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Varactor Diode

A junction diode which acts as a variable capacitor under changing reverse bias is

known as a varactor diode. When a pn junction is formed, depletion layer is created in

the junction area. Since there are no charge carriers within the depletion zone, the zone

acts as an insulator.

The p-type material with holes (considered positive) as majority carriers and n-type

material with electrons (−ve charge) as majority carriers act as charged plates. Thus the

diode may be considered as a capacitor with n-region and p-region forming oppositely

charged plates and with depletion zone between them acting as a dielectric.

This is illustrated in Fig. (i). A varactor diode is specially constructed to have high

capacitance under reverse bias. Fig. (ii) shows the symbol of varactor diode. The

values of capacitance of varactor diodes are in the picofarad (10−12

F) range.

The normal operation, a varactor diode is always reverse biased. The capacitance of

varactor diode is found as :

CT = Total capacitance of the junction

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ε = Permittivity of the semiconductor material

A = Cross-sectional area of the junction

Wd = Width of the depletion layer

When reverse voltage across a varactor diode is increased, the width Wd of the

depletion layer increases. Therefore, the total junction capacitance CT of the junction

decreases. On the other hand, if the reverse voltage across the diode is lowered, the

width Wd of the depletion layer decreases. Consequently, the total junction

capacitance CT increases.

A forward biased varactor diode would serve no useful purpose

Fig. below shows the curve between reverse bias voltage VR across varactor diode

and total junction capacitance CT. Note that CT can be changed simply by changing

the voltage VR. For this reason, a varactor diode is sometimes called voltage-

controlled capacitor.

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.

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Tunnel Diodes (Esaki Diode)

Tunnel diode is the p-n junction device that exhibits negative resistance. That means

when the voltage is increased the current through it decreases.

Tunnel Diode principles Concept of Electron Tunneling

Regular p-n Diode Tunnel Diode

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For thick barrier, both Newtonian and Quantum mechanics say that the electrons

cannot cross the barrier. It can only pass the barrier if it has more energy than the

barrier height.

For thin barrier, Newtonian mechanics still says that the electrons cannot cross the

barrier. However, Quantum mechanics says that the electron wave nature will allow it

to tunnel through the barrier.

Newtonian Mechanics Quantum Mechanics

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Tunnel Diode Operation

Under Forward Bias

Step 1: At zero bias there is no current flow

Step 2: A small forward bias is applied. Potential barrier is still very high –no

noticeable injection and forward current through the junction.

Step 3: With a larger voltage the energy of the majority of electrons in the n-region is

equal to that of the empty states (holes) in the valence band of p-region; this will

produce maximum tunneling current

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Step 4: As the forward bias continues to increase, the number of electrons in the n side

that are directly opposite to the empty states in the valence band (in terms of their

energy) decrease. Therefore decrease in the tunneling current will start.

Step 5: As more forward voltage is applied, the tunneling current drops to zero. But the

regular diode forward current due to electron – hole injection increases due to lower

potential barrier.

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Step 6: With further voltage increase, the tunnel diode I-V characteristic is similar to

that of a regular p-n diode.

Under Reverse Bias

In this case the, electrons in the valence band of the p side tunnel directly towards the

empty states present in the conduction band of the n side creating large tunneling

current which increases with the application of reverse voltage. The TD reverse I-V is

similar to the Zener diode with nearly zero breakdown voltage.

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Tunnel Diode I-V

The total current I in a tunnel diode is given by

(

)

(

)

Iexcess : is an additional tunneling current related to parasitic tunneling via impurities.

This current usually determines the minimum (valley) current, Iv . Rv and Vex are the

empirical parameters; in high-quality diodes, Rv >> R0 . Vex = 1…..5 V

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Typically, m = 1….3; V0= 0.1….0.5 V R0 is the TD resistance in the ohmic region

The maximum negative differential resistance |NDR| can be found as

The peak voltage Vp

⁄

Photovoltaic cell

The reverse biased pn junction photodiode operates in the photoconductive mode. If the

illuminated diode is used without external bias, a measurable forward voltage appears

between the p and n regions. This is called photovoltaic effect.

The solar cells convert radiation from the sun directly into electrical energy. In practice,

the open-circuit voltage of the silicon photovoltaic cells is about 0.5– 0.6 V. Their

efficiency is about 15 %.

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Definition of laser

A laser is a device that generates light by a process called stimulated emission. The

acronym LASER stands for Light Amplification by Stimulated Emission of

Radiation

Semiconducting lasers are multilayer semiconductor devices that generates a

coherent beam of monochromatic light by laser action. A coherent beam resulted

which all of the photons are in phase.

