MICROWAVE ENGINEERING

EKT 345

CHAPTER 4: MICROWAVE

SEMICONDUCTOR DEVICES

Outline:

4.1Microwave Diodes

- Shottky Diodes, Pin Diodes, Varactor Diodes, Tunnel Diodes, Gunn Diodes, IMPATT, TRAPATT

4.2Microwave Transistors

- Bipolar, Unipolar FET, HEMT

- Limitation of Transistor at High Frequencies

4.3 Parametric Amplifier

- Manley Rowe Relationship

4.1 Microwave Diodes

Two elements device which will do much more

than rectify or clamp a dc voltage.

General functions : able to switch, attenuate, mix

frequencies, detect, amplify, oscillate and

modulate signals in microwave and millimeter

bands frequencies.

Definition: two terminal electronic device that will conduct electricity much more easily in one direction than in the other.

Contd

Schottky Diodes for mixing and detection

Pin diodes for attenuation, modulation,

switching, phase shifting and limiting.

Varactor diodes for frequency multiplications,

parametric amplification and tuning

Tunnel and Gunn Diodes for oscillation

Impact Ionization Avalanche Transit Time

(IMPATT) and Trapped Plasma Avalanche

Transit Time (TRAPATT) for amplification and

oscillation

Contd

a. Schottky Diodes

Rj = resistance of metallic junctionCj = barrier capacitanceRs = bulk resistance of heavily doped Si substrate (4-6 ohm)Ls= inductance of gold whisker wire (0.4 0.9nH)Cc = Case Capacitance

Contd

Metal-semiconductor barrier diodes

Exhibits a squarelaw characteristic and have

higher burnout rating

Better reliability compared to point contact

diodes

Constructed on a thin silicon (n+ type) substrate

by growing epitaxially on n-type active layer of

about 2 microns thickness.

Metal semiconductor is formed by depositing

metal over SiO2.

Contd

b. Pin Diodes

Rj = junction resistance (variable)Cj = junction capacitance (~0.2pf)Rs = bulk semiconductor (p+ and n+) layer and contact resistance.Lp, Cp = package inductance, capacitance

Contd

Consists of a high-resistivity intrinsic semiconductor layer between highly doped p+

and n+Si layers.

Acts as an electrically variable resistor related to i layer thickness.

Intrinsic layer has a very large resistance in reverse bias and it decreases in forward bias.

When mobile carriers from p and n regions are injected into the i layer, carriers take time -diode ceases to act as a rectifier at microwave frequency and appear as a linear resistance:

variable attenuator

Contd

Pin device is designed under reverse or zero

biasing, Rj is extremely large and Cj plays the

dominant role, where Cp, Lp, Rs are small.

For reverse bias, high capacitance impedance

is presented to the microwave signal.

For forward bias, the diodes represents a very

low resistance .

Changing the bias, it acts as a switch

Single switch

Double switch use two diodes called single-pole

double-throw (SPDT)

Contd

Contd

Contd

c. Varactor Diodes

Contd

Operated under reverse bias where the

junction resistance is 10 M or more.

Equivalent circuit reverse-bias at

microwave frequencies : a simple of

capacitance and resistance in series.

Equivalent circuit of forward-bias at

microwave frequencies more complicated

must include the diffusion of the injected

carriers.

Contd

Due to skin effect, Rs increases with increase

in frequency

So, frequency used normally below 0.2fc

Contd

d. Tunnel Diodes

Rj = junction

resistance (100 ohms)

Cj = junction

capacitance (pf)

Ls = lead inductance

(nH)

Rs = bulk resistance

(few ohms)

-Rj

Equivalent circuit of tunnel diode

Contd

Heavily doped p-n junction diodes with a

negative resistance over a portion of its I-V

characteristic (refer page 416)

Used as microwave amplifiers or oscillators.

Heavy doping depletion width becomes very

thin and overlap occurs between the

conduction-band level (n-side) and the

valence-band level (p-side).

