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CHAPTER 8: MICROWAVE CHAPTER 8: MICROWAVE DIODES, QUANTUM EFFECT DIODES, QUANTUM EFFECT

& HOT ELECTRON & HOT ELECTRON DEVICES DEVICES

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QUANTUM EFFECT & HOT QUANTUM EFFECT & HOT ELECTRON PHENOMENAELECTRON PHENOMENA

Quantum effect & hot-electron phenomena to enhance circuit performancesAdvantage of quantum effect devices (QEDs) & hot electron devices (HEDs) :

Higher functionality/speed can perform relatively complex circuit functions with a greatly reduced device count

replacing large numbers of transistors or passive circuit components

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COVERAGE OF CHAPTER 8COVERAGE OF CHAPTER 8

Millimeter-wave devices over those operated at lower frequenciesThe quantum tunneling phenomenon and its related devices – tunnel diode, resonant tunneling diode (RTD) The IMPATT diode – the most powerful semiconductor source of millimeter wave powerThe Transferred-Electron Device (TED) and its transit-time domain modeThe Real-Space-Transfer (RST) transistor

© S.N. Sabki

© S.N. Sabki

© S.N. Sabki

© S.N. Sabki

Figure 8.2. Basic types of planar transmission lines: (a) microstrip, (b) coplanar waveguide stripline, and (c) suspended-substrate stripline.

A transmission line is a sub-category of waveguides that uses some physical configuration of metal and/or dielectrics to direct a signal along the desired path. Most familiar transmission lines (e.g., microstrip line) use two conductors;

C

LZ 0

L=inductance (H)C=capacitance (F)

The most commonly used transmission lines (stripline and microstrip line) aren't the only way to transmit a signal from one place to another.

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Figure 8.3. Resonant cavity: (a) resonator shape, (b) magnetic field pattern, and (c) electric field pattern.

• metal-walled chamber made of low-resistivity material enclosing a good dielectric

• Cavity supports: transverse electric (TE) & transverse magnetic (TM) modes of propagation

• Electromagnetic wave is confined by the wall of the cavity

• E in electric field capacitance C

• E in magnetic field inductance L

• LC tuned resonant tank circuit present in the cavity

• resonant mode occur in a cavity – freq with length d along Z axis (/2) – fig. 3(a)

RESONANT CAVITY

© S.N. Sabki

© S.N. Sabki

222

2

1

d

p

b

n

a

mf r

222

2

c

p

b

n

a

mcfr

General mode dependent equation for resonant freq. of the cavity:

: permeability

: permittivity

0: permeability for vacuum (=0)

0: permittivity for vacuum (=0)

c: speed of light in vacuum

100

cMode in cavity Txm,n,p:

x: E for electric dominant mode, M for magnetic dominant mode

m: no. of half-wavelength in a dimension

n: no. of half-wavelength in b dimension

p: no. of half wavelength in the d dimension

RESONANT CAVITY

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TUNNEL DIODE

The tunnel diode has a region in its voltage current characteristic where the current decreases with increased forward voltage, known as its negative resistance region. This characteristic makes the tunnel diode useful in oscillators and as a microwave amplifier.

In the TUNNEL DIODE, the semiconductor materials used in forming a junction are doped to the extent of one-thousand impurity atoms for ten-million semiconductor atoms. This heavy doping produces an extremely narrow depletion zone . Also because of the heavy doping, a tunnel diode exhibits an unusual current-voltage characteristic curve as compared with that of an ordinary junction diode.

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• Forward bias:• electrons tunnel from n-side to p-side• When V=(Vp+Vn)/3 – I reaches Ip • When V is further increased (Vp<V<Vv) tunnel I decreased (fewer available unoccupied states in p-side) until I can no longer flow• With further increased V – normal thermal I will flow (V>Vv)

• Summary:•In the forward direction the tunneling I increases from ‘0’ to a peak current Ip as the V increases• Further increase in V, the I decreases to ‘0’ when V=Vn+Vp, (V=applied voltage)

• Empirical form for the I-V characteristics is given by

kT

qVI

V

V

V

VII

ppp exp1exp 0

Figure 8.4. Static current-voltage characteristics of a typical tunnel diode. The upper figures show the band diagrams of the device at different bias voltages.

