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TOPIC 4: MICROWAVE SOURCES MICROWAVE DEVICES (EP603)

GENERATION OF MICROWAVE SIGNAL

MICROWAVES SOURCES:

a) Microwave Tubes – klystron, reflex klystron, magnetron and TWT.

b) Diode semiconductor – Tunnel, Gunn, Impatt, Varactor diodes, PIN, LSA, Schottky barrier diode.

MICROWAVE TUBES

Used for high power/high frequency combination.

Tubes generate and amplify high levels of microwave power more cheaply than solid

state devices.

Conventional tubes can be modified for low capacitance but specialized microwave

tubes are also used.

CROSSED-FIELD AND LINEAR-BEAM TUBES Klystrons and Traveling-Wave tubes are examples of linear-beam tubes

These have a focused electron beam (as in a CRT)

Magnetron is one of a number of crossed-field tubes

Magnetic and electric fields are at right angles

KLYSTRON

• Used in high-power amplifiers

• Electron beam moves down tube past several cavities.

• Input cavity is the buncher, output cavity is the catcher.

• Buncher modulates the velocity of the electron beam

PREPARED BY : ROHANA BT. IBRAHIMPOLIMASDECEMBER 2012 SESSION

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KLYSTRON CROSS SECTION

VELOCITY MODULATION

Electric field from microwaves at buncher alternately speeds and slows electron

beam

This causes electrons to bunch up

Electron bunches at catcher induce microwaves with more energy

The cavities form a slow-wave structure


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REFLEX KLYSTRON

Also known as a 'Sutton' klystron after its inventor, Robert Sutton.

The electron beam passes through a single resonant cavity. The electrons are fired

into one end of the tube by an electron gun. After passing through the resonant cavity

they are reflected by a negatively charged reflector electrode for another pass through

the cavity, where they are then collected. The electron beam is velocity modulated

when it first passes through the cavity. The formation of electron bunches takes place

in the drift space between the reflector and the cavity. The voltage on the reflector

must be adjusted so that the bunching is at a maximum as the electron beam re-enters

the resonant cavity, thus ensuring a maximum of energy is transferred from the

electron beam to the RF oscillations in the cavity. The voltage should always be

switched on before providing the input to the reflex klystron as the whole function of the

reflex klystron would be destroyed if the supply is provided after the input. The reflector

voltage may be varied slightly from the optimum value, which results in some loss of

output power, but also in a variation in frequency. This effect is used to good

advantage for automatic frequency control in receivers, and in frequency modulation

for transmitters. The level of modulation applied for transmission is small enough that

the power output essentially remains constant. At regions far from the optimum

voltage, no oscillations are obtained at all. This tube is called a reflex klystron because

it repels the input supply or performs the opposite function of a klystron.

There are often several regions of reflector voltage where the reflex klystron will

oscillate; these are referred to as modes. The electronic tuning range of the reflex

klystron is usually referred to as the variation in frequency between half power points—

the points in the oscillating mode where the power output is half the maximum output in

the mode. The frequency of oscillation is dependent on the reflector voltage, and

varying this provides a crude method of frequency modulating the oscillation

frequency, albeit with accompanying amplitude modulation as well.


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http://en.wikipedia.org/wiki/Frequency_modulation

http://en.wikipedia.org/wiki/Volt

http://en.wikipedia.org/wiki/Radio_frequency


http://en.wikipedia.org/wiki/Electron_gun


REFLEX KLYSTRON

TRAVELING-WAVE TUBE (TWT)

• Uses a helix as a slow-wave structure

• Microwaves input at cathode end of helix, output at anode end

• Energy is transferred from electron beam to microwaves


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http://en.wikipedia.org/wiki/File:Reflex.sch.enp.svg


TRAVELING-WAVE TUBE

The helix acts as a delay line, in which the RF signal travels at near the same speed

along the tube as the electron beam. The electromagnetic field due to the RF signal in

the helix interacts with the electron beam, causing bunching of the electrons (an effect

called velocity modulation), and the electromagnetic field due to the beam current then

induces more current back into the helix (i.e. the current builds up and thus is amplified

as it passes down).[2][3]

A second directional coupler, positioned near the collector, receives an amplified

version of the input signal from the far end of the helix. An attenuator placed on the

helix, usually between the input and output helices, prevents reflected wave from

traveling back to the cathode.

