Bridges, W. B.

English

Caltech Authors - Main

1965 Correspondence 309 
prel-iously by  Strum [ l ]  and Colvin [2], for 
the  system is then of the  simple  Dicke  type. 
If N antennas are directed in time se- 
quence a t  the same region of the sky, the 
noise outputs of the  channels  are  statistically 
independent when corrected for time shift, 
and  an  average of the N records  gives a least 
detectable signal temperature of 
= least detectable equivalent signal 
temperature for all N channels. 
CONCLUSIONS 
It  has been shown that a radiometer 
may be switched between several antennas 
with some loss in sensitivity. I n  radio as- 
tronomy, several applications for this type 
of system suggest  hemselves. If the ulti- 
mate sensitivity is not required for a par- 
ticular  measurement,  the  production of 
several observations  with  one receiver  would 
reduce  the  data  acquisition  time.  This  tech- 
nique is applicable if a paraboloidal  antenna 
with a multiple feed system  is  employed.  If, 
in addition,  the feeds are  such  that  the  same 
region is observed u-ith  the  several  channels 
at different times, the results may be com- 
bined to produce a sensitivity which is 
slightly  better  than  that of the simple 
Dicke system. This type of receiver could 
also be  employed  with a multiple  beam  ar- 
ray  antenna. 
R. T. XASH 
S. R. O'DONKELL 
L. FITCA 
Radio  Observatory 
Ohio  State  University 
Columbus, Ohio 
REFERENCES 
I21 
[3 1 
141 
Strum, P. D., Considerations in high sensitivity 
microwave radiometry, Proc. I R E ,  vol 46. Jan 
1958. pp 43-53. 
Colvin. R. S., A  study of radio-astronomy recriv- 
ers. Ph.D. dissertation. Stanford Universitv. 
Calif., 1961. 
Dicke.  R.  H..  The measurement of thermal radia- 
vol 17 ,  Jul 1946, pp 268-275. 
tion at microwave frequencies, Rev. Sci. Insfr. .  
Orhaug. T.. and W. Waltman. A switched load 
radiometer, Publ. 'Vufional Radio A s f r o n a y  Ob- 
smafory.  vol 1, Feb 1962. 
radio, Microwaue J . ,  vol 4, Nov 1961, pp M S .  
KO. H.  C., Temperature concepts in modern 
Goldstein. S. J. ,  Jr.. A comparison of two radiom- 
eter circuits. Proc. I R E ,  vol 43. S o v  1955. pp 
1663-1668. 
.. 
by Bennett, et al.,s and an erratum subse- 
quen t ly   a~pea red .~  However, i t  should also 
be  corrected for the  readers of the PROCEED- 
Heard and Peterson also state that the 
blue-green Ar I1 lines have been operated 
quasi-CW (100-200 p secs). True  CW  opera- 
tion (hours)  has previously been reported 
for the lines they  list,  and for five additional 
Ar I1 lines! 
W. B. BRIDGES 
Hughes Research Lab. 
hlalibu, Calif. 
INGS. 
Author's Comment6 
We acknowledge Bennett's correction* of 
the  transition  assignment for the 4880 hi 
Argon I1  and  are  indebted  to 1%'. B. Bridges 
for bringing  this to our attention. 
With respect to Bridges' comments on 
quasi-CW  operation: at the  time of publica- 
tion CW operation had been reported, but 
only at the  milliwatt level and  only for 
Argon. Heard  and Peterson' reported power 
outputs in excess of 10 watts  with no notice- 
able saturation effect for argon and xenon 
for time  durations which were limited only 
by  the  modulator. Since that  time as much 
as a 7 watt peak has been obtained for 
single-line operation at 4765 A, using tech- 
niques which do  not  require prism  selection 
of wavelength. 
J. PETERSON 
H. G. HEARD 
h nu  systems, inc. 
Palo  Alto, Calif. 
J Bennett, Jr.. W. R. ,  J. W. Knutson, Jr., G. N. 
Mercer, and J. L. D e t c h ,  Super-radiance. excitation 
