Light activated light valve with a silicon control element

In contrast to existing light addressed light valves for projection displays which use a homogeneous CdS or Se photoconductive layer as the control element for a liquid crystal cell, a silicon photo-diode array is disclosed which makes an effective control element capable of applying a spatially varying AC voltage across a liquid crystal light valve. Writing may be done with a miniature CRT in an optical or electron excitation mode. It is shown that for a video mode the ratio of diode capacitance to liquid crystal cell capacitance associated with the diode, and the frequency of the applied square wave voltage, define the buildup or decay time of the liquid crystal cell voltage. Thus, the frequency of the applied AC voltage can be used to control sensitivity and transient response and there is no lag. The dynamic range of the cell voltage is shown to depend on the ratio of diode capacitance to liquid crystal cell capacitance. The display can be used in a storage mode by erasing with an AC voltage and writing with a constant voltage. The silicon photodiode array is compatible with a CCD frame store using direct minority carrer injection for writing. This light addressed version thus represents a desirable milestone in achieving an all solid-state projection display.

FIELD OF THE INVENTION 
The present invention relates to light activated light valves and more 
particularly to an all solidstate light valve with a silicon control 
element. 
BACKGROUND OF THE INVENTION 
Light valves are a key element in projection displays. Preferably, such 
light valves are activated directly by video or data signals, using matrix 
addressing, or shift registers in combination with matrixing, to input the 
signals. Liquid crystals and electrochemichromic material have been 
studied as appropriate light valve media but liquid crystal (LC) has 
gained greater acceptance to date. For a definition of electrochromic and 
electrochemichromic material see, I. F. Chang, Nonemissive Electro-Optic 
Display, edited by A. Kmetz and F. K. Von Willisen, Plenum Press, 1978. 
Although a variety of drive circuit techniques are under study, active 
silicon integrated circuit drivers have been shown to be compatible with 
both liquid crystal and electrochemichromic media and thus presently 
represent a highly desirable approach for either a direct view or a 
projection display application (see articles by L. T. Lipton et al, SID 
Symposium Digest, 8, 64-65, 1977 and D. J. Barclay, et al, SID Symposium 
Digest, 11, 124-153, 1980). However, the complexity and size of the 
required silicon chip contribute to high cost and resolution limitations. 
An alternative that has received considerable attention, (e.g. see L. T. 
Lipton et al, SID Symposium Digest, 9, 96-97, 1978), is a valve that uses 
light addressing or activation, also called a light-activated light valve 
(see W. P. Bleha et al, Proc. SPIE 317, 179, 1981). In this case, an 
optical image is applied to the photosensitive back side of the 
light-activated light valve (LALV) and the light intensity is used to vary 
the local AC voltage applied across an LC cell. The local cell 
reflectivity is a function of the magnitude of the cell voltage. In an 
early implementation, a Se or CdS photoconductive layer was used as the 
photosensing element that varies the AC voltage, applied to the LC, point 
by point. The exciting image was produced by a CRT coupled to the 
photoconductive layer by a lens or a fiber optic plate. A more recent 
implementation involves a version of the LALV in which the photoconducting 
layer is replaced by a layer of SiO.sub.2 on a single crystal Si as the 
photodetecting control element (see V. Efron et al, SID Digest of 
Technical Papers, 142, 1981). A microdiode grid serves to isolate the 
resolution elements. The resulting MOS diodes are unilateral and must be 
recharged. Hence, the sign of the voltage across the LC is unchanged, and 
the device operated in a DC mode. Earlier work used a dynamic scattering 
mode in the liquid crystal. More recent efforts have utilized a nematic 
liquid crystal operated in a voltage controlled birefringence mode. 
The photoconductivity-controlled, AC liquid crystal LALV is, in principle, 
a good device. However, photoconductivity generally offers a poor response 
time-sensitivity tradeoff. Photoconductors exhibit lag which is light 
level dependent, a nonlinear response (gamma.perspectiveto.1/2), and are 
easily damaged by bright light. They exhibit burn-in effects. 
Photoconducting material has applications limited generally to light 
sensing. Consequently, a thorough study of the material system, 
fundamental to its performance characteristics and reliability, may be 
difficult to justify and is often lacking. The silicon diode version is 
limited to DC operation. 
