TECHNICAL REPORTS
The World s Largest-Capacity
8kV/3.6kA Light-Triggered Thyristor
by Katsumi Sato*
*Katsumi Sato is with the Power Device Division.
Mitsubishi Electric has developed an 8kV/3.6kA
light-triggered thyristor (LTT) based on a six-
inch wafer for power-converter applications in
high-voltage DC transmission and back-to-back
systems,New design features give the device
double the power-control capacity of previous
LTTs based on four-inch wafers while mitigat-
ing the problems that occur with large-capacity
devices.
Development Objectives
There is a market for high-capacity LTTs with
a high blocking voltage,since they will enable
equipment manufacturers to build power-
control systems that are simpler,more compact
and more reliable,However,increasing thyris-
tor blocking voltage and capacity generally in-
volves sacrificing on-state voltage (V
TM
),reverse
recovered charge (Q
rr
),critical rate-of-rise of on-
state current (di/dt),leakage current and other
operating characteristics,In developing the new
LTT,we needed to improve these characteris-
tics,reduce the minimum light triggering power
(P
LT
) (which helps extends the lifetime of the
light source),reduce the loss in the snubber cir-
cuit,and for reasons of equipment size,improve
critical rate-of-rise of off-state voltage (dv/dt)
capability.
Description
Fig,1 shows the 8kV/3.6kA LTT and its semi-
conductor element,The element consists of
small light-sensitive pilot thyristors at the cen-
ter that are triggered by an optical signal,and
the main thyristor,which turns on in response
to a gate current supplied by the pilot thyris-
tors,The package is designed to ensure uniform
mounting pressure,and is constructed to pre-
vent alloying between the element and the heat-
absorber disc,The ceramic seal has a clear
window to admit light.
The photocurrent induced in the pilot thyris-
tor and the displacement current generated by
dv/dt turn on the pilot thyristor,forcing a trade-
off between P
LT
and dv/dt capacity,We reduced
P
LT
without sacrificing dv/dt capability by use
of a channeled cross section for the junction
between the p-base and n-base layers,thus mak-
ing better use of the large photocurrent near the
wafer surface (Fig,2),To improve the dv/dt capac-
ity,we surrounded the main thyristor with pilot
thyristors,and developed a gate-emitter structure
that prevents dv/dt mistriggering,These struc-
tural enhancements,combined with design op-
timizations,yield a dramatic improvement in
the trade-off relationship between P
LT
and dv/dt
capability (Fig,3).
Large-capacity thyristors typically employ a
dynamic gate structure,with the pilot thyris-
tors (which supply the trigger current) arranged
in concentric circles,However,in order to turn
Fig,1 The package and semiconductor element
of the 8kV/3.6kA LTT.
Fig,2 A cross section of the light-sensitive area.
Cathode contact
(pilot thyristor)
Light irradiation
Antireflection coating
Anode contact
J1
J2
J3
NE
PE
NB
PB
Key
J1~J3 Semiconductor
junctions
NB n base
NE n emitter
PB p base
PE p emitter
June 1996 · 31
TECHNICAL REPORTS
on the main thyristor more quickly,we devel-
oped a new dynamic gate structure in which the
main thyristor area is dotted with pilot thyris-
Fig,5 The trade-off between V
TM
and Qrr.
Q
rr
(arbitrary units)
V
TM
(arbitrary units)
Key
New LTT
a71 Conventional LTT
50%
improvement
0.6 0.7 0.8 0.9 1.0 1.1
0.2
0.4
0.6
0.8
1.0
1.2
Light pulse
Units
V
D
2,000V/div
I
T
250A/div
Time 1μs/div
I
T
V
D
Fig,4 The turn-on waveforms at 8.8kV.
Fig,3 The tradeoff between P
LT
and dv/dt
capability.
P
LT
(arbitrary units)
70% improvement
0.01 0.1 1
Key
New LTT
a71 Conventional LTT
dv/dt
capability (V/μs)
100
1,000
10,000
Table 1 Major Ratings and Characteristics
Item Symbol Value
Repetitive peak off-state
V
DRM
8,000V
voltage
Repetitive peak reverse
V
RRM
8,000V
voltage
Average current I
T(AV)
3,600A
Critical rate-of-rise of
di/dt 200A/μs
on-state current
On-state voltage V
TM
2.8V
Critical rate-of-rise of
dv/dt 2,300V/μs
off-state voltage
Minimum light triggering
P
LT
8.0mW
power
Turn-off time t
q
400μs
Thermal resistance R
th(j-f)
0.004°C/W
Table 1 Major Ratings and Characteristics
32 · Mitsubishi Electric ADVANCE
tors,This arrangement has dramatically im-
proved the turn-on characteristics,With an
applied voltage of 8.8kV,a di/dt of 200A/μs is
achieved,Fig,4 shows a typical turn-on wave-
form at 8.8kV.
Lifetime control technology helps optimize
the trade-off between V
TM
and Qrr,but conven-
tional techniques such as heavy metal diffusion
and electron-beam irradiation affect the entire
wafer,We chose instead to use proton irradia-
tion,which allowed us to control the lifetime
profile in the thickness direction,This local-
ized lifetime control prevents a rise in V
TM
while
reducing Q
rr
and the subsequent continuous
leakage currents,(There is no reduction of carrier
lifetime of the n-base.) Fig,5 shows the trade-
off relationship between V
TM
and Q
rr
of a con-
ventional LTT compared with that of this device,
indicating a 50% improvement,Table 1 lists the
major specifications of the new device.
The unprecedented voltage and current ratings
of this light-triggered thyristor will make it
possible to manufacture high-capacity power-
control and power-conversion equipment that
is compact and reliable,a75