Marconi Applied Technologies Gunn Diodes Application Notes

Marconi Applied Technologies 

Gunn Diodes Application Notes 

CONTENTS 

1 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

1.1 Package Styles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

1.2 Gunn Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2 PRINCIPLE OF OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2.1 Gunn Diode Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2.2 Modes of Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

2.3 Conventional Gunn Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

2.4 Graded-gap Gunn Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

3 APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

3.1 Oscillator Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

3.2 Performance Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

3.3 Mounting and Heat Sink Considerations . . . . . . . . . . . . . . . . . . . . . 15 

4 PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . 15 

4.1 Limiting Conditions of Use . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

4.2 Typical Performance Curves . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

5 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

# 2000 Marconi Applied Technologies Limited 

This work must not be copied in whole or in part without the prior written permission of Marconi Applied Technologies Limited 

Marconi Applied Technologies Limited, Carholme Road, Lincoln LN1 1SF England Telephone: +44 (0)1522 526352 Facsimile: +44 (0)1522 545140 

e-mail: mtech.uk@marconi.com Internet: www.marconitech.com Holding Company: Marconi p.l.c. 

Marconi Applied Technologies Inc. 4 Westchester Plaza, PO Box 1482, Elmsford, NY10523-1482 USA Telephone: (914) 592-6050 Facsimile: (914) 592-5148 e-mail: mtech.usa@marconi.com 

#2000 Marconi Applied Technologies Limited A1A-Gunn Diodes AN1 Issue 2, June 2000

" 

1 DESCRIPTION 

The Gunn diode is the best known and most readily available 

device in the family of transferred electron devices (TED). They 

are employed as DC to microwave converters using the 

negative resistance characteristics of bulk Gallium Arsenide 

(GaAs) and only require a standard, low impedance, constant 

voltage power supply, thereby eliminating complex circuitry. 

The DC1200 series of GaAs Gunn diodes is designed for 

operation at fixed frequency (determined by the oscillator 

cavity) within a specified band under CW or pulsed conditions. 

Both conventional and hot electron injected (graded-gap) Gunn 

diode designs are available (see Section 2). All devices feature 

low FM and AM noise characteristics and their wideband 

negative resistance enables them to be used in voltage 

controlled and mechanically tuned oscillators. The hot electron 

injected Gunn diodes offer the additional advantages of: 

* higher fundamental frequency operation, 

* increased efficiency, 

* improved temperature stability, 

* improved turn-on characteristic and 

* reduced FM sideband noise. 

Devices are available in either polarity, but are generally 

supplied as negative heat sink. 

1.1 Package Styles 

Several standard package styles are offered covering the 

frequency range 4 to 110 GHz and designed to suit different 

applications (see Fig. 1.1). Custom packaging requirements will 

also be considered upon request. 

The following package limitations need to be taken into 

consideration: 

Thermal Resistance 

The lowest thermal resistance is offered by the screw-based 

packages (outlines 40, 86 and 106). For high volume, lower 

power applications where oscillator assembly time is more 

important, non-threaded packages such as outline 00 will be 

preferred (refer to Mounting and Heat Sink Considerations - 

Section 3.3). 

Frequency Range 

The equivalent circuit diagram of a packaged Gunn diode is 

shown in Fig. 1.2. The magnitude of the parasitic impedances 

attributed to the package element reduces with the package 

size (004864106). Consequently, the larger, more robust 

package styles are normally specified for operation at lower 

frequencies and the smaller, low parasitic impedance packages 

are recommended for the higher frequency applications. 

Magnetically Tuned Circuits 

For applications involving magnetically (YIG) tuned oscillators, 

Gunn diodes can be supplied in Kovar-free packages. 