The present invention is directed generally to replacing the 
photoconductive and associated light blocking and reflecting layers of the 
LALV with a monolithic silicon chip containing an array of junction 
photodiodes and providing an AC voltage across the liquid crystal layer. 
Prior art examples of such an approach are found in the above-noted V. 
Efron et al, SID Digest of Technical Papers, 142, 1981, wherein a 
photoconductor is replaced with a silicon diode array for implementation 
in vidicons, and in I. F. Chang et al, SID Symposium Digest 102, 1973; 
Proc. of SID, 16,227, 1975, disclosing an electron beam addressed storage 
CRT. Since the silicon photodiode array can be addressed by an electron 
beam directly or via a phosphor coupling, the photodiode array controlled 
AC liquid crystal light valve can be made as a faceplate for a miniature 
CRT. Some of the advantages of this approach are: AC liquid crystal cells 
are more reliable; the interface between the liquid crystal layer and the 
silicon control structure can be dielectric, suitably processed for liquid 
crystal alignment; AC operation provides a convenient charge/discharge 
mechanism, allowing a transient response; and electron beam addressing is 
an effective way of addressing a high resolution light valve. 
SUMMARY OF THE INVENTION 
The present invention involves a photodiode-array-controlled display light 
valve based on silicon technology and liquid crystal display technology. A 
preferred device can be fabricated with conventional silicon processing 
techniques, such as etching and diffusion or epitaxial growth on a single 
crystal silicon wafer, and the photodiode array under AC bias is capable 
of transferring AC varying voltage across a liquid crystal light valve 
according to the photoexcitation received. More particularly, a liquid 
crystal (LC) layer, which may act as a capacitor, is divided and connected 
to two photodiodes in series with an AC power supply, the circuit being 
completed with the two diodes connected back to back. Under AC operation, 
the voltage is dropped over the two half LC capacitors, with one of the 
diodes being reverse biased at a given time. In response to an applied 
square wave, each diode will be reverse biased each half cycle with the 
forward biased diode having a negligible voltage drop. When the 
photodiodes are activated by light, the voltage across the diode junction 
capacitance is transferred to the liquid crystal resulting in a field 
greater than its turn-on threshold field. Depending on the refresh rate 
and persistance of the exciting light and the RC time constant of the 
circuit, the LC cell can be operated in a video or in a storage mode. Such 
a device can easily be extended to a full matrix, say, 1000 by 1000 pels. 
If the outer electrode (viewing side) is partitioned into electrically 
floating squares each corresponding to a pel, then the pairs of diodes 
corresponding to a pel can all be connected in parallel and driven by a 
single AC source to produce an appropriate array. An implementation of the 
invention is disclosed wherein in a silicon single crystal wafer is used 
as the substrate, and by conventional diffusion or epitaxy process, p-n 
junction diodes are formed in a high resistivity layer which could be 
epitaxially grown on the substrate. The substrate is then etched away 
using a self-limiting etching process to stop the etch at the boundary of 
two different conductivity types. This membrane of silicon with diodes 
formed, and insulator and electrode regions defined, is then mounted on a 
glass plate having transparent electrodes defined as pel electrodes, in 
the manner described above. The liquid crystal is then filled and sealed. 
Only two electrode leads need to be brought out for the AC power supply 
connection. The resulting light valve can be directly addressed by a 
microCRT with a lens or with a high resolution fiber optic faceplate. 
Alternatively, this light valve can be sealed to a CRT as a faceplate, and 
can be directly addressed by the electron beam. 
Another method of fabricating this light valve is disclosed wherein the 
photodiodes are formed by planar layers of p-n type and isolation is 
provided by etching grooves into the substrate. 
It will be seen that the disclosed silicon diode array is easy to fabricate 
in a manner similar to a vidicon target, and exhibits superior performance 
to a photoconductor in controlling the liquid crystal light valve.