Periodic permanent magnet focusing

MAGNETRONPREPARED BY : ROHANA BT. IBRAHIMPOLIMASDECEMBER 2012 SESSION

5


The magnetron is a high-powered vacuum tube that generates microwaves using the

interaction of a stream of electrons with a magnetic field.

• High-power oscillator

• Common in radar and microwave ovens

• Cathode in center, anode around outside

• Strong dc magnetic field around tube causes electrons from cathode to spiral as

they move toward anode

• Current of electrons generates microwaves in cavities around outside

CAVITY MAGNETRON


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To sustain oscillations in a resonant circuit, it is necessary to continuously input energy

in the correct phase. If the instantaneous RF field, due to steady state oscillations in

the resonator, is in the direction shown, and, an electron with velocity was to travel

through the RF field such that the RF field retarded the electron velocity by an amount,

the decrease in electron energy will be exactly offset by an increase in the RF field

strength.

OPERATION In a magnetron, the source of electrons is a heated cathode located on the axis of an

anode structure containing a number of microwave resonators. Electrons leave the

cathode and are accelerated toward the anode, due to the dc field established by the

voltage source E.

The presence of a strong magnetic field B in the region between cathode and anode

produces a force on each electron which is mutually perpendicular to the dc field and

the electron velocity vectors, thereby causing the electrons to spiral away from the

cathode in paths of varying curvature, depending upon the initial electron velocity at

the time it leaves the cathode.


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The electron path under the influence of different strength of the magnetic field

As this cloud of electrons approaches the anode, it falls under the influence of the RF

fields at the vane tips, and electrons will either be retarded in velocity, if they happen to

face an opposing RF field, or accelerated if they are in the vicinity of an aiding RF field.

Since the force on an electron due to the magnetic field B is proportional to the

electron velocity through the field, the retarded velocity electrons will experience less

"curling force" and will therefore drift toward the anode, while the accelerated velocity

electrons will curl back away from the anode.

The result is an automatic collection of electron "spokes" as the cloud nears the anode

with each spoke located at a resonator having an opposing RF field. On the next half

cycle of RF oscillation, the RF field pattern will have reversed polarity and the spoke

pattern will rotate to maintain its presence in an opposing field. [7]


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The high-frequency electrical field

The "automatic" synchronism between the electrons spoke pattern and the RF field

polarity in a crossed field device allows a magnetron to maintain relatively stable

operation over a wide range of applied input parameters. For example, a magnetron

designed for an output power of 200 kW peak will operate quite well at 100 kW peak

output by simply reducing the modulator drive level.

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1

kilowatt input will generally create about 700 watt of microwave power, an efficiency of

around 65%. (The high-voltage and the properties of the cathode determine the power

of a magnetron.) Large S-band magnetrons can produce up to 2.5 megawatts peak

power with an average power of 3.75 kW. Large magnetrons can be water cooled. The

magnetron remains in widespread use in roles which require high power, but where

precise frequency control is unimportant. [4]

SLOW-WAVE STRUCTURE

• Magnetron has cavities all around the outside

• Wave circulates from one cavity to the next around the outside

• Each cavity represents one-half period

• Wave moves around tube at a velocity much less than that of light


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• Wave velocity approximately equals electron velocity

THE COMPARISON BETWEEN TWT AND MAGNETRON

a) Cost, Complexity, Maintenance and Power Consumption

Magnetron TWT

Advantages:

1. low system price(approximately %30-35 cheaper than Klystron, and %10 than TWT-pulse compressed system),

2. Magnetron market is more competitive due to a higher number of manufacturers (especially low-cost),

3. Low price of the tube itself, available from several suppliers(3 times cheaper than Klystron),

4. Low technical complexity, 5. Small size, small dimensions and

weight,6. Lower operating voltages, generally no

oil-isolated housings are needed, which simplifies maintenance significantly,

7. Excellent efficiency in terms of input/output power ratio,

8. Less electrical power consumption (Magnetrons operate on single phase power drawing approximately 12-14 amperes during normal operation),