© S.N. Sabki

Figure 8.5. Typical current-voltage characteristics of Ge, GaSb, and GaAs tunnel diodes at room temperature.

Current ratios, v

p

II

I ratios of:Ge – 8:1GaSb – 12:1GaAs – 12:1

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IMPATT DIODEIMPATT DIODE

• IMPATT: IMPact ionization Avalanche Transit-Time• Employ impact ionization & transit-time properties to produce a negative resistance at microwave frequencies• One of the most powerful solid-state sources of microwave power• Can generate the highest cw (continous wave) power output of all solid-state devices at millimeter-wave frequencies (above 30GHz).• Extensively used in radar systems & alarm systems• Noteworthy difficulty in IMPATT applications: the noise is high because of random fluctuations of the avalanche multiplication processes•1st IMPATT diode obtained from silicon p-n junction diode biased into reverse avalanche breakdown and mounted in a microwave cavity

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IMPATT diode(IMPact ionization Avalanche Transit Timediode)∗Areverse biased p-njunction capable of producing oscillations at up to100GHz

Applied voltage is slowly increased from zero. The electric field at the junction builds up until it reaches the threshold for avalanche breakdown. The avalanche produces holes which move quickly to the cathode and electrons which take much longer to reach the anode. As the electrons move towards the anode the electric field at the junction drops and so shuts off the avalanche. When the electrons reach the anode the electric field can build up again and a new avalanche develops. The frequency of the oscillation is fixed by the transit time of the electrons through the n-region.

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IMPATT DIODEIMPATT DIODE

Figure 8.6. Doping profiles and electric-field distributions at avalanche breakdown of three single-drift IMPATT diodes: (a) one-sided abrupt p-n junction; (b) hi-lo structure; and (c) lo-hi-lo structure.

• One-sided abrupt p-n junction:• Most avalanche multiplication occurs in a narrow region near the highest field between 0 & xA (width of avalanche region)

• Hi-lo structure:• Avalanche region confined within the N1 region

• Lo-hi-lo structure:• a “clump” of donor atoms is located at x=b•High field region exists from x=0 to x=b, xA=b, max. field can be much lower than hi-lo structure

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TRANSFERRED-ELECTRON DEVICES TRANSFERRED-ELECTRON DEVICES (TEDs)(TEDs)

Transferred Electron Devices (TEDs), widely know as Gunn diodes, are gallium arsenide (GaAs) or indium phosphide (InP) devices which are capable of converting direct current (DC) power into radio frequency (RF) power when they are coupled to the appropriate resonator. Typical applications for Gunn diode oscillators include local oscillators, voltage controlled oscillators (VCOs), radar and communication transmitters, Doppler motion detectors, intrusion alarms, police radar detectors, smart munitions, and Automotive Forward Looking Radars (AFLRs). Gunn Diodes are two-terminal negative-impedance semiconductors which are similiar to tunnel diodes They are mainly found in microwave oscillators in the range from ten to several hundred Gigahertz.

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TRANSFERRED-ELECTRON DEVICES TRANSFERRED-ELECTRON DEVICES (TEDs)(TEDs)

Figure 8.8. The current versus electric-field characteristic of a two-valley semiconductor. ET is the threshold field and EV is the valley field.

Negative Differential Resistance (NDR)

• Electron effect – the transfer of the conduction electrons from a high-mobility energy valley to low-mobility higher-energy satellite valley.

• Current density:

EqnJ 1aEE 0

EqnJ 2bEE

• NDR region – between ET & EV

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Transferred-electron mechanism to give rise to Negative Differential Resistance (NDR):

The lattice temp. must be low enough that in the absence of energy E most of electrons are in lower valley (conduction band minimum) – separation between 2 valleys E>kTIn the lower valley the electrons must have high mobility & small effective mass, in the upper valley electrons must have low mobility & large effective massEnergy separation between 2 valleys must be smaller than the semicond. bandgap (i.e. E<Eg) so that avalanche breakdown does not begin before the transfer of electrons into the upper valleys

N-type GaAs & InP – widely studied & used.

