SI96-06
Surging Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
EPD Transient Voltage Suppressors for Low Voltage
Electronics
metal
passivation
passivation
n+
At voltages below 5V, conventional avalanche technol-
ogy is impractical. In order to achieve stand-off volt-
ages below 5V, very high impurity concentrations
(≥10+18cm-3) must be used. This leads to detrimental
performance characteristics such as high capacitance
and very high reverse leakage current. In a joint
development effort, Semtech Corporation and the
University of California at Berkeley have developed a
proprietary device architecture called the Enhanced
Punch-Through Diode (EPD).
n+
n+
p+
p-
n+
Figure 1 - EPD TVS Device Structure
Enhanced Punch-Through Mechanism
In contrast to the traditional pn structure of silicon
avalanche TVS diodes, the EPD device employs a more
complex n+p+p-n+ 4-layer structure (Figure 1). The
structure has light doping in the p+ and p- layers so the
reversed biased n+p+ junction does not avalanche.
An npn structure was chosen over a pnp structure
because it promotes higher electron mobility, improving
device clamping characteristics. A key difference
between the EPD device and traditional punch-through
devices is the base region in the npn structure. A thin
base is used to minimize the space charge voltage,
thus lowering the clamping voltage at high transient
currents. Wide base regions employed by traditional
punch through devices result in poor clamping perfor-
mance. By carefully engineering the p-base region, the
resulting device has superior leakage, clamping, and
capacitance characteristics at voltages of 2.8V and
3.3V.
Figure 2 - EPD TVS Schematic Representation
I
PP
ISB
IPT
IR
VBRR
V
V
V
VC
RWM
PT
SB
IBRR
EPD TVS Device Operation
The IV characteristic curve of the EPD device is shown
in Figure 3. The device represents a high impedance
to the circuit up to the working voltage (VRWM). During a
transient event, the device will begin to conduct as it is
biased in the reverse direction. When the punch-
through voltage (VPT) is exceeded, the device enters a
low impedance state, diverting the transient current
away from the protected circuit. When the device is
conducting current, it will exhibit a slight “snap-back” or
negative resistance characteristic due to its structure.
To return to a non-conducting state, the current
through the device must fall below the snap-back
current (approximately < 50mA). In the reverse direc-
tion, the device looks like a diode with a reverse
breakdown voltage (VBRR) of approximately 30V.
Figure 3 - EPD TVS IV Characteristic Curve
These devices are targeted towards the protection of
today’s low voltage, sub-micron ICs. The devices are
designed primarily to protect against electrostatic
discharge (ESD) on power and transmission lines, but
are also rated for switching and low level lightning
induced transients.
www.semtech.com
Revision 11/2001
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