DETAILED DESCRIPTION OF THE INVENTION 
An array 11 of photodioes 10 are paired in rows which are shown 
perpendicular to the plane of FIG. 1. Each row is provided with a common 
potential applied to the diodes 10 in that row. Pairs of rows are 
connected to the same voltage source 20 and inter-row capacitance is held 
to a minimum. On any given half cycle of the applied square wave, AC 
voltage, the diodes in every second row are forward biased. The circuit 
elements are paired by patterning electrodes 40 into isolated, 
electrically floating rectangles opposite each vertical pair of diodes 
across liquid crystal cells 30, as seen in FIG. 2. The circuit equivalent 
for a diode pair is shown in FIG. 2A. At any instant, one diode of a pair 
is reverse biased and the other is forward biased. The circuit elements 
are the diode capacitance, C.sub.d, the LC cell capacitance, C.sub.lc, the 
resistance R of the ITO film making up electrode 40, and the current 
generators I or I' resulting from dark leakage and illumination of the 
reverse biased diode. The resistance R is designed to be sufficiently 
small such that any RC charging time is much less than a half period of 
the applied AC, and equilibrium is reached during the half cycle. R will 
be neglected in the following considerations. Leakage in the liquid 
crystal cell is small and can be ignored except for very low frequency 
operation. Propagation delay along the rows, which could be a factor for 
high frequency operation, is similarly ignored. Initially, it is assumed 
that the diode capacitance Cd is independent of voltage. 
In order to understand the operation of the array, three cases may be 
considered. 
Case I. The liquid crystal (LC) cell capacitances of a diode pair have been 
charged and the illumination is suddenly removed. The diode dark current 
is assumed to be negligible for the purpose of discussion. This exercise 
may be used to determine the time constant for decay of stored charge, and 
the background stored charge resulting from the applied voltage. 
Case II. Uniform illumination is applied to a diode pair that is initially 
uncharged. This exercise may be used to determine the buildup rate and the 
equilibrium stored charge for the case where both diodes are equally 
illuminated. 
Case III. A single diode of the pair is illuminated and the other is not. 
Case III, in combination with Case II, covers the case of nonuniform 
illuminaton of a diode pair, encountered whenever the image intensity 
varies along a vertical line. A pair of diodes constitute a vertical PEL. 
Horizontal resolution is defined by the density of diodes in a given row. 
The amplitude of the applied square wave is assumed to be V. The leakage 
current in the diodes is assumed to be zero, which will not be the actual 
case. Thus, in what follows, a constant background charge resulting from 
leakage is ignored. However, there is also a constant background charge 
resulting from charge pumped by the voltage source. It is assumed that the 
background charge can be suppressed by the threshold behavior of the LC 
cell. This is done by setting the applied voltage appropriately so that 
the background charge is near threshold. 
The charge stored in the LC cells is firstly considered. This charge, in 
turn, determines the voltage that appears across the cell. The magnitude, 
but not the sign of the voltage, determines the cell reflectivity. It 
should be noted that the charge on both cells of a pair is the same. The 
voltages across paired cells are in opposite phases at any instant. 
The analysis is straightforward. It consists of calculating the charge 
stored on each of the capacitors, C.sub.lc, after each half cycle of the 
applied square wave voltage, .+-.V, taking the charge stored from the 
previous half cycle as the initial condition. The charge generated by the 
current sources, I or I', during the condition of reverse bias on the 
respective diodes is taken as the signal charge during any half cycle. The 
forward drop of the other diode is small at the end of the charging cycle 
so that the voltage across the capacitors, C.sub.lc, is almost equal to 
the applied voltage V. For the time being, the voltage dependence of the 
diode capacitance is ignored. 
A recursion relation for the charge q.sub.n stored on C.sub.lc after the 
n.sup.th cycle becomes apparent quickly, and this can be used to develop a 
series expression for q.sub.n in terms of the initial charge. The series 
expressions are geometric and are easily summed. 
Now, considering the foregoing cases more particularly: 
CASE I 
At the end of a positive half cycle, a charge q.sub.o is stored on C.sub.lc 
and there is no illumination thereafter. 
The stored charge and the n.sup.th half cycle following the start of 
counting is given by: 
EQU q.sub.n =r.sup.n q.sub.o +(-1).sup.n C.sub.t V[(1-(-r).sup.n)/(1+r)][1] 
in which 
EQU r=C.sub.t /C.sub.d =C.sub.lc /(C.sub.lc +2C.sub.d) 
EQU C.sub.t =(2/C.sub.lc +1/C.sub.d).sup.-1 [ 2] 
The ratio r is always less than unity. The capacitance C.sub.t is the total 
series capacitance across the generator (not counting the inter-row or 
other shunt capacitance). Since one of the diodes is always forward biased 
at any instant C.sub.d is counted only once. 
