INFLUENCE OF THE INTERELECTRODIC
CHARGE DIFFUSION OUTSIDE OF THE GAP ON
THE THERMIONIC DIODE VI CHARACTERISTICS
A. Baltog, G. Musa
To cite this version:
A. Baltog, G. Musa. INFLUENCE OF THE INTERELECTRODIC CHARGE DIFFUSION
OUTSIDE OF THE GAP ON THE THERMIONIC DIODE VI CHARACTERISTICS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-457-C7-458. <10.1051/jphyscol:19797222>.
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JOVRmPL DE PHYSIQUE
CoZZoque C7, suppZdment au n07, Tome 40, JudZZet 2979,page C7- 457
NLUENCE OF THE INTERELECTRODIC CHARGE DIFFUSION OUTSIDE
OF M E GAP ON THE MERMlONIC DIODE V I CHARACTERISTICS
A. Baltog and G. Musa
Institute of Physics and TechnoZogy for Radiation Devices, P.O. Boz 5207 MagureZe-Bucharest Romanda.
The c u r r e n t v o l t a g e c h a r a c t e r i s t i c s of
a thermionic diode i s s t r o n g l y dependent
of t h e i n t e r e l e c t r o d i c charge d e n s i t y . I f
t h e r e a r e no i o n s , t h e space charge i s
p u r e l y e l e c t r o n i c and t h e V I c h a r a c t e r i s t i c s a r e space charge l i m i t e d . I f t h e r e
a r e i o n s produced by s u r f a c e o r volume
i o n i z a t i o n t h e e l e c t r o n i c space charge i s
compensated (undercompensated, compensated
o r overcompensated). I n o r d e r t o evident i a t e t h e c o n t r i b u t i o n of t h e w a l l d i f fused charges t o t h e i n t e r e l e c t r o d i c
charge d e n s i t y we used a s p e c i a l designed
thermionic p l a n a r diode, w i t h v a r i a b l e
distance.
The cathode-anode space i s surrounded
( i n c l u d i n g t h e e l e c t r d d e s ) by a g r i d ,
which i s provided with an independent
e x t e r n a l connection. This g r i d can be
biased p o s i t i v e l y o r n e g a t i v e l y a g a i n s t
t h e cathode; t h e a p p l i e d v o l t a g e being 0
o r f 3 V dc. For every of t h e s e v o l t a g e s
( v g ) a p p l i e d on t h e g r i d we measured t h e
volt-ampere c h a r a c t e r i s t i c s of t h e cathode
-anode space f o r t h e following experiment a l condition: T E = 1023-1373O~, d = 0,22,s nun, cesium p r e s s u r e
=0,13-0,9 t o r r .
Cd
~ 1 t1
h e volt-ampere c h a r a c t e r i s t i c s were
measured again a t each p r e s s u r e of t h e
added noble g a s t o t h e cesium vapours, t h e
values of t h e added xenon p r e s s u r e being
0 ; 0 , s ; 1; 3 and 10 t o r r .
The noble gas a d d i t i o n h a s a d i r e c t
e f f e c t t o t h e charge d i f f u s i o n , which must
decrease w i t h i n c r e a s e d xenon p r e s s u r e .
The choice of xenon i s due t o t h e ,approp r i a t e mass number of cesium and xenon.
Let us consider f i r s t t h e volt-ampere
c h a r a c t e r i s t i c s when t h e r e is no d i s c h a r g e
c o n d i t i o n i n t h e thermionic diode. I f I i s
t h e c u r r e n t value r i g h t b e f o r e t h e charact e r i s t i c s t u r n o u t t o t h e d i s c h a r g e mode
(sudden i n c r e a s e of t h e c u r r e n t a t const a n t v o l t a g e ) than I, (V = O V) i s t h i s
9
c u r r e n t value f o r 0 voltage on t h e g r i d ,
I (Vg=*3 V ) is t h e c u r r e n t v a l u e when t h e
g r i d p o t e n t i a l i s f 3 V.
I n f i g . 1 a r e given t h e measured v a l u e s
o r (AIII,
(vg=-3 v ) - l 0 (vg=o VJII, (vg=ov)
f o r two e m i t t e r temperatures and previousl y mentioned v a l u e s of t h e xenon p r e s s u r e s
r e p r e s e n t e d versus cesium vapow p r e s s u r e s
An i n c r e a s e of t h e e l e c t r o n i c c u r r e n t
)=i
appears when t h e g r i d p o t e n t i a l is -3 V,
due t o t h e r e p e l l i n g e f f e c t of t h e o u t s i d e c y l i n d r i c a l f o r the diffused elect r o n s o u t s i d e of t h e i n t e r e l e c t r o d i c gap.
