Tunnel Diode

     The Tunnel or Esaki diode is a thin-junction diode which exhibits negative resistance under low forward bias conditions.

       An ordinary PN junction diode has an impurity concentration of about 1 part in10⁸. With this amount of doping the width of the depletion layer is of the order of 5 microns. This potential barrier restrains the flow of carriers from the majority carrier side to the minority carrier side. If the concentration of impurity atoms is greatly increased to the level o f 1 part in 10³ . If the device characteristics are completely changed. The width of the junction barrier varies inversely as the square root of the impurity concentration and therefore, is reduced from 5 microns to less than 100 A ( 10^-8 m). This thickness is only about 1/50th of the wavelength of visible light. For such thin potential energy barriers, the electrons will penetrate through the junction rather than surmounting them. This quantum mechanical behavior is referred to as tunneling and hence, these high-impurity-density PN junction devices are called tunnel diodes.

       The V -l characteristic for a typical germanium tunnel diode is shown in Fig 1. It is seen that at first forward current rises sharply as applied voltage is increased, where it would have risen slowly for an ordinary PN junction diode(which is shown as dashed line for comparison). Also, reverse current is much larger for comparable back bias than in other diodes due to the thinness of the junction. The interesting portion of the characteristic starts at the point A on the curve, i.e. the peak voltage. As the forward bias is increased beyond this point, the forward current drops and continues to drop until point B is reached. This is the valley voltage. At 5 , the current starts to increase once again and does so very rapidly as bias is increased further. Beyond this point, characteristic resembles that o f an ordinary diode. Apart from the peak voltage and valley voltage, the other two parameters normally used to specify the diode behaviour are the peak current and the peak-to-valley current ratio, which are 2 mA and 1 0 respectively, as shown.

       The V-l characteristic of the tunnel diode illustrates that it exhibits dynamic resistance between A and B. Figure 2 shows energy level diagrams o f the tunnel diode for three interesting bias levels. The shaded areas show the energy states occupied by electrons in the valence band, whereas the cross hatched regions represent energy states in the conduction band occupied by the electrons. The levels to which the energy states are occupied by electrons on either side of the junctions are shown by dotted lines. When the bias is zero, these lines are at the same height.Unless energy is imparted to the electrons from some external source, the energy possessed by the electrons on the N-side of the junction is insufficient to permit them to climb over the junction barrier to reach the P-side. However, quantum mechanics show that there is a finite probability for the electrons to tunnel through the junction to reach the other side, provided there are allowed empty energy states in the P-side of the junction at the same energy level. Hence, the forward current is zero.

      When a small forward bias is applied to the junction, the energy level o f the P-side is lower as compared with the N-side. As shown in Fig. 2(b), electrons in the conduction band of the N-side see empty energy level on the P-side. Hence,tunneling from N-side to P-side takes place. Tunneling in other directions is not possible because the valence band electrons on the P-side are now opposite to the forbidden energy gap on the N-side. The energy band diagram shown in Fig. 2(b),is for the peak of the diode characteristic.
       When the forward bias is raised beyond this point, tunneling will decrease as shown in Fig 2(c). The energy of the P-side is now depressed further, with the result that fewer conduction band electrons on the N-side are opposite to the unoccupied P-side energy levels. As the bias is raised, forward current drops. This corresponds to the negative resistance region o f the diode characteristic. As forward bias is raised still further, tunneling stops altogether and it behaves as a normal PN junction diode.
1. Equivalent Circuit
       The equivalent circuit of the tunnel diode when biased in the negative resistance region is as shown in Fig. 3(a). In the circuit, R_s is the series resistance and L_s is the series inductance which may be ignored except at highest frequencies. The resulting diode equivalent circuit is thus reduced to parallel combination of the junction capacitance C_j and the negative resistance -R_n. Typical values o f the circuit components are R_s= 6 Ω, L_s = 0.1 nH, C_j = 0.6 pF and R_n = 75 Ω.

Applications 
1. Tunnel diode is used as an ultra-high speed switch with switching speed of the order of ns or ps
2. As logic memory storage device
3. As microwave oscillator
4. In relaxation oscillator circuit
5. As an amplifier.
Advantages 
1. Low noise
2. Ease of operation
3. High speed
4. Low power
Disadvantages
1. Voltage range over which it can be operated is 1 V less
2. Being a two terminal device, there is no isolation between the input and output circuit.

2. Comparison of tunnel diode and conventional diode

3. Specifications of Tunnel Diode
The important specifications and typical values for a tunnel diode are,