Electric Circuits and Electron Devices (Unit-V)

Electric Circuits & Electron DevicesElectric Circuits & Electron Devices

Unit VUnit V Special Semiconductor DevicesSpecial Semiconductor Devices

Prepared by

N.SHANMUGASUNDARAM,

Asst. Professor, ECE Department

Mahendra Engineering College

FIGURE - Zener diode symbol.

1. ZENER DIODE

Zener Diode:- is a silicon pn junction device that differ from rectifier diodes because

it is designed for operation in the reverse- breakdown region.

- if Zener diode is forward-biased, it operates the same as a rectifier diode.

Function:- to provide a stable reference voltage for use in power supplies, voltmeter & other instruments, voltage regulators.

FIGURE - General diode V-I characteristic.

Zener breakdown:- occurs in a Zener diode at low reverse voltages. - Zener diode is heavily doped to reduce the breakdown voltage. - This causes a very thin depletion region.

FIGURE - Tunnel diode symbols.

2. TUNNEL DIODE

A tunnel diode or Esaki diode is a

type of semiconductor diode which is

capable of very fast operation, well

into the microwave frequency region,

by using quantum mechanical effects.

Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the pn junction.

As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage.

As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier.

Thus, the most important operating region for a tunnel diode is the negative resistance region.

FIGURE - Tunnel diode characteristic curve.

FIGURE - Parallel resonant circuit.

FIGURE - Basic tunnel diode oscillator.

3. VARACTOR DIODE

The reverse-biased varactor diode acts as a variable capacitor.

FIGURE - The reverse-biased varactor diode acts as a variable capacitor.

FIGURE - Varactor diode capacitance varies with reverse voltage.

FIGURE 6 - A Resonant band-pass filter using a varactor diode for adjusting the resonant frequency over a specified range.

FIGURE - Symbol for an LED. When forward-biased, it emits light.

4. LED

FIGURE - Electroluminescence in a forward-biased LED.

FIGURE - Basic operation of an LED.

FIGURE - Examples of typical spectral output curves for LEDs.

FIGURE - Typical LEDs.

FIGURE - The 7-segment LED display.

5. LASER DIODE

A Laser diode, also known as an injection

laser or diode laser, is a semiconductor

device that produces coherent radiation (in

which the waves are all at the same frequency

and phase) in the visible or infrared (IR)

spectrum when current passes through it.

Laser diodes are used in

•optical fiber systems,

•compact disc (CD) players,

•laser printers,

•remote-control devices,

•and intrusion detection systems.

Figure: Structure of DH LASER Diode

FIGURE - Basic laser diode construction and operation.

FIGURE - Photodiode.

6. PHOTODIODE

FIGURE - Typical photodiode characteristics.

FIGURE - Operation of a photodiode.

FIGURE - PIN diode.

7. PIN DIODE

A PiN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor regions.

The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the normal function of a diode),

but it makes the PIN diode suitable for

•attenuators, •fast switches, •photo detectors, and •high voltage power electronics applications.

FIGURE - PIN diode characteristics.

FIGURE - Diode symbols.

8. SILICON CONTROLLED RECTIFIER

Two Transistor model of SCR

The switching action of gate takes place only when

(i)     SCR is forward biased i.e. anode is positive with respect to cathode.

(ii)    Suitable positive voltage is applied between the gate and the cathode.

Once the SCR has been switched on, it has no control on the amount of current flowing through it.

The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing the circuit impedance.

A minimum forward current must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to off-state or blocking state.

Note : The gate can only trigger or switch-on the SCR, it cannot switch off.

Firing Angle

The angle (in the input AC) at which the gate is triggered is known as 'firing angle'.

Holding Current

It is the minimum anode current (with gate being open) required to keep the SCR in ON condition.

Break Over voltage

It is the minimum forward voltage with gate being open, at which an SCR starts conducting heavily (i.e., the SCR is turned ON) .

Terminology

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction.

The UJT has three terminals: an emitter (E) and two bases (B1 and B2).

The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends.

The emitter is of p-type and it is heavily doped.

9. UNIPOLAR JUNCTION TRANSISTOR

Intrinsic Standoff Ratio

Unijunction transistor: (a) emitter characteristic curve, (b) model for VP .

Application of UJT – RELAXATION OSCILLATOR

REVIEW:

• A unijunction transistor consists of two bases (B1, B2) attached to a resistive bar of silicon, and an emitter in the center.

• The E-B1 junction has negative resistance properties; it can switch between high and low resistance.

• The intrinsic standoff ratio is η= RB1 /(RB1 + RB2), for a unijunction transistor. The trigger voltage is determined by η.

• Unijunction transistors and programmable unijunction transistors are applied to oscillators, timing circuits, and Thyristor triggering.