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Jake Richards, Jay Johnson, Rebekah Nelson, Teresia Djunaedi, Cameron Dahl

The effects of high frequencies limit the effectiveness and applicability of transistors. As such, special devices are required in order to operate at high frequencies.

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• High Frequency Microwave Devices

o Esaki Diodes

o Gunn Diodes

o Impact Avalanche Transit-Time Diodes

• High Power Switching Devices

o PNPN Diodes

o Semiconductor-Controlled Rectifiers

o Modern Power Devices

• Negative Conductance Devices (NCDs) o Negative Differential Resistance (NDR)

Characteristics

o High Frequency Cutoff

o Esaki Diodes (Tunneling Diodes)

o Gunn Diodes

o Impact Avalanche Transit-Time Diodes (IMPATT Diodes)

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Also know and Esaki Diodes Named after Japanese physicist Leo Esaki who discovered electron tunneling, invented the tunneling diode, and won the 1973 Nobel Prize.

When doping is increased beyond what is found in normal diodes the Fermi-level can move into the conduction band on the n-type side or into the valence band on the p-type side.

This is called a degenerate n-type and degenerate p-type.

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In either the n-type semiconductor or the p-type semiconductor the conditions are met for electron tunneling. ◦ That is filled and empty states separated by a

narrow potential barrier of finite height.

This allows current to pass through without the time delays associated with drift and diffusion current.

Before bias is applied Tunneling occurs on

both sides and averages out.

Zero net current EV and EC overlap. With no bias the

Fermi-level is constant.

Compared to a standard p-n junction this has a larger built-in electric field and a smaller depletion region.

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More tunneling occurs from p to n regions.

Current flows from n to p side

Very little reverse bias required.

This is similar to the Zener effect, except that no bias is required for the bands to overlap.

A

B

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More tunneling occurs from n to p regions.

Current flows from p to n side

Very little forward bias required.

At first the current increases with the voltage A

B

C

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The bottom of the EC band begins to pass the top of the EV band.

Less electrons are able to tunnel.

dV/dI is negative.

The diode has negative resistance.

The bottom of the EC band begins to pass the top of the EV band.

Less electrons are able to tunnel.

dV/dI is negative.

The diode has negative resistance.

A

B

C D

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Eventually, diffusion current dominates and current increases again.

Once diffusion current dominates the diode I-V characteristic returns to that of a normal diode.

A

B

C

D E

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Note: The symbol for a tunneling diode.

R1 and R2 Bias the tunneling diode in it’s negative resistance region.

Initially current through the inductor is zero.

Therefore, VAB is also zero.

A B

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As current starts to flow through the inductor VAB increases.

Vout moves up along the diode’s I-V curve.

A B

Eventually VAB reaches the point that the diode enters reverse saturation.

However, the charged inductor holds Vout and slowly discharges backwards along the diode’s I-V characteristic.

A B

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VAB eventually reaches the point that the diode leaves it’s reverse reverse saturation region.

This starts the oscillation back at the beginning.

The inductor charges and Vout increases back up the diode’s I-V characteristic.

A B

http://www.falstad.com/circuit/e-tdrelax.html

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Semiconductor that consists of only N-Type SC material (No P-N junction or Schottky metal-semiconductor contact).

Also known as a TED (transfer electron device).

Consists of three regions:

- Two heavily N-doped regions on each

terminal.

- One lightly doped material in the middle.

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At high field values the middle region becomes a region of negative differential resistance.

This relation combined with the timing properties in the intermediate layer make construction of RF relaxation oscillators.

All that’s required is a DC

current applied through

the device.

This negative differential resistance negates the positive resistance of a load and creates a “zero” resistance circuit that can sustain oscillations indefinitely.

This allows the creation of oscillators in the 10GHz range (up to THZ).

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Police Radar

Automatic Door Openers

Cheap Microwaves

Radar Jammers

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IMPATT – Impact avalanche transit-time diode. ◦ Ex: Read diode.

Originally proposed by W.T. Read & involved n+-p-i-p+ structure.

A type of microwave negative-conductance device.

Operates by injecting carriers into drift region.

n+-p region – avalanche multiplication occurs.

i (essentially intrinsic) region – generated holes drift in moving to p+ contact.

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Operates in negative-conductance mode when a-c component of current is negative over a portion of cycle and a-c voltage is positive & vice versa.

Negative-conductance cause current to to lag behind voltage in time: ◦ Delay due to avalanche process

◦ Further delay due to transit time of carriers across drift region

Sum of delay times is approx. ½ cycle to occur.

Assume p region is very narrow.

All avalanche multiplication takes place in thin region near n+-p junction.

If critical field of avalanche Ea just met n+-p space charge region, avalanche multiplication begins at t = 0.

Electrons generated in avalanche region move to n+ region, and holes enter i drift region

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More holes generated in avalanche region as applied a-c voltage goes positive

Pulse of holes continues to grow as E is above Ea.