00 

" 

5.3/5.9 

" 

" 

11.52/1.63 

" 

" 

11.98/2.08 

" 

1.52/1.63 1.70/1.85 1.52/1.63 13.00/3.15 

" 

" 

" 

" 

" 

" 

" 

40 

" 

" 

0.75/0.85 

3-48 UNC 2A THREAD 

0.22/0.28 

" 

" 

" 

0.53/0.63 

" 

12.05/2.15 

12.95/3.00 

" 

" 

" 

" 

" 

12.45/2.55 

5.28 MAX 

" 

" 

" 

Fig. 1.1 Standard Gunn Diode Package Outlines (continued on page 3) 

Gunn Diodes AN1, page 2 

#2000 Marconi Applied Technologies

86 

" 

4.0/4.5 

" 

0.63/0.78 

" 

" 

0.46/0.56 

" 

" 

" 

" 

11.22/1.32 

" 

12.49/2.59 

" 

" 


106 

" 

3.5/3.7 

" 

" 

" 

0.47/0.57 

" 

12.89/3.00 

" 

" 

10.72/0.90 

" 

" 


" 

" 

0.31/0.47 

1.2 Gunn Oscillators 

Marconi Applied Technologies designs and manufactures a 

range of oscillator products using Marconi Gunn diodes. 

Standard products include a range of fixed frequency and 

mechanically or electronically tuned oscillators. Enquiries are 

welcomed for oscillator products that are not in the standard 

range. 

7504 

R S L P 

C P 

2 PRINCIPLE OF OPERATION 

2.1 Gunn Diode Theory 

In order to understand the nature of the transferred electron 

effect exhibited by Gunn diodes, it is necessary to consider the 

electron drift velocity versus electric field (or current versus 

voltage) relationship for GaAs (see Fig. 2.1). 

Below the threshold field, E th , of approximately 0.32 V/mm, the 

device acts as a passive resistance. However, above E th the 

electron velocity (current) decreases as the field (voltage) 

increases producing a region of negative differential mobility, 

NDM (resistance, NDR). This is the essential feature that leads 

to current instabilities and Gunn oscillations in an active device 

and is due to the special conductance band structure of direct 

band gap semiconductors such as Gallium Arsenide (see Fig. 

2.2). 

7R G 

C G 

" 

7486 

" 

DIODE ELEMENT 

PACKAGE ELEMENT 

C P = PACKAGE CAPACITANCE 

L P = BOND LEAD INDUCTANCE 

R S = SERIES RESISTANCE 

7R G = GUNN DIODE RESISTANCE 

C G = GUNN DIODE CAPACITANCE 

" 

DRIFT VELOCTY v (CURRENT) 

E th 

0.32V/mm 

ELECTRIC FIELD E (VOLTAGE) 

E H 

" 

Fig. 1.2 

Simple Equivalent Circuit for a Packaged Gunn 

Diode 

Fig. 2.1 The Electric Field Dependent Average Drift 

Velocity for N-type GaAs 

#2000 Marconi Applied Technologies Gunn Diodes AN1, page 3

4 

7487 

T = 300K 

3 

CONDUCTION BAND 

SATELLITE X 

VALLEY 

2 

SATELLITE L 

VALLEY 

" 

" 

" 

DE = 0.31 eV 

" 

" 

" 

1 

LOWER G 

VALLEY 

E g = 1.42 eV 

0 

" 

VALENCE BAND 

71 

ENERGY (eV) 

72 

L [111] G " [100] 

" 

X 

Fig. 2.2 

The Band Structure of GaAs at 300 K 

The energy-momentum relationship contains two conduction 

band energy levels, G and L (also known as valleys) with the 

following properties: 

* In the lower G valley, electrons exhibit a small effective 

mass and very high mobility, m 1 . 

* In the satellite L valley, electrons exhibit a large effective 

mass and very low mobility, m 2 . 

* The two valleys are separated by a small energy gap, DE, 

of approximately 0.31 eV. 

In equilibrium at room temperature most electrons reside near 

the bottom of the lower G valley. Because of their high mobility 

(* 8000 cm 2 V -1 s -1 ), they can readily be accelerated in a strong 

electric field to energies in the order of the G-L intervalley 

separation, DE. Electrons are then able to scatter into the 

satellite L valley, resulting in a decrease in the average electron 

mobility, m, as given below: 

m =(n 1 .m 1 +n 2 .m 2 )/(n 1 +n 2 ) 

where n 1 = electron density in G valley 

n 2 = electron density in L valley 

Above the high field, E H , most electrons reside in the L valley 

and the device behaves as a passive resistance (of greater 

magnitude) once again. 

