Semiconductor timing device with radioactive material at the floating gate electrode of an insulated-gate field-effect transistor

A semiconductor timing device having a charge storage region, the charge state of which may be detected by field effect action, the charge storage region containing radioactive material which decays by emitting charged nuclear particles, so that the charge state of the charge storage region varies progressively with time.

This invention relates to semiconductor devices, particularly but not 
exclusively insulated gate field-effect devices for use as timing devices. 
Nowadays, numerous systems incorporate a timer. Washing machines and 
cookers require timers operating over seconds or hours without the need 
for very great accuracy. Electronic watches however require time keeping 
to be accurate to about 1 part in 10.sup.4. Bank vaults, strong-rooms and 
other chambers could use timers operating over days, months or even years. 
According to the present invention there is provided a semiconductor device 
suitable for use as a timer in which radioactive decay is employed to 
control an operating condition of the device with time. 
One advantage of a device in accordance with the invention is that since 
radioactive material decays without an external power source, no external 
power is dissipated when such a device is counting time. 
Such devices can be made for a variety of timing operations, by 
appropriately choosing the quantity of radioactive material present and 
its half-life. Thus, by providing a large quantity of an element having a 
short half-life, the operating conditions of the device can be made to 
change rapidly so providing a short-interval timer. However, if it is 
desired to use the device to time a plurality of intervals throughout the 
life of the device, it is generally desirable for the device operating 
conditions to change at a substantially constant rate throughout the life 
of the device. This can be achieved by selecting a radioactive isotope 
having a long half-life compared with the life of the device, so that the 
radioactive decay which occurs during the life of the device produces a 
negligible change in the level of radio activity during that life. Thus, 
for a device having a life of the order of 10 years, the half-life chosen 
will generally be at least of the order of 10.sup.2 years. 
One form of semiconductor device in accordance with the invention may 
comprise a semiconductor body portion, a charge-storage region on the 
semiconductor body portion and spaced therefrom by insulating material so 
that, by field effect action across the insulating material, the charge 
state of the charge-storage region influences the surface potential of the 
semiconductor body portion under the charge-storage region, and in the 
vicinity of which charge-storage region a radioactive element is present 
the radioactive decay of which changes the charge state of the 
charge-storage region to control with time the said surface potential. The 
radioactive decay may increase or decay the said surface potential, and be 
used in a variety of semiconductor devices. 
One specific form of semiconductor device in accordance with the invention 
is an insulated-gate field-effect device comprising a semiconductor body 
portion, a gate region on the semiconductor body portion and spaced 
therefrom by insulating material so that, by field-effect action across 
the insulating material, the charge state and potential of the gate region 
influences a channel in which charge carriers can be made to flow in the 
semiconductor body portion under the gate region, in the vicinity of which 
gate region there is present a radioactive element, the radioactive decay 
of which changes the charge state and potential of the gate region to 
control with time the conductance in the channel. The channel conductance 
may be increased or decreased by the radioactive decay, so that the device 
may operate in the enhancement-mode or in the depletion-mode. A measure of 
time elapsed can then be obtained from the change in current through the 
channel. The radioactive decay may be employed to take the potential of 
the gate region through a threshold value to either form or pinch-off the 
channel of the device so switching the device either on or off. In one 
preferred form the gate region is wholly surrounded by insulating material 
and has no supply conductor connection. 
Preferably, the radioactive decay emits electrically-charged particles for 
instance beta (.beta.)particles. In a preferred form, the radioactive 
element decays directly into a stable product or into a product having a 
much longer half-life so simplifying the reactions influencing the charged 
state during the life of the device. 
The semiconductor device may be for example a transistor and may be an 
element of an integrated circuit.

The IGFET shown in FIG. 1 is a p-channel depletion mode device. The device 
comprises part of a semiconductor substrate 1 of n-type conductivity 
having a major surface 2. An insulating layer structure 3, 4 and 5 is 
present at the surface 2. A gate region in the form of an electrode layer 
6 is present on the thin insulating layer portion 4 and is separated 
thereby from the surface 2. The gate electrode 6 is sandwiched between and 
wholly surrounded on all sides by the insulating layer portions 4 and 5. 