9. Less heat dissipation (lower costs for air conditioning),

10. Long expected lifetime of the tube,11. Lower maintenance costs (lower costs

for spare parts and lower mean time to repair),

12. Better for higher powers like 1000kW (cooling easier than Klystron)

Advantages:

1. Low peak power(~7-8kW) with pulse compression technique

2. Higher duty cycle;3. Long pulse width possible (~ 50 us);4. Smaller in dimensions and weight;5. Low power consumption6. Low Initial Price7. Some methods have been developed

for problems like “Range Side-lobe” and “Blind Range”, but more research may be expected for operational and common usage of PC TWT Radars from leading manufacturers. There is a probability that full solid state transmitters can be used instead of TWTs in the.

8. TWT may operate at lower cathode (beam) voltage, which simplifies the construction of the modulator.

Disadvantages:

1. No significant disadvantages compared with klystron and TWT

Disadvantages:

1. Higher initial and operational costs than magnetron systems

2. Supply is limited(only one manufacturer)


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3. Still not commonly used in operational networks.

4. Operating costs still not clear (TWT price and life time)

5. Higher training costs

b) SensitivityMagnetron TWT

Advantages

1. High peak and average power,2. High sensitivity(very close to Klystron)3. Higher peak power with new

applications (500kW) brings better target illumination and better reflectivity detection.

4. Suitable for long range measurements and country-wide coverage if clutter has a negligible impact

Advantages:

1. Low power TWT tubes are used with pulse compression technique. With this technique sensitivity is increased up to 15 dB. This would mean that a PC radar with peak power of 8 KW would have the same performance in regard to detect ability as a conventional 250 KW radar

2. Better range resolution can be achieved(up to 50m)

Disadvantages

1. No long-term experience available from testing in the field that new 500kW magnetrons

Disadvantages:

1. No significant disadvantages compared with magnetron and klystron

c) Coherency

Magnetron TWTAdvantages:

1. Although there is frequency drift but it is still able to measure velocity to 3.28 feet accuracy with the magnetron transmitter.

2. Second trip echo identification and recovery can be done in magnetron systems by producing a coded train of pulses and other methods

3. Some of the magnetron disadvantages may also be compensated (or at least mitigated) by modern technologies.

Advantages:1. TWTs work as an amplifier like

Klystron. They are coherent transmitter types also.

2. Less susceptible to multi-trip echoes


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Today a state-of-the-art digital receiver allows phase-locking with highest accuracy resulting in a good clutter suppression value for magnetron systems. A modern solid-state modulator employing sophisticated current stabilization schemes together with modern coaxial magnetron further increases the transmitter stability.

4. Instability source, are minimized. Moreover long-term frequency drift is reduced and can be compensated by means of automatic receiver frequency control

Disadvantages:

1. lower clutter suppression capability,2. inherent frequency drift and frequency

modulation,3. frequency can be adjusted

mechanically only,4. insufficient stability for generating very

long pulses,5. 40 dB Improvement Factor Limitation6. Doppler radar images are not so

widely used as the conventional reflectivity images. Nonetheless, the Doppler data is important for all radar outputs because the clutter suppression, which is used in all images.[5][6]

Disadvantages:

1. Higher initial and operational costs than magnetron systems

d) Bandwidth & Frequency Occupation


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1. Larger frequency range to work with in a magnetron transmitter, where you klystron is usually limited to ~50 MHz adjustment only. (~ 5.4 – 5.8 GHz sample for C-Band)

Advantages:

1. Higher resistance to interferences and spurious emissions by other external RF sources

2. A wide bandwidth is necessary in applications where good range-resolution is required or where it is desired to avoid deliberate jamming or mutual interference with nearby radars

Disadvantages:

1. Larger occupied RF bandwidth (higher spurious power level), thus potential problems with frequency allocation and interferences with other systems.

