AN8

Application Note 8
Issue 2 January 1996
The ZTX415 Avalanche Mode Transistor
An Introduction to Characteristics, Performance and Applications
Neil Chadderton
Introduction
Avalanche mode devices can provide
extremely high switching speeds and
are capable of producing current outputs
far in excess of that obtained from
conventional circuits.
These attributes lend themselves to
many applications, including; laser
diode
drivers
for
ranging,
measurement, and collision avoidance
systems, Pockel cell drivers for laser Q
switches, SAW device exciters, streak
cameras and fast high voltage/ high
current pulse generators.
The Zetex Semiconductors ZTX415 is an
avalanche transistor that ideally suits
this mode of operation and application.
The device is available in the proven
silicone E-Line package that assures
excellent thermal properties to
complement the high performance
avalanche characteristics. It is also
available in the SOT-23 surface mount
package (as FMMT415) to enable very
low inductance designs.
This Application Note outlines the
principle of avalanche mode operation,
gives details on important parameter
and operating conditions, and suggests
a few application circuit examples.
(Please refer to Appendix A for a list of
reference material).
Figure 1
Transistor Characteristics in the
Avalanche Region.
What Is An Avalanche
Transistor?
Avalanche transistors are characterised
by a negative resistance region in their
V-I breakdown curve (usually called
secondary breakdown) as illustrated in
Figure 1. This region permits controlled
switching of very high currents in
nanoseconds when appropriate external
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Application Note 8
Issue 2 January 1996
circuitry is employed. The output pulse
generated is limited by the primary
breakdown BVCBO , the transistor’s ‘Onstate’ voltage and the mean dissipation
that can be tolerated.
Characteristics
This section reviews the important
parameters of an avalanche mode
transistor with particular reference to
the ZTX415.
The current passed in the secondary
breakdown condition, defined as IUSB ,
is dependent on the supply to the device
and it’s ‘On-state’ characteristics. The
ZTX415 is capable of producing very
high avalanche current as indicated by
Figure 2. This shows the maximum peak
current as a function of pulse width for a
sinusoidal-like pulse. The area under
curve 1 has been verified by extensive
reliability work. For example, devices on
a life test have produced current pulses
of 60A peak and 10ns pulse width for
over 4 x 1011 times without failure.
The diagram also shows a recorded 107
operations to failure (curve 2), and a 103
operations to failure, (curve 3, observed
sudden failure points). The test circuit
used to derive the measurements for
these curves is shown in Figure 3, and is
basically
a
single
capacitor
arrangement.
+HT
10k
CURRENT
SENSE
51
ZTX415
Figure 3
Single Capacitor Test Circuit for Iusb
Measurements.
The avalanche current is again illustrated
in Figure 4 for a typical device, showing
its temperature dependency for the
specification conditions.
Figure 2
Maximum Avalanche Current
v Pulse Width.
Figure 4
IUSB v Temperature.
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Application Note 8
Issue 2 January 1996
When using avalanche devices, attention
to the base bias and therefore the
possible gain variation can be important.
Figure 5 shows the gain variation versus
collector current over the operating
temperature range for the ZTX415.
Figure 5
hFE v Collector Current.
The circuit techniques employed, and
ultimately the usable operating range,
depend somewhat on the static voltage
characteristic of the avalanche device,
e.g. to obtain a high voltage pulse the
transistor must have a high BVCES. The
ZTX415 has been designed using high
voltage technology to produce a device
possessing a minimum BVCES of 260V
over the specified temperature range (the
25°C minimum actually being 280V). This
makes possible the generation of high
current pulses with the minimum of
circuitry.
Furthermore, the ZTX415 process is very
closely controlled to yield a product with
a relatively narrow band of BVCES (Viz:
1.2:1 compared with 2:1 for conventional
high voltage product) - an important
attribute when designing high voltage
stacks employing many devices in
series, to produce kilovolt range output
pulses. This feature also releases the
customer from expensive selections and
matching operations, otherwise
required to guarantee performance and
reliability.
