Download pnpn Switching Devices

p-n-p-n Switching Devices
The basic pnpn device consists of four back to back alternate p and n
regions. Thus it has three p-n junctions: j1, j2 and j3. The two
terminals are known as anode and cathode according to the region to
which they are connected. The junctions can be forward bias
junctions or reverse bias. When the anode is positive with regard to
cathode the device is in forward bias and reverse configuration is
known as the reverse bias. However, the forward bias state has two
modes of operation: the forward blocking mode and the forward
conducting mode. Switching between these two modes takes place at
the onset of a critical peak forward voltage.
In the forward blocking mode (v positive) the j1 and j3 junctions are
both forward biased while j2 is reverse biased. But after a critical
forward peak voltage, the device goes into forward conducting mode.
In forward conduction mode (v positive) the device exhibits low
impedance state and allows current to flow. All the three junctions j1,
j2 and j3 are forward biased. Whereas in reverse blocking mode the
junctions j1 and j3 are reverse biased while junction j2 is forward
biased. The reverse biased junction leads to blocking of current
through the device terminal. Hence no current flows in such a
reverse blocking mode.
The working of pnpn as a switching device can be visualized as a
combination of two transistors. One is a pnp BJT formed by junctions
j1 and j2 and the other is npn BJT formed by junctions j2 and j3. In
this configuration, the base of pnp forms the collector of npn and the
base of npn forms the collector of pnp. Thus the base current of pnp
equals collector current of npn and vice-versa.
For the device to get into the operating mode there are some
triggering mechanisms. Thermal agitation: charge carriers are
produced by thermal effects. Optical injection: by EHP generation
through optical methods, one can increase the current through the
device. However these methods are complex and require external
tools. Voltage triggering: it is a simple triggering method. When
voltage increases the critical peak voltage or a steady state triggering
voltage, complete switching takes place at this voltage.
Semiconductor controlled Rectifier (SCR)
While a pnpn diode is switched by controlling the voltage across its
terminal, the SCR is switched by controlling through a third terminal.
The SCR is used in power switching applications like dimming and
can handle current which ranges from a few mA to hundreds of
amperes.
The structure of a three terminal SCR is similar to the pnpn diode,
except that it has a third metal contact attached to one of the base
regions. This third pin is known as gate. The gate current helps in
sweeping the carriers across junctions, thus increasing the injection
efficiency. Hence the switching state is achieved at lower values of
Vp. Once the device switches to forward conducting state, gate
current no longer has any effect on the working and the device can
remain in the conducting state for an infinitely long duration. Hence
SCRs can be widely used in switching applications. Switching devices
like the SCR or pnpn can function for switching applications only in
the forward blocking mode.
The bilateral switching device can be switched to both the forward
and reverse biased mode by making suitable modifications in the
structure. These can be used in the switching application, which
involves a-c waveforms. If a bilateral switch is properly designed it
shows symmetrical characteristics in the forward and reverse bias
regions.