There is no supply conductor connection to the gate electrode 6 which thus 
has a floating potential depending on its charge state. The gate electrode 
6 is a charge-storage region of the device. 
The gate electrode 6 overlies a p-type channel 11 at part of the surface 2 
between source (7, 9) and drain (8, 10) of the IGFET. The source and drain 
may include separate p-type source and drain regions 7 and 8 respectively 
which are present in the n-type substrate 1 adjacent the surface 2, and 
which are contacted by metal electrodes 9 and 10 respectively via windows 
in the insulating layer structure 3, 4, 5. However the source and drain 
may consist of metal electrodes 9 and 10 which form a rectifying Schottky 
contact with the n-type substrate 1 at the windows in the insulating 
structure 3, 4 and 5. The electrodes 9 and 10 extend away over the 
insulating layer portion 3. 
By pulsing the junction between the substrate 1 and drain 8, 10 into 
avalanche breakdown, hot electrons can be injected into the insulator 4 to 
write a negative charge and potential on the gate electrode 6. This 
negative charge changes the surface potential of the semiconductor 
substrate 1 under the electrode 6 and increases the conductivity of the 
p-type channel 11 between the source 7, 9 and drain 8, 10. The channel 11 
permits a hole current to flow between source and drain electrodes 9 and 
10. The magnitude of the negative charge on the gate electrode 6 controls 
the conductance of the channel 11. Furthermore, the gate electrode 6 
surrounded by the insulating layer portions 4 and 5 can be made so that 
this negative charge can be retained on the gate electrode 6 for more than 
ten years at 125.degree. C., only insignificant leakage occurring through 
the insulator 4 and 5. 
An IGFET structure similar in several respects to that described so far 
with reference to FIG. 1 is known for a binary-logic Read-only memory 
application, see for example, "Electronics", May 10, 1971, pages 91 to 95. 
However, in accordance with the present invention, radioactive material 12 
is also present in the device of FIG. 1, in the vicinity of the gate 
electrode 6. The material 12 is indicated by criss-cross hatching in FIG. 
1. 
In this example, the radioactive material 12 may be an isotope which emits 
low energy .beta. particles, for example 32.sub.14 Si at an energy of 
approximately 90 KeV, or 63.sub.28 Ni at an energy of approximately 65 
KeV, or 3.sub.1 H (tritium) at an energy of approximately 12 KeV. 
Each .beta. particle escaping from the top surface of the transistor device 
will in principle reduce the negative charge on the gate 6 by one unit. 
However, it will be appreciated that while the .beta. particle passes 
through the insulator 5 ionization occurs, and the gate electrode 6 may be 
discharged by an amount dependent on the life of charge carriers in the 
insulator 5 and the energy dissipated by the escaping .beta. particle. If 
the carrier mobility in the channel 11 is constant with temperature and 
gate voltage, then, this unit charge reduction of the gate electrode 6 
will reduce the conductance of the channel 11 by one unit. Therefore, at 
any time after the IGFET has been set by the avalanche operation, the 
channel conductance should be directly proportional to the current which 
would flow through the channel 11 if a specific potential difference were 
applied between the source and drain electrodes 9 and 10. Therefore a 
measure of the channel current variation is a measure of the time interval 
during which that variation was produced by the radioactive decay. 
The radioactive material 12 is preferably selected to have a long half-life 
compared with the device life, so that the rate of decay remains 
substantially constant over the device life. Under these circumstances, 
the same variation in gate-potential and channel-current will correspond 
to a substantially identical time interval from setting the device, at any 
stage during the life of the device. 32.sub.14 Si has a half-life of 
approximately 10.sup.3 years, and 63.sub.28 Ni has a half-life of 
approximately 125 years. Either of these is therefore suitable for use in 
a device having a life of 10 years or less. 
