A light-activated semiconductor device usable as an optoelectronic switch, pulse generator or optical detector is provided. A semiconductor device is disclosed which provides back-to-back metal-thin oxide-silicon (MTOS) capacitors. Each capacitor includes a thin, light-absorptive aluminum electrode which overlies a thin oxide layer and a lightly doped region implanted in an intrinsic silicon substrate

Maserjian, J.

English

NASA Technical Reports Server

United States Patent [191 
Maserjian 
[54] LASER ACTIVATED MTOS MICROWAVE 
[76] Inventor: Joseph Maserjian, 5668 Pine Cone 
[21] Appl. NO.: 542,232 
[22] Filed: Oct. 14, 1983 
[Sl] Int. C1.3 ............................................. HOlL 29/78 
[52] U.S. C1. ..................................... 357/23.6; 357/30; 
357/58; 357/59 
[58] Field of Search ................ 357/30, 23.6, 58, 30 F, 
357/30 D,  30 I, 59 
[561 References Cited 
DEVICE 
Rd., LaCrescenta, Calif. 91214 
U.S. PATENT DOCUMENTS 
3.523.190 SA970 Geotzberger et al. ........... 357/30 X 
3,852,119 12/1974 Gosney et al. ................... 357/58 X 
3,882,531 5/1975 Michon et ai. ................ 357/23.6 X 
3,917,943 11/1975 Auston .............................. 357/30 X 
4,231,050 10/1960 Casey, Jr. et al. ................ 357/30 X 
4,376,285 3/1963 Leonberger et al. ............. 357/30 X 
4,363,267 5/1983 Webb .................................... 357/30 
p i ]  Patent Number: 4,531,143 
[45] Date of Patent: Jul. 23, 1985 
OTHER PUBLICATIONS 
Jacobus et al., “Optical Detector for Short-Wavelength 
Light”, IBM Technical Disclosure Bulletin, vol. 14, No. 
9, Feb. 1972, p. 2624. 
LeBlanc, “One-Device Storage Cell Method”, IBM 
Technical Disclosure Bulletin, vol. 16, No. 3, Aug. 1973, 
Thaniyavarn et al., “Metalflunnel-Barrier/Semicon- 
ductorflunnel-Barrier/Metal Fast Photodetector”, 
Appl. Phys. Lett. 40(3), Feb. 1982, pp. 255-257. 
Primary Examiner-Andrew J. James 
Assistant Examiner-William A. Mintel 
WI ABSTRACT 
A metal thin oxide silicon (MTOS) optically-activated 
semiconductor device having respective thin, optically 
absorptive aluminum layers deposited over a thin oxide 
layer formed on a silicon substrate over a lightly doped, 
implant diffused region. 
pp. 956-957. 
4 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19860010045 2020-03-23T06:52:52+00:00Z
u. s. Patent Jul. 23,1985 4,531,143 
r d 25 N 
I;; I 
I t 
FIG 2 
iwt v e  
I t--25 
FIG 3 
r-----1 r---1 
cox I I I cox c d  I 
I 
I 
I 
4,53 1,143 
1 
LASER ACTIVATED MTOS MICROWAVE DEVICE 
ORIGIN OF THE INVENTION 
DESCRIPT1oN OF THE 
The just-summarized invention will now be described 
in detail in conjunction with the drawings of which: 
5 FIG. 1 illustrates a cross-section of the preferred 
FIG. 2 illustrates a stripline embodiment of the inven- 
The invention described herein was made in the per- 
formance of work under a NASA Contract and is sub- 
ject to the provisions of Section 305 of the National 
(72 STAT 435; 43 USC 2457). 
embodiment Of the invention; 
tion; and 
10 embodiment. 
Aeronautics and ’pace Act Of 1958, Law 85-568 FIG,  3 illustrates a circuit equivalent of the preferred 
BACKGROUND OF T H E  INVENTION 
1. Field of the Invention 
The subject invention relates to a semiconductor 
device usable as an optoelectronic switch, pulse genera- 
tor or optical detector. 
2. Background of the Invention 
Prior art optoelectronic switches are known which 
are based upon structures employing silicon or 111-V 
semiconductor substrates. See Auston, U S .  Pat. No. 
3,917,943 and Leonberger et al., U.S. Pat. No. 
4,376,285. Such devices present several drawbacks in- 
cluding relatively slow turn-off speeds and reduced 
efficiency due to uncontrolled surface effects. For  ex- 
ample, Auston requires a separate laser pulse to turn off 
his device as a result of the long minority carrier recom- 
bination time. Leonberger et al. propose a faster time 
without a laser turn-off pulse by using an indium-phos- 
phide substrate. Such a device is still limited by the 
minority carrier recombination problem and therefore 
is relatively slow in turning off. 
In Applied Physics Letters, 40, p. 255 (1982), 
Thanivavarn and Gustafson describe a fast optical de- 
tector fabricated of metal/tunnel oxide/silicon/tunnel 
oxide/metal. The tunneling oxide structure would ap- 
pear to limit performance of the device and preclude it 
from operating in a steady high frequency mode. 
It is an object of the subject invention to provide a 
device usable as a high frequency optoelectronic switch 
which avoids the long relaxation times and surface ef- 
fects of prior art devices. 
It is another object of the invention to provide such a 
switch employing an established fabrication technol- 
ogy. 
It is yet another object of the invention to provide an 
optically activated, semiconductor microwave device 
having general application in microwave science, engi- 
neering and communication systems. 
15 
20 
25 
30 
35 
40 
45 
The preferred embodiment is an MTOS (metal-thin 