Contd Tunnel-diode oscillator

Consists of a tank circuit coupled with the diode by means of a capacitive divider

When the power switched on, a surge current produces oscillationin the tank circuit

The R-C values make the dc bias at the centre of the negative resistance characteristic of the diode

Sustained oscillation occurs if magnitude of the negative resistance of the diode is equal or greater than the resistance of tank circuit.

The oscillator circuit can generate microwave signals up to frequencies: 100GHz

Contd

Contd

Contde. Gunn Diodes

Contd

Negative resistance devices which normally

used as low-power oscillator at the microwave

frequencies in transmitter and as local oscillator

in receiver front ends.

Two main principles of operation:

Gunn Mode or Transit-Time (TT) mode

Limited-Space Charge Accumulation (LSA) mode

Other modes:

Quenched domain mode

Delayed mode

Contd

TT mode

Low efficiency of power generation

Uncontrollable frequency by external circuit

LSA mode

Produce several watts of power with minimum

efficiency 20%.

Power output decreases with frequency:

example:

1 Watt at 10 GHz

1 mW at 100 GHz

Contd

Contdf. IMPATT

IMPact Avalanche and Transit Time

diodes

An efficiency of 3% CW power and 60%

pulsed power.

Operated from 500 MHz to 100GHz.

Power output between 1W (CW) and over

400W (pulsed).

Many forms:

n+ pip + or p + nin + Read device

p + nn + abrupt junction

p + in + diode

Manufactured from Ge, Si, GaAs or InP,

The highest efficiency, highest operating

frequency and least noise figure but

complicated fabrication process: GaAs

Contd

Contd

Reverse-biased condition n+ pip +

When reverse bias voltage exceeds the breakdown voltage VB, a maximum electric field of very high value (MV/m) appears at the n+p junction.

The holes moving in the high field region acquire sufficient energy to excite valence electrons of the atom into the conduction band resulting in avalanche multiplication of electron hole-hole pairs

To prevent burning diode: a constant diode current bias source is used to maintained the average current at safe limit I0.

The diode current is contributed by the conduction electrons which move to n+

region and associated holes which drift through i space charge region to the p+

region, under the influence of a lower but steady electric field.

Contd

Contd

IMPATT disadvantages:

Since dc power is drawn due to induced electron

current in the external circuit, IMPATT diodes

have low efficiency.

Tend to be noisy due primarily to the avalanche

process and to the high level of operating

current.

Typical noise figure is 30dB which is worse than

Gunn Diodes.

Contd

g. TRAPATT

TRApped Plasma Avalanche Triggered

Transit Diode

Suitable for low frequency (1-3GHz) applications

with pulse output of several hundreds watts

Efficiency 20-60%

Manufactured from Si and have p+n n+ or n+ pp+

configurations

p-n junction reverse-biased beyond the

breakdown junction current density is higher

Contd

TRAPATT diode is mounted inside a coaxial resonator at position of maximum RF voltage swing.

When avalanche occurs at the dc reverse-bias plus RF swing beyond the threshold of breakdown, a plasma of holes and electrons are generated

The plasma density results in a high potential difference across the junction in opposition to the dc reverse bias voltage

At this low voltage, the plasma gets trapped

Since the external circuit current flow, the voltage rise and the trapped plasma is released producing current pulse across the drift space.

Total transit time is the sum of the delay time in releasing the trapped plasma and the drift time.

Since the transit time is longer due to low voltage, the operating frequency is limited below 10 GHz current pulse is associated with low voltage, the power dissipation is low and efficiency is higher

Contd

TRAPATT disadvantages:

High noise figure ~60dB limits it use as an

amplifier

Generates strong harmonics due to the short

duration current pulse

Contd

COMPARISON BETWEEN GUNN, IMPATT, TRAPATT AND BARITT

4.2 Microwave Transistors

Ordinary transistors are pnp or npn junction transistors :

bipolar and unipolar FET

High frequency operations of these devices are limited by:

Diffusion capacitance at base-emitter junction

Space charge capacitances at base-emitter and base-collector

junction

Base spreading resistance

Bulk resistance of the base

Microwave transistors (MT) are miniaturized design to

reduce device and package parasitic capacitances and

inductances and to overcome the finite transit time of

charge carriers in the semiconductor materials.

