Diode Biasing

Q1: What is biasing?
A1: Biasing is when you establish a potential difference across an element so that it operates in a specific way.

Q2: Okay, so I have a general sense of what biasing is. But how does it apply to a diode that makes use of a PN junction?
A2: Well, you can apply a voltage drop across the diode in 2 ways: in the same direction as the drop in internal junction voltage or in the opposite direction.

Q3: What happens with the first case?
A3: When the applied voltage drop is in the same direction as the drop in junction voltage, the diode is said to be reverse biased. Since the applied voltage drop reinforces the internal junction voltage, the insulating depletion layer widens. Hence, a reverse-biased PN junction diode conducts negligible current. However, once that voltage drop reaches a high enough magnitude, the electric field is strong enough to upset the covalent bonds at the depletion zone, generating electron-hole pairs. When this threshold voltage or breakdown voltage is exceeded, there is a massive surge in conduction. Exceeding breakdown voltage unintentionally could be catastrophic if it causes the diode to melt. On the other hand, special diodes called Zener diodes are designed to make use of this breakdown voltage.

Q4: What about the other case?
A4: When the applied voltage drop is in the opposite direction as the drop in junction voltage, the diode is said to be forward biased. Negligible current flows through the diode, until the externally applied voltage overcomes the internal junction voltage. This threshold value is called knee voltage. Once applied voltage exceeds voltage, the diode is able to conduct much more current.