In a practical Gunn diode, electrons are accelerated from the 

cathode by the prevailing electric field. When they have 

acquired sufficient energy, they begin to scatter into the low 

mobility satellite valley and slow down. This charge fluctuation 



esults in a localised increase in the electric field and a 

corresponding decrease (to below the threshold value) 

elsewhere within the sample. Electrons ahead of the region still 

exhibit high mobility and move away from the charge 

fluctuation causing a depletion of carriers. Electrons behind 

the charge fluctuation, are also moving faster and accumulate 

behind the depleted region causing a dipole (or high field) 

domain (see Fig. 2.3), which grows in amplitude. 

A fall in current is associated with the domain growth due to its 

reduced mobility. The domain propagates through the sample 

at a constant velocity (*1 x 10 7 cm.s 71 for GaAs) until it 

reaches the anode where it collapses. As the domain collapses, 

the electric field outside the domain rises and the current in the 

external circuit increases until the threshold field is reached 

again and a new domain is formed. 

For a given doping density there will be a minimum device 

length that will support a domain due to the finite time required 

for domain growth. If this time is longer than that required for 

the domain to traverse the sample, then domain formation will 

not occur. Similarly, if the doping density is too high, then the 

current (and hence the temperature) in the semiconductor 

becomes too great and the life expectancy of the device is 

reduced. In practice, the product of doping density, n, and 

device length, l, is maintained between the following limits: 

1x10 12 cm 72 4 n.l 4 2x10 12 cm 72 

SPACE CHARGE 

" 

7488 ACCUMULATION LAYER 

" 

DEPLETION LAYER 

" 

0 

CATHODE 

" 

DISTANCE 

ANODE 

ELECTRIC FIELD 

" 

E th 

E H 

E L 

ANODE 

0 

CATHODE 

DISTANCE 

" 

Fig. 2.3 

The Band Structure of GaAs at 300 K 


2.2 Modes of Oscillation 

2.2.1 Transit Time Mode 

This is the basic diode oscillation mode and is independent of 

the external circuit. Current peaks are obtained when a domain 

is quenched at the anode, after which another is nucleated near 

the cathode. The frequency is determined by the domain transit 

time, T t : 

T t = l/v D 

where l = length of device 

v D = domain velocity (* 1x10 7 cm.s -1 ) 

so frequency, f t : 

f t = 1/T t =v D /l 

This mode of operation was first reported by (and is named 

after) J. B. Gunn in 1963. Its main characteristics are (see Fig. 

2.4): 

* The total electric field across the device at any time is 

above the threshold value. 

* The current waveform consists of narrow spikes, indicating 

a high harmonic content and low efficiency at fundamental 

frequency. 

* The RF field across the device is small, indicating low 

impedance. 

* The transit time frequency is a strong function of operating 

voltage and temperature. 

Therefore the transit time mode has poor stability and 

efficiency. 

7489 

I 

" 

I p 

E 0 

T t 

t 

I v 

" 

" 

" 

E th 

" 

E 

t 

" 

Fig. 2.4 

Current and Field Waveforms for Transit Time Mode 



2.2.2 Delayed Domain Mode 

Fig. 2.5 shows the basic voltage and current waveforms for this 

mode of operation. Unlike the transit time mode, the total 

electric field across the device drops below the threshold value, 

E th , during part of the RF cycle, such that nucleation of a new 

domain is delayed. 

As soon as the field rises above E th , a domain nucleates at the 

cathode and travels across the device. As the field swings 

below E th , the domain arrives at the anode and decays its 

charge. A new domain cannot form at the cathode until the 

field rises above E th again. This delay time between extinction 

and creation of domains modifies the operating frequency, f d , 

to: 

f d = 1/(T t +T d ) 

where T t = transit time 

T d = delay time 

The transit time is a fixed quantity for a given device, but the 

delay time is a function of the RF voltage, which is determined 

by an external circuit. It follows that the operating frequency is 

always below the transit time frequency. 

The important characteristics of this mode of operation are: 

* The total electric field across the device is below the 

threshold field, E th , over a part of the RF cycle. 

