Gunn Diodes engineering electronics and communication

Gunn Diode

1. J. B. Gunn, "Microwave Oscillation of Current in III-V Semiconductors",
Solid State Commun., 1 88 (1963)
Gunn Diodes
n-type GaAs
Metal Metal
In 1960’s GaAs was a new emerging semiconductor material
John Gunn research objective was to study the ohmic contacts to GaAs

2. V
I
GaAs sample I-V characteristic in Gunn experiments
n-type GaAs
Metal Metal
5V

3. V
I
GaAs sample I-V characteristic in Gunn experiments
n-type GaAs
Metal Metal
15V

4. V
I
GaAs sample I-V characteristic in Gunn experiments
30V
n-type GaAs
Metal Metal

5. .
0
4
8
12
16
20
0 20 40 60 80 100 120 140 160
Current (mA)
Time (ps)
j
s
=
qn
o
v
s
j
p
=
qn
o
v
p
Short-pulse current waveform in Gunn experiment

6. Electron drift velocity – Electric field dependence in GaAs
2 4 6 8 10 12 14
0.5
1
1.5
2
Electric field (kV/cm)
μ= 0.85 m
2
/Vs
μ= 0.5 m
2
/Vs
Physical mechanism of the Gunn effect
Si
GaAs

7. Such an assumption is wrong.
2 4 6 8 10 12 14
0.5
1
1.5
2
Electric field
(
kV/cm
)
μ= 0.85 m
2
/Vs
μ= 0.5 m
2
/Vs
Current voltage characteristic of GaAssample
in strong electric fields
I = q ×n ×v(F)
×
Area
Since F = V/L, one can expect that I-V characteristic would be
similar in shape to the v(F) curve
2 4 6 8 10 12 14
0.5
1
1.5
2
Electric field
(
kV/cm
)
μ= 0.85 m
2
/Vs
μ= 0.5 m
2
/Vs
Current
Voltage

8. 2 4 6 8 10 12 14
0.5
1
1.5
2
Electric field (kV/cm)
μ=0.85 m
2
/Vs
μ=0.5 m
2
/Vs
Space chargeinstabilityin semiconductors
with negative differential mobility (NDM)
F
C
In GaAs, at electric fields exceeding the critical value of F
C
≈3.2 kV/cm
the differential mobility is negative.
When the field exceeds F
C
and further increases, the electron drift velocity decreases.

9. x
x
F
0
≈F
c
v
0
= v
m
x
n
0
= N
D
F
v
F
c
v
m
-
+
F
v
n
Space charge instability in semiconductors with NDM
Initially uniform
electric field and
concentration
distribution in
the sample.

10. x
F
0
≈F
c
F
v
F
c
v
m
-
+
F
x
v
0
= v
m
v
x
n
0
= N
D
n
00
D
F
nN
q
x
ρ
ε
εεε
∂
−
=− =
∂

11. x
F
x
v
F
0
≈F
c
v
0
= v
m
x
n
n
0
= N
D
F
v
F
c
v
m
-
+

12. x
F
x
v
F
0
≈F
c
v
0
= v
m
x
n
n
0
= N
D
F
v
F
c
v
m
-
+

13. x
F
x
v
F
0
≈F
c
v
0
= v
m
x
n
n
0
= N
D
F
v
F
c
v
m
-
+
v
s
v
s
High-field, or
Gunn domain

14. x
F
x
v
F
0
≈F
c
v
0
= v
m
x
n
n
0
= N
D
F
v
F
c
v
m
-
+
v
s
v
s

15. x
F
x
v
F
0
≈F
c
v
0
= v
m
x
n
n
0
= N
D
F
v
F
c
v
m
-
+
v
s
v
s

16. x
F
vF
0
≈F
c
F
v
F
c
v
m
-
+
v
sCurrent – time dependence in the sample with high-filed domain
Current at the device electrodes:
I
V
= q n v
s
When the domain is moving between the cathode and anode:

17. F
v
F
c
v
m
-
+
v
sCurrent – time dependence in the sample with high-filed domain
Current at the device electrodes:
I
m
= q n v
m
When the domain dissipates in the anode and new domain did not form yet:
x
x
F
0
≈F
c
v
0
= v
m
F
v

18. v
F
c
v
m
v
sCurrent – time dependence in the sample with high-filed domain
I
m
= q n v
m
.
0
4
8
12
16
20
0 20 40 60 80 100 120 140 160
Current (mA)
Time (ps)
j
s
=
qn
o
v
s
j
p
=
qn
o
v
p
I
V
= q n v
s