Disadvantages:

1. Occupied Bandwidth. PC radars require a larger occupied bandwidth(here TWTs discussed with pulse compression technique)

2. Range side-lobes and blind range

e) Suitability for New Technologies (Future Scope)


1. Supports dual polarization

Advantages:

1. Multi-trip echo recovery (less false echoes and ambiguities),

2. Frequency agility (more independent samples, higher data update rate),

3. Pulse compression (lower peak power, higher sensitivity),

4. Pulse shaping (optimized spectrum, lower bandwidth),

5. Supports dual polarization

Disadvantages:

1. Doesn’t support techniques need coherency like pulse compression.

Disadvantages:

TWT's cannot be calibrated conventionally by using a test signal generator providing a test signal of fixed, known power and frequency. The distortion of the phase information of received echo signals may result in the fact that fast moving targets are assigned to the wrong range gate and therewith to a wrong geographical


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location. This effect is named the “Range-Doppler-Coupling”. [3][6]

Conclusion

Many millions of cavity magnetrons have been employed for radar the vast majority

has been for microwave ovens. The use in radar itself has dwindled to some extent, as

more accurate signals have generally been needed and developers have moved to

klystron and traveling-wave tube systems for these needs. Unfortunately the problem

still remains that TWT's are very difficult to acquire. Nevertheless, their high power

output, high gain, and ease of operation make them the ideal way to run power at very

high frequencies.

MICROWAVE SOLID-STATE DEVICES (SEMICONDUCTOR DIODE)

Quantum Mechanic Tunneling – Tunnel diode


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Transferred Electron Devices – Gunn, LSA, InP and CdTe

Avalanche Transit Time – IMPATT, Read, Baritt & TRAPATT

Parametric Devices – Varactor diode

Step Recovery Diode – PIN,

Schottky Barrier Diode.

Designed to minimize capacitances and transit time.

NPN bipolar and N channel FETs preferred because free electrons move

faster than holes

Gallium Arsenide has greater electron mobility than silicon.

TUNNEL DIODE (ESAKI DIODE)

Symbol

Heavy doping of the semiconductor material creates a very thin potential barrier in the

depletion zone which leads to electron tunneling through the barrier.

Note the negative resistance zone between Vp and Vv.


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Equivalent Circuit Characteristic Curve

http://commons.wikimedia.org/w/index.php?title=File:Tunnel_diode_symbol.svg&page=1


The tunnel diode uses a heavily doped abrupt pn junction resulting in an extremely

narrow junction that allows electrons to tunnel through the potential barrier at near-zero

applied voltage. This results in a dip in the current-voltage ( I-V) characteristic, which

produces NDR (negative differential resistance). Because this is a majority-carrier

effect, the tunnel diode is very fast, permitting response in the millimeter-wave region.

Tunnel diodes produce relatively low power

TRANSFERRED ELECTRON DEVICES (TEDs)

TEDs are made of compound semiconductors such as GaAs.

They exhibit periodic fluctuations of current due to negative resistance effects when a

threshold voltage (about 3.4 V) is exceeded.

The negative resistance effect is due to electrons being swept from a lower valley

(more mobile) region to an upper valley (less mobile) region in the conduction band.

GUNN DIODE

• Slab of N-type GaAs (gallium arsenide)

• Sometimes called Gunn diode but has no junctions

• Has a negative-resistance region where drift velocity decreases with increased

voltage

• This causes a concentration of free electrons called a domain


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GUNN DIODE (CROSS SECTION)

N-, the active region, is sandwiched between two heavily doped N+ regions. Electrons

from the cathode (K) drifts to the anode (A) in bunched formation called domains.

Note that there is no p-n junction.

The diode behaves as an amplifier due to negative resistance effect.

A Gunn diode is typically an n-type compound semiconductor, such as GaAs or InP,

which has a conduction band structure that supports negative differential mobility.