This e f f e c t i s more e v i d e n t f o r low emitt e r temperatures when s u r f a c e i o n i z a t i o n
is p r a c t i c a 1 , l y n e g l i g i b l e . I n t h i s case,
due t o t h e s i g n i f i c a n t e l e c t r b n i c space
charge and n e a r l y zero p o t e n t i a l d i f f e rence between cathode and anode,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797222
t h e d i f f u s i o n of e l e c t r o n s o u t s i d e t h e
i n t e r e l e c t r o d i c space i s important. A t
h i g h e r e m i t t e r temperature t h e r e i s a
p a r t i a l e l e c t r o n i c space charge ComPensat i o n due t o s u r f a c e i o n i z a t i o n of t h e
cesium atoms which subsequent lowering of
t h e values of t h e r e l a t i v e diode c u r r e n t
i n c r e a s e A I / I o . The xenon a d d i t i o n lower
t h e e l e c t r o n d i f f u s i o n due t o t h e dec r e a s e of t h e m o b i l i t y of t h e e l e c t r o n s .
For a p o s i t i v e voltage ( + 3 V) on t h e
g r i d , t h e behaviour of A I / l o is changed
d r a s t i c a l l y a s it i s shown i n f i g . 2. The
main e f f e c t i s t h e e l e c t r o n e x t r a c t i o n
t h e i n t e r e l e c t r o d i c gap with p r a c t i c a l l y
no charge d i f f u s i o n o u t s i d e t h e gap / 2 / .
I n t h e f i g . 3, a r e given t h e value of t h e
breakdown v o l t a g e s V d taken a s usual and
t h e voltage V a a s b e f o r e defined. Because
V a i s a r e a l appearance of anode glow f i g .
3 g i v e s us t h e e r r o r i n t h e a p p r e c i a t i o n
from t h e i n t e r e l e c t r o d Y c space w i t h subsequent r e d u c t i o n of t h e diode c u r r e n t .
A t t h e considered temperature of t h e
e m i t t e r an a d d i t i o n a l e f f e c t appears a s a
r e s u l t of t h e i o n l o s s r e d u c t i o n which i s
important o n l y a t low cesium and xenon
p r e s s u r e s . Because one ion can compensate
t h e e f f e c t of n e a r l y 500 e l e c t r o n s t o t h e
space charge, s l i g h t change i n i o n l o s s
can g i v e t o s i g n i f i c a n t change i n t h e
e l e c t r o n i c c u r r e n t . A t higher cesium or.
gas p r e s s u r e s t h e d i f f u s i o n of i o n s i s
reduced f a s t e r than t h a t of e l e c t r o n s / l / .
The changes i n t h e V I c h a r a c t e r i s t i c s
of t h e thermionic diode a t t h e a p p l i c a t i o n of p o s i t i v e o r n e g a t i v e v o l t a g e s on
t h e g r i d can be u s e f u l i n o r d e r t o d e f i n e
t h e r e a l breakdown p o i n t . Indeed, a t t h i s
p o i n t of the diode p o t e n t i a l V a t h e
d i f f e r e n c e s between c h a r a c t e r i s t i c s w i t h
0 V, -3 V and + 3 V on t h e g r i d , a r e
p r a c t i c a l l y n e g l i g i b l e , due t o t h e anode
glow which producesa p o t e n t i a l t r a p i n
of t h e breakdown voltage f o r v a r i o u s
parameters of t h e diode.
I n conclusion, t h e use of t h e g r i d
g i v e s information on t h e charge l o s s outs i d e t h e e m i t t e r c o l l e c t o r space of
thermionic diodes, p o i n t i n g o u t t h e range
where such l o s s e s a r e important. Conseq u e n t l y , a b e t t e r geometry which lower t h e
charge l o s s i s a long c y l i n d r i c a l e l e c trode structure.
References
/1/
G.
Musa, D .
Roum. P h y s . ,
/2/
G.
Popescu, A.
13, n r .
Musa e t a l .
-
Baltog
-
Rev.
1 , 7 3 (1968)
Proc.
3rd I n t . Conf.
Therm. E l e c t r . Pow. Gen.,
JuliCh(1972)