalanche process
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Structure
of
BARITT
W1 W2
Neff
z
z
E
p+ n p+
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Widths of BARITT
1
1
2
( )
D bi
W
qN V V
ε
=
−
2
2
2
( )
D bi
W
qN V V
ε
=
−
for forward bias junction
for reverse bias junction
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Structure
of
BARITT
at reach-through condition 1 2
W W W
= +
p+ n p+
E
z
1
2
3
1: depletion region
2: low field region
3: saturated velocity region
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Microwave
generation
in
BARITT
n+ p p+
E
z
Vac
z
z
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Microwave generation in BARITT
Carriers are thermionically injected over the barrier
in presence of ac field
Voltage reaches the maximum
Peak current is delayed w.r.t ac voltage by T/4
External current induced in the circuit when the charge
bunch travels through the reverse-biased depletion layer
takes ¾ of the time-period to reach the negative
terminal

3
2
2
d
f
π
π τ =
3
4 d
f
τ
=
3
4
s
v
f
w
=
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Frequency of BARITT

Advantages of BARITT diode
less noisy due to thermionic emissions
offers noise figure of about 15 dB
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Disadvantages of BARITT diode
relatively narrower bandwidth
lower power handling capability
efficiency of the BARITT diode decreases with
increase in the frequency
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Applications of BARITT diode
 Mixer
 Large signal Oscillator
 Small signal amplifier
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TRAPATT
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Features of TRAPATT
produces high microwave power with very high dc-to-RF
conversion efficiency (40-60%)
operated under pulsed condition
characterized by lower oscillation frequency

Structure of TRAPATT
p+ n n+
E
z
z
Neff
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Trapping
of
plasma
plasma of very high density is created by
avalanche electric field
this makes collapse of field owing to sharp
increase of conductivity in the region
carriers drift very slowly, i.e., plasma is trapped

Propagation of avalanche shock-front
τ1
W
z
E
p+ n n+

W
z
E
τ2
p+ n n+

W
z
E
p+ n n+
τ3

W
z
E
p+ n n+
τ4

. D
D qN
∇ =
 
D
E
qN
z
ε
∆
=
∆
D
qN
E
z ε
∆
=
∆
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Velocity of Avalanche Shock-Front

Field rises in the carrier free drift region during shock
Front propagation
z
E I
v
t ε
∆
= =
∆
Shock-front velocity
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Velocity of Avalanche Shock-Front

( )
( )
E
z t
E
t
z
∆
∆ ∆
=
∆
∆
∆
( )
D
I
z
qN
t
ε
ε
∆
=
∆  
 
 
z
D
I
v
qN
=
2/7/2021 48
Velocity
of
Avalanche
Shock-Front

Operation of TRAPATT
A
B
C
D
E
F
G
A
τ
t
0.5τ
charging
plasma formation
residual extraction
plasma extraction
charging
voltage
/
current

at point ‘A’, electric field is uniform throughout the sample
and its magnitude is large but less than the value required
for avalanche breakdown
at ‘A’, diode current is turned ON
diode behaves like a linear capacitor and reaches at
point ‘B’ owing to charging

after generation of sufficient carriers, electric field is
depressed throughout the depletion region, causing
voltage to decrease; shown from ‘B’ to ‘C’
as few carriers are drifted out, field is further depressed
and traps the remaining plasma, so voltage reaches at
point ‘D’
at point ‘E’, plasma is removed

residual charge remains, which, when removed, voltage
increases from ‘E’ to ‘F’
at point ‘F’, all the generated charges are removed
from pint ‘F’ to ‘G’, charges are again raised like a fixed
capacitor
at point ‘G’, current goes to zero for half-a-period, and
voltage remains constant until the cycle repeats

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Advantages of TRAPATT diode
offers higher efficiency compare to IMPATT diode
efficiency of about 40-60 % can be achieved
very low power dissipation
most suitable for pulsed operation
can operate from 3 - 50 GHz

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Disadvantages of TRAPATT diode
not used for continuous operation mode as it
offers high power densities (10 - 100 W/m2)
very high noise figure which is about 60 dB
supports frequencies below mm-wave band

2/7/2021 55
Applications of TRAPATT diode
Microwave beacons
Local oscillators in Radar
ILS (Instrument Landing System)
S-Band pulsed transmitters for phased array radar
Radio altimeter

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