Although this device is referred to as a Gunn diode, after its inventor, the device does

not contain a pn junction and can be viewed as a resistor below the threshold electric

field (Ethres). For applied voltages that produce electric fields below Ethres, the electron

velocity increases as the electric field increases according to Ohm's law. For applied

voltages that produce electric fields above Ethres, conduction band electrons transfer

from a region of high mobility to low mobility, hence the general name "transferred

electron device." Beyond Ethres, the velocity suddenly slows down due to the significant


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electron transfer to a lower mobility band producing NDR. For GaAs, Ethres is about 3

kV/cm. The Gunn effect can be used up to about 80 GHz for GaAs and 160 GHz for

InP. Two modes of operation are common: nonresonant bulk (transit-time) and

resonant limited space-charge accumulation (LSA). See also Electric field.

DRIFT VELOCITY IN GALIUM ARSENIDE

TRANSIT-TIME MODE

• Domains move through the GaAs till they reach the positive terminal

• When domain reaches positive terminal it disappears and a new domain forms

• Pulse of current flows when domain disappears

• Period of pulses = transit time in device

GUNN OSCILLATOR FREQUENCY

• T=d/v

T = period of oscillationd = thickness of devicev = drift velocity, about 1 105 m/s


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• f = 1/T

AVALANCHE TRANSIT-TIME DEVICES

If the reverse-bias potential exceeds a certain threshold, the diode breaks down.

Energetic carriers collide with bound electrons to create more hole-electron pairs.

This multiplies to cause a rapid increase in reverse current.

The onset of avalanche current and its drift across the diode is out of phase with the

applied voltage thus producing a negative resistance phenomenon.

IMPATT DIODE

Operate by a combination of carrier injection and transit time effects. There are several

versions of IMPATT diodes, including simple reverse-biased pndiodes, complicated

reverse-biased multidoped pn layered diodes, and reverse-biased PIN diodes. The

IMPATT must be connected to a resonant circuit. At bias turn-on, noise excites the

tuned circuit into a natural oscillation frequency. This voltage adds algebraically across

the diode's reverse-bias voltage. Near the peak positive half-cycle, the diode

experiences impact avalanche breakdown. When the voltage falls below this peak

value, avalanche breakdown ceases. A 90° shift occurs between the current pulse and

the applied voltage in the avalanche process. A further 90° shift occurs during the

transit time, for a total 180° shift which produces NDR. An IMPATT oscillator has

higher output power than a Gunn equivalent. However, the Gunn oscillator is relatively

noise-free, while the IMPATT is noisy due to avalanche breakdown.

• IMPATT stands for Impact Avalanche And Transit Time

• Operates in reverse-breakdown (avalanche) region

• Applied voltage causes momentary breakdown once per cycle

• This starts a pulse of current moving through the device

• Frequency depends on device thickness


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IMPATT DIODE

The current build-up and the transit time for the current pulse to cross the drift region

cause a 180o phase delay between V and I; thus, negative R.

IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are

possible.

They must operate in conjunction with an external high-Q resonant circuit.

They have relatively high output power (>100 W pulsed) but are very noisy and not

very efficient.


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PARAMETRIC DEVICES

VARACTOR DIODES

The variable-reactance (varactor) diode makes use of the change in capacitance of a

pn junction is designed to be highly dependent on the applied reverse bias. The

capacitance change results from a widening of the depletion layer as the reverse-bias

voltage is increased. As variable capacitors, varactor diodes are used in tuned circuits

and in voltage-controlled oscillators.

Typical applications of varactor diodes are harmonic generation, frequency

multiplication, parametric amplification, and electronic tuning. Multipliers are used as

local oscillators, low-power transmitters, or transmitter drivers in radar, telemetry,

telecommunication, and instrumentation.