TRANSFERRED-ELECTRON DEVICES TRANSFERRED-ELECTRON DEVICES (TEDs)(TEDs)

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Figure 8.9. Two cathode contacts for transferred-electron devices (TEDs) (a) Ohmic contact and (b) two-zone Schottky barrier contact.

Device operation

• Have epitaxial layers on n+ substrates

• Typical donor conc. range: 1014 to 1016cm-3

• typical device lengths L range: few to several hundreds

• Fig (a) & (b): energy band diagram at thermal equilibrium & electric-field distribution when V=3VT

• VT: product of threshold field ET & device length L

• To improve performance: use 2-zone cathode contact (consists of high-field zone & n+ zone – fig. (b)(similar to lo-hi-lo IMPATT diode)

• electrons are heated in the high-field zone injected to the active region

© S.N. Sabki

Figure 8.10. Formation of a domain (dipole layer) in a medium that has a negative differential resistivity (NDR).

• Operational of TED depends on 5 factors:

• Doping concentration

• Doping uniformity

• Length of the active region

• Cathode contact characteristics

• Type of circuit

• Operating bias voltage

• Important mode of operation for TED: transit-time domain mode (+ve & -ve charges separated by a small distance – dipole formation/domain)

• Electric field E in dipole > E on either side of it

• Because of NDR, I in low field region > I in high field region

•Time required for domain to travel from cathode to anode : L/v (L:active device length, v: average velocity)

•Freq. for transit time domain mode: f=v/L

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QUANTUM-EFFECT DEVICES (QED)QUANTUM-EFFECT DEVICES (QED)

Figure 8.12. Band diagram of a resonant-tunneling diode.

• QED uses quantum mechanical tunneling to provide carrier transport – active layer thickness is very small (10nm).

• Give rise to a quantum size effect that can alter the band structures & enhance device transport properties

• Resonant Tunneling Diode (RTD) (Basic QED)

• Semicond. double-barrier struc. contains 4 heterojunctions (GaAs/AlAs/GaAs/AlAs/GaAs), 1 quantum well in the conduction band

• Important parameter: energy barrier height E0, energy barrier thickness LB, quantum well thickness LW

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Figure 8.13. (a) Schematic illustration of AlAs/GaAs/AlAs double-barrier structure with a 2.5 nm barrier and a 7 nm well. (b) Transmission coefficient versus electron energy for the structure.

• If LW small (10nm) – a set of energy levels will exist inside the well (E1, E2, E3, E4) –fig.13 (a)

• If LB small – resonant tunneling will occur

• incident electron (has energy E = one of the discrete energy levels in the well) – it will tunnel thru the double barrier with a unity (100%) transmission coeff.

• Transmission coeff. decreases rapidly as energy E deviates from the discrete energy levels

• eg.: electron with energy 10meV higher or lower than the level E1 105 times reduction in the transmission coeff. (Tt) – fig.13 (b)

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Figure 8.15. A mesa-type resonant tunneling diode (cross-section)

• GaAs/AlAs layers are grown on an n+ GaAs substrate

• LB = 1.7nm

• LW = 4.5nm

• active regions are defined with ohmic contacts

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Figure 8.16. Measured current-voltage characteristics of the diode in Fig. 8-15.

• Note: I-V curve is similar to that of a tunnel diode (fig.4)

• At thermal equilibrium (V=0) energy diagram is similar to fig.13(a)

• Increase the applied V – electrons in the occupied energy states near the fermi level to the left side of the 1st barrier tunnel into the quantum well

• the electrons tunnel thru the 2nd barrier into the unoccupied states in the right side.

