Gunn Effect

The high-frequency oscillation of the electric current that is flowing through the semiconducting solids is known as the Gunn effect. This is used in a solid-state device called Gunn diode in the production of short radio waves called microwaves. In the early 1960s, it was discovered by J.B.Gunn. The materials such as gallium arsenide or cadmium sulfide can exhibit the Gunn effect in such types of materials the electrons can exist in two states of mobility. The electrons that have a higher mobility rate can move through the solids easily than those electrons that have a lower mobility rate. In the absence of electrical voltage, the electrons are in a high mobility state. When the voltage has applied the electrons just as in the conductors they start to move. This motion of electrons can cause an electric current. In the case of some solids, if the voltage applied is more then the movement of electrons increases and thus the increase in the flow of current. In the materials that exhibit the Gunn effect, the strong electrical voltage can make the electrons move into a lower mobility state. This makes the electrons move slowly and thus decreases the electric conductivity. In electric circuits such as the Gunn diode, the relation between the voltage and current can result in the generation of high-frequency AC from the direct source. 

Gunn Diode

Gunn diode is a form of the two-terminal semiconductor electronic component that is used in high-frequency electronics and has negative resistance. It is also known as TED (Transferred Electron Device). It is discovered by J.B. Gunn depending on the Gunn effect in 1962. It is used in oscillators in the generation of microwaves whose application is found in automatic door openers, radar seed guns, and microwave relay data transmitters. 

The internal construction of these diodes is different from the other diodes where it consists of only N-doped semiconductor and most of the diodes consist of N and P-doped regions. Like other diodes, it cannot conduct in one direction and it cannot rectify the alternating current that’s why the term TED. There are three regions present in the Gunn diode: two of these regions are doped heavily on each terminal and a thin layer is present in between these two layers that are made of n-doped material which is light. The electrical gradient will be more across this thin layer when the voltage is applied to the device. The current in the middle layer will increase when the voltage is increased and when the high-field values the middle layer conductive properties are altered thus the resistivity increases and the current will fall. By this, we can tell that the Gunn diode has a region known as negative resistance in the curve of current versus voltage where the increased voltage value can cause a decrease in the current flow. This is the property used to amplify the radio frequency or it can become unstable and it oscillates when it is biased. 

Working of Gunn Diode

In some of the semiconductor materials such as Gallium arsenide (GaAs), the electronic band structure of these materials consists of valence and conduction bands along with these bands there is another energy band or sub-band. Compared to the conduction band, this third band is at higher energy and it is empty until the energy is passed to it to promote the electrons. This energy is obtained from the kinetic energy of the ballistic electrons. These are the electrons that are present in the conduction band but are moving with some kinetic energy and are capable of moving to the third band. These electrons are either injected by a cathode with energy or they start below the Fermi level by applying strong energy; they are provided with a long mean free path to obtain the needed energy. With the application of forwarding voltage, the Fermi level present in the cathode moves to the third band and by matching the density of states and by the usage of additional interface layers the reflections of ballistic electrons starting around the Fermi level will be minimized.

In GaAs, the effective mass of the electrons that are present in the third band is said to be higher than that of the conduction band. Thus the drift velocity or mobility of the electrons present in that band is less. By increasing the forward voltage, the number of electrons that reach the third band will increase thus it slows down the movement this, in turn, leads to the decrease in the current. This cause can create negative differential resistance. 

The charge carrier density that is present along the cathode becomes unstable and develops the small segments that are of low conductivity by the application of a high potential to the diode and the remaining cathode portion will have high conductivity. The cathode voltage drops will occur along the segment having a high electric field region. By the influence of this electric field, these segments will move from cathode to anode. There will always be a thin slice of high field strength that is present at the background of the low field strength thus it is difficult to balance the population of both bands. A low conductive region is formed at the cathode by the small increase in the forward voltage and due to the increase in the resistance this segment moves along the side of the bar to reach the anode. When the segment reaches the anode, it is absorbed to form a new segment in the cathode by maintaining the constant voltage. Any existing slice will be quenched by decreasing the resistance if the voltage is decreased.

Application of Gunn Diode

Let us take a look at the various application of Gunn diode. These Gunn diodes are mainly used at the frequency of microwave or above that due to their capability of high frequency. They can produce the highest output powers of the semiconductor devices and are commonly found using the oscillator. They are also used in microwave amplifiers to amplify the signals.

• Sensors and Measuring Devices: The Gunn diode oscillators are used in the generation of microwave power for applications such as airborne collision avoidance radar, sensors to monitor the flow of traffic, anti-lock brakes, car radar detectors, automatic door openers, burglar alarms, motion detectors, pedestrian safety devices, sensors to avoid derailment of trains, moisture content monitors, and remote vibration detectors. 

• Radio Amateur Use: By the virtue of low voltage operation these Gunn diodes serve as generators of microwave frequency for the low power microwave transceivers these are called Gunnplexers. These were first used in the late 1970s by the British amateur radio and many designs of these Gunnplexers are published in journals. The diode is mounted into the three-inch waveguide and to drive the diode it is approximately modulated with the supply of low voltage say less than 12v power supply of direct current is used. One end of the waveguide is fed to the horn antenna and another end is blocked to form a resonant cavity. 

To enable the listening of the other amateur stations an additional mixer diode is inserted into the waveguide and is often connected to a modified FM broadcast receiver. These are commonly used in the frequency range of 10 GHz and 24 GHz ham bands and sometimes 22 GHz security bands are modified to use as diodes. Typically if the mixed diode is reused in the existing waveguide then these parts are known for their static sensitivity. Commercially this part is protected with the parallel resistor and a variant is being used in the Rb atomic clocks. Even the Gunn diode is weakened for the usage the mixer diode is used for the applications of lower frequency. 

• Radio Astronomy: For milli-meter and sub-milli-meter wave radio astronomy transmitters, these Gun oscillators are used as the local oscillators. The Gunn diode is mounted in such a way that the cavity is tuned to resonate at the rate of twice the fundamental frequency of the diode. By adjusting the micrometre the cavity length is changed. Over a tuning range of 50%, these Gunn diodes are capable of generating power over 50mW. For the application of the sub-milli-meter wave radio astronomy transmitter, the Gunn oscillator frequency is multiplied by the diode frequency.

Conclusion

Gunn diodes are mainly used at the range of microwave frequencies or above due to their capability of high frequency. The Gunn diodes are usually built to generate the microwave frequencies at the range of 10 GHz to THz. Gunn diode semiconductor material is GaAs. The other material used is Ge, InAs, ZnSe, CdTe and many more. In electric circuits such as the Gunn diode, the relation between the voltage and current can result in the generation of high-frequency AC from the direct source. The advantage of these is high bandwidth, high reliability, the manufacturing cost is low, the operating voltage is relatively low, etc.