The charge, q.sub.n, has two components. One originates from the initial 
charge, q.sub.o, and is reduced in magnitude by the factor on each 
successive half cycle. On a time average basis, taking time instances 
corresponding to the end of a half cycle, the parameter n may be replaced 
by: 
EQU n=2ft.sub.n 
in which f is the frequency of the square wave, and t.sub.n the time 
corresponding to the end of the n.sup.th half cycle. Hence, the 
relationships: 
EQU r.sup.n =exp-t.sub.n /.tau. 
EQU .tau.=(-2fln r).sup.-1 [ 3] 
As f is increased, the rate of decay .tau.-1, is increased. As C.sub.d 
approaches zero, r approaches unity and the time constant approaches 
infinity. Thus, the diode capacitance plays a key role in providing a 
transient capability. 
The second component in q.sub.n is an alternating charge proportional to 
C.sub.t V. This represents the alternating background charge of magnitude 
EQU q.sub.background =C.sub.t V/(1+r) [4] 
The full term containing C.sub.t V represents the transient buildup 
behavior of the background charge, were the device just turned on. This 
would be the case with the simple choice of q.sub.o =0. The transient term 
in nonalternating. Indeed, had this situation been anticipated and the 
background charge been included in q.sub.o, the transient term would have 
been absent. Thus, 
EQU q.sub.n =r.sup.n (q.sub.o -C.sub.t V/(1+r))+(-1).sup.n C.sub.t V/(1+r) 
is a more illuminating representation in which the steady decay of the 
initial signal charge, and the constant background charge, are clearly 
distinguished. 
CASE II 
There is uniform illumination; the diodes produce signal charges q.sub.s 
and -q.sub.s on subsequent half cycles; and there is no initial charge. 
The charge after the n.sup.th half cycle following a positive half cycle is 
EQU q.sub.n =(-1).sup.n [r q.sub.s +C.sub.t V][(1-(-r).sup.n)/(1+r)][5] 
The background charge is the same as before. The signal charge has a steady 
state value, which alternates in sign, of 
EQU q.sub..infin. =rq.sub.s /(1+r) [6] 
and a non-alternating transient component which decays as r.sup.n. The 
transient time constant is similarly .tau.=(-2fln r).sup.-1. 
The achievable contrast in the liquid crystal cell depends on the ratio of 
maximum signal charge to background charge. This may be called the dynamic 
range. The maximum value of q.sub.s =1/2 C.sub.lc. Thus, the dynamic 
range, D.R., is 
##EQU1## 
To achieve a large dynamic range, it is desirable to maximize the ratio 
C.sub.lc /C.sub.d. This implies r approaching unity, which increases the 
response time constant. Thus, there is a tradeoff between dynamic range 
and response time. 
CASE III 
Only one diode is illuminated and produces a signal charge q.sub.s. 
The charge at the end of the n.sup.th half cycle following a positive half 
cycle is, 
(n=odd) 
EQU q.sub.n =-rq.sub.s (1-r.sup.n+1)/(1-r.sup.2)-C.sub.t V(1+r.sup.n)/(1+r) [8] 
(n=even) 
EQU q.sub.n =-r.sup.2 q.sub.s (1-r.sup.n)/(1-r.sup.2)+C.sub.t 
V(1-r.sup.n)/(1+r) 
The background charge is the same as before. The signal charge has a steady 
state value 
EQU q.sub..infin. =rq.sub.s /(1-r.sup.2) (n=odd) 
EQU q.sub..infin. =r.sup.2 q.sub.s /(1-r.sup.2) (n=even) [9] 
and a transient term with the same buildup time. The magnitude of the 
signal charge, for the second half of a full cycle, is reduced by the 
factor r, as one might expect. The average signal charge during a full 
cycle is 
EQU q.sub..infin. .vertline.average=1/2rq.sub.s /(1-r) [10] 
This is different from the uniform illumination case. For r=1/3, the two 
have identical responses. For r.gtoreq.1/3, the nonuniform illumination 
component is enhanced and vice versa. This part can be summarized by 
noting that there is a steady background component, and that transient 
behavior is characterized by the exponential time constant .tau.=-(2f ln 
r).sup.-1 which for r=1/3 is about equal to one half period of the applied 
AC voltage. The choice r=1/3 equalizes the response to uniform and the 
average of nonuniform illumination and provides a signal charge of 1/4 the 
photo charge plus leakage charge produced by the diode during a half 
cycle. The choice r=1/3 corresponds to C.sub.d =C.sub.lc. The dynamic 
range is r/(1-r) and for the choice r=1/3, has the value 1/2. To achieve a 
larger dynamic range requires r closer to unity. 
