OPTIMUM TRIGGERING CONDITIONS FOR A

TRAPA TT OSCILLATOR

Wolfgang J. R. Hoefer and Yvon Depratto *

1. Introduction

The high efficiency mode of oscillation in avalanche diodes

(TRAPA TT-mode) has been studied extensively in recent years.

Most authors agree basically on the theory of the steady state

of such oscillations, but there exist different theories and

observations concerning the initiation of the high efficient

mode, i. e. the process in which the current density in the diode

rises to a value necessary to launch an avalanche shock front

into the depletion region.

Three different starting mecanisms have been reported

to date:

i. The generation of voltage swings at the TRAPA TT

(rr - transit-angle) frequency by trapping the IMPATT oscillation. h '

h Q. 1,2,3 .

ln a 19 - cavlty.

ii. The buildup of space-charge independent transit-time

oscillations of small transit-angle. 4

*Department of Electrical Engineering,University of Ottawa,Ottawa, Ontario,.Canada

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iii. The direct launching of a shock front by application of a

very steep voltage pulse to a diode, whose parasitic impedances

h b ". d5

ave een minimize.

In this study, the triggering behaviour of the low cost FD-300

TRAPA TT diode has been investigated in coaxial and microstrip

circuits of the Evans 2 type. The purpose of this study was to

determine the mecanisms that initiate the TRAPA TT mode in

this particular diode, and to identify the circuit parameters that

govern the triggering process. No attempt was made to modify

the active part of the device itself, however, the rC'le of package

parasitics has been taken into consideration. The results

obtained in this study lead to a number of statements about the

optimum triggering conditions for this TRAPATT oscillator.

2. The diode

All experiments reported in this paper were carried out

using a low cost, commercially available computer diode (FD-300).

This p + -n-n + structure (silicon) is capable of oscillating in

the pulsed TRAPA TT mode at frequencies ranging from about

250 MHz to 1. 5 GHz. Peak power levels approach 100 watts in

this range.' Table 1 summarizes the essential parameters of a

typical FD-300 diode. 6, 7

MT --210

Depletion width (at breakdown) (W c)

Doping concentration in the n-region (Nd)

Junction diameter Dj

Static breakdown voltage (VB)

10 \JIm

1O15/cm3

210 ~m

Punch-through voltage (Vp)

Saturation current Is (250 C,

Thermal resistance (RJC)

Package series inductance Ls

Junction capacitance Cj (below breakdown)

230 V

"" 60 V

10 V reverse bias) 50 x 10-12 A

1O00C /Watt

"" 4 nH

"" 0.4 pF

Table 1 Parameters of a typical FD-300 (TRAPATT) diode

(After Chudobiak6 and Chaffin 7)

3. The TRAPATT circuit

The diodes were studied in a coaxial TRAPATT circuit of the

type described by Evans2 and a microstrip circuit reported by

Chudobiak8. Fig. 1 shows the microstrip TRAPATT circuit and

it's essential parameters. Fig. 2 presents the diode load impe-

dance, on-voltage, peak output power and efficiency as a function

of frequency for a O. 3 ~sec, 1. 25 A bias pulse, as measured in

Chudobiak's circuit. P'~rformance was optimized by tuning the

output low pas s filter until maximum output power and a stable

output spectrum were obtained at the desired frequency. It is

reasonable to assume that these conditions provide optimum

triggering of the TRAPATT mode in this circuit. The performance

of the diode in toe coaxial circuit was practically identical, except

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Fig. I

Id BiasMicrostrip

Oscillatorc:J--

RF(After Chudobiak8)I

":-

t. , , . . .R~

.f~~.0Q

~V~'.. :o~~~, .

!w~~t~.-~ ,:_, ,

, 300 400 500 600. 700 800f'~ 1000MHZ

.~j~

Fig. Z

Microstrip

Oscillator

Pout

performance.