19. Transit-time oscillations in Gunn diodes
.
0
4
8
12
16
20
0 20 40 60 80 100 120 140 160
Current (mA)
Time (ps)
j
s
=
qn
o
v
s
j
p
=
qn
o
v
p
GD
L
R
L
Domain transit time: t
tr
= sample length /domain velocity
t
tr
= L/v
s
In GaAs, v
s
≈
10
7
cm/s
For the sample with the length L = 100
μ
m,
t
tr
= 100 ×10
-4
cm / 10
7
cm/s = 10
-9
s
The frequency of transit –time oscillations:
f
tr
= 1/t
tr
= 10
9
1/s = 1 GHz
For L=10
μ
m, f
tr
= 10 GHz

20. .
0
4
8
12
16
20
0 20 40 60 80 100 120 140 160
Current (mA)
Time (ps)
j
s
=
qn
o
v
s
j
p
=
qn
o
v
p
GD
L
R
L
1. Operating frequency controlled by the sample length:
no tuning, varies from sample to sample, sensitive to sample non-uniformities.
2. Current waveform consist of short pulses with the width << half-a-period:
low efficiency
Transit-time oscillation issues:

21. 1. Resonator voltage controls the
domain nucleation and dissipation.
2. Current waveform pulses are wider
as compared to transit-time mode:
higher efficiency
Resonator-controlled oscillations in Gunn diodes
Gunn diode in the
LC-resonator

22. Highly-efficient Limited –Space charge- Accumulation mode
Approach:
Domain formation requires certain time t
d
.
If the resonator frequency f
r
>> (1/t
d
), the domain cannot completely develop
The filed and concentration in the sample remain nearly uniform.
The “dynamic” I-V curve of the Gunn diode reproduces the v(F) dependence

23. Highly-efficient Limited –Space charge- Accumulation mode
Achieved frequencies: up to 100 GHz

24. Kroemercriterion in the Gunn effect
Concentration
Distance
Cathode
Anode
Field
Characteristic time of the domain formation can
be evaluated by effective RC- circuit charging
time:
0
0
||
ddd
d
tRC
qn
ε
ε
μ
≈=
Domain formation time is equal to
t
d
(so-called Maxwell relaxation time);
n
0
is the equilibrium electron concentration,
μ
d
is the differential electron mobility.
In GaAs, typically, |
μ
d
|
≈
2000 сm
2
/(V×s)
C
d
=
εS
L
R
d
=
L
qμ
d
n
o
S
0
d
d
S
C
L
ε
ε
=
0
d
d
dL
R
qn S
μ
=

25. Kroemercriterion in the Gunn effect
Characteristic domain transit time in the sample of the length L:
tr
sL
t
v
≈
If domain formation time t
d
is greater
than the domain transit time t
tr
, the domain
does not have enough time to develop – the
diode is stable. Gunn diode is stable if t
d
> t
tr
;
Gunn diode may oscillate in one of the Gunn-
domain modes if t
d
< t
tr
Concentration
Depletion
Layer
Accumulation
Layer
Distance
Field
Anode Cathode
L
(
)
()
0
,
||
ooCR
s
oCR
d
nL nL
v
where n L
q
εε
μ
>
=
0
0
ddd
d
tRC
qn
ε
ε
μ
≈=
Kroemer criterion for
domain formation:

26. Stable Gunn diodes -amplifiers
Field/concentration distributi ons and impedance –frequency
dependence in stable Gunn diode
If the Kroemer criterion is not met:0
||
s
o
d
v
nL
q
ε
ε
μ
<
High-field domains do not form and Gunn diodes are stable.

27. Stable Gunn diodes -amplifiers
Reflective type microwave diode amplifier:
When the diode resistance R
d
<0, the amplitude of reflected e/m wave A
refl
is
greater than that of incident wave A
inc

28. Stable Gunn diodes –travelling space-charge
wave amplifiers
Space-charge amplitude increases from cathode to anode: unidirectional
amplification.

29. Gunn diode mode of operation –parameter map
0
||
s
o
d
v
nL
q
ε
ε
μ
>=
Gunn diode works as an oscillator
f
0
< 1/t
d
– Gunn diode operates in the Gunn domain mode.
f
0
> 1/t
d
– Gunn diode operates in the limited space charge accumulation
(LSA) mode – no domains are formed.
For the LSA mode, f
0
> 3×1/t
d
if f
0
>1/t
d
but f
0
< 3 ×1/t
d
, Gunn diode operates in a mixed Gunn
domain/LSA mode
0
||
s
o
d
v
nL
q
ε
ε
μ
<
Gunn diode works as a stable amplifier. No Gunn
domain or LSA oscillations
0
0
d
d
t
qn
ε
ε
μ
=
The mode of operation depends on the relationship between
the resonant frequency of the attached resonant circuit f
0
and the domain formation time:
I.
II.