siliconchip.com.au Australia’s electronics magazine July 2019 17
- Lattice displacement effects; de-
scribed above - Total ionising dose effects; a cu-
mulative effect of radiation causing
long term damage - Transient effects, such as the short
but intense pulse caused by a nu-
clear explosion which may or may
not cause permanent damage - System-generated EMP effects
which can result in destructively
high currents - Single-event effects (SEE) – proba-
bly the most significant events elec-
tronics are subject to
Single Event Effects
SEE is the general term for a vari-
ety of phenomena such as the ionisa-
tion effects described above, in which
a single energetic radiation event has
some effect on the electronic state of
an electronic structure. Single Event
Effects can be classified as follows:
Single Event Upset (SEU) – “soft”
errors which result in no permanent
electronic damage. SEU errors often
manifest as ‘bit flips’ in memory, ie, a
zero changing to a one or vice versa. In
some cases, multiple bits can be affect-
ed. This can also result in inappropri-
ate pulses in circuitry (see Figs. 5 & 6).
SEU can potentially place the affect-
ed circuitry in some undesired mode
such as a test mode, a program execu-
tion halt or some other unwanted state.
An SEU can be cleared, if detected,
by a computer or equipment reset, or
by re-writing the affected bit with its
original value, which was famously
done in the Voyager spacecraft; see
below.
Single Event Latchup (SEL) – this
can be either a “soft” or a “hard” error.
A hard error can lead to the destruc-
tion of the device.
In an SEL, a circuit element is forced
into a high-current state, causing ex-
cessive heating beyond a device’s op-
erational limits (see Fig.7). This could
result in its destruction (hard error) un-
less the fault is quickly detected and
the device is reset by power cycling.
This type of effect was first noted in
1979, and it can be caused by heavy
ions or protons.
Note that the commercial radiation-
hardened chip (GR712RC) mentioned
below has circuitry to monitor junc-
tion temperatures which can shut
down and reset the device in this case.
Single Event Burnout (SEB) – this
is a “hard” error which destroys the
device.
Devices such as power metal ox-
ide semiconductor field effect tran-
sistors (Mosfets) were thought to be
the only ones affected by this, but it
is now known that other devices such
as power bipolar junction transistors
(BJTs), insulated gate bipolar transis-tors (IGBT), thyristors, high-voltage di-
odes and CMOS PWM controllers and
drivers are also susceptible.
This destructive mode of failure is
due to the passage of heavy ions or
other particles, which may originate
in solar radiation, through sensitive
regions of the device.
SEBs in power Mosfets have been
known to occur in space-based electron-
ics since 1986, but more recently, have
been recognised as a possible source of
failure for terrestrial devices as well.
An SEB event occurs when a high-
voltage semiconductor device is bi-
ased in an off state with a voltage close
to its maximum rated value applied.
A single ionising particle then
strikes the depletion region of the de-
vice, generating a series of electron-
hole pairs.
If the electric field in that region
is strong enough, an avalanche or re-
generative feedback effect is initiated,
causing destructively high currents in
the device.Fig.7: CMOS circuits contain parasitic bipolar structures which can be triggered
by transient signals from radiation. Such circuits are protected by guard bands
and clamps, but radiation signals can bypass these. Two parasitic transistors are
shown in a four-layer device. If triggered, several hundred milliamps can flow,
leading to rapid heating and destruction if this is not detected and stopped within
milliseconds. SEL is more likely at higher temperatures. Figure courtesy NASA.Figs.5 & 6: a Single Event Upset, whereby a heavy ion or
proton passes through a memory element, creating electron
and hole pairs due to ionisation within the crystal lattice.
This creates a parasitic current which can alter the value of
the bit stored in memory (a bit flip). In the case of a proton
passing through the structure, secondary nuclear reactions
can lead to further effects. Source: NASA.