What is GUNN Diode?
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| Gunn diode |
GUNN diode is a semiconductor diode made up of N type semiconductor material only. It does not have PN Junction like the other diodes. So it has only electrons as the majority charge carriers and it has two electrodes. Mainly used as low power oscillator to generate microwave.
GUNN Effect:
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| Gunn effect |
The GUNN effect was discovered by John Battiscombe Gunn of IBM in 1963. He noticed some noise in some semiconductor materials during his experiments and it is named after him as GUNN effect. High frequency oscillation of the electric current flowing through certain semiconducting material produces short radio waves called microwaves. This effect is seen in only few materials like gallium arsenide or cadmium sulphide.
Symbol of GUNN Diode:
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| Symbol of Gunn diode |
Most commonly used GUNN diode symbol is shown in the figure above. It consists of two filled triangles and the two pointed tip of the triangle meeting each other. The terminals are marked as Anode and Cathode.
Construction of GUNN diode:
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| Construction of Gunn diode |
As mentioned earlier the GUNN diode is made up of only N type material. Lighty doped material which is called active layer is placed in between two heavily doped N type material. The heavily doped material provides conductivity to the diode. It is placed over the conducting base which acts as the heat sink.
Mostly GUNN diode material are poor conductors, so it produces lot of heat during the operation and thus the heat sink is used. Anode is formed by placing a gold film at the top of the surface.
Working of GUNN diode:
When the no external voltage is applied the free electrons which is the majority charge carriers in the N type material move freely with high mobility.
When it is forward biased the movement of electrons results in the current. In normal conductors when the voltage is increased the mobility of the electrons increases, thus current increases. But in GUNN diode when the applied voltage increases , due to GUNN effect the electrons with high mobility are forced into low mobility state.
Thus the conductivity decreases. Because of this unusual relationship between voltage and current, high frequency oscillating current is produced.
When the current pulse formed reaches the end of the active region the next pulse starts. This decides the operational frequency. It depends on the thickness of the active region. GUNN diodes in the market need external voltage of about 9V and produces microwaves of frequency from 1 to 200 GHz.
V-I Characteristics of GUNN Diode:
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| V-I characteristics of Gunn diode |
When the voltage increases the current also increases and reaches a peak point. When voltage is increased further the current decreases and that is called as negative resistance region. This results in a oscillation.
Advantages:
- Size is smaller
- Manufacturing cost is low
- It reliable and stable at higher frequencies
- Noise to signal ratio is better
- Operates at high bandwidth
Disadvantages:
- Efficiency is low
- Operating current is high, so the power dissipation
is also high
- High turn ON voltage
- Temperature stability is low
Application of GUNN Diode:
- Used in microwave instruments and receiver circuits
- Used in automatic door openers , traffic signal etc
- Used in radio communications
- Used in military systems
- Used in tachometers
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