© s.n. sabki revision revision chapter 8 chapter 8

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© S.N. Sabki Revision Revision CHAPTER 8 CHAPTER 8

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© S.N. Sabki

Revision Revision CHAPTER 8CHAPTER 8

© 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.

© S.N. Sabki

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

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

© S.N. Sabki

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.

© S.N. Sabki

• 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

© S.N. Sabki

IMPATT DIODEIMPATT DIODE

• IMPATT: IMPact ionization •Avalanche •Transit-•Time

© S.N. Sabki

IMPATT diode(IMPact ionization Avalanche Transit Timediode)∗A reverse biased p-n junction 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.

© S.N. Sabki

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

© S.N. Sabki

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.

© S.N. Sabki

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

© S.N. Sabki

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

© S.N. Sabki

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

© 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