What is a Diode?
The semiconductor is one of the fascinating concepts in the world of electronics compared to the passive components like the capacitors, resistors, and inductors. A diode is a type of semiconductor electronic device which is most widely used.
The semiconductor device, which acts as a one-way switch for current, is known as a diode. The device allows the easy flow of the current in one direction only; it puts a lot of restriction if the current tries to flow in the opposite direction.
The semiconductor diodes are also sometimes called rectifiers as the pulsating direct current (dc) is obtained by changing the alternating current (ac). The rating of the device is done based on its current and voltage capacity.
The polarity of the diode is determined by the anode (i.e., the positive lead) and the cathode (i.e., the negative lead). In most of the diodes, the current flow is allowed only when a positive voltage is applied to the anode. The configuration of a variety of diodes is displayed in the graphic below.
The current flow allowed by the diode is known as forward biased. When the diode does not permit the flow of the current, it is known as reverse biased, here the diode acts as an insulator.
Fun Fact: The arrow of the diode's symbol points against the direction of the electron flow, the reason being the symbol was conceived by the engineers, and their schematic diagram shows the current flowing from anode to cathode, i.e., from positive voltage to negative voltage source.
Diode Symbol
Below, the symbol of the diode is shown. The arrowhead indicates the direction of the conventional current, which is in the forward biased condition. The literal meaning is that the cathode is connected to the n side, and the anode is connected to the p side.
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V - I Characteristics of a Diode
The below circuit diagram shows the arrangement for the PN junction diode. The voltmeter is connected in parallel, and an ammeter is a connector in series to the diode, whereas the variable resistor controls the supply. The diode diagram is shown below:
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When the circuit is operational, if the diode is in forward biased condition, then at some point of a particular voltage, the potential barrier will get eliminated. Such voltage is known as the Cut-off voltage or Knee Voltage. The forward current will rise exponentially if the forward voltages go beyond the limit, the continuous rise in voltage of this will cause the device to get damaged due to overheating.
The following graph shows the state of diode conduction in forward and reversed biased conditions.
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In reverse bias, the minority current carriers produce reverse current when there is an increase in the reverse voltage. Due to a further increase in reverse voltage, the reverse current also increases, which will cause a sudden breakdown at a certain point, causing permanent damage to the junction.
What is the non-ideal diode equation?
I = I₀(\[e^{\frac{qv}{kT}}\] - 1)
I0 is directly proportional to recombination, and thus, inversely proportional to material quality.
Ideal Diodes
The below-shown diode equation provides you with the expression for current flowing through the diode as a function of voltage. The Ideal Diode Law is shown as:
I = I₀(\[e^{\frac{qv}{kT}}\] - 1)
Where:
I = through the diode the net current flowing;
I0 = the diode leakage current density in the absence of light, known as the dark saturation current
V = voltage applied across the terminals of the diode;
q = absolute value of electron charge;
k = Boltzmann's constant; and
T = absolute temperature (K).
One of the critical parameters which differentiate the one diode from another is the dark saturation current (I0). I0 measures the recombination of the device. A diode will have a more considerable I0 value if the recombination of the diode is large.
FAQs on Diode
1. What is a diode and what is its primary function in an electronic circuit?
A diode is a two-terminal electronic component that primarily acts as a one-way gate for electric current. Its fundamental function is to allow current to flow easily in one direction (known as forward bias) while severely restricting its flow in the opposite direction (reverse bias). This property of unidirectional conduction is crucial for many electronic applications as per the CBSE 2025-26 syllabus.
2. What is the standard circuit symbol for a P-N junction diode?
The standard circuit symbol for a P-N junction diode consists of an arrow pointing towards a vertical line. The arrow represents the P-type (anode) terminal and indicates the conventional direction of current flow. The vertical line represents the N-type (cathode) terminal. Current flows from the anode to the cathode when the diode is forward-biased.