OPERATING MODES 
The photodiode LALV of the present invention can be operated in a video or 
in a storage mode (FIG. 2B). In the video mode, the applied square wave is 
run at a constant frequency. If the light valve is driven by a CRT (that 
is, if an image produced by a CRT is projected onto the light valve), the 
CRT frame rate should be twice the square wave frequency. In this case, 
integration time is not relevant since the illumination is not steady. 
However, setting the frame rate at higher multiples of twice the square 
wave frequency does allow charge accumulation. For slowly varying 
illumination, the integration time is half the period. In this case, 
sensitivity and transient response can be traded off simply by varying the 
drive frequency. For viewing without flicker, the drive frequency should 
not fall below 60 Hz, although a much lower rate could prove to be 
acceptable since the flicker is likely to be reduced for a square wave 
drive. 
The photodiode array is also sensitive to electron excitation. Thus, the 
diode array LALV can be directly activated by the electron beam of a CRT 
without the intervening phosphor screen and optics. 
In the storage mode, the LALV is erased by operation without input light 
using several cycles of a high frequency square wave. This leaves the 
target with only the steady state background charge. Then, a steady DC 
voltage is applied. A single frame scan of the writing CRT imparts the 
stored charge image to the liquid crystal cell. For this case, n=1 applies 
and there is no distinction between Case II and Case III. The stored 
charge is .+-.rq.sub.s in either case. A value of r close to unity is 
desirable. The longer time constant is not relevant except that it 
increases the number of cycles required for erase. However, the frequency 
of the square wave during erase can be made high, keeping the erase time 
low. 
The choice r close to unity is probably not especially deleterious to the 
video mode. It does increase the response time and enhances the response 
the high spatial frequencies. However, both effects can probably be 
tolerated. Hence, a display optimized for the storage mode is also useful 
in the video mode. The greater dynamic range is highly desirable. 
For storage, the image is maintained for a relatively long time if the 
liquid crystal cell is allowed to float in an open circuit mode. 
Eventually though, the stored charge in the LC layer will leak off and 
redistribute. However, in a closed circuit mode, dark current leakage in 
the reverse biased photodiode of each pair gradually adds more charge to 
the liquid crystal cell and the field eventually saturates. The time 
constant for storage depends on the diode leakage and capacitance. A time 
constant of several seconds is possible. The erase-write cycle is then 
repeated, this time with the opposite sign of DC voltage on the cell to 
provide field cycling in the cell. The erase-write cycle can be 
sufficiently fast so that it should not be particularly annoying. The CRT 
frame time for writing can be longer than in the conventional case, 
thereby allowing a larger number of PELS. 
An alternative way of operating this light valve in storage mode is to use 
long persistence phosphor in the addressing CRT. In this mode, the light 
valve is operated under AC and the storage time is controlled by the 
phosphor persistence time. 
SENSITIVITY 
The sensitivity of a LALV depends on the photosensor. For silicon diodes 
with an AR coating, and good geometry, the quantum efficiency can approach 
100% over the range 0.4-0.9 micron with the maximum near 0.8 microns. The 
spectral response drops off rapidly beyond 1 micron. The response is 
linear (gamma=1) and the diode has no lag. The transient response time 
constant of the photodiode LALV is roughly equal to the period of the 
applied square wave voltage. The photodiode is generally more sensitive 
than a photoconductor except at low light levels. The photodiode has a 
gain of unity. In a photoconductor, the lifetime of a conduction promoting 
trap is high at low light levels. This allows high photoconducting gain, 
but causes high lag. Hence, use at low light levels is not always 
desirable. The quantum efficiency of a photoconductor is only a few 
percent. The gain of a photoconductor decreases with increase in light 
level, so that the charge-light characteristic is not linear 
(gamma.perspectiveto.1/2). The sensitivity of a photodiode is essentially 
independent of voltage. The gain of a photoconductor increases with 
voltage. 
