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EFFECT. OF ELECTRON ... For a study of the effect of electron irradiation on the current-voltage, current-capacitance, and pulsed ... In 1/Io ~ ~ (Uv. -- 1R~). (3).
EFFECT OF

OF

GALLIUM

ELECTRON

IRRADIATION

ARSENIDE

PULSED

V. N. B r u d n y i , A. A. P. Vyatkin, M. a n d S. V. M a l y a n o v

A. A.

ON

THE

PARAMETERS

DIODES

Vilisov, Krivov,

UDC 621.382.018.756

It is shown that 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 diodes produced by various methods v a r y in approximately the s a m e m a n n e r . The behavior of the capacitance C (U) during i r radiation can be explained on the basis of the theories developed for planar p - n junctions and Schottky b a r r i e r s . The r e c o v e r y time of diodes always i n c r e a s e s during i r r a d i a t i o n with l a r g e doses (~v ~ 10 is e l e c t r o n s / c m 2 ) . At s m a l l e r doses for diodes of the Schottkyb a r r i e r type (weakly formed), Treco v always i n c r e a s e s with irradiation, p e r h a p s due to a d e c r e a s e in the concentration n; for strongly formed diodes (having p a r a m e t e r s a p p r o x i mating those of diffused diodes), the Treco v behavior is governed by the nature of the i m purity distribution and by the r a t i o of the lifetime ~ of the m i n o r i t y c a r r i e r s to the diode time constant R~. With T > RC, a d e c r e a s e in T r e c o v m a y be o b s e r v e d as a r e s u l t of a d e c r e a s e in r . F o r a study of the effect of electron i r r a d i a t i o n on the c u r r e n t - v o l t a g e , c u r r e n t - c a p a c i t a n c e , and pulsed c h a r a c t e r i s t i c s of point-contact gallium a r s e n i d e pulsed diodes, diodes having the following p a r a m e t e r s w e r e produced f r o m n-type gallium a r s e n i d e : a r e s i s t i v i t y of 0.06 or 0.9 2 9cm, a c h a r g e - c a r r i e r c o n c e n tration of (3-5) 91016 o r (1-2) 91015 cm ~3, and a mobility of 4500 o r 5500 cm2/(V 9see). Gallium a r s e n i d e plates d -< 0.5 m m in thickness w e r e soldered with tin to a nickel s u b s t r a t e ; e l e c t r i c a l contacts w e r e formed f r o m beryllium b r o n z e needles. After synthesis, the diodes underwent e l e c t r i c a l forming by h a l f - c y c l e forward c u r r e n t pulses. The peak c u r r e n t s ranged from 0.05 to 0.8 A, and the pulse length was ~0.5 sec. The diodes w e r e i r r a d i a t e d by 1.5 MeV electrons at r o o m t e m p e r a t u r e on an e l e c t r o s t a t i c g e n e r a t o r . The p - n junctionS w e r e i r r a d i a t e d in m e t a l - g l a s s envelopes, so all the data r e f e r to the incident doses (~). 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 w e r e m e a s u r e d with a d i r e c t c u r r e n t and a relative e r r o r of 5%. The differential r e s i s t i v i t y was determined from the slope of the forward b r a n c h of 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 at d i r e c t c u r r e n t s of 2 and 3 mA. The capacitance m e a s u r e m e n t s w e r e c a r r i e d out at 30 MHz, within 10%. The r e c o v e r y time Treeo v and the switching charge Q w e r e determined f r o m the t r a n s i ent switching c h a r a c t e r i s t i c s of the diodes as the forward c u r r e n t was switched to a r e v e r s e current. 1.

Current-Voltage

Characteristics.

The forward branch of 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 can be d e s c r i b e d by [1] I = I0 {exp ( U ~ - - 1Rs) -- 1}, w h e r e I 0 = 10-15-10 -i~ A, Upn is the voltage a c r o s s the p - n junction, and Rs is the leakage r e s i s t a n c e . s m a l l I and for I / I 0 >> 1, the forward branch can be written as In I/Io ~ a Urn.

(1) For (2)

In the case of gallium a r s e n i d e point diodes this e x p r e s s i o n holds even with I > 10 -~ A; for l a r g e values of I and Rs, and taking account of the voltage drops in the diode base, we can r e w r i t e Eq. (1) as [2] In 1/Io ~ ~ (Uv. -- 1R~). V. D. Kuznetsov Siberian P h y s i c o t e c h n i c a l Institute, T o m s k State University. T r a n s l a t e d f r o m Izvestia VUZ. Fizika, No. 4, pp. 109-113, A p r i l , 1970. Original a r t i c l e submitted June 18, 1969. 9 197,3 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced [or any purpose whatsoever without permission o[ the publisher. A copy of this article is available [rom the publisher for $15.00.