Ibias: 1. 25A

0--Bias pulse

duration: O. 3 j.Ls

Pulse repetition

frequency: I KHz

( Courtesy of W. J. Chudobiak)

at frequencies above IGHz, where the efficiency was 3% higher

than that of the microstrip oscillator due to the inherent losses

of the latter.

4. ExEeriments and observations

In a first experiment, spectrum analysis of the diode current

and voltage was performed. The voltage drop in a I ohm disc re-

sistor between ground and positive diode terminal provided the

current signal. The voltage at the diode was measured through a

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3 kG resistor into 50 G. (Fig. 1). It was verified that both probes

had no measurable influence on the oscillator performance.

Particular attention was paid to the natural IMPA TT frequency

(rr - transit-angle), which was often reported to trigger the

avalanche shock front. For the given device, this frequency was

calculated to be in the vicinity of

f = =7

10 cm/s

2 x 10 -3cm= 5 GHz

vns

IMPA TT 2 We

Where v is the saturated electron velocity and W the depletionns cwidth of the diode.

Harmonics of the TRAPA TT frequency were detected in a

range extending into the X -band, but they all disappeared when

the bias pulse duration was reduced to 100 ns so that the TRAPA TT

mode was not yet established and only transitory oscillations were

present. However, a small signal ( I db above noise level) could

be detected at 4 GHz (:t 200 MHz, depending on the diode under

test). The frequency of this signal was independ~nt of circuit

tuning over the whole TRAPATT frequency range. This suggests

a spurious resonance of the package series inductance Ls with the

junction capacitance Cj' since the values for Ls and Cj in table 1

indicate that resonance should occur at 4GHz. However, the si-

gnal amplitude was so small (corresponding to a current density

in the diode of about 0.1 AI cm2) that it could not playa significant

role in the triggering of the avalanche shock front, which requires

1700 A/cm2. It was concluded that IMPATT oscillatlons did not

contribute to the build-up of the TRAPATT mode in this oscillator.

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In order to complement the spectrum analysis of the diode

waveforms, which yields no information of the time behaviour of

the different frequency components, the growing oscillation was

recorded at various TRApA TT frequencies nsing a sampling

oscilloscope. The following figures show the most characteristic

cases which will be ana1ysed and.discussed in the next section.

>

~>010

a. Diode voltage 10 ns / dive

>

~«10

0

b. IOns / di v .Diode current

Fig. 3 Triggering of a TRAPA TT oscillation

fTRAPATT ::: 330 MHz IBias ::: 1. 25A

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.~~>010

a. Diode voltage 10 ns / div.

.>

~<10.0

b. Didde current 10 ns /div.

Fig. 4

Triggering of a

TRAPATT oscilla-

tion

f -TRAPATT - 550 MHz

IBias = 1. 25 A

:>."

~0U')

Fig. 5

fTRAPA TT =660 MHz

Triggering of a TRAPATT oscillation (Diode voltage)

IBias = 1. 25 A

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>."

~0I.(')

TRAPA TT oscillation

Fig. 6

Triggering of a

a. Diode voltage 5 ns / div. fTRAPA TT =810 MHz

IBias = 1. 25 A

>

~~I.(')

d

b. Diode current 5 ns/div.

5. Discussion

The analysis of the TRAPATT process by Clorfeine3 as applied

to the FD-300 by Chaffin7 shows that it's optimum TRAPATT

frequency lies between 630 and 700 MHz. The period of this fre-

quency corresponds to one cycle of plasma triggering and extrac-

tion at twice the critical current density. assuming a rectangular

current waveform. Thus. breakdown would occur natura1y at this

rate provided that sufficient current is delivered promptly by the

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resonant circuit. The above conditions are closely met in our

circuit.