* The current waveform consists of broad spikes, indicating 

a low harmonic content and higher efficiency at 

fundamental frequency. 

* The RF field across the device is large, indicating high 

impedance. 

* The operating frequency is determined mainly by the 

resonant frequency of the external circuit and can be made 

very stable. A device can also be used over a much 

broader bandwidth below the transit time frequency. 

This mode of operation is therefore most commonly used in the 

majority of commercial applications. 

7490 

I 

" 

I p 

I v 

t 

E 0 

" 

T d T t 

" " 

" 

E th 

" 

E 

t 

" 

Fig. 2.5 

Current and Field Waveforms for Delayed Domain Mode 


2.3 Conventional Gunn Diode 

A conventional Gunn diode generally consists of three layers; a 

relatively low doped transit region sandwiched between two 

highly doped contact regions, forming an n + -n- + structure 

(see Fig. 2.6). The device is defined basically by five 

parameters: 

n The doping concentration in the transit region of the 

Gallium Arsenide. 

l The thickness of the transit region. 

R 0 The low field resistance of the diode measured close to 

the origin. 

I th Threshold current. This is the maximum current through 

the device and can exceed the operating current by as 

much as 50%. 

NB. The power supply must be capable of passing 

through this point. 

V th Threshold voltage. The voltage at the current 

maximum. 

The ratio 7R G /R 0 is an important circuit design parameter. V th 

and I th are independent of the external circuit and may be 

measured with the diode in any good heat sink. Because V th is 

measured on the flat part of the curve, the measuring technique 

must be carefully specified to avoid error. 

7491 

I 

I th 

" 

R 0 

The frequency of the Gunn diode is determined primarily by the 

transit region length, l. However, a portion of this region is used 

to accelerate the electrons from the cathode until they have 

sufficient energy to enter the low mobility state and does not 

support domain formation. This ‘dead zone’ may be as much as 

0.25 mm in a transit region length of 1.5 mm (for a millimetric 

diode) and, because it acts as a parasitic resistance, results in 

reduced efficiency. For conventional Gunn diodes there is a 

rapid fall-off in power at frequencies above 60 GHz, where the 

less efficient second harmonic component of the power has to 

be utilised. 

The starting voltage must be well below the required operating 

voltage (typically V op = 3 x V th ), particularly when low 

temperature operation is required, as the starting voltage rises 

with falling temperature. This can be controlled to some extent 

by the correct choice of n and l, but severely restricts the use of 

conventional Gunn diodes at temperatures below about 725 8C. 

The turn-on voltage, V on (the voltage above threshold at which 

coherent RF power is obtained), increases to the point where it 

equals the peak power voltage V pk (see Fig. 2.7a). This forces 

the diode to be operated at a higher voltage with corresponding 

loss in power, reduced efficiency, poor FM sideband noise and 

the increased possibility of device failure. 

2.4 Graded-gap Gunn Diode 

The limitations of the conventional Gunn diode (described 

earlier) can be overcome by injecting high energy, ‘hot 

electrons’, into the transit region. The concept is to introduce 

electrons into a region so that the temperature, which describes 

their energy distribution, is much greater than that of the 

semiconductor lattice. In the graded-gap Gunn diode, electrons 

are injected into the transit region with an energy equal to that 

of the G-L intervalley separation so that stable domains will 

form very near to the cathode and move across the transit 

region as soon as the field is high enough to sustain 

accumulation and propagation. The dead zone is effectively 

eliminated and the transit length is fixed and independent of 

bias. Therefore, coherent power can be generated over a wide 

range of operating voltages (see Fig. 2.7b). 