• Resonance occurs when the energy of the injected electrons = E1

• V=V1=Vp – the conduction band edge on the left side is lined up with E1

• When V=V2, the conduction band edge is above E1 – electrons that can tunnel decreases – small I

• Iv due mainly to the excess I components: electrons that tunnel via an upper valley in the barrier

© S.N. Sabki

To minimize the Iv – must improve the quality of the heterojunction interfaces & eliminate impurities in the barrier & well regionsFor higher applied V (V>Vv) – Ith due to electrons injected thru higher discrete energy levels in the well or thermionically injected over the barrierIth increases with inceasing V (similar to tunnel diode)To reduce Ith: increase the barrier height & design a diode that operates at relatively low bias voltagesResonant Tunneling Diodes can be operated at very high freq. – smaller parasitics

© S.N. Sabki

HOT ELECTRON DEVICESHOT ELECTRON DEVICES

Figure 8.18. Energy band diagram of a hot electron heterojunction bipolar transistor.

• Hot electrons: electrons with kinetic energies substantially above kT (k is Boltzman’s constant & T is lattie temp.)

• As the dimensions of semicond. devices shrink & internal fields rise, a large fraction of carriers in the active regions of the device during its operation is in states of high kinetic energy

• Fig.18: an AlInAs/GaInAs HBT

•Electrons are injected by thermionic emission over the emitter-base barrier at an energy Ec=0.5eV above the conduction band-edge in the p-GaInAs base

© S.N. Sabki

Figure 8.19. (a) A heterostructure with alternate GaAs and AlGaAs layers. (b) Electrons, heated by an applied electric field, transfer into the wide-gap layers. (c) If the mobility in layer 2 is lower, the transfer results in a negative differential conductivity.

Real-Space-Transfer Transistor

• In thermal equilibrium: mobile electrons reside in the undoped GaAs quantum wells & spatially seperated from their parent donors in AlGaAs layers – fig.19(a)

• Give power input to the structure – the carriers heat up and undergo partial transfer into the wide-gap layer – fig.19(b)

• If the mobility in layer 2 is lower, negative differential resistance will occur in the 2-terminal circuit – fig.19(c)

• Transferred-electron effect, based on the momentum-space intervalley transfer named real-space transfer

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Figure 8.20. Cross section and band diagram of a real-space-transfer transistor in a GaInAs/AlInAs material system.

• Source & drain contacts are to undoped GaInAs channel

• Collector contact is to a doped GaInAs conducting layer – separated from the channel by a larger bandgap material (i.e. AlInAs; Eg=1.45eV)

• At VD=0, electron density is induced in the source-drain channel by +ve Vc – but no Ic flows because of the AlInAs barrier

• VD increases, ID begins to flow & the channel electrons heat up to some effective temp. Te

• The injected electrons are swept into the collector by the Vc – induced electric field, giving rise to Ic

Transistor action results from control of the Te in the source-drain channel

Real-Space-Transfer Transistor

© S.N. Sabki

Exercise (Microwave)Exercise (Microwave)

Find the characteristic impedance of a nearly lossless transmission line (R is very small) that has a unit-length inductance of 10nH and a unit-length capacitance of 4pF

Solution

Impedance

50105.2104

1010 312

9

0 C

LZ

© S.N. Sabki

Exercise (microwave)Exercise (microwave)

For a cavity of the dimensions a=5cm (0.05m), b=2.5cm (0.025m) and d=10cm (0.1m), find the resonant frequency in the dominant TE101 mode

Solution:

GHz354.3

5002

m/s103

)10()0()20(2

2

8

22

222

c

c

p

b

n

a

mcfr

© S.N. Sabki

Exercise (Transferred Electron Exercise (Transferred Electron Devices)Devices)

A GaAs TED is 10m long and is operated in the transit-time domain mode. Find the minimum electron density n0 required and the time between current pulses

Solution:For transit-time domain mode, we require n0L1012 cm-2

n0 1012/L = 1012/10x10-4 = 1x1015cm-3

The time between current pulses is the time required for the domain to travel from the cathode to anode:

t = L/v = 10x10-4/107 = 10-10s = 0.1ns

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INFORMATION!INFORMATION!

TEST 2

DATE: 5th October 2007 (FRIDAY)

TIME: 8.30pm – 9.30pm

VENUE: DKG 1 (will confirm later)

TOPICS:CHAPTER 5,6,8,9

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DON’T FORGET!!DON’T FORGET!!

Submit Mini Project Report NEXT WEEK (19th Sep 2007) during lecture.

Mini Project Presentation on 24th & 25th Sep 2007 during lab session