In this lecture, we are going to learn about the Tunnel diode, its definition, how the tunnel diode work, and how it can be used in many applications. We will discuss the whole topic in every detail. So let’s start with the basic definition of Tunnel Diode.

**Tunnel Diode**

Generally, a normal p-n junction has an impurity concentration of about 1 part in 108. This amount of doping has a depletion layer width of about 5 microns i.e. 5 x 10-4 cm. The diodes in which the concentration of impurity atoms greatly increased up to 1 part in 103, to get completely changed characteristics, are called Tunnel diodes. These diodes are first introduced by Leo Esaki in 1958.

Due to heavy doping the depletion region gets reduced considerably, of the order of 10-6 cm i.e. about 1/100 the width of the depletion region in a normal p-n junction diode. Due to the thin depletion region, an electron penetrates through the barrier. This is called tunneling and hence such high impurity density p-n junction devices are called tunnel diodes.

Many carriers in tunnel diodes penetrate the barrier at velocities far more than the velocities available in the conventional diodes, at low forward bias voltages. Due to such an effect, it shows a negative resistance region in its volt-ampere characteristics. This negative resistance region is the most important feature of the tunnel diode.

The circuit symbol of the tunnel diode and its equivalent circuit in the negative resistance region is shown in the figure below.

**Characteristics of Tunnel Diode**

The below figure shows the volt-ampere characteristics of a tunnel diode.

For small forward voltage (up to 50 mv for germanium) the resistance remains small, of the order of 5\Omega, and current increases. The current attains a peal value I_{p} corresponding to the voltage V_{p} which is about 600 mV. The Ip can vary from a few microamperes to several hundred amperes.

At the peak point, the slope dI/dV of the characteristics becomes zero. If now the forward voltage is increased further, beyond V_{p}, then the current starts decreasing rather than increasing. Tus the dynamic conductance dI/dV becomes negative. Hence the dynamic resistance dV/dI is negative and it shows negative resistance characteristics, Thus negative resistance continues till a voltage V_{v} is called **valley voltage**.

At the valley voltage V_{v}, the current I_{v}, and slope dI/dV became again zero.

After V_{v}, if the voltage is increased, the current again increases. Thus resistance again becomes positive and remains positive thereafter.

At the so-called peak forward voltage V_{f}, the current again reaches the value equal to peak current I_{p}.

The value of current between Ip and Iv can be obtained with three different voltage values. For the value of current between Ip and Iv the characteristics are triple values. These multi-value features make the stunning diode useful in pulse and digital circuits.

**construction of Tunnel diode**

The most common commercially available tunnel diodes are made from germanium or gallium arsenide. The basic construction of an advanced design tunnel diode is shown in the figure.

**working of tunnel diode**

Consider a circuit as shown ninth figure below. The supply voltage is V and R is the load resistance.

The drop across the tunnel diode is Vt. Applying KVL,

V=I_TR + V_T

I_T=-\frac{1}{R}V_T+\frac{V}{R} ……Eq.1

So when I_T=0, V_T=V while when V_T=0, I_T=\frac{V}{R}. Using these two points a load line can be obtained on the V-I characteristics of the tunnel diode as shown in the figure below.

The load line intersects the characteristics at the three points A, B, and C. The load line position and the slope is completely dependent on the network elements and tunnel diode characteristics.

Points A and C are stable operating points located in the positive resistance region of characteristics. while point B is in the negative resistance region and hence an unstable operating point.

An important point regarding stable points A and C is that if there is a slight change in the network conditions there will not be any change in the circuit behavior and position of the Q point for the circuit. If the supply voltage increases then point C will move up on the curve as the voltage across diode V_{T} will increase. When voltage decreases to the original value, point C will regain its original position back.

But if the operating point is defined at point B in the unstable region and if supply voltage slightly increases then correspondingly V_{T} increases but I_{T} decreases due to negative resistance characteristics. This further reduces V_{T} and the process continues till the point shifts into a stable region as point C. If the supply voltage slightly decreases, point B moves up to achieve stable point A.

Thus point B can be defined as an operating point using the load line concept but in practice, it will get stabilized at locations A or C.

This concept is used in a negative resistance oscillator using a tunnel diode.

**Tunnel diode oscillator circuit**

The below figure shows a negative resistance oscillator using a tunnel diode.

The network elements are so designed to obtain the operating point Q at position O as shown in the figure below. This is the intersection point of the load line with tunnel diode characteristics. the design is such that the load line interests the characteristic only at one point which is the Q point. A stable operating point is not at all defined.