V th 

" 

V 

7492 

n ++ n ++ 

DOPING DENSITY 

" 

" 

l 

" 

n 

OHMIC 

CONTACT 

TRANSIT REGION 

SUBSTRATE 

OHMIC 

CONTACT 

Fig. 2.6 

Structural Schematic and Current-Voltage Relationship for a Conventional Gunn Diode 



V on V pk 

7493 

POWER (mW) 

50 

" 

" 

25 

0 

0 

a) 

1 2 3 4 5 6 

VOLTAGE (V) 

V pk 

50 

" 

POWER (mW) 

25 

V on 

" 

0 

0 

b) 

1 2 3 4 5 6 

VOLTAGE (V) 

Fig. 2.7 

Typical Turn-on Characteristic of: a) Conventional Gunn Diode and b) Graded-gap Gunn Diode 

There are a variety of possible structures which can be used for 

hot electron injection. To produce microwave power with as 

little bias dependence as possible, the optimum injector shape 

has a slowly increasing potential with an abrupt drop back to 

the transit region value. This is best realised with a graded-gap 

injector (see Fig. 2.8). Simulations have shown that a thin n + 

layer between the injector and the transit region is critical for 

controlling the electric field, while retaining the hot electron 

properties. Fig. 2.8b shows the n + spike depleted giving an 

injection energy of DE. 

An additional benefit from using a graded-gap Gunn diode is 

the much improved temperature stability. This is due to the 

electron temperature being set by the injection energy, typically 

equivalent to 2000 K. Changes in the substrate temperature in 

the 130 8C range usually required for military specifications, etc, 

are relatively small in comparison. 

The structural schematic and current-voltage relationship for a 

graded-gap Gunn diode are shown in Fig. 2.9. Under forward 

bias, the peak current is much less defined due to the action of 

the cathode injector (i.e. significant numbers of electrons 

already reside in the upper valley when entering the transit 

region) and the gradient of the curve beyond threshold is 

shallower. 


7494 

n + SPIKE 

BIAS = 0 

" 

CONTACT INJECTOR TRANSIT REGION CONTACT 

" 

" 

" 

" 

" 

" 

a) 

e 7 

" 

" 

GRADED 

GAP 

" 

*DE 

BIAS = V 

V 

" 

" 

n + n + n 7 

n + 

b) 

Fig. 2.8 A Schematic Diagram of a Hot Electron Injector Gunn Diode under a) Zero Bias, and b) Forward Bias V. 

7495 

REVERSE 

1 

FORWARD 

CURRENT (A) 

0 

0 

1 2 3 4 

BIAS (V) 

7496 

n ++ n + n ++ 

DOPING DENSITY 

" 

" 

l 

" 

n 

OHMIC 

CONTACT 

GRADED 

AlGaAs 

TRANSIT REGION 

SUBSTRATE 

OHMIC 

CONTACT 

Fig. 2.9 

Structural Schematic and Current-Voltage Relationship for a Graded-gap Gunn Diode 



Graded-gap Gunn diodes have demonstrated room temperature 

performance of up to 80 mW and 2.4% efficiency at 90 GHz; 

the best results achieved for mm-wave GaAs devices at this 

frequency. 50 - 60 mW at 94 GHz, with efficiencies of 1.6%, is 

reproducibly achieved (see Table 2.1). FM sideband noise is 

better than 780 dBc/Hz, 100 kHz from carrier, equal to that 

obtained from the best conventional devices. Significantly 

these devices exhibit a turn-on voltage very close to threshold 

(see Figure 2.7b), which allows coherent oscillations around 

peak power over the full military specification temperature 

range. Further evidence of the improved temperature stability 

can be seen from the power, frequency and peak power 

variations across this temperature range, generally a factor of 

two or more below that exhibited by devices without hot 

electron injection. This is an added bonus for VCO designers 

who can then utilise a larger bandwidth, because there is no 

longer need to compensate for frequency drift with 

temperature. 

Freq 

(GHz) 

Temp 

(8C) 

V on 

(V) 

V op 

(V) 

I op 

(mA) 

Power 

(mW) 

Eff 

(%) 

Noise * 

(dBc/Hz) 

90 740 3.9 4.9 680 58 1.75 

90 +25 3.2 4.7 660 50 1.6 786 

90 +80 3.1 4.8 640 42 1.4 

3 APPLICATIONS 

Gunn diodes are reliable, relatively easy to install and the lower 

output power levels fall well below the safety exposure limits. 

They are ideally suited for use in low noise sources such as local 

oscillators, locking oscillators, low and medium power 

transmitter applications and motion detection systems. Higher 

power varieties can be used in phase-locked oscillators or as 

reflection amplifiers in point-to-point communication links and 

telemetry systems. 