• Lower frequencies: used as voltage-variable capacitor

• Microwaves: used as frequency multiplier

– this takes advantage of the nonlinear V-I curve of diodes

Varactors are used as voltage-controlled capacitors

Varactors are operated in a reverse-biased state. No current flows, but since the

thickness of the depletion zone varies with the applied bias voltage, the capacitance of

the diode can be made to vary. Generally, the depletion region thickness is

proportional to the square root of the applied voltage; capacitance is inversely

proportional to the depletion region thickness. Thus, the capacitance is inversely

proportional to the square root of applied voltage


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http://en.wikipedia.org/wiki/Capacitance

http://en.wikipedia.org/wiki/Square_root

http://en.wikipedia.org/wiki/Depletion_zone

http://en.wikipedia.org/wiki/Reverse-biased

http://en.wikipedia.org/wiki/Capacitors

http://commons.wikimedia.org/w/index.php?title=File:Varicap_symbol.svg&page=1



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http://upload.wikimedia.org/wikipedia/commons/8/89/Varactor.svg


STEP RECOVERY DIODE

PIN DIODE

Is a step recovery diode in which charge storage is used to produce oscillations. When

a diode is switched from forward to reverse bias, it remains conducting until the stored

charge has been removed by recombination or by the electric field. A step recovery

diode is designed to sweep out the carriers by an electric field before any appreciable

recombination has taken place. Thus, the transition from the conducting to the

nonconducting state is very fast, on the order of picoseconds. Because of the abrupt

step, this current is rich in harmonics, so these diodes can be used in frequency

multipliers.

• P-type --- Intrinsic --- N-type

• Used as switch and attenuator

• Reverse biased - off

• Forward biased - partly on to on depending on the bias

PIN DIODE


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LSA

SCHOTTKY BARRIER DIODE

A Schottky barrier diode (SBD) consists of a rectifying metal-semiconductor barrier

typically formed by deposition of a metal layer on a semiconductor. The SBD functions

in a similar manner to the antiquated point contact diode and the slower-response pn-

junction diode, and is used for signal mixing and detection. The point contact diode

consists of a metal whisker in contact with a semiconductor, forming a rectifying

junction. The SBD is more rugged and reliable than the point contact diode. The SBD's

main advantage over pn diodes is the absence of minority carriers, which limit the

response speed in switching applications and the high-frequency performance in

mixing and detection applications. SBDs are zero-bias detectors. Frequencies to 40


24


GHz are available with silicon SBDs, and GaAs SBDs are used for higher-frequency

applications.

Schottky diode

Main article: Schottky diode

Another type of junction diode, the Schottky diode, is formed from a metal–

semiconductor junction rather than a p–n junction, which reduces capacitance and

increases switching speed.

[edit] Current–voltage characteristic

A semiconductor diode’s behavior in a circuit is given by its current–voltage

characteristic, or I–V graph (see graph below). The shape of the curve is determined

by the transport of charge carriers through the so-called depletion layer or depletion

region that exists at the p–n junction between differing semiconductors. When a p–n

junction is first created, conduction-band (mobile) electrons from the N-doped region

diffuse into the P-doped region where there is a large population of holes (vacant

places for electrons) with which the electrons "recombine". When a mobile electron

recombines with a hole, both hole and electron vanish, leaving behind an immobile

positively charged donor (dopant) on the N side and negatively charged acceptor


25

http://en.wikipedia.org/wiki/Dopant


http://en.wikipedia.org/wiki/P%E2%80%93n_junction

http://en.wikipedia.org/wiki/Depletion_region

http://en.wikipedia.org/wiki/Depletion_region

http://en.wikipedia.org/wiki/Depletion_zone

http://en.wikipedia.org/wiki/Current%E2%80%93voltage_characteristic

http://en.wikipedia.org/wiki/Current%E2%80%93voltage_characteristic

http://en.wikipedia.org/w/index.php?title=Diode&action=edit&section=12

http://en.wikipedia.org/wiki/Metal%E2%80%93semiconductor_junction

http://en.wikipedia.org/wiki/Metal%E2%80%93semiconductor_junction

http://en.wikipedia.org/wiki/Schottky_diode


http://commons.wikimedia.org/w/index.php?title=File:Schottky_diode_symbol.svg&page=1


(dopant) on the P side. The region around the p–n junction becomes depleted of

charge carriers and thus behaves as an insulator.

However, the width of the depletion region (called the depletion width) cannot grow

without limit. For each electron–hole pair that recombines, a positively charged dopant

ion is left behind in the N-doped region, and a negatively charged dopant ion is left

behind in the P-doped region. As recombination proceeds more ions are created, an

increasing electric field develops through the depletion zone that acts to slow and then

finally stop recombination. At this point, there is a "built-in" potential across the

depletion zone.