oxide-silicon) device as shown in cross-section in FIG. 
1. The device includes first and second thin aluminum 
layers 11, 13 flanked by thicker aluminum layers 15, 17. 
These aluminum layers 11,13,15,17 are deposited on a 
thin silicon dioxide layer 19, which in turn resides on a 
relatively thick layer 21 of intrinsic (silicon (> 5000R 
cm). A small gap 23 is provided between the two thin 
aluminum layers 11, 13, exposing a portion of the silicon 
dioxide layer 19. To complete the structure shown in 
FIG. 1, an implant diffused region 25 and a uniform 
aluminum underside layer 27 are provided. This struc- 
ture provides two capacitors with respective electrodes 
formed by the thin aluminum layers 11, 13. These layers 
11, 13 are also light absorptive. 
The dimensioning of the structure of FIG. 1 will now 
be described. The thick aluminum layers 15, 17 are 
preferably of a thickness greater than 100 nanometers 
(100 nM). The thin aluminum layers 11, 13 are no 
greater than 10 nM in thickness such that they are light 
absorptive. These thin layers 11, 13 may be replaced 
with a thicker polysilicon layer. The thin oxide layer 19 
is about 10 nM in thickness and provides a large oxide 
capacitance per unit area coupled to the silicon deple- 
tion capacitance. The oxide layer 19 is thick enough to 
be nonconducting below about 7 volts across the capac- 
itor and to avoid tunneling effects. The implant-diffused 
region 25 should be light doping, e.g., - 10l6 cm-3, N 
type, to provide a reasonably large depletion region at 
inversion (- 1 pM) and, correspondingly, low capaci- 
tance per unit area. The depth of the diffused region L 
should be thick enough ( > 2  pM) to provide a conduc- 
tive path between the two capacitors at microwave 
frequencies. 
FIG. 2 shows use of the structure of FIG. 1 in a strip 
line 31 embodiment. The dimensions shown in FIG. 1 
and FIG.  2 may be optimized for microwave operation. 
The width of the gap 23, denoted “a” in FIG. 2, is as 
50 narrow as possible (1 to 3 pM). The width c of the metal 
strip is chosen to give the desired impedance of a strip SUMMARY OF T H E  INVENTION 
The foregoing objects are achieved by a new combi- line on the semiconducting (I) silicon chip of thickness 
nation of elements defining a laser-operated solid state d. The metal pattern may be structured into any desired 
microwave circuit component composed of back-to- configuration as part of a microwave network, as long 
back capacitors on an intrinsic silicon chip. Light from 55 as the silicon behaves as a dielectric at frequencies 
a laser is absorbed through thinned areas of the capaci- > I . lrp/es where PI represents resistivity of the intrinsic 
tor electrodes. A thin oxide layer under the electrodes silicon and E , ~  represents the dielectric constant. 
provides a large oxide capacitance per unit area, COU- The following discussion will further illustrate selec- 
pled to the silicon depletion capacitance and is thick tion of the parameters of the device of FIGS. 1 and 2 to  
enough to avoid tunneling effects and to exhibit non- 60 function as a laser-activated microwave switch. 
conductivity at a suitable voltage level across one ca- The principle of operation is based on the well- 
pacitor. known illumination effect phenomena of MOS capaci- 
The dimensions of the metallization on the semi- tors. There are two mechanisms: (1) change in genera- 
insulating silicon substrate may be chosen for the de- tion rate in the depletion region and (2) change in the 
sired impedance in a stripline construction of a micro- 65 population of minority carriers in the neighborhood of 
wave device. Such a device may function as a laser- the depletion region. The first mechanism is of interest 
operated microwave switch, a laser-activated pulse for the fastest possible response time. The first mecha- 
generator, or a fast-response optical detector. nism can be described in terms of the equivalent circuit 
4,53 1,143 
3 
under instantaneous bias of an ac signal Vejw‘ as shown 
in FIG. 3. 
The left capacitor is (at the indicated time) reverse- 
biased into inversion and the right capacitor forward- 
biased into accumulation (becoming CU.J. Therefore the 5 
using R, 
p.\l - 
bc , I = / l a .  b. L )  
total capacitance (dark) is: and 
C S - = o . r  cm e b c  
w,>- - 
Cdud = ,c Cm Cd 2do.r - ri + c”.r C”, > > Cd > cd. 
2 dm 
10 
J/ ,  is the generation current of minority carriers (holes). %.YP.V 1 
The thermal contribution is totally negligible at high 
frequencies. By irradiation with laser light, with photon 
energy hv greater than the silicon band gap: 
Choosing 
dOx= IO  nM, piy=0.5R cm 
15 
4 
wc> 1.2 IO1Osec-l where F is the laser intensity (watts/cm2), hv is the 2o 
photon energy (joules), and 7 is the quantum efficiency 
(fraction of incident photons which generate holes in 
depletion region). Clearly, if Jh is large enough, c d  
becomes bypassed and, in the limit, the total capxi -  