Microwave sources have the advantages over ultrasonic 

detectors of size and beamwidth, and over optical systems of 

working in dusty and adverse environments. The low voltage 

requirements of Gunn oscillators mean that battery or regulated 

mains supplies may be used, (battery drain can be reduced by 

using low current devices or by operation in a pulsed mode). 

However, microwaves are reflected from metal surfaces and 

partially reflected from many others e.g. brick, Tarmac and 

concrete, and they are attenuated by oxygen, water or water 

vapour. Figure 3.1 shows attenuation effects in the frequency 

range of interest. 

94 +25 3.0 4.5 600 60 2.0 788 

60 +25 3.0 4.5 600 120 5.0 790 

35 +25 3.0 4.5 1300 350 5.5 795 

* 100kHz offset frequency 

Table 2.1 Typical Performance of Graded-gap GaAs Gunn 

Diodes 

FREQUENCY (GHz) 

10 15 

30 

60 70 95 100 150 220 300 

100 

183 GHz 

60 GHz 

7497 

ATTENUATION (dB/km) 

10 

1.0 

0.1 

SEA LEVEL 

" 

22 GHz 

" 

" 

0 2 

118 GHz 

0 2 

" 

H 2 0 

0.01 

0.001 

" 

" 

H 2 0 

4km 

ITU REGION 2 RADIOLOCATION BANDS 

" 

" 

IEEE RADAR LETTERBANDS 

X Ku K Ka mm 

30 20 

10 

5 

3 

2 1 

WAVELENGTH (mm) 

Fig. 3.1 

Atmospheric attenuation at millimetre wavelengths with IEEE radar letterbands and ITU radiolocation bands for 

region 2 (after Altshuler et al) 


The range of application of Gunn sensors for industrial and 

commercial use is extensive and the following is only a brief list: 

Collision avoidance radar 

Vehicle ABS 

Traffic analyser sensors 

‘Blind spot’ car radar 

Pedestrian safety systems 

Elapsed distance meters 

Automatic identification 

Presence/absence indicators 

Movement sensors 

Distance measurements 

Slow-speed sensors 

Level sensors 

Traffic signal actuators 

Proximity movement detectors 

Door opening sensors 

Barrier operation 

Process control devices (object counting) 

Intruder/burglar alarms 

Perimeter protection 

Train derailment sensors 

Contactless vibration transducers 

Rotational speed tachometers 

Linear distance indicators 

Moisture content measurement 

Table 3.1 indicates the most commonly used Gunn diode types 

by application. 

APPLICATION 

Local Oscillators: 

Radar 

Fast tunable ECM 

Diode noise measurement 

Telecommunications: 

Transmitters 

Low noise oscillators 

Point-to-point links 

Control devices: 

Railway crossings 

Traffic control 

Vehicle ABS 

Door openers 

Motion detectors: 

Speed control 

Radar detectors 

Intruder alarms 

Shoreline navigation 

Low Power 

Fixed Frequency CW Broadband CW Pulsed 

High Power 

Low Frequency High Frequency 

* 

* 

* 

* 

* 

* 

* 

* 

* * 

* 

* 

* 

* 

Transmitters: 

Radar transponders 

Missile beacons 

Radio link exciters * 

Injection locked amplifiers * 

Paramp pump sources * 

Instrumentation * 

* 

* 

* 

* 

Table 3.1 

Gunn Diode Selection Chart 



7498 

" 

RF 

DIODE 

" 

RF 

IRIS 

DIODE 

" 

*lg/2 

" 

" 

*lg/2 

" 

Fig. 3.2 

Basic Microwave Cavity Design 

3.1 Oscillator Design 

The natural frequency of oscillation of the Gunn diode can be 

altered to some extent by the external circuit. The formation or 

premature collapse of a domain within a cycle can be 

controlled, and hence the power, frequency and efficiency 

adjusted. Control of the frequency by external means rather 

than by device parameters only is essential for a stable 

oscillator. The external cavity used to resonate the device 

negative impedance can be coaxial, waveguide, microstrip, YIG 

crystal, etc, depending on the required application. 