Another important parameter is the
minimum voltage required for
avalanche operation, below which the
device has the switching characteristic
of a device in non-avalanche mode. This
‘starting’ voltage is dependent on the
external circuitry employed with the
device, but for a simple single capacitor
arrangement is seen to vary as shown in
Figure 6.
Figure 6
Minimum Starting Voltage as a Function
of Capacitance.
Th is show s the minimum voltage
required for avalanche operation as a
function of capacitance, for different
drive currents. The rate of change of
drive current is also relevant, as Figure 7
illustrates.
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Application Note 8
Issue 2 January 1996
performance of silicone packaging,
Zetex devices out-perform any other
TO92 style packages available. Referring
to Figure 9 will show that the ZTX415 is
usable without heatsinking up to an
ambient temperature of 175°C. This
enables higher currents (or voltages),
and higher pulse repetition frequencies
to be realised than with alternative
types.
Figure 7
Minimum Starting Voltage as a Function
of Drive Current.
This demonstrates that a lower starting
voltage may be achieved by using a
faster changing drive current.
Temperature plays a less significant
role, with a typical coefficient of 0.11V/°C
over the whole operating range, but as
low as 0.05V/°C over a more usual range
as shown in Figure 8.
Figure 9
Dissipation De-rating Curves.
Reliability
Figure 8
Minimum Starting Voltage as a Function
of Temperature.
Any device is eventually limited by the
maximum permitted power dissipation
a l lowed fo r a partic ular p ac k a ge .
However, due to the superior thermal
Extensive reliability research has been
performed on all of the DC and
Avalanche characteristics of the ZTX415,
confirming both the power rating and
the inherent quality produced by proven
device
design
techniques.
Environmental, life test, and high
temperature reverse bias (HTRB ) trials
are carried out routinely to guarantee
continuing quality.
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Application Note 8
Issue 2 January 1996
+220V
Applications
10k
Avalanche mode characteristics can be
realised in a wide variety of circuits from
simple single capacitor arrangements
through shaped pulse circuits, fast
monostables and high current/high
voltage pulse generators. The following
gives some typical application
examples.
The delay line pulse generator, shown in
alternative configurations in Figures 10a
and 10b, is so called because it depends
on a charged delay line - the coaxial
cable. The circuit produces a rectangular
pulse, dependent on the length and
characteristics of the cable.
ZO=50R
51
ZTX415
51
Figure 10b
Delay Line Pulse Generator.
+170-260V
10k
51
+220V
ZTX415
10k
ZO=50R
51
Figure 11a
High Current Pulse Generator for Laser
LEDs.
ZTX415
10A/DIV
51
Figure 10a
Delay Line Pulse Generator.
Similar to the delay line generator but
using a single capacitor arrangement, is
the circuit shown in Figure 11(a). Using
a ZTX415, this circuit is capable of
producing sinusoidal-like pulses of tens
of amperes, as reproduced in Figure
11(b). This form of circuit is ideal for
driving laser LEDs.
10ns/DIV
Figure 11b
Current Pulse Generated by Circuit
shown In Figure 11a, showing 46A Peak
8ns Wide Sinusoid.
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Application Note 8
Issue 2 January 1996
Pulse forming networks may be added to
the basic configuration (as shown in
Figure 12(a)) for specific pulse shape
requirements.
Large stacks of avalanche devices are
possible as shown in Figure 13, which
also shows an additional capacitor
included on the lower devices. This is
sometimes necessary, depending on
+135V
10k
2.2uH
51
It is also possible to use the ZTX415
avalanche device in series to allow a
higher supply, and therefore generate
very high voltage pulses.
2.2uH
ZTX415
7.5
ZTX415
Figure 12a
High Current Pulse Generator with Pulse
Forming Network.
