Unipolar diodes are a type of diode that conduct current in one direction only. They are commonly used in various electronic circuits and devices. Among the given options, the Schottky diode is a type of unipolar diode.

Schottky diode:
- A Schottky diode is a semiconductor device that is formed by the junction of a metal and a semiconductor material, typically silicon or gallium arsenide.
- It is also known as a hot carrier diode or a Schottky barrier diode.
- The metal-semiconductor junction in a Schottky diode is responsible for its unique characteristics.
- It has a low forward voltage drop, typically around 0.3 to 0.5 volts, making it suitable for applications where low power loss is desired.
- The low forward voltage drop is due to the absence of a P-N junction in the diode structure.
- It has a fast switching speed and low reverse recovery time, which makes it suitable for high-frequency applications.
- Schottky diodes are commonly used in power supplies, voltage clamping circuits, and RF (radio frequency) applications.

Zener diode:
- A Zener diode is a type of diode that operates in the reverse breakdown region of its voltage-current characteristics.
- It is used for voltage regulation and protection in various electronic circuits.
- Zener diodes are designed to have a specific breakdown voltage, known as the Zener voltage.
- When the voltage across a Zener diode exceeds its breakdown voltage, it starts conducting in the reverse direction, allowing current to flow through it.
- Zener diodes are commonly used in voltage reference circuits, voltage regulators, and surge protectors.

Step recovery diode:
- A step recovery diode (SRD) is a special type of diode that exhibits a unique switching characteristic.
- It is primarily used in high-frequency applications, such as pulse generators and frequency multipliers.
- The SRD has a very fast switching speed and can generate short duration pulses with fast rise and fall times.
- It is called a step recovery diode because it recovers quickly from its conducting state to its non-conducting state, resulting in a step-like voltage waveform.
- SRDs are commonly used in applications where precise timing and fast switching are required.

Therefore, among the given options, the Schottky diode is the unipolar diode.