The radioactive material 12 may be deposited on or with the gate electrode 
6. However, very precise control of small quantities and their location 
can be achieved by ion implanting the radioactive isotope. This 
radioactive ion implantation may be effected after providing the 
insulating layer part 5, by forming an implanted buried layer 12. 
By implanting a dose of 10.sup.16 32.sub.14 Si atoms/cm..sup.2 into the 
vicinity of the gate electrode 6, the change of charge on the gate 
electrode 6 would be of the order of 6.times.10.sup.11 electron 
charges/cm.sup.2 /month; in a structure such as that shown in FIG. 1 and 
having typical IGFET dimensions, this charge variation would correspond to 
a potential variation in the gate electrode of approximately 3 
volts/month. This value is expected to vary less than 1% over 10 years. 
However, for a similar implantation dose of 63.sub.28 Ni, the gate voltage 
would change by approximately 1 volt/day. Thus it will be apparent that by 
varying the half-life and dose, the characteristics can be altered to give 
an IGFET timer the sensitivity and consistency required for a given 
application. 
Preferably, a thin metal layer 13 is present on the insulating layer part 5 
over the gate electrode 6 and radioactive material 12. When the material 
12 is provided by ion implantation, layer 13 may be provided before or 
after the implantation. The layer 13 can serve to maintain the whole of 
the top surface of the insulating layer part 5 at the same potential, 
during the radioactive decay. It can also be provided with a supply 
conductor connection, and during avalanching of the drain junction the 
layer 13 may be held at a more positive potential to assist in injecting 
hot electrons from the avalanche into the gate electrode 6. 
Preferably, measures are taken to inhibit absorption of radiated 
.beta.-particles in the semiconductor substrate 1 where they might cause 
undesirable radiation damage of the semiconductor crystal lattice. Thus, 
the radioactive material 12 may be provided near only the top surface of 
the gate electrode 6, and the gate electrode 6 can be chosen to have a 
thickness sufficiently large to absorb most of the .beta.-particles 
radiating towards the semiconductor substrate 1. When radioactive ion 
implantation is employed, the energy of the bombarding ions and hence 
their range relative to the thickness of layer 5 (and layer 13 if present) 
can be chosen so that the ions are implanted in a distribution having a 
peak concentration at the interface between the gate electrode 6 and the 
overlying insulating layer part 5. 
The device of FIG. 1 may be manufactured using conventional techniques, for 
example thermal oxidation, photolighographic masking and etching 
techniques, layer deposition, thermal diffusion of impurity and ion 
implantation. Thus, for example the substrate 1 may be of silicon and the 
insulating layer 3 of thermally grown silica. The insulating layer parts 4 
and 5 may be of deposited silica. The gate electrode 6 may be of high 
conductivity polycrystalline silicon deposited on the layer part 4 and 
doped, for example by diffusion, at the same time as source and drain 
regions 7 and 8. However the gate electrode 6 may be a metal, for example 
molybdenum. The layers 13, 9 and 10 may be of aluminium. 
It will be evident that radioactive decay may be employed in other forms of 
IGFET structure to form a timing device. Thus, the IGFET shown in FIG. 2 
is a p-channel enhancement mode device having a gate region 6 formed by 
trapping centres at an interface between a silica layer 4 and a silicone 
nitride 20 layer. Parts of the device of FIG. 2 corresponding to parts of 
the device of FIG. 1 are denoted by the same references in FIG. 2 as in 
FIG. 1. 
It is known that charge can be stored for long periods of time at trapping 
centres formed at a silica/silicon-nitride interface, see for example 
"Electronics", July 5, 1971, pages 53 to 56. The device of FIG. 2 employs 
such a charge storage area to replace the buried gate electrode 6 of the 
device of FIG. 1. Apart from this modification, the transistor of FIG. 2 
is similar in structure and operation to that of FIG. 1. A thin silica 
layer 4 is present on the substrate surface 2 over the region of the 
channel 11. The silicon nitride layer 20 is deposited on this silica layer 
4, to form the charge storage area 6 at the interface. The radioactive 
material which is a low energy beta (.beta.) emitter is ion implanted 
either through the layer 20 or before formation of the nitride layer 20. 
