A tunnel diode has characteristics different from other diodes. When
reverse biased it has a linear voltage-to-current curve. In other
words, it looks much like a resistor when reverse biased; double the
voltage and you double the current. When forward biased a tunnel diode
also looks like
a resistor at low voltages. Once a certain voltage is reached, further
increasing the voltage causes a decrease in current. This is the
opposite of practically any other device, where an increase in voltage
causes an increase in current. The range of voltage where this occurs
is called the "negative resistance" region. When the voltage approaches
the threshold voltage (0.3 volts for germanium and 0.7 volts for silicon) it begins to act like a normal forward-biased diode.
Characteristic curve of a tunnel diode.
The characteristics of a tunnel diode are caused by quantum tunneling. This
phenomenon is where charge carriers with insufficient energy to cross the
depletion region nevertheless cross the region, causing significant conduction
where ordinary diodes will conduct little. This quantum tunneling occurs when
reverse-biased and at low voltages when forward-biased. When quantum tunneling
occurs, the diode acts like a resistor with a linear voltage-to-current
relationship. When the forward-biased voltage reaches the Peak Point Voltage
(around 50 to 100 millivolts for a germanium tunnel diode), the quantum
tunneling starts to decrease. The result is that the current decreases linearly
as voltage increases. This continues until the Valley Point Voltage is reached
(around 300mV for a germanium tunnel diode), after which the tunnel diode
follows the characteristics of a typical germanium diode. The tunnel diode can
be thought of as a voltage-sensitive variable resistor. When reverse-biased and
below the Peak Point Voltage when forward-biased, it has a low resistance of
around five to ten ohms. After passing the Peak Point Voltage, the resistance
increases exponentially, which causes the current to decrease linearly as the
voltage increases. This is the so-called negative resistance region. This
continues until the Valley Point Voltage is reached, where the resistance
decreases such that the current increases exponentially as with a typical diode.
LGM model
Like the zener diode you can imagine a tunnel diode using the Little Green
Man model. In the case of the tunnel diode, the diode is forward biased and the
little green man has a chart of the tunnel tiode charastic curve. His job is to
make the resistance of the diode whatever it takes to cause to current to match
the cureve at the specified voltages.
Uses
Tunnel diodes are often used as the active component of very high-frequency
oscillators. Consider a circuit consisting of a tunnel diode in series with a
tank circuit. When the tank circuit is caused to oscillate, Following
Kirchhoff's Voltage Law, the voltage across the tunnel diode will drop as the
voltage across the tank circuit rises, and vice versa; the voltage across the
tunnel diode will be 180 degrees out of phase with the voltage across the tank
circuit. If the tunnel diode is biased correctly, it will be put into its
highest current state (the Peak Point Voltage) when the tank circuit is in its
highest voltage state. This will push current into the tank circuit, forcing the
voltage even higher, thus compensating for losses, and keeping the tank circuit
oscillating. When the tank circuit is in its lowest voltage state, the tunnel
diode will be in its lowest current state (at the Valley Point Voltage),
allowing the tank circuit voltage to drop, thus not putting current into the
tank circuit at the wrong time. Think of it like a little green man controlling
a variable resistor in series with the tank circuit. He controls the resistance
to feed current into the tank circuit at just the right time to compensate for
losses.
