Practical Electronics/Diodes

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[edit] Introduction

A diode is a device made by joining n-type and p-type semiconductor material to form a "PN junction". This allows current to flow from the p-type to the n-type material, but not the other way around. The mechanism for this is explained in the Semiconductors Wikibook. A diode has two terminals: the "anode", connected to the p-type semiconductor and "cathode", which is connected to the n-type. A diode necessarily has polarity – it must be used the correct way around for its purpose, or it will not behave as expected.

A PN junction and the equivalent diode.

[edit] I-V characteristics

When a diode is connected with the anode at a positive voltage relative to the cathode, it is said to be forward biased. Forward curent flow is exponential with voltage, and starts to become significant around a voltage known as the cut-in or turn-on voltage, v_on. This is an arbitrarily defined point, above which the diode is considered to be conducting. The definition of turn-on voltage depends very much on type of diode and the application. For a normal silicon signal diode at rated currents, it is generally 0.6–0.7V, for LEDs it can be from around 1V for a red LED to 4V for a blue one.

A real diode actually conducts very slightly in the reverse direction, as well as strongly in the forward direction. The current that flows when a small reverse bias is applied is called the "diode leakage current", which is on the order of 1 nanoampère for every volt of reverse bias, bus is also non-linear.

When the reverse bias exceeds a level known as the breakdown voltage, v_br, the diode begins to conduct very well in reverse as well, due to semiconductor effects. In some types of diode, such as Zener diodes, this effect is exploited. For a "normal" diode, the breakdown voltage is likely to be around −20V.

A diode has a nonlinear I-V characteristic, which is not shown to scale here. The turn-on voltage, v_on is the arbitrarily defined voltage above which the diode is deemed to be in a conductive state.

Because of a diode's strongly non-linear I-V behaviour, one must be careful not to overload them. For example, when a silicon signal diode has 0.7V of forward bias across it, it may conduct 1mA of forward current. If, however, 1V of forward bias is applied, 100mA may flow. For every additional volt, the current flow becomes exponentially larger, and due to the voltage drop across the diode, exponentially more heat is dissipated inside the diode. This can easily melt the silicon and destroy the diode, even at modest voltages.

Rectification diodes are designed to carry large currents and voltages and are used to convert AC power supplies to DC. Signal diodes cannot be used as rectification diodes, and rectification diodes should not generally be used as signal diodes, due to their poor small-signal performance.

You should always read the datasheet of any device you are unfamiliar with to find the rated maximums and standard operating conditions. As a rough guideline for silicon signal diodes, 150mA is the maximum current and 70V is the maximum reverse voltage.

[edit] Types of Diodes

[edit] P-N Diode

P-N Diode is made from joining a negative-typed semiconductor with a positive-typed semiconductor .

[edit] DC Voltage

When connected in series with voltage source . Output voltage will be equal to voltage souce minus diode's voltage .

V --|>|-- V_o = V - 0.7|0.3

When connected in parallel with voltage source . Output voltage will be limited at diode's voltage therefore diode can be used as voltage limiter

V --|>|-- V_o = 0.7|0.3

[edit] AC Voltage

When diode conducts current, it allows only positive or negative cycle of current to flow depends on how diode is connected with AC voltage

V (AC) --|>|-- only positive cycle current can flow in the diode . Negative current is at ground potential or zero

V (AC) --|<|-- only negative cycle current can flow in the diode . Positive current is at ground potential or zero

In this manner, diode acts as a Voltage Rectifier

V --|>|-- only positive current can flow in the diode with the value clamped at diode's voltage. Negative current is at ground potential or zero

V --|<|-- only negative current can flow in the diode with the value clamped at diode's voltage. Positive current is at ground potential or zero

In this manner, diode acts as a Voltage Clampping

Diode needs to be biased with the right voltage in order to conduct or non conduct current ie to turn diode on or turn diode off. This makes diode act like a Switch

To AC voltage or signal, diode acts like a rectifier so only half wave cycle is allowed to pass this introduce harmonics in the circuit since the output wave form does not look the same as the input wave form . For this reaon, diode and a tuned LC can be used for tuning harmonic frequency

[edit] Light Emitting Diode - LED

The flat side is cathode!

Light emitting diodes emit light when a small current passes through them in the forward direction. They are manufactured in different shapes and colours:

Green, yellow, red, blue and white are the most common colors, but there are also multi-coloured LEDs (with 4 wires - red, green and blue component and cathode).

Every LED has a maximal current, which should not be exceeded. Therefore we must always put a resistor before the diode. A common LED uses 20 mA current. When using a 5 V source, we can compute the resistance R this way:

In this case, we would use 160 Ω.

Voltage drop on a LED is relatively high, between 1.5 and 2.5 V (generally this voltage increases going from red to blue).

The LEDs cannot be used for rectifying.

[edit] Variable Capacitor Diode

Diode made to conduct in the backward region . When Variable Capacitor Diode conducts current, it will behave like a capacitor of capacitance C in the range of pF at an apllied voltage V = - v

For this reson, Variable Capacitor Diode can be used to build a Voltage Controlled Oscillator (VCR), a device that will oscillate at precise voltage

[edit] Zener diodes

Diode made to conduct in the backward region . When Zener diode conducts current, the voltage across the diode is always at a constant value approximately -17 v regardless of the value of the applied voltage . For this reason, Zener diode is often used as a Voltage Stablizer

[edit] Avalanche Diode

[edit] Schottky diodes

The Shockley ideal diode equation or the diode law (named after transistor co-inventor William Bradford Shockley, not to be confused with tetrode inventor Walter H. Schottky) is the I–V characteristic of an ideal diode in either forward or reverse bias (or no bias). The equation is:

where

I is the diode current,

I_S is the reverse bias saturation current,

V_D is the voltage across the diode,

V_T is the thermal voltage,

and n is the emission coefficient, also known as the ideality factor. The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted).

The thermal voltage V_T is approximately 25.85 mV at 300 K, a temperature close to “room temperature” commonly used in device simulation software. At any temperature it is a known constant defined by:

where

q is the magnitude of charge on an electron (the elementary charge),

k is Boltzmann’s constant,

T is the absolute temperature of the p-n junction in kelvins

The Shockley ideal diode equation or the diode law is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn’t account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn’t describe the “leveling off” of the I–V curve at high forward bias due to internal resistance.

Under reverse bias voltages (see Figure 5) the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of -I_S. The reverse breakdown region is not modeled by the Shockley diode equation.

For even rather small forward bias voltages (see Figure 5) the exponential is very large because the thermal voltage is very small, so the subtracted ‘1’ in the diode equation is negligible and the forward diode current is often approximated as

The use of the diode equation in circuit problems is illustrated in the article on diode modeling.

[edit] Additional Reading

• Diode testing
• The Wikipedia article on diodes has further detail on the history, theory, and types of diodes.