ITT - Index of

__ .
ITT
FOUR-LAVER DIODE
_____________________ **_A_PP_L_IC_A_T_IO __ N_N_O_T_E_S
SEMICONDUCTORS
TWENTY-FOUR CIRCUIT APPLICATIONS FOR ITT 4-LAYER DIODES
THEORY OF OPERATION
The ITT Semiconductor Four-Layer Diode is a
two-terminal semiconductor switch, sometimes
referred to as a negative-resistance diode.
These PNPN silicon devices are useful in many
applications including pulse generators, oscillators,
telephone switching, sweep generators,
and multivibrators.
The diode has two stable states; the ON or
low impedance state, and the OFF or high impedance
state. To turn the device ON, voltage
across the terminals must exceed the switching
voltage (V,). The device can be turned OFF by
reducing the current through the device below
the holding current ((h).
Ih
Is
I
I
I
Vh
Figure 1
:m
----I II
I
Vs
Figure 1 is the V-I characteristic of a typical
device. V, is the point at which the device
breaks down and switches to the ON state
where current is limited only by the external
circuit impedance.
Ih is the minimum current that must be passed
through the diode to keep it in the ON state.
If the current drops below this value the diode
will switch to the 0 FF state.
I
Figure 2B
IC2t
I L,
~I
~
I/32
TI.
t ICI
The PNPN diode can be thought of as a
complimentary pair of transistors, one a PNP
and the other an NPN as shown in Figure 2A.
Figure 2B shows the two transistors as they
would appear schematically. The collector of
T2 drives the bose of T1 giving rise to a positive
feedback loop with a gain of B1 B2. As long as
B1 B2 is less than 1 the equivalent circuit is
stable with the upper and lower PN junctions
biased forward' and the middle junction reversed
biased. In this condition only a small
leakage current flows.
The diode will remain in this condition until
either the temperature or the anode voltage is
increased to a point where B1 B2 = 1. At this
point, the middle junction breaks down and
the circuit becomes regenerative. This occurs
because the collector of each transistor supplies
current to the base of the complimentary
transistor in the equivalent circuit and their
Betas are current dependent. As sOOn as they
reach unity the device effectively "runs away"
or "fires" and the current through the circuit is
limited only by external impedance.
Vac
Figure 3
ANODE
Figure 2A
p
p
N N N
p p p
N
N
CATHODE
**Application notes will be found in the last pages of this section.
15-7