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CHAPTER - V

0

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5.0 SUMMARY CONCLUSION AND FUTURESCOPE

The thesis work addresses temperature behavior and compensation technique

of PIN diode, Schottky barrier diode and FET based microwave circuits. The

research work proposed and successfully demonstrates a novel temperature-

compensation technique, based on optimum bias load line technology, which

compensates temperature behavior of PIN and Schottky ba

microwave circuits, which is more accurate, simpler and more reliable than

previous reported circuits. The work shows that the same

applicable for temperature compensation of light emitting diode. The l

also presents and demonstrates practical and theoretical dc.>

temperature compensation mechanisms of various microwave

systems based on MESFET, HEMT, PIN diode and Schottky barru

diodeY*\ pI lv r

tpp h nIvv1 M OOM sV

s

I I rr

r *' i

L &

I «

lUUCu

SUMMARYand CONCLUSION5.1

A hitherto unexplored mathematical property of PIN diode, Schottky ban

diode and Light Emitting Diode has been discussed. This property is exploited in

a simple and easily adjusted control circuit that provides excellent setting

accuracy and temperature stability. It is shown that this approach can be used for a

wide range of practically available diodes. This circuit uses no separate

temperature sensor or compensating mechanism, but responds directly to the

junction temperature of the diodes. This prevents any errors caused by

temperature gradients, or by self-heating of the diodes due to high RF levels. The

proposed novel technology provides setting and temperature accuracy, which is

i Pr1C1

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better than accuracy available lrom MMIC based passive digital attenuator

circuits.

5.1.1 Temperature Dependency of Bandgap

Potential

Temperature compensation technique of PIN diode and light emitting diodes

based on the optimum bias load line technology, where open circuit voltage of the

control circuit depends upon bandgap potential of the semiconductors. All the

above discussion assumes that band gap potential is independent on temperature

or any other environment condition. However, bandgap potential of

semiconductor is a function of temperature and pressure. At room temperature and

under normal atmospheric pressure, the values of the bandgap are 1.12 eV for Si,

and 1.42 eV for GaAs. These values are for high-purity materials. For highly

doped materials, the bandgaps become smaller. Experimental result shows [1] that

the bandgaps of most semiconductors decreases with increasing temperature. The

bandgap approaches 1.17 eV, and 1.519 eV, respectively for Si and GaAs

semiconductor at 0 K. The variation of bandgap with temperature can be

expressed approximately by a universal function:

or2E,(T) = Et(0)—T + /}

where Eg(0), a and [3 are 1.17 eV, 4.73xl0'4 and 636 K for Si and 1.519 eV,

5.405x10 4 and 204 K for GaAs respectively.

(5.1)

Near room temperature, the bandgap of Si decreases with pressure, dEg/dP = -

2.4 xlO'6 eV/(kg/cm2). The bandgap of GaAs increases with pressure, dEJdP - -

12.6 xlO'6 eV/(kg/cm2).

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5.1.2 Temperature Dependency of BarrierHeight

Temperature compensation technique of Schottky barrier diodes based on the

optimum bias load line technology, where open circuit voltage of the control

circuit depends upon Schottky barrier height (ife) of the metal-semiconductors

junction of the diode. The change in the Schottky barrier height [52] with

temperature is given by:

#.(rW,(r.)+mk(r)-£!t(r.)] (5.2)

where m is in between 0 to 1, depending upon the surface state energy level.

Thus, Schottky barrier height decreases with the increase ot temperature as of

bandgap potential on the semiconductor.

5.1.3 Temperature Dependency of Bias Resistor

Temperature coefficient of bias resistors contribute to the temperature

coefficient of RF performance of the diode based circuits. Moreover, on-

TEMPERATURE CONTROLLED THERMAL CHAMBER

VRFFIREF Diodebasedcircuit(2ÿ Analog

Switch forR

B A

L

Fig.-5-l: Measurement to consider temperature dependency of

analog-switch, resistor, Eg, <1>B

resistance of the analog switches used in the driver circuit for stepwise control of

the RF performance has also some temperature coefficient. Figure-5.1 shows the

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schematic block diagram of a diode based circuit. The driver circuit contains

analog switch and series resistor, both have finite temperature coefficient. To

determine the optimum open circuit voltage of the circuit, the voltage and current

of the circuit is measured at point ‘C’ instead at the diode terminal point ‘A’.

Value of the resistor R is kept approximately equal to the actual value required for

the circuit. The diode (PIN, Schottky or LED) based circuit, analog switch and the

biasing resistor are all kept in the temperature controlled thermal chamber

shown in the figure. Three sets of voltage and currents are measured at three

different temperatures (two extreme operating temperatures and another at the

middle of the operating temperature) of the chamber, adjusting voltage/current to

the circuit for same performance, and are plotted as shown in the Figure-5.2. The

load line will be the best fitted straight line passing all these three points. The

intercept of the load line with the voltage axis will be the optimum open circuit

voltage of the bias network and inverse slope will give the extra resistor value

required which will be algebraically added to the resistor R.

as

The presented practical procedure takes into account the temperature

coefficient of the analog switch as well as bias resistor since measurement

reference point taken after these components.

(VREFI, IREFI)

(VREF2» IREF2)

< ' (VREF3> IREF3)a4.4J Load Line

VREF (Volts) VOPT

Fig.-5.2: Plot of load line

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Today, very precession voltage source, based on the bandgap reference are

available with excellent temperature stability. Thus, the above procedure with the

temperature stable voltage source provides highly accurate temperature stable

performance of the diode (RF performance of PIN and Schottky barrier diode and

illumination of LED) based circuits.

The proposed optimum bias load line technology for PIN and Schottky barrier

diode definitely solve the problems of temperature variation of RF performance ol

the diode based circuits. RF circuit manufactures definitely encouraged to use PIN

diode and Schottky barrier diode for dynamically RF signal control circuit such

attenuator, phase shifter linearizer, etc. Similarly, using optimum bias load line

technology, lighting industry will provide temperature compensated LED

illumination without increasing circuit complexity.

as

5.1.4 Self-Heating and Thermal Runaway

The proposed driver circuits for diode based circuit uses no separate

temperature sensor or compensating mechanism, but responds directly to the

junction temperature of the diodes. This prevents any errors caused by

temperature gradients, or by self-heating of the diodes due to high power

dissipation of the diode.

In case of fixed current bias condition, the device current remains constant

irrespective of the diode temperature. However, when diodes are operated under

temperature compensation condition, the diode current will increase with the

increase of diode temperature this will further increase the diode temperature and

diode current. This effect may cause thermal runaway of the device. Though, the

series resistance of the bias network limit the current to the diode, for more

precaution, it should be ensured that the device operation is within the limited

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temperature variation environment to keep the device current and power

dissipation within the allowable limit.

5.2 FUTURE SCOPE OF THE WORK

The thesis work invented and successfully demonstrated the “optimum bias

load line technology” for achieving temperature invariant performance of the

junction diode-based circuits. Although optimum bias load line technology solve

the temperature variation problem of the diode-based circuits, there is no such

type of technology evolved for microwave transistor based circuits. As discussed

in the previous chapters that many ECPs are function of temperature and these are

also vary from one device to another device. Thus, in the design phase it is nearly

impossible to incorporate accurate temperature compensation circuit to meet the

performance requirement over the broad operating temperature environment.

use of separate gainThough, the thesis work proposed and demonstrate the

variation block to compensate the overall gain variation of the amplifier it is noi

suitable to compensate the output power variation of the power amplifier.

5 C ‘Bent, Qujarat ‘University, India.Heg. 9{p.: 5024. 19,k January, 2004.Thesis

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