A diode is an electronic component that allows electric current to flow in only one direction and offers much greater resistance in the other direction. The most widely used diode type in modern circuit design is the semiconductor diode due to its usefulness and low cost.

Working Principle of Diodes

When a diode is connected in a battery-lamp circuit, the diode either allows or prevents current flow to the lamp, depending on the polarity of the applied voltage.

1. Forward Bias Diode: In the forward-bias state, the diode conducts current. The gap region formed at the P-N junction narrows when voltage is applied and the diode shows less resistance to current. In this case, the voltage drop across the diode is constant and is typically around 0.7 V for silicon diodes and 0.3 V for germen diodes.

2. Reverse Bias Diode: In the reverse-bias state, the diode prevents current flow. The gap region at the P-N junction becomes wider and makes it difficult for current to pass. In this case, the current through the diode is usually very small, which is called leakage current.

Foward and reverse bias diode

Diode Symbol and Current Direction

The diode symbol is usually represented by a triangle and a line at the tip of the triangle. The tip of the triangle indicates the conventional current direction (from anode to cathode). If we are using electron flow, the electron flow will be opposite to the arrow of the symbol, since the current direction will be the opposite.

The behaviour of diodes can be likened to the control valve used in hydraulic systems. The control valve allows flow in one direction only, while blocking flow in the other direction. This is similar to the behaviour of the diode in the reverse-bias state.

Diode Symbol

Forward Voltage and Current Relationship of Diodes

When forward voltage is applied, a very small voltage drop occurs across the diode. In silicon diodes, this drop is approximately 0.7 V and in germen diodes it is 0.3 V. Diodes operate with this constant voltage drop and this feature allows diodes to be used in rectifier applications.

The voltage and current relationship on the diode is described by the diode equation. This equation takes into account the current flowing through the diode, voltage drop, temperature and some physical constants. However, when analysing simple diode circuits, the forward voltage drop of diodes is usually assumed to be close to a constant value (0.7V for silicon, 0.3V for germen).

^{The Diode Equation}

Reverse Voltage (PIV) and Diode Collapse

Reverse-bias diodes can be damaged under high reverse voltage. If a diode exceeds a certain inverse voltage threshold, collapse occurs and the diode is damaged. This reverse voltage value of diodes is called PIV (Peak Inverse Voltage) and is usually specified by the manufacturer. Silicon diodes usually have PIV values above 50 V.

Diodes are critical components in modern electronic circuits. By passing current in the forward-bias state and blocking current in the reverse-bias state, diodes are widely used, especially in rectification, signal processing and protection circuits. Key characteristics of diodes include factors such as forward voltage drop, PIV withstand at reverse voltage, and leakage current.