What is Tunnel Diode
A tunnel diode is a pn junction that exhibits negative resistance between two values of forward
voltage (i.e., between peak-point voltage and valley-point voltage). A conventional diode exhibits *positive resistance when it is forward biased or reverse biased. However, if a semiconductor junction diode is heavily doped with impurities, it exhibits negative resistance (i.e. current decreases as the voltage is increased) in certain regions in the forward direction. Such a diode is called tunnel diode.
Theory of Tunnel Diode.
The tunnel diode is basically a pn junction with heavy doping of p-type and n-type semiconductor materials. In fact, a tunnel diode is doped approximately 1000 times as heavily as a conventional diode. This heavy doping results in a large number of majority carriers. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion layer. As a result, the depletion layer is very narrow. In comparison with conventional diode, the depletion layer of a tunnel diode is 100 times narrower. The operation of a tunnel diode depends upon the tunneling effect and hence the name.
The heavy doping provides a large number of majority carriers. Because of the large number of carriers, there is much drift activity in p and n sections. This causes many valence electrons to have their energy levels raised closer to the conduction region. Therefore, it takes only a very small applied forward voltage to cause conduction. The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling. Valence electrons seem to tunnel through the forbidden energy band. As the forward voltage is first increased, the diode current rises rapidly due to tunneling effect. Soon the tunneling effect is reduced and current flow starts to decrease as the forward voltage across the diode is increased. The tunnel diode is said to have entered the negative resistance region. As the voltage is further increased, the tunneling effect plays less and less part until a valley-point is reached. From now onwards, the tunnel diode behaves as ordinary diode i.e., diode current increases with the increase in forward voltage.
V-I Characteristic of Tunnel Diode.
Fig. 7.18 (i) shows the V-I characteristic of a typical tunnel diode.
(i) As the forward voltage across the tunnel diode is increased from zero, electrons from the region “tunnel” through the potential barrier to the p-region. As the forward voltage increases, the diode current also increases until the peak-point P is reached. The diode current has now reached peak current IP (= 2.2 mA) at about peak-point voltage VP (= 0.07 V). Until now the diode has exhibited positive resistance.
(ii) As the voltage is increased beyond VP, the tunneling action starts decreasing and the diode
current decreases as the forward voltage is increased until valley-point V is reached at valley-point
voltage VV (= 0.7V). In the region between peak-point and valley-point (i.e., between points P and V), the diode exhibits negative resistance i.e., as the forward bias is increased, the current decreases. This suggests that tunnel diode, when operated in the negative resistance region, can be used as an oscillator or a switch.
(iii) When forward bias is increased beyond valley-point voltage VV (= 0.7 V), the tunnel diode behaves as a normal diode. In other words, from point V onwards, the diode current increases with
the increase in forward voltage i.e., the diode exhibits positive resistance once again. Fig. 7.18. (ii)
shows the symbol of tunnel diode. It may be noted that a tunnel diode has a high reverse current but operation under this condition is not generally used.
Tunnel Diode Oscillator
A tunnel diode is always operated in the negative resistance region. When operated in this region, it works very well in an oscillator. Fig. 7.19 (i) shows a parallel resonant circuit. Note that RP is the parallel equivalent of the series winding resistance of the coil. When the tank circuit is set into oscillations by applying voltage as shown in Fig. 7.19. (ii), damped oscillations are produced. It is
because energy is lost in the resistance RP of the tank circuit.
If a tunnel diode is placed in series with the tank circuit and biased at the centre of the negative resistance portion of its characteristic as shown in Fig. 7.20, undamped oscillations are produced at
the output. It is because the negative-resistance characteristic of the tunnel diode counteracts the
positive-resistance characteristic of the tank circuit. The circuit shown in Fig. 7.20 is called tunnel diode oscillator or negative resistance oscillator. The negative resistance oscillator has one major drawback. While the circuit works very well at extreme high frequencies (upper mega hertz range), it cannot be used efficiently at low frequencies. Low-frequency oscillators generally use transistors.
What is Varactor Diode ?
A junction diode which acts as a variable capacitor under changing reverse bias is known as a
varactor diode. When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p-type material with holes (considered positive) as majority carriers and n-type material with electrons (−ve charge) as majority carriers act as charged plates. Thus the diode may be considered as a capacitor with n-region and pregion forming oppositely charged plates and with depletion zone between them acting as a dielectric. This is illustrated in Fig. 7.21 (i). A varactor diode is specially constructed to have high capacitance under reverse bias. Fig. 7.21 (ii) shows the symbol of varactor diode. The values of capacitance of varactor diodes are in the picofarad (10−12 F) range
where CT = Total capacitance of th junction
ε = Permittivity of the semiconductor material
A = Cross-sectional area of the junction
Wd= Width of the depletion layer
When reverse voltage across a varactor diode is increased, the width Wd of the depletion layer increases. Therefore, the total junction capacitance CT of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the width Wd of the depletion layer decreases. Consequently, the total junction capacitance CT increases.