The energy of the bombarding radioactive ions is chosen so that the 
implantation peak occurs at the interface between the layers 20 and 4. 
The device of FIG. 2 can be set by avalanching the source or drain 
junctions to inject hot electrons into the silica layer 4; these electrons 
are then trapped at the silica/silicon-nitride interface as a negative 
charge state. This negative charge state induces the p-type inversion 
channel 11. In another form, however the silica layer 4 may be thin enough 
to permit electron tunnelling therethrough from the n-type substrate 1; in 
this case, the metal layer 13 may be provided with a supply conductor 
connection by which a large positive potential relative to the substrate 1 
is temporarily applied to the layer 13 to aid the electron tunnelling to 
write the negative charge state at the interface. Rather than the metal 
layer 13, it is then this charge storage region 6 at the interface which 
acts as the gate region of the IGFET to control conduction in the channel 
11. The negative charge at this storage region 6 is then reduced with time 
by the .beta.-particle emitting radioactive decay. The time is measured 
from the value of channel current, in a manner similar to that for the 
IGFET device of FIG. 1. 
Such IGFET timers can be used in either an analogue or a digital manner. In 
the analogue case, the value of the channel current can be monitored and 
read off on a scale or dial calibrated in units of time. In another form, 
however, the IGFET can be used in conjunction with circuitry which senses 
the fall (or rise) of the channel current to a certain level (which 
corresponds to a certain time interval). Such circuitry on sensing that 
level can switch to a different state so as to perform a desired operation 
on an appliance or other apparatus controlled by the timer. However, such 
sensing circuitry can also be employed for digital operation of the IGFET 
timer to reset the IGFET timer (for example by re-avalanching a drain 
junction) when the channel current reaches a specified level; in this 
latter manner, a circuit arrangement including the IGFET could form a 
pulse generator. 
Radioactive decay may be used to vary the potential of the gate region 
through a threshold value, so as to turn-on the IGFET in the 
enhancement-mode or turn-off the IGFET in the depletion-mode. Thus, the 
p-channel depletion-mode and enhancement-mode devices of FIGS. 1 and 2, 
will be turned off when the radioactive decay of the .beta.-emitting 
element has sufficiently reduced or reversed the negative charge state set 
in the gate region 6. The time required to do this depends on the 
half-life and quantity of the radioactive material 12 chosen, as described 
hereinbefore. 
It should be noted that by choosing the radioactive material 12 to be an 
alpha (.alpha.) particle emitter rather than a .beta.-emitter, a modified 
charge release can be obtained from the gate region 6 in either the device 
of FIG. 1 or the device of FIG. 2. 
The IGFET shown in FIG. 3 is an n-channel enhancement mode device. The 
device structure is similar to that of FIG. 1 except that the substrate 1 
is of p-type conductivity, and source and drain regions 7 and 8 are of 
n-type conductivity. It is known that there is a tendency for an n-type 
inversion skin to form on a high resistivity p-type silicon substrate 
below a silica layer. The presence of such an n-type inversion skin would 
provide a current carrying channel between the n-type source and drain 
regions 7 and 8 so that the device would be a depletion mode rather than 
enhancement mode device. The n-type inversion skin around the IGFET area 
would also connect the n-type source and drain regions 7 and 8 of the 
IGFET with neighbouring n-type regions of any adjacent circuit elements 
provided in the p-type substrate 1. However, in accordance with the 
invention described and claimed in our British Pat. No. 1,261,723 (PHB 
31848), an acceptor impurity (for example boron) is implanted in the 
substrate 1 adjacent the surface 2. The acceptor ion dose is chosen to 
counteract the formation of the natural n-type inversion skin both in the 
channel area 11 and the surface area 21 around the transistor and to 
determine a precise threshold voltage for the n-channel enhancement IGFET. 