Initially, when the switch is closed, the supply voltage starts increasing, and at V=V_{p}, the current attains the maximum value I_{p}. Now for V > V_{p}, according to the characteristics, the current must decrease.

But, V = I_TR+I_T(-R_T) …… Eq.2

As the tunnel diode is operating with Q point is negative resistance region hence RT is taken negatively.

\therefore V = I_T(R-R_T) ……Eq.3

Hence I_{T} is decreasing and R-R_{T} is also decreasing. But V is increasing beyond V_{p}. This is contradictory according to Eq.3. hence diode operating point switches from A to B in that region when current can increase as voltage increases. But a point B, V = V_{F} which is greater than V. To Achieve this, the polarity of the transient voltage across the coil must reverse, and the current starts decreasing from B to C. Energy stored in the inductor while the current has reaches I_{P, }starts decreasing.

This decrease in current continues till the point of operation shifts to point C where the current is I_{v} while the voltage is V_{v} Now this voltage is still more than V and hereafter current starts increasing again. But as the inductor is discharging, the current must decrease as voltage decreases. Hence it is not possible in practice to increase the current after point C but the point of operation shifts from point C to D where voltage is V_{D} and current is L_{r}

This is a stable region where the current can further decrease rather than increase. But from point D onwards, tunnel current can again increase to Tp. Thus the process repeated on its own again and again, never settling at an operating point defined in an unstable region of characteristics. The result is the oscillator circuit with the help of a fixed supply and negative resistance characteristics of the tunnel diode.

The voltage and current waveforms are shown in the below figure.

Thus the square type waveform can be obtained using a tunnel diode. The oscillator waveforms are not necessarily exact symmetrical i.e. T_1\neq T_2. This is because portions DA and BC are not identical.

**Expression for Time Period T**

let

R1 = Tunnel diode resistance of the portion passing through the origin

R2 = Tunnel diode resistance of the second positive resistance region

Then RT = R + R2 while R’T = R + R1 and R = load or circuit resistance.

The linear piecewise approximation of the tunnel diode is shown in the below figure.

Now V_Y=V'-V=V'_V-V-I_vR_2 ……Eq.4

Then the time T1 is given by,

T_1=\frac{L}{R_L}\ln \left[ \frac{V_Y + I_pR_T}{V_Y+I_vR_T} \right ] ……Eq.5

While the time T2 is given by,

T_2=\frac{L}{R'_T}\ln \left[ \frac{V-I_vR'_T}{V-I_pR'_T} \right ] ……Eq.6

The total period is given by,

T = T_{1} + T_{2}

while f=1/T=frequency of oscillations

**Advantages of tunnel diode**

The advantages of a tunnel, diode are:

- Environment immunity i.e. the peak point (V
_{p}, I_{p}) is not a sensitive function of temperature. - low cost
- Simplicity i.e. a tunnel diode can be used along with a d.c. supply and few passive elements to obtain various applications circuits.
- Low noise.
- High speed. i.e. the tunneling takes place at the speed of light hence the switching times of the order of nanosecond are easily obtained and switching times as low as 50 psec also can be obtained.
- Low power consumption

The only disadvantages of this diode are its low output voltage swing ad it is two-terminal device. hence there is no isolation between input and output. Hence transistor is used along a tunnel diode for frequencies below 1 GHz.

**Applications of Tunnel diode**

The various other applications of tunnel diode are,

- As a high-speed switch
- In pulse and digital circuits
- In negative resistance and high frequency (microwave) oscillators
- In switching networks
- In timing and computer logic circuitry
- Design of pulse generators and amplifiers.

**Comparison of Tunnel Diode and Conventional Diode**

Sr. No | Tunnel Diode | Conventional p-n junction diode |
---|---|---|

1. | Impurity concentration is high about 1 part in 10^{3} atoms. | Impurity concentration is low about 1 part in 10^{8} atoms. |

2. | The depletion region width is about 5 microns which is 1/100th the width of a typical p-n junction diode. | The width of the depletion region is high compared to the tunnel diode. |

3. | The carrier velocities are very high at low forward bias, hence can punch through the depletion region. | The carrier velocities are low at low forward bias, hence can not penetrate the depletion region. |

4. | The V-I characteristics show the negative resistance region. | The V-I characteristics do not show the negative resistance region. |

5. | The materials used for construction are germanium or gallium arsenide. | Silicon is the most popular used. |

6. | The switching time is very low in the order of nano to picoseconds. | The switching time is high. |

7. | Used for high-frequency oscillators, high-speed applications such as computer pulse and digital circuits, and switching networks. | Used in rectifiers and other general-purpose applications. |

**Frequently Asked Questions on Tunnel Diode**

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