PNPN diode, also known as a Shockley diode, is a four-layer diode comprising of four alternate layers of P-type and N-type materials as shown in figure below. The P-type and N-type semiconductor regions on the extreme are called anode and cathode, respectively
PNPN diode
Operation of a PNPN diode is explained below.
• When anode (P-type) is made positive with respect to the cathode (N-type), junctions J1 and J3 are forward-biased.
• The applied voltage effectively appears across junction J2, thus reverse biasing junction J2.
• If the applied voltage is increased such that it exceeds the peak inverse voltage of semiconductor junction J2, then the semiconductor junction J2 breaks down.
• At this break-over voltage, the current increases all of a sudden from a very small value to a very large value and the PNPN diode switches from its OFF state (blocking state) to its ON state. This increase in current is accompanied by reduction in the voltage giving rise to a negative resistance region.
Figure below shows a PNPN diode represented as back-to-back connected NPN and PNP transistors. However, two transistors connected back-to-back do not make a PNPN diode.
PNPN diode represented as back-to-back connected NPN and PNP transistors
Both the transistors are in active region because the collector junctions of both of them (i.e., J2) are reverse-biased and their emitter junctions (J1 of PNP transistor and J3 of NPN transistor) are forward-biased.
The collector current of a bipolar transistor in the active region is given by
Where, IC is the collector current α the short-circuit gain of CB configuration ICO the reverse saturation current.
IE1 = Emitter current of Q1 = +I and IE2 = Emitter current of Q2 = -I
Also, the leakage current for Q1 (ICO1) is negative and for Q2 (ICO2) is positive. Let ICO be the total leakage current of the device. Therefore,
Hence
Now in the case of transistor Q1, substituting the sum of all currents entering the transistor equal to zero, we get
Substituting different values, we get
Here a1 and a2 are the forward current gains of the two transistors in common-base configuration. Their values are between 0 and 0.95 depending upon the currents flowing through them. When we are increasing the voltage, effectively we are doing nothing but increasing the value of a1 and a2 and when a1 + a2 reaches unity, there is a sudden increase in anode current as
The moment (a12) tries to exceed unity, current becomes exceedingly large. This brings all three junction diodes into saturation region. Therefore, Q1 and Q2 go to saturation due to their collector junctions being forward-biased. Due to this, magnitudes of a1 and a2 decrease so that
(a1 + a2) never exceeds unity.
In nutshell, transistors Q1and Q2 enter saturation only up to the extent that (a1 + a2) remains unity.
The V–I characteristics of a PNPN diode are shown in Figure below.
V–I characteristics of PNPN diode
The characteristic curve can be divided into the following three regions
• Cut-off region or the forward-blocking state
• Saturation region
• Transition region (shown as the dotted line)
In the cut-off region, current through the device is ideally zero. Practically, this current is extremely small, equal to the current that would flow through a reverse-biased PN junction. Cut-off region extends to a voltage equal to the break-over voltage marked VB in the V-I characteristics.
As the anode-to-cathode voltage exceeds VB, the device switches rapidly from the cut-off region to the saturation region. The dotted line in the V-I characteristics indicates this rapid switching action. The PNPN diode cannot operate in this region. Hence, the V-I characteristics here is shown as dotted line. Once the device has broken-over, it stays in that state as long as the current remains above a value called the holding current marked IH in the V-I characteristics.
To bring the device to the cut-off state, the current should be brought below the holding current value. The voltage corresponding to the holding current is the holding voltage marked as VH. The value of VH is approximately 0.7 V. VH is a increases with increase in the magnitude of current flowing through the device.
A PNPN diode behaves like an ON–OFF switch. The switch is open in the forward-blocking state and is closed in the saturation region. The characteristics of a PNPN diode for a negative anode-to-cathode voltage are similar to that of a reverse-biased diode.
Silicon unilateral switch (SUS) is a device very similar to a PNPN diode, except that its forward voltage drop after firing is higher than the PNPN diode.
Figure below shows the basic relaxation oscillator circuit configured around a PNPN diode.
PNPN diode relaxation oscillator
The circuit functions as follows.
• Initially the PNPN diode is in the cut-off state. It therefore behaves like an open switch. Capacitor C begins to charge exponentially through resistor R towards the applied DC voltage V.
• As the voltage across the capacitor reaches a value equal to the break-over voltage of the PNPN diode, the diode breaks down and rapidly switches to the saturation region.
• PNPN diode is now in the ON-state and behaves like a closed switch. The capacitor rapidly (almost instantaneously) discharges through the PNPN diode. The discharge process continues as long as the current through the PNPN diode remains above the holding current value.
• The moment it falls below the holding current value, the PNPN diode rapidly switches to the OFF-state. The capacitor begins to charge again through R and the process is repeated.
• Thus, the capacitor repetitively charges and discharges through R and PNPN diode, respectively.
The charging time is determined by the product of R and C. Charging equation is given below
The discharge time is determined by the product of C and ON-resistance of the PNPN diode. The waveform across the capacitor resembles a sawtooth, more so when the charging voltage V is large as compared to the break-over voltage of the PNPN diode. Figure below shows the waveform.
The waveform becomes a perfect sawtooth if resistor R were replaced by a constant current source. The charging process in that case would be represented by
Where,
I is the magnitude of the constant charging current.
The semiconductor material used for making PNPN diodes is always silicon. Germanium is never used for construction of PNPN diodes. With germanium as the semiconductor material, the magnitudes of a may become large enough to give (a1 + a2) equal to unity for a very small value of applied voltage thus leading to an unstable “OFF”-state.
The effect of rate of change of applied voltage on the break-over voltage is termed as rate effect. When the PNPN diode is in the forward-blocking state, the center diode junction J2 is reverse-biased while the two outermost diode junctions J1 and J3 are forward-biased. PNPN diode represented by two forward-biased and one reverse-biased diode is shown in Figure below. All reverse-biased diodes have some capacitance across them
Rate effect in PNPN diodes
When rate of change of applied voltage is low, capacitances C offers very high reactance with the result that the current through the capacitance can be ignored. When the rate of change of applied voltage becomes so large that the current through C, when added to the total current, causes the sum a1 + a2 to reach unity at an applied voltage less than the break-over voltage. This higher rate of change of applied voltage thus effectively reduces the break-over voltage leading to premature firing of the device. This phenomenon is called rate effect.
