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Tunnel Diode: Characteristics & Applications

Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

In this lesson, we describe the characteristics of the tunnel diode. This device finds use at high frequencies. Applications explored are the relaxation oscillator and the harmonic oscillator.

What is a Tunnel Diode?

A tunnel is a passageway. In quantum theory, the tunneling effect describes the passageway for particles through a barrier. The barrier is the P-N junction, and the tunneling is made possible by heavily doping the junction. Doping is the process of adding impurities to the semiconductor material to change its properties.


See the top portion of a T in the symbol?
tunnel_diode_symbol


Just like the common P-N junction diode, the current, i, is measured through the diode from the anode (labeled with a ''+'') to the cathode (labeled with a ''-''). The voltage, v, is measured across the diode from anode to cathode.

Plotting the i-v Characteristic


Three regions defined by the slope
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Unlike the P-N junction diode, the tunnel diode does not go into breakdown at some negative voltage. Nor does the tunnel diode shut off the current flow for voltages below a turn-on voltage. On the left in the characteristics, in the region labeled ''a'', as the voltage across the tunnel diode increases, the current increases nearly linearly. The slope is positive.


See the region with a negative slope?
negative_resistance_region


In the region labeled ''c'' the characteristic resembles the growing exponential of the forward-biased P-N junction diode. However, the region labeled ''b'' is drastically different.

After the current reaches a peak value, further increases in voltage will reduce the current flow. This is contrary to most devices. Usually, an increase in voltage leads to an increase in current. In the ''b'' region, a positive change in voltage will produce a negative change in current. This is due to the tunneling effect. A changing resistance is produced which is the change in voltage divided by the change in current. In the tunneling region, this change in resistance is negative. Thus, we say the tunnel diode exhibits negative resistance.

Applications of the Tunnel Diode

The Relaxation Oscillator

An oscillator is a circuit which produces a time-varying repetitive waveform. With just a few components and the tunnel diode, we can wire-up a relaxation oscillator. A relaxation oscillator has a waveform with sharp edges.


Tunnel diode, two resistors, a battery and an inductor
relaxation oscillator


The two resistors, R1 and R2, along with the battery set the tunnel diode to a target voltage in the negative resistance region. This target voltage is never reached.

The oscillator waveform is the voltage across the diode. The switch is turned on, and the current through the diode increases as the voltage is headed towards the target voltage. Because of the positive change in current, the voltage across the inductor continues to increase. This is the green portion of the curve.


null


Once the diode current peaks, the voltage across the inductor is positive which means the change in current is still positive. However, the current cannot increase past this peak. The diode is just entering the negative resistance portion with a negative slope. But the current through the inductor cannot change instantaneously from a positive to a negative value. Also, the diode is constrained to have values of i and v on its characteristic curve. The only way this can happen is if the voltage jumps (the yellow line) to a more positive value. Note, for an inductor, the current must be continuous but the voltage does not have to be.

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