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Submitted on 1 Jan 1979
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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 OUT-
SIDE 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�. �jpa-00219204�
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 f o r 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 s p a c e charge i s compensated (undercompensated, compensated o r overcompensated). I n o r d e r t o eviden- t 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 c h a r g e s 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 d i s t a n c e .
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 w i t h an independent e x t e r n a l connection. This g r i d can be b i a s e d 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 e v e r y o f 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 f o l l o w i n g experimen- t a l c o n d i t i o n : T E = 1023-1373O~, d = 0,2- 2,s nun, cesium p r e s s u r e =0,13-0,9 t o r r .~ 1 1 t h e volt-ampere c h a r a c t e r i s t i c s were Cd
measured a g a i n 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 v a l u e s 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 d e c r e a s e w i t h i n c r e a s e d xenon p r e s s u r e . The c h o i c e of xenon i s due t o t h e ,appro- p r i a t e mass number of cesium and xenon.
L e t us c o n s i d e r 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 i s 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 v a l u e r i g h t b e f o r e t h e charac- t 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 cons- t a n t v o l t a g e ) than I, (V = O V) i s t h i s c u r r e n t v a l u e f o r 0 v o l t a g e on t h e g r i d , 9
I (Vg=*3 V ) i s 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,
)=i
(vg=-3 v ) - l 0 (vg=o VJII, (vg=ov) f o r two e m i t t e r temperatures and previous- l 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 v e r s u s cesium vapow p r e s s u r e sAn i n c r e a s e o f t h e e l e c t r o n i c c u r r e n t
a p p e a r s when t h e g r i d p o t e n t i a l i s -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 t h e d i f f u s e d e l e c - t 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 emit- t e r t e m p e r a t u r e s when s u r f a c e i o n i z a t i o n i s 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 c a s e , 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 - r e n c e 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 t h e i n t e r e l e c t r o d i c gap w i t h p r a c t i c a l l y i n t e r e l e c t r o d i c space i s important. A t no charge d i f f u s i o n o u t s i d e t h e gap / 2 / . h i g h e r e m i t t e r temperature t h e r e i s a I n t h e f i g . 3, a r e given t h e v a l u e o f t h e p a r t i a l e l e c t r o n i c space charge ComPensa- breakdown v o l t a g e s V d taken a s u s u a l and t 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 v a l u e s 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 de- c 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 v o l t a g e ( + 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
from t h e i n t e r e l e c t r o d Y c space w i t h sub- s e q u e n t 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 c o n s i d e r e d 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 i m p o r t a n t o n l y a t low cesium and xenon p r e s s u r e s . Because one i o n 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 h i g h e r cesium or.
g a s 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 t h e r m i o n i c diode a t t h e a p p l i c a - t i o n o f 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 d i o d e 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
t h e v o l t a g e V a a s b e f o r e d e f i n e d . Because V a i s a r e a l appearance o f 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 of t h e breakdown v o l t a g e f o r v a r i o u s parameters of t h e diode.
I n c o n c l u s i o n , t h e use of t h e g r i d g i v e s i n f o r m a t i o n on t h e charge l o s s out- s 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 d i o d e s , 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. Conse- q 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 - t r o d e s t r u c t u r e .
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