As explained in the said British Patent this threshold voltage can be 
precisely controlled by appropriately choosing the acceptor ion dose 
implanted in the channel area 11. 
The radioactive material 12 is a low energy .beta.-emitter implanted at the 
top surface of the buried gate electrode 6. As this material 12 decays, a 
positive charge state (and potential) is built up on the buried gate 
electrode 6. When this gate potential reaches the threshold voltage of the 
device which was determined by the acceptor ion implant in channel area 
11, an n-type inversion channel forms in the area 11. An elecron current 
can then flow from the source (7, 9) to the drain (8, 10) when a potential 
difference is applied between the source and drain electrodes 9 and 10. 
Thus there is provided an IGFET timing device which will only switch on 
after a certain time has elapsed from setting. This time is determined by 
the threshold voltage chosen for the device, and the quantity and 
half-life of the radioactive material 12. Such a device could be employed 
in conjunction with a lock on a vault or chamber, to prevent the vault or 
chamber being opened before a certain time. The IGFET device of FIG. 3 can 
be reset by avalanching the drain or source junction to inject hot 
electrons into the insulator 4. 
FIG. 4 shows a modified form of the IGFET of FIGS. 1 and 3 in a 
cross-sectional view transverse to that of FIGS. 1 and 3. The gate 
electrode 6 on a thin insulating layer portion 4 extends laterally outward 
over part of the thicker surrounding insulating layer 3. The channel 11 is 
present under the portion of the gate electrode 6 on the thinner layer 
portion 4. The charge state and potential of the gate electrode 6 
influences principally and significantly the semiconductor surface 
potential under this electrode portion on the layer 4, and thus influences 
the conductance of the channel 11 as described previously with reference 
to FIGS. 1 and 3. However, in this modified form, the radioactive material 
12 is provided in the vicinity of only that portion of the gate electrode 
6 on the thicker layer 3, and the layer 3 is sufficiently thick to inhibit 
absorption of .beta.-particles radiated towards the semiconductor 
substrate 1. Because of its good conductance, the charge state and 
potential of the whole gate electrode 6 is changed by the radioactive 
decay. In this modified form, the gate electrode 6 need not be thick, and 
the radioactive material need not be provided only near the top of the 
electrode 6 to inhibit this absorption. 
Thus, the accompanying drawings show an insulated-gate field-effect device 
in which a radioactive element 12 is present in the vicinity of a gate 
region 6, and the radioactive decay changes the charge state and potential 
of the gate region 6 to control with time the conductance of a current 
channel 11 under the gate region 6. Such a gate region 6 is a charge 
storage-region which influences the underlying semiconductor body surface 
potential. An important advantage of such a device is that since the 
radioactive material 12 decays without employing an external power source, 
no external power is dissipated when such a device is counting time. For 
solid state watch and clock applications such an advantage would be 
important; however, for such an application errors arising from charge 
drift in the insulated gate system and from temperature variations would 
be undesirable. 
It will be evident from what is described hereinbefore that many 
modifications and variations are possible within the scope of the 
invention. Thus, for example, a timing device may be formed in which the 
gate or charge-storage region consists of a charge-storage area buried in 
a thick insulating layer and formed by radiation damage arising from the 
radioactive material implant; in such a case, the radioactive material may 
be metallic so that the buried charge-storage area formed by the 
radioactive implant has a metallic nature. 
A radioactive timing device in accordance with the present invention may be 
integrated with other circuit elements in a common semiconductor 
substrate. It may be, for example, a stage of a so-called "charge-coupled 
device" (C.C.D.), "charge-transfer device", or "bucket-brigade device" in 
which charge is transferred between different surface locations in a 
semiconductor substrate by field effect action on applying various 
potentials to insulated gates in sequence.