Diode

SEMICONDUCTORS DIODES A device having a PN junction mounted in a container, suitable for conducting and dissipating the heat generated in operation, and having connecting leads: anode & cathode

Cathode

Anode

Symbol

P

N

Construction

Diodes • Diodes are suitable crystal (usually silicon) which have a ‘junction” of p- and n-type material. • When forward biased, a diode acts as a conductor and current flows. • When reverse biased, a diode acts as an insulator and no current flows.

Rectifiers • Early methods of changing a.c to d.c used thermionic tubes or metal form devices. • Silicon diodes are now widely used to rectify a.c to d.c.

Zener diodes • A Zener diode is a pn silicon junction diode that is reverse biased and with the supply voltage sufficient to produce the ‘avalanche’ or ‘breakdown’ effect. • A zener diode must be used in conjunction with a ‘current-limiting’ resistor to keep the current flow through the diode to a safe level. • A zener diode is used to produce a stable voltage from a supply which is fluctuating.

Zener diodes • A zener diode will have a negative temperature coefficient. • Zener diodes are produced with varying degrees of ‘doping’ to obtain a range of working voltages. • Zener diodes may be connected in series to obtain a desired value of voltage. • Sketches curves of voltage and current for a zener diode.

Zener diodes

Common

Zener

Curves for Current and Voltage in Zener Diode

Circuit Diagram using Zener Diode

Light-emitting diodes (LEDs) • Crystals manufactured from gallium phosphide and gallium arsenide have light-emitting properties when a small electric current is passed through them. • LED’s are essentially pn diodes operated in forward bias. • The colour emitted from such a diode depends upon the materials used in its manufacture, and that the usual colours are yellow, green and red.

Light-emitting diodes (LEDs) • The maximum permitted reverse bias voltages across a LED are low, so a silicon diode is usually connected in parallel with the LED to protect it from excessive reverse bias voltages. • A resistor is always used in series with a LED as a ‘current limiter’. • LED’s are used for numeric displays and as indicators in electrical supply systems

Physical structures

Terminal identifications

IDEAL DIOED When the diode is ON, it acts like a short circuit and current flow

When the diode is OFF, current is zero, so it acts like an open circuit

V/I CHARACTERISTICS

When diode is FB and applied voltage increase from zero, hardly any current flos through the device in beginning. As soon as VB is reach, current through the diode increase with increase of battery voltage. When the diode is RB, majority carriers are blocked and a small current flow through diode. As the reverse voltage is increase from zero, the reverse current quickly reach saturation value, Io or leakage current. When reverse voltage exceeds a certain value called zener voltage Vz, leakage current increase sharply and the curve indicating zero resistance at this point.

FORWARD When voltage is applied across a diode in BIASED such a way that the diode allows current, the diode is said to be forward-biased

Short circuit

D

D

I

V

R

V

R

REVERSE BIASED When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased Open circuit

D

D

I=0 V

R

V

R

METER CHECK OF A DIODE Multimeter without “Diode Check”

Forward biased

Reverse biased

Measure a diode's forward voltage (0.7 V for Si and 0.3 V for Ge)

Multimeter with “Diode Check”

DIODE RATINGS Maximum DC reverse voltage Maximum forward voltage Maximum (average) forward current Maximum total dissipation Maximum reverse current Storage temperature range Reverse recovery time Operating junction temperature

APPLICATIONS

The most popular application of the diode: rectification Rectification is the conversion of alternating current (AC) to direct current (DC) Rectification is the conversion of alternating current (AC) to direct current (DC)

Positive half cycle

Negative half cycle

Positive half cycle

Negative half cycle

Full-wave rectification of three-phase AC

ZENER DIODE Diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage known as "Zener knee voltage" or "Zener voltage"

Symbol

FORWARD & REVERSE BIASED When forward-biased, zener diodes behave much the same as standard diodes In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that zener voltage point. So long as the power dissipated by this reverse current does not exceed the diode's thermal limits, the diode will not be harmed

R

V

Dz

Please take note of the zener diode's orientation in the above circuit: the diode is reverse-biased, and intentionally so. If we had oriented the diode in the "normal" way, so as to be forward-biased, it just like a regular rectifying diode. If we want to exploit this diode's reverse breakdown properties, we must operate it in its reverse-bias mode.

LIGHT EMITTING DIODES, LED A forward-biased PN junction which emit visible light when energized. The color of the emitted light depends on the type of the semiconductor materials used: GaAs – infrared radiation GaP – red or green light GaAsP – red or yellow light

Application of LED a b c d e f g It possible to light any segment by forwarding the particular LED