Gunn Diode | Advantages and Disadvantages

Microwave devices that operate by transferred electron mechanism are called Gunn Diodes. Some materials like GaAs show the behaviour of negative mobility with increase in electric field. This phenomenon is caused by the transfer of conduction band electrons from lower energy high mobility state to high energy low mobility state. This is known as ‘transferred electron effect’ or Ridley Watkins Hilsum (RWH). The devices based on this effect are called transferred electron devices. Gunn diodes are negative resistance devices which are normally used as low power oscillators at microwave frequencies. The basic structure of Gunn diode consists of n type GaAs semiconductor. Heavily dopped n+ regions are formed over the substrate. If the voltage or electric field is applied to GaAs initially the current will increase with voltage. When the voltage exceeds the threshold voltage a high electric field is produced and the electrons are excited from initial lower state to higher state. If the rate at which the electrons transferred is high the current will decrease with increase in voltage producing negative resistance.

C_j – Diode Capacitance

R_j – Diode Resistance

R_s – Total Resistance

C_p – Package Capacitance

L_p – Package Inductance

The conduction band has two valley’s

1. Central Valley with low energy and high mobility

2. Satellite Valley with high energy and low mobility

Under normal conditions electrons are in the central valley. When the electric field increases, the velocity of electrons increases. This happens only till the energy reaches the threshold value. Above the threshold value the mobility is non linear.

When the electric field increases beyond the threshold value the velocity of electrons in the central valley increases and gain enough energy to transfer to satellite valley. Such a transfer is defined as transferred electron mechanism. The effective mass of electrons in the satellite valley is higher than the effective mass of electron in the central valley. This results in decreased mobility of electrons in the satellite valley. Since the mobility is decreased, velocity decreases.

Current Density, J = qμE = qV.

When velocity decreases, current density decreases. Thus, when the field becomes more than the critical value the reduction in current indicate negative resistance. This is also known as ‘negative differential mobility’ or ‘bulk negative differential conductivity’ or ‘Gunn effect’ or ‘RWH Theory’.

Modes of Operation:

1. Gunn Oscillation Mode:

This mode is defined in the regions where the product of frequency and length is about 10⁷ cm/sec and the product of doping and length is greater than 10¹² cm/sec. In normal conditions, Gunn diode is operated in this mode with E > E_TH.

The three possible modes of Gunn Oscillation are,

i. Transit Time Domain Mode:

Applied Velocity is equal to drift velocity.

Oscillation period is equal to transit time.

ie, τ₀ = τ_t

Efficiency is below 10%.

Length of the domain is 10⁷ cm/sec.

ii. Delayed Domain mode:

In this mode, the length of the domain is between 10⁶ cm/sec and 10⁷ cm/sec.

The domain is created when E < E_TH.

New domain cannot be formed until the field rises above the threshold.

Oscillation period is greater than transit time.

ie, τ₀ > τ_t

It is also known as inhibited mode.

Efficiency is about 20%.

iii. Quenched Domain Mode:

Length of the domain is greater than 2 x 10⁷ cm/sec.

If the field drops below the minimum value, the domain collapses before reaching the anode.

Oscillation period is less than transit time.

ie, τ₀ < τ_t

New domain is created before the field swings back above the minimum value.

Efficiency is about 13%.

2. Limited Space Charge Accumulation Mode:

When the frequency is very high the domains do not have sufficient time to form while the applied electric field is above the threshold value. As a result, the charge carriers accumulate near the cathode and collapse with time.

Efficiency is about 20%

Oscillation time, τ₀ = 3 x τ_t

Gunn Diode Oscillator:

One of the main applications of Gunn diode is Gunn diode Oscillator. It is used to generate and control microwave frequencies. They are mainly applied in relays, radars etc.

When the gunn diode is biased in negative resistance region, it will produce oscillations. These oscillations can be in the range of GHz. The nature of oscillations depends on the diode area.

Advantages of Gunn Diode:

1. Highly Stable

2. Higher band width and reliability

3.  Small Size

4. Low Supply Voltage

5. Low Cost

6. Highly immune to noise.

Disadvantages of Gunn Diode:

1. Efficiency decreases below 10 GHz

2. Poor Temperature Stability

3. Small tuning range

4. More power dissipation