mechanisms, and quasi-CW oscillation in the visible Ar+laser. Appl.  Phys. Leu., vol 4. May 1%4.  pp 
180-182. ... 
4 Erratum: laser oscillation in singly ionized argon 
in the visible spectrum. Appl. Phys. L e k ,  vol 5. Jul 
1963. P 39. 
6 Gordon, E. I . .  E. F. Labuda. and W .  B. Bridges, 
Continuousvisiblelaseraction in singly ionized argon. 
krypton and xenon, Appl.  Phys.  Leff. ,  vol 4, May 
1964. pp 178-180. 
6 Manuscript received December 2, 1964. 
Manuscript received November 6, 1964. 
1 Heard, H.  G. and J.  Peterson. Mercury-rare gas 
visible-UVlaser. Proc.  IEEE (Concspondcncc). vol 52. 
Sep 1964, pp 1049-1050. October 19. 1964. The research reported here was 
Manuscript received October 8. 1964; revised 
f Bridges, W. B.,  Laser oscillation in singly ion- sponsored by the U. S. Army Electronic Labs., Fort 
ized argon in the visible spectrum, Appl.   Phys.  Lett.. Monmouth. N. J., under Contract DA28-043-AMC- 
vol4. Apr 1964, pp 12&130. 00231(E) and RCA Labs., Princeton, N. J. 
INSULATED - GATE 
DEVICE 
I I 
Fig. 1. Experimental method used f o r  
transient measurements. 
However, it  has been pointed out  by 
Zaininger [3] that if the  measurement is to 
be meaningful, one must accurately know 
the donor or acceptor density in the semi- 
conductor. If the  donor  density is not  known 
accurately, the above method will lead to 
errors  in  determining  the  surface-state 
density. 
Knowledge of the doping density in a 
semiconductor used in insulated-gate  de- 
vices [4], [SI, is difficult to obtain by con- 
ventional means. Hall  measurements on the 
semiconductor prior to completion of the  de- 
vice may be misleading  since subsequent 
processing may affect the  donor  doping 
densities. This is particularly true in thin 
films. The  same  measurements  are  not con- 
venient  in completed structures  and  can  be 
difficult to  interpret  due to variation of 
carrier density in the surface space-charge 
region [6]. I t  is the purpose of this corre- 
spondence to describe a technique for deter- 
mining the doping density in a completed 
M-I-S device. The  method used is similar to 
the one used by  Many [ 7 ]  in studying the 
Schottky barrier at a CdSelectrolyte con- 
tact. The experimental set-up is shown in 
Fig. 1. I t  consists of the device, a pulse 
generator,  and a large capacitor  to  measure 
the  charge  induced  into  the M-I-S contact. 
The theory of the  measurement is as 
follows: assume that  by  applying a dc  bias  on 
the  gate  electrode, a depletion region is 
formed at   the surface of the semiconductor. 
The  charge in the  depletion region can be ex- 
pressed as 
where QD is the  space  charge in the  depletion 
region per unit  area, p is the  electronic 
charge, ND is the  donor  impurity, N A  is the 
acceptor impurity density, e, is the semi- 
conductor  permittivity, 4 is the  surface 
potential with the dc bias applied. The as- 
sumption of uniform donor  and  acceptor 
density  in  the  space-charge region has been 
made. If now a narrow-voltage pulse is ap- 
plied to the gate in a direction so as to 
further deplete the surface, all charge re- 
moved will come  from the  conduction  band 
and  not from surface or deep  traps.  This is 
true if the pulse is narrow enough so that 
surface states and  deep  traps  are  not  in 
thermal equilibrium with  the  conduction 
band. The new charge  in  the  depletion region 
is now 
where V, is the new surface potential. The 
analysis which follows is similar to  the  one 


Mercury-rare gas visible-UV laser

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