Question:

Published on: 12 October, 2024

**Explain the tunneling action in a tunnel diode.**

Answer:

Tunnel diode is specially made p-n junction device which exhibits negative resistance over part of the forward bias characteristics. It has extremely heaving doping on both sides of the junction and an abrupt transition from the p – side to the n- side.

Under unbiased condition, there is just the same probability of electrons going from states in the conduction band on the n side to the states in the valence band on the p- side, as in case of p material.

Fig. 25(a) Tunnel diode under no biased condition and (b) Tunnel diode under forward biased condition

The main criterions for tunneling are

- Fermi level is within the band.
- The barrier of the deletion layer at the junction is very thin (≈100A°) and which can be achieved by means of heavy doping (≈10
^{21}atoms/c.c) in both the p and n layer. Thus the particle have the lower kinetic energy cannot cross the barrier height but have the probability that they can tunnel through the potential barrier. - At 0 K, all the states above Fermi level are empty and below Fermi level all the states are occupied.
- There must be filled energy states on the side from which particle will tunnel and allowed empty states on the other side into which particle penetrates through at the same energy level.

Fig. 25 shows the energy band diagram of a tunnel diode under no bias and forward biased condition. Initially at zero bias condition as no filled states are available in any side of the junction, hence no tunneling takes place. With the application of forward bias voltage, the conduction band upward and creates vacancy level in the p- side and as a result tunneling takes place.

The I-V characteristics of Tunnel diode is given in Fig. 26.

Fig. 26 I-V characteristic of Tunnel diode.

From the forward characteristics curve we see that with the increase of forward biasing voltage from zero the tunneling current starts to follow from n- side to p- side and reaches the peak value I_{A}. With further increase of forward bias voltage i.e. for V>V_{A}, the conduction band rises further up and as a result no corresponding allowed empty state is available in the p- side, hence no electro can tunnel through the barrier and the tunneling current drops nearly equal to zero as shown by I_{B} and the corresponding voltage V_{B} in the I-V characteristic curve. This valley current results due to the presence of defect states within the semiconductor energy gap. Such a current in the valley region can also be called as the excess current.

When the forward bias voltage V is increased above the valley voltage V_{B}, the ordinary injection current I at the p – n junction starts flow. This injection current increases exponentially in its normal course with the increase of forward biasing voltage.

Total current density in tunnel diode is

J=J_{Tunn}+J_{Exces}+J_{Ther}

Tunneling current density is

J_{Tunn}=J_{A}(v/v_{A})exp(1-V/V_{A})

Where J_{A} is the peak current density corresponds to peak voltage V_{A} at point A.

Excess current density is

J_{Exces}= J_{B}exp[α(V-V_{B})]

Where J_{B} is the valley current density at point B and α is constant.

The thermal current density is

J_{Ther}=J_{S}[exp(q_{B}/nkT)]

Where 1<n<2 and J_{S} is the saturation current density. One of the available empirical curve fitting expressions to represent the complete tunnel diode I-V characteristics is given by

\(I=\frac{(I_A-I_B)}{{(V_B-V_A)}^5}\left[5\left(V-V_A\right)\left(V-V_B\right)^4-\left(V_B-V_A\right)^5\right]+I_B\)

Where V_{B}=Valley voltage at point B

V_{A}=Peak voltage at point A

I_{A}=Peak current at point A

I_{B}=valley current at point B

This equation does not yield I=0 for v=o, unless a specific relation exists between I_{A}/I_{B} and V_{B}/V_{A}. A simpler approximation may be derived on the basis of the fact that between the peak and valley points of the I-V curve of the tunnel diode resembles a sine wave. The empirical relation then follows to be

I=A+Bsin\ (C In V+D)

Where A, B, C and D are arbitrary constants.

Subjects

Trending

**Can a circulator be used as an isolator? If so, how? Difference between circulator and isolator**

View : 737

12 October, 2024

Random questions

**Write short note on PCM ( Pulse Code Modulation).**

12 October, 2024

**Why Schottky diodes are suitable for microwave region?**

12 October, 2024

**Explain the tunneling action in a tunnel diode.**

12 October, 2024

**Establish the equivalence between Thevenin’s and Norton’s theorems.**

12 October, 2024

**State and explain Biot – Savart law.**

22 June, 2022