Gunn Diodes are named after a researcher J. B. Gunn from an IBM, he discovered that the materials form group (III-V) of predict table such as Gallium arsenide (GaAs), and Indium Phosphide (InP), when applied voltage increases up to the certain value the mobility of electrons in these materials decreases, thereby producing negative differential resistance region. A diode made up of these materials can generate microwaves frequencies.
What is a Gunn Diode?
Gunn diode is two terminals electronic device, which is composed of only one type of doped semiconductor i.e N-region. The unique property of Gunn diode is that it works in Negative differential resistance region, which means it can be used to generate microwaves frequencies of 0 to 100 GHz.
The Gunn Diode is also known as Transferred Electronic Device (TED) because it is composed of N-region, and N-region semiconductor has electrons as a majority carrier, and the Transferred Electronic Devices (TEDs) uses such materials which have electrons in the majority.
Note: A negative differential resistance means the relationship between Voltage and current is out of phase (180°).
Symbol of Gunn Diode:
There are a number of symbols assigned for Gunn diode that may be seen in different circuit diagrams, one of the most widely used standard symbol for Gunn diode is shown below in which two simple diodes touching at the common point.
What is the Gunn Effect?
Sir John Battiscombe Gunn (J.B Gunn) in 1963, observed something useful while working on noise properties of semiconductors, he observed that material of group (III-V) of predict table have the ability to generate microwaves frequencies and oscillation. Gunn Effect can be summarized into that whenever the voltage applied to the semiconductor material of group (III-V), increase up to the critical voltage value they generate microwave power of few Gigahertz (GHz).
Construction of Gunn Diode:
The Gunn diode is fabricated from a single N-type semiconductor layer. It has three layers of N-type semiconductor. Most widely used material for the construction of the Gunn diode is Gallium arsenide (GaAs), and Indium Phosphide (InP).
Note: It can also be constructed from other materials of a group (III-V) of predict table.
Among these three layers of the Gunn diode, the first layer and third layer is widely doped of the n-type semiconductor. While the in-between second layer of this Diode is lightly doped compared to 1st and 3rd layer. During the manufacturing process of the Gunn Diode, the first and third layer are formed by the ionization process and the middle layer is an epitaxial layer grown on the N-type substrate.
In order to use this Diode in electronic circuits, the metallic connection in the first and third layer is provided during the manufacturing process.
• The heat sink is used to make the Diode stable for the excessive heat and to prevent damages.
• The range of generation of microwave frequencies depends on the amount of doping in the first and third layers of the Diode.
Working of Gunn Diode:
The Gunn diode is unique diode it is different from an ordinary P-N junction diode because there is no P-region and no junction in Gunn Diode. But still, it is called a diode due to the presence of two electrodes in the construction of this Diode. When the external voltage is applied to this diode, the entire voltage appears in the active region. Here active region is referred to as a middle layer of the device.
Due to which the electrons from the 1st layer of the conduction band (having almost zero resistivity) are transferred into the third layer of the valence band. Because applied voltage has made the electrons to flow from conduction band to valence band. The third layer of Gallium arsenide has the mobility of electrons which is less than that of the conduction band of the first layer.
When the electrons have transferred from the conduction band to the valence band, after some threshold value the current through the device starts decreasing, Due to this the effective mass of electrons starts increasing and thus mobility starts decreasing due to which the current starts decreasing, And this creates the negative differential resistance region in the Gunn diode.
In this negative differential resistance region, the current and voltage have an inverse relationship, which means when the current starts to increase the voltage starts to fall. And when voltage starts to increase the current start to decrease. Thus, it generates pulses with 180° phase reversal and thus this device is able for the operation of amplifier and oscillator circuits.
Gunn Diode Oscillator:
Gunn diodes are widely used as oscillators to generate microwaves with frequencies range of 1 to 100 GHz. It is a Negative Differential Resistance device as explained above and also they are called as transferred electron device oscillator.
There are two types of Gunn Diode Oscillators, TEO oscillators, and Microstrip oscillators.
When the DC bias is applied to this diode it behaves in negative differential resistance and generates microwave frequencies. Consequently, the circuit provided below is able to oscillate at low frequencies with the presence of tuned circuit inductance and other circuit connections.
Characteristics of Gunn Diode:
The characteristic of Gunn Diode is almost similar to the tunnel diode characteristics.
The graph below shows the V-I characteristics of a Gunn Diode with the negative differential resistance region.
Initially the Current starts to increase in Gunn diode with the applied bias voltage, At a particular instant, the current starts to decrease and this point is known as peak point. After crossing peak point the current starts decreasing and this creates a negative differential resistance region in the Gunn diode. And because of this negative differential resistance region, the diode acts as the oscillator.
Gunn Diode Operations Mode:
In 1963 the J.B Gunn first declared his observation of microwave oscillator; various modes of operation have been introduced, depending on their operating conditions and material parameters.
Here we will cover the present four modes of operations of the Gunn Diode.
- Gunn oscillation mode: in the Gunn oscillation mode, In this region the device is unstable and In this case, the oscillation frequency is almost entirely determined by the resonant frequency of the cavity and has a value of several times the intrinsic frequency.
- Stable amplification mode: In the stable amplification mode, Gunn Diode exhibits amplification at the distinct-time frequency rather than the oscillation. This is known as stable amplification mode.
- LSA oscillation mode: LSA or Limited-Space-Charge Accumulation mode is the simplest mode of operation, and it consists of a uniformly doped semiconductor without any internal space charges. Due to this, the internal electric field of the device would be uniform to the applied voltage. The current in the device is then proportional to the drift velocity.
- Bias-circuit oscillation mode: This mode occurs only when there is either Gunn or Limited-Space-Charge Accumulation oscillation when a semiconductor diode is biased to the threshold; the value of average current in a device suddenly drops as the oscillations begin in Gunn Diode. Due to the drop of the current in diode can lead to oscillations in the bias circuit.
Advantages of Gunn Diode:
- Gunn’s are cheaper to construct.
- It provides better SNR or Noise to Sound Ratio.
- Gun’s are very small in size and rigid in nature.
- This diode is applicable to be used for amplification and oscillations.
- It can have a good bandwidth of 1 to 100 GHz.
- It requires a very low operating voltage.
- Oscillator circuit is simple to construct.
Disadvantages of Gunn Diode:
- It is thermal sensitive hence require heat sinks.
- It is less efficient than other frequency generator devices.
Applications of Gunn Diode:
- Gunn’s are used for amplification and oscillation.
- These are used as a sensor in the Collision avoidance radar systems in electronic communication.
- These are used in Vehicle ABS system.
- They are used as Traffic analyzer sensors
- They are used in commercial applications of electronic instruments and devices such as, `Blindspot’ car radar, Pedestrian safety systems, Elapsed distance meters, Automatic identification, Presence/absence indicators, Movement sensors, Distance measurements.
This was all about Gunn Diode, Gunn Effect, construction of Gunn, characteristics of Gunn diode, Modes of Gunn Diode, Gunn diode oscillator and its working with applications in brief, and if you have any query or information regarding the Gunn diodes, please commenting below.