Contd

Three categories of MT:

Low-noise transistor which is employed in first stage

Low-level transistor which is used to drive power stage

Power transistor which amplifies final power output.

MT constructions:

Hybrid Microwave Integrated Circuits (HMIC)

The transmission lines and matching networks are

realised by microstrip circuit elements and the discrete

components.

Monolithic Microwave Integrated Circuits (MMIC)

All active devices and passive circuit elements are

fabricated on a single semiconductor crystal

Contd

a. Bipolar Transistor

Planar in form, Si n-p-n type operating up to

5GHz

GaAs used for performance improvements in

the operating frequency in high temperature

and radiation field

Devices Geometry:

Inter-digitated

Overlay

Matrix form with wide emitter area

Contd

Contd

Contd

Epitaxial n layer is formed by condensing a

single crystal film of semiconductor material

upon a low resistivity Si wafer of substrate

n+.

A p-type diffused base and n+-type diffused

emitter are formed

Typical value:

emitter width is 1 micron

base thickness is 2 microns

emitter length is 25 microns

Contd

High frequency limitations:

At high frequencies, the reactance due to junction

capacitances limit the gain. The values of these

capacitance depend on the depletion layers width or

the bias voltage and provides feedback paths. The

feedback effect is reduced in CB mode of operation. The

junction capacitance must be reduce as much as

possible.

Signal loss in the lead inductance can be minimized by

reducing the length in packaging

The transit time taken by the majority of charge

carriers to cross from the emitter to collector limits the

high frequency

Contdb. Unipolar Field Effect Transistor (FET)

Manufactured from GaAs for higher frequency of operation which

is achieved due to higher electron mobility compared to Si.

Frequency up to 40GHz, Noise figure 3dB.

Operates with only one type of carrier: electrons

Earliest FET

Junction FET (JFET)

Metal oxide semiconductor FET (MOSFET)

Insulated gate FET (IGFET)

Metal semiconductor FET (MESFET) Competitor to bipolar

Different design from bipolar

Bipolar : 10V 12V on the collector, biased the base and attached

emitter to ground

Unipolar: 10V 12V on the drain, biased the gate and attached source

to ground.

Contd

MESFET Construction MESFET equivalent circuit

Contd

c. High Electron M0bility Transistor (HEMT)

Latest addition to the line up of

solid state devices design.

Looks similar to GaAs FET

Single stage gain 15dB at 8 GHz,

noise figure 0.4dB

4.3 Parametric Amplifier

Parametric amplifier Named because its operation due to the periodic variation of the devices parameter such as capacitance of varactor diode under the influence of a suitable pump signal.

If a small input signal at a frequency fg and the ac power source operated as a pumping signal at a frequency fp are applied together to the varactor diode, linear amplification of a small signal results, due to time varying capacitance of the diode.

Pump signals provides the power required for amplification and the power output is either at the input frequency fg or at the idler frequency fi = fp-fg

Contd

Manley-Rowe Relations Derived a set of general energy relations regarding power

flowing into and out of an ideal nonlinear reactance.

These relations are useful in predicting whether power gain

is possible in a parametric amplifier.

Sometimes called Power-Conservation Relations (PCR)

Contd Advantages and limitations of Parametric Amplifiers

Noise figure

Less thermal noise due to minimum resistive elements

Range 1-2 dB

Frequency

Upper frequency limit (40-200GHz), set by difficulty of obtaining source power at the pump frequency and by the frequency at which the varactor capacitance can be pumped.

Lower frequency is set by the cut-off frequency of the microwave components used in the circuit

Bandwidth

Small due to the presence of tuned circuit

Gain

Limited to (20-80dB) by the stabilities of the pump source and the time varying capacitance