Reverse bias is a condition in which the positive terminal of a power supply is connected to the cathode of a diode, and the negative terminal is connected to the anode. In this condition, the diode is not conducting current, and a reverse voltage is applied across it.

The diodes that work under reverse bias condition are:

1. Zener Diode:
- A Zener diode is specifically designed to operate under reverse bias condition.
- It is primarily used for voltage regulation and voltage reference applications.
- When the reverse voltage across a Zener diode exceeds its breakdown voltage (also called the Zener voltage), it starts conducting in the reverse direction and maintains a nearly constant voltage drop across it.
- The breakdown voltage can be adjusted by controlling the doping level during the manufacturing process.

2. Varactor Diode:
- A varactor diode, also known as a voltage variable capacitor or tuning diode, works under reverse bias condition.
- It is primarily used in electronic circuits for frequency tuning and voltage-controlled oscillators (VCOs).
- When a reverse bias voltage is applied to a varactor diode, the width of the depletion region changes, resulting in a change in capacitance.
- By varying the reverse bias voltage, the effective capacitance of the varactor diode can be controlled, allowing it to be used for frequency tuning.

3. Photodiode:
- A photodiode is a semiconductor device that operates under reverse bias condition.
- It is designed to convert light energy into electrical current.
- When photons (light particles) strike the photodiode, it generates electron-hole pairs, which then contribute to the current flow.
- The reverse bias voltage applied across the photodiode helps in reducing the capacitance and increases the response time.

4. LED (Light-Emitting Diode):
- An LED is a semiconductor device that emits light when forward biased.
- However, when a reverse bias voltage is applied to an LED, it does not emit light.
- The reverse bias condition is used to protect the LED from excessive current flow, which could damage the device.

In conclusion, the diodes that work under reverse bias condition are Zener diode, varactor diode, photodiode, and LED.