3. How is a P-N junction formed and what is a depletion region?
A P-N junction is formed at the interface where a P-type semiconductor (with an excess of holes) is joined with an N-type semiconductor (with an excess of free electrons). Due to the concentration difference, electrons from the N-side diffuse to the P-side, and holes from the P-side diffuse to the N-side. This diffusion creates a thin layer near the junction that is depleted of free charge carriers, known as the depletion region or space charge region. This region contains only immobile ions and acts as a potential barrier.
4. How does a P-N junction diode work under forward and reverse bias conditions?
The working of a P-N junction diode depends on the external voltage applied:
- Forward Bias: When the positive terminal of a battery is connected to the P-type side and the negative terminal to the N-type side, the diode is in forward bias. The applied voltage opposes the potential barrier, reducing its width. This allows majority charge carriers to cross the junction easily, resulting in a significant forward current.
- Reverse Bias: When the negative terminal is connected to the P-type side and the positive terminal to the N-type side, the diode is in reverse bias. The applied voltage supports the potential barrier, increasing its width. This prevents the flow of majority carriers, and only a very small leakage current due to minority carriers flows through the diode.
5. What does the V-I characteristic curve of a typical silicon diode show?
The V-I (Voltage-Current) characteristic curve of a diode graphically represents its behaviour under different biasing conditions. For a typical silicon diode, it shows that in forward bias, the current remains negligible until the voltage crosses a threshold called the cut-in or knee voltage (around 0.7V for silicon), after which the current increases exponentially. In reverse bias, it shows a very small, constant leakage current until the reverse breakdown voltage is reached, at which point the current increases dramatically.
6. What are some important types of diodes and their specific uses?
Besides the standard P-N junction diode, several special-purpose diodes exist, each designed for a specific function:
- Zener Diode: Used as a voltage regulator by operating in its reverse breakdown region.
- Light Emitting Diode (LED): Converts electrical energy into light energy. Used for indicators, displays, and lighting.
- Photodiode: Converts light energy into electrical energy. Used in light sensors, smoke detectors, and optical communication.
- Varactor Diode: Acts as a voltage-controlled capacitor. Used in tuning circuits of radios and TVs.
7. How do diodes facilitate the process of rectification (converting AC to DC)?
Diodes are the core components in rectifier circuits that convert alternating current (AC) to direct current (DC). Since a diode allows current to pass in only one direction, it effectively "chops off" one half of the AC waveform.
- In a half-wave rectifier, a single diode allows only the positive (or negative) half-cycles of the AC input to pass through, blocking the other half.
- In a full-wave rectifier (using multiple diodes), both positive and negative half-cycles of the AC input are converted into a pulsating DC output, making the process more efficient.
8. What is a Zener diode and how is its function as a voltage regulator different from a standard diode?
A Zener diode is a specially doped silicon diode designed to operate reliably in the reverse breakdown region. Unlike a standard diode, which may be destroyed by reverse breakdown, a Zener diode can handle this current without damage. Its key feature is maintaining a nearly constant voltage across its terminals (the Zener voltage) over a wide range of reverse currents. This property makes it ideal for use as a voltage regulator, providing a stable voltage reference in circuits.
9. Why is a diode often compared to a one-way valve for electricity?
The analogy of a one-way valve (like a check valve in plumbing) is used to explain a diode's primary function of unidirectional current flow. A valve allows fluid to flow in one direction but closes to block flow in the reverse direction. Similarly, a diode offers a low-resistance path for current in the forward direction (valve open) and a very high-resistance path in the reverse direction (valve closed), effectively controlling the direction of electron movement.
10. What are some key real-world applications of diodes?
Diodes are fundamental components found in nearly all electronic devices. Their key applications include:
- Power Supplies: As rectifiers to convert household AC voltage to DC voltage for powering electronic gadgets.
- Voltage Regulation: Zener diodes are used to maintain a steady voltage.
- Signal Demodulation: Used in radios to extract audio signals from radio waves.
- Over-voltage Protection: To protect circuits from voltage spikes.
- Lighting and Displays: LEDs are used for everything from indicator lights to household lighting and TV screens.