ZTX415
In this way, rectangular pulses are easily
produced, the form of which is
dependent on the number of LC
sections. Figure 12(b) shows a -6.7A
peak, 90ns pulse generated by the two
section generator of Figure 12(a).
ZTX415
-70V
ZTX415
-60V
-50V
-40V
Figure 13
High Voltage Stack (or Series) Operation
of ZTX415.
-30V
-20V
-10V
0
0
40ns
80ns
120ns
160ns
200ns
Figure 12b
Current Pulse Generated by Circuit in
Figure 12a, showing -6.7A Peak, 80ns
Pulse Width.
application, to ensure avalanche. This
particular topology can, by optimising
the voltage shared across each
transistor, generate voltage steps of
many kilovolts. Similar configurations
w i t h p u l s e f o r m i n g n etw o r ks ar e
possible as shown in Figure 14.
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Application Note 8
Issue 2 January 1996
10k
ZTX415
ZTX415
7.5
Figure 14
Series operation of ZTX415 with Pulse Forming Network.
When even higher currents are required,
it is possible to devise circuitry using
parallel ZTX415s as shown in Figure 15.
Adjustable biasing makes possible the
ZTX415
simultaneous avalanche of the transistor
array, allowing current pulses of over
one hundred amperes to be produced.
ZTX415
ZTX415
Load
Figure 15
High Current Parallel operation of ZTX415.
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Application Note 8
Issue 2 January 1996
Appendix A
Papers Detailing Applications Of Avalanche Transistors
1): Background:
Application Of Avalanche Transistors To Circuits With A Long Mean Time To Failure
Werner B.Herden.
IEEE Transactions on Instrumentation and Measurement, Vol.25, No.2 June 1976.
Properties of Avalanche Injection and its Application to Fast Pulse Generation and
Switching
Yoshihiko Mizushima and Yoshiharu Okamoto.
IEEE Transactions On Electron Devices, Vol. ED-14, No. 3, March 1967.
Static and Dynamic Behaviour of Transistors in the Avalanche Region
Paolo Spirito.
Istituto Elettrotecnico, Universita di Napoli, Piazzale Tecchio, 80125 Napoli, Italy
IEEE Journal Of Solid-State Circuits, April 1971.
An Analysis of the Dynamic Behaviour of Switching Circuits Using Avalanche
Transistors
P. Spirito and G.F. Vitale.
IEEE Journal Of Solid-State Circuits, August 1972.
2): Parallel Operation:
A Fast Risetime Avalanche Transistor Pulse generator for Driving Injection Lasers
James P.Hansen, William Schmidt. US Naval Research Lab, Washington DC.
IEEE Proceedings Feb 1967
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Application Note 8
Issue 2 January 1996
Appendix A (Continued)
3): High Voltage Pulse Generation:
Subnanosecond High-Voltage Pulse Generator
David Brown and Don Martin.
Los Alamos National Laboratory, MS J957, Los Alamos, New Mexico 87545
Rev. Sci. Instrum. 58 (8), August 1987.
Fast High Voltage Pulsers For Nuclear Instruments
Yoneichi Honsono, Satoshi Takeuchi, Ken-ichi Hasegawa, and Masaharu Nakazawa.
Journal Of The Facility Of Engineering, The University Of Tokyo, Vol. XL, No.1 1996.
Avalanche Transistor Pulser For Fast-Gated Operation of Microchannel Plate
Image-Intensifiers
Arvid Lundy, James S.Lunsford, and A. Don Martin.
Los Alamos Scientific Laboratory, University Of California, Los Alamos, NM 87545
IEEE Transactions On Nuclear Science, Vol. NS-25, No.1, Feb 1978.
Design of Reliable High Voltage Avalanche Transistor Pulsers
E.Stephen Fulkerson. Lawrence Livermore National Laboratory, Livermore,
CA 94551
Rex Booth. Kaiser Engineering Livermore Inc. Livermore, CA 94551
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