If an external voltage is placed across the diode with the same polarity as the built-in

potential, the depletion zone continues to act as an insulator, preventing any significant

electric current flow (unless electron/hole pairs are actively being created in the

junction by, for instance, light. see photodiode). This is the reverse bias phenomenon.

However, if the polarity of the external voltage opposes the built-in potential,

recombination can once again proceed, resulting in substantial electric current through

the p–n junction (i.e. substantial numbers of electrons and holes recombine at the

junction). For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for

Germanium and 0.2 V for Schottky). Thus, if an external current is passed through the

diode, about 0.7 V will be developed across the diode such that the P-doped region is

positive with respect to the N-doped region and the diode is said to be "turned on" as it

has a forward bias.

A diode’s I–V characteristic can be approximated by four regions of operation.


26

http://en.wikipedia.org/wiki/I%E2%80%93V_characteristic

http://en.wikipedia.org/wiki/P%E2%80%93n_junction#Forward_bias

http://en.wikipedia.org/wiki/P%E2%80%93n_junction#Reverse_bias

http://en.wikipedia.org/wiki/Photodiode


http://en.wikipedia.org/wiki/Electron%E2%80%93hole_pair

http://en.wikipedia.org/wiki/Depletion_width

http://en.wikipedia.org/wiki/Nonconductor

http://en.wikipedia.org/wiki/Charge_carrier


Figure 5: I–V characteristics of a p–n junction diode (not to scale—the current in the

reverse region is magnified compared to the forward region, resulting in the apparent

slope discontinuity at the origin; the actual I–V curve is smooth across the origin).

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called

reverse breakdown occurs that causes a large increase in current (i.e., a large number

of electrons and holes are created at, and move away from the p–n junction) that

usually damages the device permanently. The avalanche diode is deliberately

designed for use in the avalanche region. In the Zener diode, the concept of PIV is not

applicable. A Zener diode contains a heavily doped p–n junction allowing electrons to

tunnel from the valence band of the p-type material to the conduction band of the n-

type material, such that the reverse voltage is "clamped" to a known value (called the

Zener voltage), and avalanche does not occur. Both devices, however, do have a limit

to the maximum current and power in the clamped reverse-voltage region. Also,

following the end of forward conduction in any diode, there is reverse current for a

short time. The device does not attain its full blocking capability until the reverse

current ceases.


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http://en.wikipedia.org/wiki/Zener_diode

http://en.wikipedia.org/wiki/Avalanche_diode

http://en.wikipedia.org/wiki/Avalanche_breakdown

http://en.wikipedia.org/wiki/Peak_Inverse_Voltage

http://en.wikipedia.org/wiki/File:Diode-IV-Curve.svg


The second region, at reverse biases more positive than the PIV, has only a very small

reverse saturation current. In the reverse bias region for a normal P–N rectifier diode,

the current through the device is very low (in the µA range). However, this is

temperature dependent, and at sufficiently high temperatures, a substantial amount of

reverse current can be observed (mA or more).

The third region is forward but small bias, where only a small forward current is

conducted.

As the potential difference is increased above an arbitrarily defined "cut-in voltage" or

"on-voltage" or "diode forward voltage drop (Vd)", the diode current becomes

appreciable (the level of current considered "appreciable" and the value of cut-in

voltage depends on the application), and the diode presents a very low resistance. The

current–voltage curve is exponential. In a normal silicon diode at rated currents, the

arbitrary cut-in voltage is defined as 0.6 to 0.7 volts. The value is different for other

diode types—Schottky diodes can be rated as low as 0.2 V, Germanium diodes 0.25 to

0.3 V, and red or blue light-emitting diodes (LEDs) can have values of 1.4 V and 4.0 V

respectively.

At higher currents the forward voltage drop of the diode increases. A drop of 1 V to 1.5

V is typical at full rated current for power diodes.


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http://en.wikipedia.org/wiki/Light-emitting_diode



http://en.wikipedia.org/wiki/Exponential_function

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