506

(3)

I,A 10-~ ,fa 84

I,i r

/o"

I

/

ffs

m

/

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Y

~4

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e.e

ttv

Fig. I

r

/

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g

4, Fig. 2

Fig. 1. Effect of e l e c t r o n i r r a d i a t i o n on the forward b r a n c h of 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 the diodes. I r r a d i a t i o n d o s e s of (2.2, 4.5, 6.7, and 9). 10 ts e l e e t r o n s / c m 2 : 0) diodes based on m a t e r i a l having n = (3-5) 910 is cm-a; • diodes made from a m a t e r i a l having n = (1-2) 91015 cm -3. F r o m left to right the c u r v e s a r e a r r a n g e d in o r d e r of i n c r e a s i n g dose. Fig. 2. Differential r e s i s t a n c e of diodes as a function of the i r r a d i a t i o n dose. The notation is the s a m e as in Fig. 1. On the basis of the f o r m i n g c u r r e n t s and the electrical p r o p e r t i e s , we can classify these diodes into the following three groups: a) Unformed Contact: ~ = 15---20 V-1, ~ = 2

at

300~

The dependence of the capacitance on the applied r e v e r s e voltage is d e s c r i b e d by

C = A U 1I', w h e r e n _ 3. As was shown in [3], 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 of such a contact can be d e s c r i b e d on the basis of the theory developed for a planar p - n junction [4]. b) Weakly F o r m e d Contact: q a = -~---1. KT' The voltage dependence of the capacitance is a p p r o x i m a t e l y quadratic. Schottky b a r r i e r .

This is the case of a contact with a

c) Strongly F o r m e d Contact: U = 0 , 6 - - 1 , 2 V, ~ 2 ,

C=AU

1/3.

A junction with a smooth impurity distribution r e s u l t s . The basic p r o p e r t i e s of the diodes in these three groups change in essentially the s a m e m a n n e r during e l e c t r o n irradiation. F i g u r e 1 shows the change in the forward c u r r e n t - v o l t a g e branch as a function of the i r r a d i a t i o n dose. When gallium a r s e n i d e diodes a r e i r r a d i a t e d , both the n-type and p - t y p e regions a r e compensated [5]. It follows from relation (2) that

Up,, = }~cT In l/lo. q

(4)

Using the data of [6], we can write a relation for the change in the c a r r i e r lifetime in the b a s e during i r radiation: 1 _

1 +~, tO

(5)

/n

507

Ubrk,

C,pF ,,~1,~."~~ "

9

t e2

4,5

!~

, ~7,"

f 9,gt # . t a t % i / c r n

_,

~4 2

2

4

Fig. 3

6

U,V

Fig. 4

Fig. 3. Dose dependence of the breakdown voltage. (3) Unformed diodes based on a m a t e r i a l having n = (3-5) 9l0 is cm-3; otherwise the notation is the s a m e as in Fig. 1. Fig. 4. The C (U) c h a r a c t e r i s t i c for one diode; the p a r a m e t e r is the i r r a d i a t i o n dose, whose values a r e given in the Fig. 1 caption. The upper c u r v e is C (17) before irradiation. where TOis the lifetime of m i n o r i t y c a r r i e r s before irradiation, ~- is that after irradiation, and m is the r a d i a t i o n - r e s i s t a n c e coefficient. Substituting e x p r e s s i o n (5) into (4), and differentiating with r e s p e c t to r

dV,•

,~r 1.

we find

(6)

i.e., the higher the radiation dose, the s m a l l e r the fraction of the applied voltage which a p p e a r s a c r o s s the p - n ]unction. Similarly, we find the following relation for the voltage applied to the diode base: dub

a~,

_

(7)

B'~[

(n -

~w)

'

where n is the c a r r i e r concentration in the base, y is the rate at which c a r r i e r s a r e removed, found from n = n 0 - y r and B is a constant which depends on the p r o p e r t i e s of the base. It follows f r o m this relation that the voltage drop a c r o s s the base i n c r e a s e s with i n c r e a s i n g dose. F r o m Fig. 1 we see that the voltage redistribution between the p - n junction and the base has the g r e a t e s t effect on the forward branch: the higher the incident-electron dose, the lower the forward c u r r e n t at which the effect of the base on the forward branch m u s t be taken into account. This figure also shows c h a r a c t e r istics for diodes produced from m a t e r i a l s having c a r r i e r concentrations of (1-2) 910 i5 and (3-5) 910 l~ cm -~. F i g u r e 2 shows that the differential r e s i s t a n c e R d of the diodes i n c r e a s e s quite rapidly with i n c r e a s i n g radiation dose. Study of the dose dependence of the breakdown voltage Ubr k shows that Ubr k i n c r e a s e s because of the i n c r e a s e in the base r e s i s t a n c e during the i r r a d i a t i o n (Fig. 3). We used as Ubrk the voltage c o r r e s p o n d i n g to a r e v e r s e c u r r e n t of 10 -4 A (Fig. 3). The data f o r Figs. 1-3 w e r e a v e r a g e d for five-diode lots. 2.

Voltage-Capacitance

Characteristics

We m e a s u r e d the v o l t a g e - c a p a c i t a n c e c h a r a c t e r i s t i c s of diodes having a l a r g e contact a r e a in o r d e r to obtain m o r e stable c h a r a c t e r i s t i c s . The m e a s u r e d C values include the holder capacitance, whose value is ~0.15 pF. F o r unformed contacts, for which we have C = A u i / ~ or a C n e a r l y independent of the v o l t age (for a small C), we o b s e r v e that C d e c r e a s e s with i n c r e a s i n g dose. F o r diodes whose capacitance is an approximately quadratic or cubic function of the voltage, we obs e r v e an i n c r e a s e in n in the equation C = AU 1/n a f t e r irradiation. In this c a s e the modulation M = (Cmax - C m i n ) / ( C m a x + Cmin) of the capacitance d e c r e a s e s for given diodes. Data for one of the diodes a r e shown

508

C, pF

O,Snmc/div

{6

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2,2