Resonant Tunnelling Diode

RESONANT TUNNELLING DIODE

Introduction

So far we have discussed about the quantum tunnelling through a single potential barrier. Now we are going to discuss about the quantum tunnelling through two potential barriers (or) a potential profile.

Resonant tunnelling occurs through a potential profile which consists of two potential barriers, so called double barrier structure which are located very close to each other.

Principle

Resonant tunnelling diode works on the principle of tunnelling effect, in which the charge carriers cross the energy barrier(s) even with lesser energy than the barrier potential, quantum mechanically. The probability of tunnelling increases with the decreasing barrier energy.

Symbol and Circuit diagram

The symbol and circuit diagram of a resonant tunnelling diode is as shown in Fig. 3.13 and Fig. 3.14 respectively.

Theory

A resonant tunnelling diode also called as Essaki diode is formed using p and n‒materials, with heavy doping say 1000 times larger than the conventional p‒n‒junction diode. Due to heavy doping, the barrier potential decreases drastically, in turn will help the charge carriers to easily tunnel the junctions, quantum mechanically.

Quantum Well Structure

A resonant tunnelling diode (RTD) consists of a quantum well structure with discrete energy values E₁, E₂ etc, surrounded by two thin layer of potential barriers (V₁ and V₂) with emitter (in n‒region) and collector (in p‒region) on either side as shown in Fig. 3.15.

V ‒ I Characteristics

The diode is forward biased as shown in Fig. 3.14. During forward bias, when voltage is increased, then the current in the diode varies at different resistance regions as follows.

Positive Resistance Region

When a voltage is applied across the resonant tunnelling diode, a terahertz wave is emitted and therefore at resonance, the energy value (E₁) in the quantum well becomes equal to the energy value (E_e) in the emitter side.

 i.e., At low voltage and at resonance ⇒ E₁≈ E_e

Thus at E₁ = E_e, i.e., at resonance, the charge carriers tunnel the potential barriers (V₁ and V₂) and reaches collector region by the process called resonant tunelling.

Therefore the current increases rapidly due to tunnelling effect and reaches the peak point 'P' as shown in Fig.3.16 and this current, is called peak current (I_p). The voltage at which the diode reaches peak current is called peak voltage (V_p).

This region, where the current increases due to the increase in applied voltage is called positive resistance region.

Negative Resistance Region

When the voltage is further increased, then the terahertz wave dies out and now the energy value (E₁) in the quantum well becomes lesser than the energy value (E_e) in the emitter side.

i.e., At higher voltage ⇒ E₁ < E_e

However, since the quantum well has discrete energy values, the energy value (E₂) in the quantum well is still larger than the energy value (E_e) in the verse the emitter side as shown in Fig.3.15.

 i.e., E₂ > E_e

Therefore, the charge carriers cannot tunnel the potential barriers and thus the current in the diode decreases, and reaches the valley point 'V as shown in Fig. 3.16.

This region where the current decreases due to increase in applied voltage is called negative resistance region.

This minimum current is called valley (I_V) and the corresponding voltage is called valley voltage (V_V).,

Normal Diode

Now, when the applied voltage is further increased beyond the valley point voltage in such a way that energy value (E2) in the quantum well becomes equal to the energy value (Ee) in the emitter side, then the current again increases and therefore the resonant tunnelling diode behaves as a normal diode as shown in Fig. 3.16.

Thus, the current in resonant tunnelling diode is due to 3 components viz.,

(i) Tunnelling current (I_T)

(ii) Diode current (I_D)

and (iii) Excess current (I_E)

  Total current, I_Total = I_T+I_D+I_E

Advantages

1. Cost and noise is low.

2. Fabrication is very simple.

3. Operation speed is very high.

4. Power dissipation is low and hence it is environmental friendly device.

Disadvantages

1. Since it is a two terminal device, it is difficult to isolate the input and output.

2. It is a low output swing device.

Applications

1. As resonant tunnelling diode has both positive resistance [From point O to P] and negative resistance [From point P to V, it has many applications in the switching devices,

2. It can be used as normal diodes also.

3. They are used as high frequency microwave oscillators.

4. When resonant tunnelling diode is operated under negative resistance region, then it can be used as an oscillator (or) a switch.

5. Resonant tunnelling diode (RTD) are used in memory calls, multivalued logic circuit devices etc.

Note: RTD is will replace transistors as the workhorse of IC's is near future.