Inspection of Fig. 3 to 6 actually reveals that in all cases

the oscillation starts at a harmonic which is closest to this

natural frequency. When-the circuit is tuned to 330 :MHz (Fig~ 3),

the growing waveform at the second harmonic is virtually sinusoi-

dal. Late in the process, higher harmonics grow and shape the

signal in the typical TRAPA TT fashion. In the circuit tuned for

550 MHz (Fig. 4), the first voltage swings occur at the fundamen-

tal frequency which is close to the optimum range. The current.waveform reveals that the second harmonic soon triggers a

premature breakdown of limited significance until wave shaping

enhances the first harmonic again. At 660 MHz (Fig. 5), the

rather clean triggering waveform at the fundamental fr~quency

underlines the significance orth~s frequency as a.natural parameter

of the device. At 810 MHz (Fig. 6), seizable oscillations occur

at the fundamental frequency only. The secondary breakdown is

even more limited than at 330 MHz and 550 MHz. This is

pr.obably due to the series package inductance Ls, which acts as

a low pass filter.

In order to assess the role of this inductance, the diode was

unpackaged, and several values of inductance were added in series

with the chip. It was found that for inductance values differing by

more than 30% from the package inductance Ls' triggering became

very erratic and intermittent, reducing the output power typically

by 3 db.

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The following conclusions can be drawn from the above

observations:

i. The presence of a series inductance of about 4 nH

provides an optimum condition for triggering the TRAPATT mode

between 300 MHz and 1000 MHz by reducing high frequency

ringing at the diode and limiting premature avalanching at fre-

quencies higher than 660 MHz. This is in agreement with

observations made by Carroll and Crede9.

ii. Triggering occurs always at the harmonic frequency that

is closest to the natural TRAPA TT frequency of 660 MHz. The

resonant circuit must therefore poesess 'sufficient Q at this fre-

quency in order to support the triggering oscillations.

The authors would like to thank Dr. W. Chudobiak for advice

and discussions. The work was financially supported by the

National Research Council of Canada under grant No. A 7620.

6. References.

1 Deloach, B.C. - Scharfetter, D.L.: Device physics ofTRAPATT oscillators, IEEE Trans. on Electron Devices, Vol.ED-17, No.!, Jan. 1970, pp. 9-21

2 Evans, W.J. : Circuits for high-efficiency avalanche diodeoscillators', IEEE Trans. on Microwave Theory and Techniques,Vol. MTT-17, No. 12, Dec. 1969, pp. 1060-67

3Clorfeine, A.S. - Ikola, R.J. - Napoli, L.S. : A theory for

the high-efficiency mode of oscillation in avalanche diodes, RCAReview, Vol. 30, Sept. 1969, pp. 397-422

4 Culshaw, B. : High-efficiency low frequency operating modesin avalanche diodes under low-current-density conditions, Proc.IEEE, Vol. 117, No. 12, Dec. 1970, pp. 2221-2227

MT - 218

5 Yanai, H. - Torizuka, H. - Yamada, N. : Large amplitudehigh-efficiency oscillation using Si-avalanche diode and it ISexperimental analysis, 8th Int. Conf. on Microwave and opticalgeneration. Amsterdam, 1970

6 Chudobiak, W. : Microstrip TRAPA TT oscillator, Electro-nics Letters, -Yolo 6, No. 14, July 1970, pp 438-9

7 Chaffin, R.J. : High-power TRAPA TT oscillations fromparallel connected low-cost diodes, IEEE Trans. on MicrowaveTheory and Techniques, Yolo MTT-18, No. 11, Nov. 1970.pp. 985-6

8 Chudobiak, W. : TRAPATT Microstrip Oscillator, PaperNo. 71201, International Electronics Conference, Toronto,Oct. 1971

9 Carroll, J.E. - Crede, R.H. : A computer simulation ofTRAPATT circuits, Int. J. Electronics-, Yolo 32, No.3, 1972,pp. 273-296

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ON MICROWAVE COMMUNICATION

Budapest, 24-30 June, 1974

15IV. MT - 219