Fig. 7.22 shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT. Note that CT can be changed simply by changing the voltage VR. For this reason, a varactor diode is sometimes called voltage-controlled capacitor.
Application of Varactor Diode
We have discussed that we can increase or decrease the junction capacitance of varactor diode simply by changing the reverse bias on the diode. This makes a varactor diode ideal for use in circuits that require voltage-controlled tuning. Fig.7.23 shows the use of varactor diode in a tuned circuit. Note that the capacitance of the varactor is in parallel with the inductor. The varactor and the inductor form a parallel LC circuit. For normal operation, a varactor diode is always operated under reverse bias. In fact, this condition is met in the circuit shown in Fig. 7.23. The resistance RW in the circuit is the winding resistance of the inductor. This winding resistance is in series with the potentiometer R1. Thus R1 and RW
form a voltage divider that is used to determine the amount of reverse bias across the varactor diode D1 and therefore its capacitance. By adjusting the setting of R1, we can vary the diode capacitance. This, in turn, varies the resonant frequency of the LC circuit. The resonant frequency fr of the LC circuit is given by;
If the amount of varactor reverse bias is decreased, the value of C of the varactor increases. The
increase in C will cause the resonant frequency of the circuit to decrease. Thus, a decrease in reverse bias causes a decrease in resonant frequency and vice-versa.
Example 7.5. The LC tank circuit shown in Fig. 7.23 has a 1 mH inductor. The varactor has capacitance of 100 pF when reverse bias is 5V d.c. Determine the resonant frequency of the circuit
for this reverse bias.
What is Shockley Diode ?
Named after its inventor, a Shockley diode is a PNPN device having two terminals as shown in Fig.
7.24 (i). This *device acts as a switch and consists of four alternate P-type and N-type layers in a single crystal. The various layers are labelled as P1, N1, P2 and N2 for identification. Since a P-region adjacent to an N-region may be considered a junction diode, the Shockley diode is equivalent to three junction diodes connected in series as shown in Fig. 7.24 (ii). The symbol of Shockley diode is shown in Fig. 7.24 (iii).
Working of Scockley Diode
(i) When Shockley diode is forward biased (i.e., anode is positive w.r.t. cathode), diodes D1 and D3
would be forward-biased while diode D2 would be reverse-biased. Since diode D2 offers very high resistance (being reverse biased) and the three diodes are in series, the Shockley diode presents a very high resistance. As the *forward voltage increases, the reverse bias across D2 is also increased.
At some forward voltage (called breakover voltage VBO), reverse breakdown of D2 occurs. Since this breakdown results in reduced resistance, the Shockley diode presents a very low resistance. From now onwards, the Shockley diode behaves as a conventional forward-biased diode; the forward current being determined by the applied voltage and external load resistance. This behaviour of Shockley diode is indicated on its V-I characteristic in Fig. 7.25.
(ii) When Shockley diode is reverse biased (i.e., anode is negative w.r.t. cathode), diodes D1 and D3 would be reverse-biased while diode D2 would be forward-biased. If reverse voltage is increased
sufficiently, the reverse voltage breakdown (point A in Fig. 7.25) of Shockley diode is reached. At
this point, diodes D1 and D3 would go into reverse-voltage breakdown, the reverse current flowing through them would rise rapidly and the heat produced by this current flow could ruin the entire device. For this reason, Shockley diode should never be operated with a reverse voltage sufficient to reach the reverse-voltage breakdown point.
Conclusion of shockley diode
. The above discussion reveals that Shockley diode behaves like a switch. So long as the forward voltage is less than breakover voltage, Shockley diode offers very high resistance (i.e., switch is open) and practically conducts no current. At voltages above the break-over value, Shockley diode presents a very low resistance (i.e. switch is closed) and Shockley diode conducts heavily. It may be noted that Shockley diode is also known as PNPN diode or four layer diode or reverse blocking diode thyristor. Note. Once Shockley diode is turned ON (i.e., it starts conducting), the only way to turn it OFF is to reduce the applied voltage to such a value so that current flowing through Shockley diode drops below its holding current (IH) value. Diode D2 then comes out of its reverse-breakdown state and its high-resistance value is restored. This, in turn, causes the entire Shockley diode to revert to its high resistance (switch open) state.
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