IMPLEMENTATION 
Two alternative embodiments for achieving the light valve structure 
schematic of FIG. 1 will now be described. The first embodiment is shown 
in FIG. 3. 
The substrate, which may be in the form of a silicon single crystal wafer, 
is prepared by growing a thick semi-insulating epi layer 11 on a 
conventional substrate 14 (see FIG. 3A). A deep, patterned n-layer 12 is 
then diffused producing conducting rows. Alternate rows are connected in 
common at the periphery. After the n-diffusion, the oxide mask is stripped 
and a new oxide layer is grown. Holes are opened for the p-diffusion and 
p-portions 13 are formed. After the p-diffusion, a metal layer is 
deposited and patterned to place square, isolated metal reflecting 
electrodes 17 centered over each diode. These squares serve three 
functions: (1) as electrodes for the LC cell; (2) as reflectors; and (3) 
as a light blocking layer to protect the photodiode from the projection 
light. The light path between the metal squares must be blocked. 
The original substrate layer is then etched off uniformly down to the grown 
epi layer in the area opposite the diode array. The region around the 
diode array is kept thick and provides the supporting member for the thin, 
light-sensitive area. Uniformity of the thinning is assured by virtue of 
the large differential etching rate for the two layers that can be 
achieved. A membrane as thin as 10 microns, stretched over the supporting 
thick rim, will be structurally sound, yet thin enough to allow collection 
of the light generated minority carriers (holes for the geometry 
described) by the photodiode. The exciting light should be in the near IR 
so that it is absorbed near or within the diffused n-layer 12. The space 
between metallurgical n-layers, under the openings in the metal, can be 
subjected to ion bombardment (for example, gold) using the metal as the 
mask. The light blocking means would then be unnecessary since gold is a 
strong recombination center. The visible projection light would be 
absorbed close to the surface, within the implanted region, and virtually 
none of the minority carriers would reach the diodes. 
If desired, the membrane can be stiffened by filling in the etched region 
with transparent material 15 (see FIG. 3A). The photodiode array is then 
covered with a dielectric layer 16 which serves two purposes: (1) it 
becomes one boundary of the liquid crystal cell; and (2) it stiffens the 
membrane. This membrane of silicon with diodes formed and insulator and 
electrode regions defined, may then be mounted on a glass plate 50. 
One of the features of the choice of sign of conductivity is that the 
photodiode array can function as a target for imaging as a camera tube. 
This allows diagnostics, during development, of dark current, sensitivity, 
spectral response, resolution, diode capacitance, and processing problems 
leading to defects. In this configuration, both sets of rows are tied 
together electrically and function as the target backplate. This 
particular approach has the advantage that direct electron beam scanning 
of the silicon can be used as an alternative to photon imaging with a CRT 
as the source. 
Another alternate embodiment is shown in FIG. 4. In this case the silicon 
substrate would be shown on the left if it were not etched off later in 
the process. It is chosen to be n-type. A uniform p-layer is grown, 
followed by a several micron thick n-layer. A final, thin n+ layer is 
either grown or diffused in. The silicon is then mounted and bonded on a 
sapphire or glass substrate, which serves as the window and as a support 
for the silicon. The substrate is then etched away down to the p-player. 
Metal and thin dielectric layers are then deposited uniformly. The 
horizontal elements of a rectangular grid are then patterned and the 
silicon etched through to the support. This isolates the rows in an 
interdigitated finger pattern so that alternating potentials can be 
applied to the n-layers. The vertical separations are then patterned and 
the silicon etched part way through. This provides continuity for the 
rows, yet isolates the diodes in any given row. A dielectric might be 
applied to provide passivation for the mesa diodes. 
The thin n+-layer serves to produce a low surface recombination interface 
assuring very high collection efficiency for holes produced by visible 
light. The n+-layer reduces the series resistance of the diode rows. 
