Schottky-Barrier Diode and Metal-Oxide-Semiconductor Capacitor

11 Schottky-Barrier Diode and Metal-Oxide-Semiconductor Capacitor Gas Sensors Comparison and Performance S. J. Fonash and Zheng Li

Downloaded by UNIV OF NEW HAMPSHIRE on March 5, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch011

Engineering Science Program, Pennsylvania State University, University Park, PA 16802

Schottky-barrier diode and metal-oxide-semiconductor (MOS) capacitor gas sensors have established themselves as extremely sensitive, versatile solid state sensors. In this review the basis for the chemical sensitivity of these devices will be explored and the various device structures used for these sensors will be discussed. A survey of the performance of the diode-type and capacitor-type structures will be presented and a comparison of characteristics of these two classes of solid state gas sensors will be given. S c h o t t k y - b a r r i e r type d i o d e s , i n t h e form o f m e t a l - s e m i c o n d u c t o r (M-S), m e t a l - i n t e r f a c i a l l a y e r - s e m i c o n d u c t o r (M-I-S), o r degenerate s e m i c o n d u c t o r - i n t e r f a c i a l l a y e r - s e m i c o n d u c t o r s t r u c t u r e s , can be c o n f i g u r e d t o be e x t r e m e l y s e n s i t i v e s o l i d s t a t e gas d e t e c t o r s . I f t h e s e c h e m i c a l l y s e n s i t i v e diode s t r u c t u r e s a r e used as a g a t e , one can f a b r i c a t e c h e m i c a l l y s e n s i t i v e MESFET (metal-semiconductor f i e l d e f f e c t t r a n s i s t o r ) s e n s o r s . M e t a l - o x i d e - s e m i c o n d u c t o r (M-O-S) o r , to be more g e n e r a l , m e t a l - i n s u l a t o r - s e m i c o n d u c t o r (M-I-S) c a p a c i t o r s t r u c t u r e s , as w e l l as degenerate s e m i c o n d u c t o r - i n s u l a t o r - s e m i c o n d u c t o r c a p a c i t o r s t r u c t u r e s , can a l s o be c o n f i g u r e d t o be e x t r e m e l y s e n s i t i v e s o l i d s t a t e gas d e t e c t o r s . I n t h i s case, i f these chemi c a l l y s e n s i t i v e c a p a c i t o r s t r u c t u r e s a r e used as a g a t e , one can f a b r i c a t e c h e m i c a l l y s e n s i t i v e MOSFET ( m e t a l - o x i d e - s e m i c o n d u c t o r f i e l d e f f e c t t r a n s i s t o r ) s e n s o r s . The c h e m i c a l s e n s i t i v i t y o f MESFET s e n s o r s i s due t o t h e c h e m i c a l s e n s i t i v i t y o f t h e d i o d e - t y p e s t r u c t u r e and t h e c h e m i c a l s e n s i t i v i t y o f MOSFET s e n s o r s i s due t o t h e chemical s e n s i t i v i t y o f the capacitor-type s t r u c t u r e . Consequently, t h i s r e v i e w w i l l focus i t s a t t e n t i o n on t h e b a s i c p h y s i c s and chemi s t r y o f s e n s i n g t a k i n g p l a c e i n d i o d e - t y p e o r c a p a c i t o r - t y p e configurations. S i n c e both metals and degenerate semiconductors have been used as the c o u n t e r - e l e c t r o d e t o t h e semiconductor i n b o t h diode and c a p a c i t o r - t y p e d e v i c e s , a more g e n e r a l n o t a t i o n than t h a t u s u a l l y found i n t h e l i t e r a t u r e w i l l be employed i n t h i s r e v i e w . T h i s more g e n e r a l i z e d n o t a t i o n w i l l r e f e r t o t h e c o u n t e r - e l e c t r o d e as t h e cond u c t o r ( c ) . Hence, M-S, M-I-S, and degenerate s e m i c o n d u c t o r - i n t e r f a c i a l l a y e r - s e m i c o n d u c t o r d i o d e d e v i c e s a l l become C-S o r C-I-S 0097-6156/ 86/ 0309-0177$07.50/ 0 © 1986 American Chemical Society

In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

178

diode s e n s o r s , as a p p r o p r i a t e . F o l l o w i n g t h i s scheme, a l l i t o r - t y p e s t r u c t u r e s become C-I-S c a p a c i t o r s e n s o r s . As we gas d e t e c t i o n o c c u r s when t h e presence o f a gaseous s p e c i e s the c u r r e n t - c a r r y i n g c a p a b i l i t y o f a C-S o r C-I-S diode o r c h a r g e - s t o r a g e c a p a b i l i t y o f a C-I-S c a p a c i t o r s t r u c t u r e .