tance under illumination C approaches Co,/2. 
The  frequency dependence for this effect can be writ- 
ten as: 
o r  
fc>2.0CH 
This cutoff can be increased as required by appropri- 
25 ate adjustments of L, Do,, p,vor a+b.  However, some 
constraint exists on do, and a +  b (a should be small 
compared with b to minimize R,) because of impedance 
matching requirements. For  example, selecting a 50R 
_ = -  1 2 , 1 W-T-  7 1  ( 2 )  impedance strip line, the desired “on” capacitance state 
c cox c d  I + ~ 2 ~ 2  30 may be 50a, corresponding to approximately 3 dB 
transmission. That is 
where T is the time constant for minority carrier (hole) 
generation in the inversion layer, and can be estimated 
from: 
35 
(3) 
which is just the time for the current density Jh to 4o 
charge Cox to the full voltage (V/2), which would 
occur if c d  were absent. From Eq. (2): 
which are the limiting cases already mentioned. There- 
fore, the laser intensity F required for switching the 50 
capacitance states at frequency o becomes: 
hwC,, Vw 
F’ 27ebc 
5 5  
Choosing 
w=lO’Osec-l,~=O.l. hv=1.4 eV 
Cox/bc= 3.4. 10-7//cm2. V= 2 volts, 
60 
one obtains F>4.8.104 watt/cm2. Such intensities are 
readily achievable with lasers. 
The parasitic cutoff frequency must also be consid- 
ered. This frequency can be estimated from: 
65 
I 
w -- 
- R,C 
2 
w cox 
-= 50 
Choosing w =  lolo, b=5 p M  (a=2 pM), do,= 10 nM, 
one obtains c=240 pM. This value is quite reasonable 
or may be tapered to the width of the strip line. Like- 
wise, an interdigitated structure may be used to opti- 
mize the area for optical coupling. 
A second mechanism to be considered involves 
change in minority population near the depletion re- 
gion. This effect would increase the quantum efficiency 
but at the expense of slower response. If one wishes to 
suppress this effect to avoid a long additional tail on the 
response, one may apply a d c  bias between the top and 
back contacts to sweep the holes away from the capaci- 
tor towards the back contact. Alternately, one may 
choose to enhance the effect (and increase the quantum 
efficiency) by biasing in the opposite direction to sweep 
the holes towards the capacitor. With such a drift field 
applied, the frequency response may still be sufficient 
for most applications, and the required laser intensity 
would be substantially smaller (- 1/10). 
A final point concerns ac impedance switching. If the 
MOS flat-band voltage is near zero, the two back-to- 
back capacitances will exhibit in the “off” state a com- 
bined capacitance peak (at zero voltage) which will 
degrade the performance. This can be easily corrected 
with a small dc bias to the N-region. 
The  device shown in FIGS. 1 and 2 may also serve as 
a laser-activated pulse generator. By applying a d c  bias 
V to the back-to-back capacitors, they can be charged 
in the off state to a small level (V Cd). When illuminated 
with a laser pulse the capacitance increases to C,/2 and 
charges to VCox/2. Therefore, a current pulse must 
flow in response, thus generating a pulse along the strip 
4,53 1,143 
5 6 
line. Other considerations remain essentially the same as 
for impedance switching. 
Thus, a back-to-back capacitor MTOS chip configu- 
ration is disclosed which is sensitive to an impinging 
optical flux. The metallization, doping, and substrate 5 
dimensions have been selected to be compatible with 
microwave frequencies so that various component ef- 
fects can be realized and controlled, for example, by a 
solid-state laser control pulse source. 
the subject invention is subject to numerous adaptations 
and modifications without departing from the scope 
thereof. Therefore, it is to be understood that, within 
the scope of the appended claims, the invention may be 
practiced other than as specifically described herein. 
a lightly doped region in said silicon substrate; 
a dielectric layer formed on said silicon substrate and 
over said lightly doped region; and 
first and second light absorptive, conductive layers, 
each residing on said dielectric layer and extending 
over said lightly doped region, said first and second 
conductive layers being spaced apart by a gap, said 
gap residing over said lightly doped region. 
2. The semiconductor device of claim 1 wherein said 
It will be appreciated by those skilled in the art that 10 dielectric layer is an oxide layer of a thickness sufficient 
to prevent tunneling effects. 
3. The semiconductor device of claim 2 wherein said 
first and second conductive layers each step up to a 
thicker nonlight absorptive structure near the edge of 
4. The semiconductor device of claims 1, 2 or 3 
15 said lightly doped region. 
What is claimed is: 
1. A semiconductor device comprising: 
an intrinsic silicon substrate; 
wherein said lightly doped region is implant diffused. * * * * *  
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 


Laser activated MTOS microwave device

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