DESIGN OF THE SILICON PHOTODIODE ARRAY 
As indicated above, it is desirable to design the photodiode to have a 
junction capacitance under reverse bias that is smaller than the 
capacitance of the LC cell. Typically, an LC cell may operate with a 
potential of several volts, say 5 volts at 30 Hz. The cell thickness would 
be 10 microns. Thus, the liquid crystal cell, for a pel size of 25 microns 
square and a dielectric constant of 5 for the LC, would have a capacitance 
of about 2.times.10.sup.-15 F. In order to obtain a smaller photodiode 
junction capacitance, the design parameters may be determined from the 
following equation (abrupt junction) 
##EQU2## 
where .epsilon..sub.Si is the dielectric constant of silicon; 
N.sub.Si is the doping level in silicon; 
V.sub.bi is the built-in voltage.perspectiveto.19 volts for N.sub.Si 
=10.sup.15 to 10.sup.16 cm.sup.-3 ; 
V is the bias voltage; 
q is the electron charge; and 
A is the area of a diode junction. 
If a doping level of 2-3.times.10.sup.15 cm.sup.-3 is selected, a diode 
should be designed to have a junction size less than 4 to 5 micron square. 
This can be done with present day lithography. In general, the diode may 
be made smaller if higher doping is chosen and vice versa. 
ALL SOLID STATE SIGNAL INPUT 
The silicon photodiode array has properties which potentially make it a 
good choice for a LC array control element. As described earlier, the 
photo-excitation of minority carriers in the bulk silicon substrate and 
the subsequent diffusion to a reverse biased diode represent the input 
means. As already indicated, excitation by energetic electron bombardment 
is also an option. Hence a miniature CRT is a suitable choice for writing. 
However, there are other options. 
It was early recognized by applicants that the charge coupled device (CCD) 
could be used for display as well as imaging. Specifically, it was 
appreciated that a frame store CCD in a direct bandgap semiconductor would 
produce light, if all the depletion region wells in which minority 
carriers are stored were simultaneously collapsed. The in-rushing majority 
carriers would combine radiatively with the minority carriers and produce 
light in proportion to the stored charge. The local variations in stored 
charge would produce an optical image. 
Silicon, being an indirect bandgap semiconductor, would be unsuitable for 
the write CCD, but GaAs, which has been demonstrated to be suitable for 
CCDs, would be effective. The emitted radiation at 0.88 microns would be 
near the peak of the silicon photodiode sensitivity. A sandwich structure, 
consisting of a GaAs CCD frame store and the silicon diode array LALV 
would provide an all solid-state solution. 
The long diffusion length of minority carriers in silicon, which makes it 
an inefficient light emitter and an efficient photodiode, provides a 
mechanism for directly injecting minority carriers into the photodiode 
array substrate. One implementation may have a silicon substrate with a 
CCD frame store opposing the photodiode array. Operation of the CCD during 
write would be independent of the operation of the light valve control 
element. 
Following application of voltage to the diode array, the depletion regions 
of the fully filled frame store are collapsed and replaced by an 
accumulation region at the substrate interface. This injects the stored 
minority carriers into the substrate and keeps them from returning to the 
interface. The minority carriers diffuse virtually without loss to reverse 
biased diodes. Lateral diffusion, in combination with the nonzero 
thickness of the substrate, causes a loss of resolution. This is minimized 
by keeping the thickness of the bulk region small compared to the spatial 
extent of the diode pair, implying substrate thickness in the range of 
25-50 microns. Double-sided processing under these circumstances while 
difficult may be achievable. 
The video rate for a CCD can greatly exceed what is posible for a CRT. This 
makes possible a high resolution light valve operating at a standard frame 
rate. 
The CCD version described here differs from the Hughes CCD light valve 
described by Little et al, in SID Digest, 13,250, 1982, in the mechanism 
of using the injected minority carriers to control the state of the liquid 
crystal and the use of AC, rather than DC across the LC. 
It will be seen that a new silicon control element for a light-activated 
light valve has been described which replaces a homogeneous 
photoconductive layer in a conventional, AC light activated light valve. 
Although it is more complex than the homogeneous photoconductor, it is 
comparatively simple in terms of state-of-the-art silicon technology. 
Using the silicon control element as described herein one can realize a 
number of advantages relating to basic understanding of the material 
system (liquid crystal) compatibility with the requirements of the light 
valve-sensitivity, lag, linearity of light response, ruggedness, and 
ability to withstand high overloads. In addition, the simple structure of 
silicon diodes offers the potential for high yield and low cost and thus 
it can be the basis for an effective light-activated, light valve system. 
This control element combined with a CCD frame store provides an all 
solid-state light valve.