Tunnel diodes and their applications

Diodes are fundamental building blocks in electronics. Their most basic function is controlling the flow of current. Unlike a wire that conducts in both directions, a diode acts as a one-way valve for electricity. They are made from semiconductors like silicon with two terminals — an anode (positive) and a cathode (negative). In general, when a diode is forward-biased (positive voltage on anode and negative on cathode), current flows easily. In reverse bias (opposite voltage), the current flow is very minimal.

What is a tunnel diode?

A tunnel diode is a special type of diode known for its unique characteristic — negative resistance. Unlike regular diodes, tunnel diodes are very heavily doped with impurities, creating a very narrow depletion region (the area between p and n regions in a semiconductor). This heavy doping allows electrons to tunnel through the depletion region due to a quantum mechanical effect. Due to heavy doping, the energy bands (valence and conduction) of the p and n regions get squeezed very close together in the depletion region. This allows electrons to tunnel through the barrier, even though they don't have enough classical energy to overcome it entirely. This tunneling effect enables current flow even at low voltages. Due to tunneling, current flow in a tunnel diode can decrease as voltage increases in a specific voltage range. This negative resistance property is what makes tunnel diodes distinct.

How does current decrease with increasing voltage level?

At low voltages, the bands are close enough for efficient tunneling. As voltage increases, these things happen:

• Band misalignment: The applied voltage pushes the energy bands further apart, reducing the overlap between the valence and conduction bands. This means fewer electrons can find a suitable energy state on the other side to tunnel through.
• Increased energy: While some electrons can still tunnel with higher voltage, the overall tunnelling probability decreases.
• Negative resistance region: When the voltage increases beyond the optimal range, the decrease in tunnelling probability due to band misalignment outweighs the effect of increased energy for some electrons. This leads to a negative resistance region in the current versus voltage (I-V) characteristic curve of the tunnel diode. In this region, current actually decreases as voltage increases.

Comparison of tunnel diodes with regular diodes

Applications

Due to their fast-switching speed and negative resistance, tunnel diodes are often used in:

• High-frequency oscillators
• Amplifiers
• Detectors
• Fast switching circuits

Let’s explore some of these applications. In high-frequency oscillators, tunnel diodes can generate stable and fast electrical oscillations. A typical oscillator circuit relies on positive feedback. It takes a small signal, amplifies it, and feeds some of that amplified signal back to the input, creating a continuous oscillation. However, with a regular resistor (positive resistance), the amplified signal weakens as it passes through the resistor due to voltage drop. Tunnel diodes, in their negative resistance region, can act like an amplifier with a gain greater than 1. When a small AC signal is applied to a tunnel diode biased in its negative resistance region, the current increases slightly as the voltage of the signal increases, and vice versa. This can amplify the AC signal.

Tunnel diodes are used in detectors to convert high-frequency signals (like radio waves) into a usable baseband signal (carrying the information). It can act as a rectifier, similar to a regular diode. It allows current to flow more easily in one direction (forward bias) compared to the other (reverse bias). However, unlike a regular diode, a tunnel diode doesn't require any external biasing voltage for detection in many cases (zero-bias operation). When the high-frequency signal and the local oscillator signal are applied to the tunnel diode, the non-linearity in its current response creates a mixing effect. This mixing generates the desired beat frequency signal at the difference of the two input frequencies.

In fast switching circuits, the heavy doping and narrow depletion region in tunnel diodes allows electrons to tunnel through the barrier with minimal delay, leading to incredibly fast switching times in the nanosecond range (billionths of a second) or even picosecond range (trillionths of a second). In comparison, regular diodes have a larger depletion region and slower carrier diffusion processes, limiting their switching speed. Moreover, when a voltage step is applied to a circuit, there's usually a transition time for the current to reach its steady state. In circuits with positive resistance, this transition can be slow. However, the negative resistance of a tunnel diode can help speed up this transition.

Conclusion

A tunnel diode is a diode with negative resistance, which is its distinctive feature. Tunnel diodes are extensively doped with impurities, resulting in a small depletion region, unlike normal diodes. They are valuable because of their fast-switching speeds, a wide operating bandwidth, fast response times and zero-bias operation.