t h e capacw i l l see, modifies the


S t r u c t u r e s and P r i n c i p l e s o f O p e r a t i o n Diode S e n s o r s . I n g e n e r a l , C-S and C-I-S S c h o t t k y d i o d e - t y p e gas s e n s o r s have t h e c o n f i g u r a t i o n s i n d i c a t e d i n F i g u r e s 1 and 2. As seen i n F i g u r e 1A, t h e c o n d u c t o r - s e m i c o n d u c t o r (C-S) s t r u c t u r e i s the s i m p l e r c o n f i g u r a t i o n . The e s s e n t i a l f e a t u r e o f t h i s s t r u c t u r e i s t h e c o n d u c t i n g f i l m / s e m i c o n d u c t o r i n t e r f a c e and t h e concomitant b a r r i e r (space charge) r e g i o n i n t h e semiconductor. We note t h a t whenever d i s s i m i l a r m a t e r i a l s a r e j o i n e d t o g e t h e r ( i n t h i s case a c o n d u c t o r and a s e m i c o n d u c t o r ) , a double l a y e r ( i . e . , space charge) i s c r e a t e d a t t h e i n t e r f a c e (1_) . S i n c e a semiconductor has v e r y few f r e e c a r r i e r s t o use t o e s t a b l i s h i t s p a r t o f t h e double l a y e r , t h e space charge ( b a r r i e r ) r e g i o n i n a semiconductor extends r e l a t i v e l y f a r i n t o i t s b u l k as seen i n t h e f i g u r e . I t i s t h i s extended b a r r i e r r e g i o n i n t h e semiconductor which c o n t r o l s t r a n s p o r t a c r o s s t h e c o n d u c t o r / s e m i c o n d u c t o r i n t e r f a c e as seen s c h e m a t i c a l l y i n F i g u r e I B . Any change i n t h e double l a y e r a t t h e c o n d u c t o r / s e m i c o n d u c t o r i n t e r f a c e would r e f l e c t i n t o a change i n t h e e x t e n t and h e i g h t o f t h e b a r r i e r seen by charge c a r r i e r s i n t h e semiconductor. Any change i n t h e b a r r i e r o r , more g e n e r a l l y , any change i n t r a n s p o r t a c r o s s t h e b a r r i e r r e g i o n w i l l produce a change i n t h e c u r r e n t - c a r r y i n g c a p a b i l i t i e s o f t h e C-S j u n c t i o n seen i n F i g u r e 1. The b a r r i e r (space charge) r e g i o n i n t h e semiconductor can be m o d i f i e d by a p p l y i n g a b i a s t o t h e s t r u c t u r e as seen i n F i g u r e I B . T h i s causes t h e c u r r e n t f l o w t o change. The semiconductor b a r r i e r can a l s o be m o d i f i e d due t o t h e presence o f some c h e m i c a l s p e c i e s i n the environment i f t h a t c h e m i c a l s p e c i e s can modify t h e double l a y e r at t h e c o n d u c t o r / s e m i c o n d u c t o r i n t e r f a c e . I f a chemical species can e f f e c t a change i n t h e double l a y e r , t h e n t h e b a r r i e r i n t h e semicond u c t o r i s c o r r e s p o n d i n g l y changed and c o n s e q u e n t l y t h e c u r r e n t f l o w i n g a c r o s s t h e c o n d u c t o r / s e m i c o n d u c t o r i n t e r f a c e , a t some v o l t a g e , i s changed. Such a change i n t h e c u r r e n t f l o w i n g a c r o s s t h e s t r u c t u r e a t some b i a s due t o t h e presence o f a c h e m i c a l s p e c i e s c o n s t i tutes detection o f that species. We note t h a t t h e p r e s e n c e o f a c h e m i c a l s p e c i e s , i f i t a f f e c t s the double l a y e r , c a n cause an i n c r e a s e o r d e c r e a s e i n t h e b a r r i e r height i n t h e s e m i c o n d u c t o r depending on t h e i n t e r a c t i o n between the s p e c i e s and t h e C-S s t r u c t u r e . Those s p e c i e s t h a t cause t h e b a r r i e r t o i n c r e a s e would enhance r e c t i f y i n g c u r r e n t - v o l t a g e b e h a v i o r i n t h e C-S s t r u c t u r e ; those s p e c i e s t h a t cause t h e b a r r i e r t o d e c r e a s e would enhance ohmic ( l a c k o f r e c t i f i c a t i o n ) c u r r e n t - v o l t a g e b e h a v i o r i n t h e C-S s t r u c t u r e . A c h e m i c a l s p e c i e s i n t h e environment o f a C-S s t r u c t u r e c a n i n t e r a c t w i t h t h e d e v i c e and t h e r e b y modify i t s double l a y e r i n a number o f ways. These a r e t h e f o l l o w i n g : (1) t h e s p e c i e s can modify the work f u n c t i o n ( e l e c t r o c h e m i c a l p o t e n t i a l ) o f t h e C - l a y e r which c o n s e q u e n t l y r e s u l t s i n a m o d i f i c a t i o n o f t h e double l a y e r (2^ 3 ) , (2) t h e s p e c i e s can i n t r o d u c e d i p o l e s a t t h e c o n d u c t o r / s e m i c o n d u c t o r i n t e r f a c e t h e r e b y m o d i f y i n g t h e charge d i s t r i b u t i o n o f t h e double