Laser Operating Principles

Two things are required to operate a laser: (1) gain medium that can amplify the

electromagnetic radiation propagating inside it and provide the spontaneous emission

noise input and (2) a feedback mechanism that can confine the electromagnetic field

through the well-defined optical modes.

the gain medium for a semiconductor laser consists of a semiconductor material.

A simple, practical and most commonly used method employs current injection through

the use of a forward-biased p-n junction. Such semiconductor lasers are referred to as

injection lasers or laser diodes.

The excited atoms eventually return to their normal “ground” state and emit light in the

process. Light emission can occur through two fundamental processes known as

spontaneous emission and stimulated emission.

1. Spontaneous emission represents the case of an electron in the conduction band

recombining spontaneously with a hole (missing electron) in the valence band to

generate a photon.

2. Absorption stimulates the generation of an electron in the conduction band while

leaving a hole in the valence band.

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3. Stimulated emission is stimulating the recombination of an electron a hole and

simultaneously generating a new photon. This is the all-important positive gain

mechanism that is necessary for lasers to operate.

Producing intense laser beam or amplification of light through stimulated emission

requires

higher rate of stimulated emission than spontaneous emission and self-absorption,

which is only possible f or N2 > N1 even though E2 >E1 (opposite to the Boltzmann

statistics). It means that one will have to create the condition of population inversion

In a semiconductor laser, population inversion is accomplished by injecting

electrons into the material to fill the lower energy states of the conduction band.

Background Physics

Consider the „stimulated emission‟ as shown previously.

Stimulated emission is the basis of the laser action.

The two photons that have been produced can then generate more photons, and the 4

generated can generate 16 etc… etc… which could result in a cascade of intense

monochromatic radiation.

Population Inversion

Therefore we must have a mechanism where N2 > N1

This is called population inversion

Population inversion can be created by introducing a so call metastable centre where

electrons can piled up to achieve a situation where more N2 than N1

The process of attaining a population inversion is called pumping and the objective

is to obtain a non-thermal equilibrium.

It is not possible to achieve population inversion with a 2-state system.

If the radiation flux is made very large the probability of stimulated emission and

absorption can be made far exceed the rate of spontaneous emission.

But in 2-state system, the best we can get is N1 = N2.

To create population inversion, a 3-state system is required.

The system is pumped with radiation of energy E31 then atoms in state 3 relax to

state 2 non radiatively.

The electrons from E2 will now jump to E1 to give out radiation.

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Laser diode

Forward electrical bias across the laser diode causes holes and electrons to be

"injected" from opposite sides of the p-n junction into the depletion region.

When an electron and a hole are present in the same region, they may recombine or

"annihilate" with the result being spontaneous emission

The difference between the photon-emitting semiconductor laser and conventional

phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use

of a different type of semiconductor, one whose physical and atomic structure

confers the possibility for photon emission. These are the "direct bandgap"

semiconductors.

Laser Diode Principle

Consider a p-n junction

In order to design a laser diode, the p-n junction must be heavily doped.

In other word, the p and n materials must be degenerately doped

By degenerated doping, the Fermi level of the n-side will lies in the conduction

band whereas the Fermi level in the p-region will lie in the valance band.

Diode Laser Operation

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Population Inversion in Diode Laser

P-n junction must be degenerately doped.

Fermi level in valance band (p) and

conduction band (n).

No bias, built n potential; eVo barrier to

stop electron and holes movement

Forward bias, eV> Eg

Built in potential diminished to zero

Electrons and holes can diffuse to the

space charge layer

EFn-EfP = eV

eV > Eg

eV = forward bias voltage

Fwd Diode current pumping

injection pumping

There is therefore a population inversion

between energies near EC and near EV

around the junction.

This only achieved when degenerately

doped p-n junction is forward bias with

energy > Egap

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The Lasing Action

The population inversion region is a layer along the junction also call inversion

layer or active region

Now consider a photon with E = Eg

Obviously this photon cannot excite electrons from EV since there is NO electrons

there

However the photon can stimulate electron to fall down from CB to VB.

Therefore, the incoming photon stimulates emission than absorption

The active region is then said to have „optical gain‟ since the incoming photon has

the ability to cause emission rather than being absorbed.

Pumping Mechanism in Laser Diode

It is obvious that the population inversion between energies near EC and those near

EV occurs by injection of large charge carrier across the junction by forward

biasing the junction.

Therefore the pumping mechanism is forward diode current Injection pumping

Laser Diode Characteristics

Nanosecond & even picosecond response time (GHz BW)

Spectral width of the order of nm or less

High output power (tens of mW)

Narrow beam (good coupling to single mode fibers)

Laser diodes have three distinct radiation modes namely, longitudinal, lateral and

transverse modes.

In laser diodes, end mirrors provide strong optical feedback in longitudinal

direction, so by roughening the edges and cleaving the facets, the radiation can be

achieved in longitudinal direction rather than lateral direction.