Reaches peak value at wt = π

Phase delay: π/2 ◦ inherent from

avalanche

◦ Further delay from drift

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Avalanche multiplication stops (wt > π)

Pulse of holes drifts towards p+ contact.

a-c terminal voltage is negative

Dynamic conduction is negative

Energy is supplied to a-c field

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• Primarily interested in devices which are only turned on due to an external signal o This is accomplished due to a high impedance

when the device is under forward bias, and once switched to the conducting state the device displays low impedance

• PNPN Diodes

• Thyristors

• Semiconductor-Controlled Rectifiers (SRCs)

• Four-Layer structure

Referred to as a Shockley diode

This section looks at the effects on Two-Terminal PNPN diodes

p

+ v -

p n n

i i

A K

j1

j2

j3

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• Off state

o Forward Blocking

o Reverse Blocking

• On state

o Forward Conducting

• Off state

o Forward-blocking state

When device is forward-biased but has not been triggered, such as not having reached the critical peak forward voltage (Vp)

o Reverse-blocking state

When device is reverse-biased

• On state

o Forward Conducting state

When the device is forward-biased and the critical peak forward voltage (Vp) has been reached

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• Forward Blocking o j1 and j3 are forward

biased

o j2 is reverse biased

the majority of the forward voltage in the device can be found across the j2 junction as v is increased

p

+ v -

p n n

i i

A K

j1

j2

j3

• Reverse Blocking o j1 and j3 are reverse

biased

o j2 is forward biased

supply of holes and electrons is limited due to being surrounded on both sides by reverse biased junctions

results in a small saturation current

p

- v +

p n n

A K

j1

j2

j3

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• Forward Conducting o j1, j2, and j3 are all

forward biased

o once switched to this state, v will be very small

p

+ v -

p n n

i i

A K

j1

j2

j3

Two-Transistor Analogy

• A PNPN Diode can be thought of as being made up of two transistors (one is a pnp and one is an npn transitor) which are coupled

o collector of npn and base of pnp are shorted

o base of npn and collector of the pnp are shorted

p

p

p

n

n

n E B C

C B E

ib1 = ic2

ic1 = ib2

i

i

a1

a2

A

K

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Two-Transistor Analogy

• ic1 = collector current of pnp transitor

• ic2 = collector current of npn transitor

• ib1 = base current of pnp transistor

• ib2 = base current of npn transistor

• a = emitter-to-collector current transfer ratio

p

p

p

n

n

n E B C

C B E

ib1 = ic2

ic1 = ib2

i

i

a1

a2

A

K

p

p

p

n

n

n E B C

C B E

ib1 = ic2

ic1 = ib2

i

i

a1

a2

A

K

Two-Transistor Analogy

• Can solve for the current, i

• set a1=an and a2=an

• Using this equation (7-37b)

• Ico1 and Ico2 are collector saturation currents

• collector currents:

Ic = an*Ie-Ico*(e^(q*Vcb/KT - 1)

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p

p

p

n

n

n E B C

C B E

ib1 = ic2

ic1 = ib2

i

i

a1

a2

A

K

Two-Transistor Analogy • Total current through the

device:

Triggering Mechanisms

• Voltage triggering

o When forward bias voltage is raised to Vp

o causes breakdown of the j2 junction

• Base Width Narrowing

o Base width decreases as a1 and a2 increase

o Once a1 and a2 are increased enough, switching occurs

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Triggering Mechanisms

• Avalanche Multiplication and Base Width Narrowing

o avalanche multiplication causes breakdown of the j2 junction

causes an inreased number of carriers in the p and n regions surrounding j2, which will switch the device on

• dv/dt Triggering

o when a device is forward-biased very quickly

o if dv/dt is large enough, it can cause an increase in current which can cause switching

dv/dt Triggering

• The rate at which voltage is applied can create a transient current relationship

• Cj2 = junction capacitance of j2

• vj2 = instantaneous voltage across j2

i(t) = =C +v d C v d v dC

dt dt dt

j2 j2 j2

j2 j2

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• Breakdown will occur in a collector junction of a transistor

o when Ma = 1 and ib = 0

• Breakdown of coupled transistors occurs at j2 when

o Mp*a1 + Mn*a2 = 1

o Mp = hole multiplication factor

o Mn = electron multiplication factor

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The SCR is used for: ◦ Power switching

◦ Analog switches

◦ Motor control switches

◦ These handle large current and

voltages

The SCR has three modes: ◦ Forward Blocking

◦ Reverse Blocking

◦ Forward conducting

http://en.wikipedia.org/wiki/Silicon-controlled_rectifier

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

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The SCR uses what is called a floating gate. This gate is disconnected and this creates a permanent switch feature over a single transistor.

This switch can be turned on and remain on using a small voltage and applying it to the gate and the cathode.

Doing this operation (force) turns on the NPN transistor portion and this eliminates VBE_on being necessary

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The SCR is used because the rectifier can be turned on with either a very small current or a very large current.

To control the rectifier from turning on, on accident, you would connect a resistor between the Gate and the Cathode

This creates design limitations for specific

turn off voltages

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

The SCR’s output is a triggered pulse wave

A positive gate current will make the SCR be in Forward Conducting mode, and a negative current runs it in Reverse Conducting mode

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The IGBT is used for fast switching and high power efficiency, and is more efficient than the SCR

The SCR has difficulty turning off since the voltages need to be so far apart, while the IGBT has a MOSFET in place of the using a resistor to control the turn-off of the switch

This device is easier to turn off than the SCR

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Due to the structure, the IGBT cannot conduct in reverse bias mode

But as a tradeoff, it can handle larger amounts of currents

It features high input impedance and low output capacitance

With the doping properties of this FET, it maximizes the conductivity of the part and it is able to be modulated in use ◦ This decreases the overall voltage drop across the

structure in the on state

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What are the two regions are in the IMPATT Diode?

What is the benefit of tunneling current?

Which is better, the IGBT or the SCR?

"Leo Esaki." Wikipedia. Wikimedia Foundation, 05 June 2012. Web. 13 May 2012. <http://en.wikipedia.org/wiki/Leo_Esaki>.

"Tunnel Diode Relaxation Oscillator." Tunnel Diode Relaxation Oscillator. Web. 13 May 2012. <http://www.falstad.com/circuit/e-tdrelax.html>.

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

http://www.youtube.com/watch?v=AiVVrxgNSFg&feature=topics

Streetman, Ben G. "Chapter 10." Solid State Electronic Devices. Sixth ed. Englewood Cliffs, NJ: Prentice-Hall, 1980. 526-31. Print.