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11. FONASH AND LI

Solid State Gas Sensors

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179

J—J—OLMH SEMICONDUCTOR

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F i g u r e 1. Schematic o f a C-S d i o d e . (A) p h y s i c a l c o n f i g u r a t i o n ; (B) energy band diagram i n thermodynamic e q u i l i b r i u m ( z e r o b i a s ) and under f o r w a r d b i a s . l a y e r (£,5), (3) the s p e c i e s can s i g n i f i c a n t l y change the i n t e r f a c e s t a t e d e n s i t y i n the s e m i c o n d u c t o r i n t e r f a c i a l r e g i o n ( p i n n i n g o r u n p i n n i n g the Fermi l e v e l ) t h e r e b y changing the c h a r g e , and hence the b a r r i e r , i n the s e m i c o n d u c t o r ( 6 ) , o r (4) the s p e c i e s can d i f f u s e i n t o the s e m i c o n d u c t o r where i t may m o d i f y the doping d e n s i t y o f t h e semiconductor t h e r e b y changing the charge d e n s i t y i n the b a r r i e r r e g i o n and, c o n s e q u e n t l y , the b a r r i e r ( 7 ) . I n p r i n c i p l e , i f t h i s f o u r t h p o s s i b l e mechanism were t o o c c u r i n a p a r t i c u l a r s e n s i n g s i t u a t i o n , i t c o u l d o c c u r because (1) the d i f f u s i n g s p e c i e s a c t e d as a donor o r a c c e p t o r l e v e l i n the semiconduct o r r e s u l t i n g i n augmentation or compensation o f t h e d o p i n g d e n s i t y o r (2) t h e d i f f u s i n g s p e c i e s d e - a c t i v a t e d the s e m i c o n d u c t o r dopant ( 7 ) . We note t h a t , i n g e n e r a l , the s p e c i e s t o be d e t e c t e d need n o t be, by i t s e l f , the e n t i t y t h a t causes t h e work f u n c t i o n change i n the c o n d u c t o r , produces the i n t e r f a c e s t a t e d e n s i t y change, i n t r o d u c e s the d i p o l e s , or d i f f u s e s i n t o t h e s e m i c o n d u c t o r . R a t h e r , i t may be


180

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

CONDUCTOR

LAYER

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SEMICONDUCTOR

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CHARGE

LAYER

BIAS

E N E R G Y BAND

DIAGRAM

F i g u r e 2. Schematic o f a C-I-S d i o d e . (A) p h y s i c a l c o n f i g u r a t i o n ; (B) energy band diagram i n thermodynamic e q u i l i b r i u m ( z e r o b i a s ) and under f o r w a r d b i a s . some r e a c t i o n p r o d u c t generated, perhaps c a t a l y t i c a l l y , a t t h e cond u c t o r s u r f a c e which i n t e r a c t s w i t h t h e s e n s o r s t r u c t u r e . The f o u r mechanisms we have j u s t enumerated g i v e r i s e t o d e t e c t i o n due t o m o d i f i c a t i o n o f t h e double l a y e r . Each o f t h e s e f o u r mechanisms l e a d s t o d e t e c t i o n by changing t h e c a r r i e r t r a n s p o r t a c r o s s t h a t b a r r i e r r e g i o n due t o a change i n t h e b a r r i e r h e i g h t

Schottky-Barrier Diode and Metal-Oxide-Semiconductor Capacitor

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