Generally, coaxial and microstrip circuits offer low Q values 

with poor noise and stability performance, resulting in the 

frequency of oscillation changing with load and environmental 

variations. YIG crystal tuned circuits are usually too expensive 

for commercial and industrial applications and therefore the 

most common cavity is waveguide. Usually the Gunn diode is 

mounted on a post structure between the waveguide walls, 

either lg/2 from an iris or lg/2 from a short circuit (see Fig. 

3.2). Some alteration is necessary to set the exact frequency to 

allow for diode and package parasitics and manufacturing 

tolerances. Tuning screws of either metal or dielectric are used 

to modify the cavity resonant frequency. Power output 

variations are achieved by adjusting the coupling between 

diode and load using variations in post size or tuning screws. 

A more detailed schematic of a packaged Gunn diode mounted 

in a radial disc, microwave cavity and its equivalent circuit 

diagram are given in Figs. 3.3 and 3.4. 

BACKSHORT 

GUNN DIODE 

" 

7499 

" 

v 

" 

" 

" 

BIAS CHOKE 

FILTER 

WAVEGUIDE 

RADIAL DISC 

Fig. 3.3 The Packaged Gunn Diode Mounted in a 

Microwave Cavity (approximately half scale). 

7500 

BIAS 

CHOKE 

FILTER 

WAVEGUIDE 

BACKSHORT 

L P 

DISC 

LOAD 

-R G 

L B 

C P 

C D 

C S 

Fig. 3.4 

An Equivalent Circuit Diagram for the Gunn Diode, Package, Disc and Cavity 


Most simple Gunn oscillators are designed empirically using a 

few ground rules and a lot of experience. The designs have to 

tolerate batch to batch variations in diode parameters and have 

the required stability. They must not present the diode with an 

RF impedance such that the device takes DC power but 

produces no RF output, thus leading to a possible turn-on or 

switch-on failure. 

A recommended Gunn diode driver circuit is shown in Fig. 3.5 

V S 

Fig. 3.5 

R 

I R 

I B 

I Z 

" 

" 

" 

" 

7501 

I R =I B +I Z 

I B =I sat /h fe 

R=(V S 7V Z1 )/I R 

V Z1 =V G0 + 0.7 V 

C 1 = 0.1 mF 

G 1 = Gunn diode 

Z 1 ,Z 2 = Zener diode 

V Z2 =V G max 

V Z1 VZ2 V G0 

Z 1 Z 2 C 1 

G 1 

A Recommended Gunn Diode Drive Circuit 

3.2 Performance Considerations 

3.2.1 Matching 

The Gunn diode is a current generator and needs matching into 

the circuit for optimum output power, noise, smooth tuning 

and power-temperature variation. Small differences may exist 

even between cavities which are nominally identical. Testing in 

the customer’s own cavity design is always the preferred 

solution. 

The impedance matching problem becomes increasingly critical 

as the cavity Q goes up. The impedance of the diode is 

determined to the first order by the thickness, doping and 

operating voltage. However, second order trimming can be 

obtained by varying the voltage to alter the domain width, and 

therefore the domain capacitance, by field control. The voltage 

at which maximum power is obtained is cavity dependent. If 

the diode is not well matched to the cavity, there may be a 

conflict between the voltage for correct match and the voltage 

at which the diode would want to operate in a low Q cavity. 

Trimming the diode parameters, therefore, becomes more 

critical as Q increases and the tolerances on device 

specifications become correspondingly tighter. It follows that it 

is more difficult to design a diode to work in a high Q cavity 

without having access to the cavity. 

3.2.2 Starting Voltage 

The conventional Gunn diode is a broadband negative 

resistance device and random noise is required to start it. 

Starting becomes more difficult at low temperature and in high 

Q cavities. High Q operation also means higher voltage 

operation for reliable starting. These limitations can be 

overcome (at higher frequencies) by the use of a graded-gap, 

current injected Gunn diode. 

3.2.3 Operating Current 

Increasing the operating current increases the output power. 

However, the maximum safe operating current for an 

encapsulated device is limited by the corresponding increase in 

the transit region temperature (9250 8C). This, in turn, is 

determined by the thermal resistance of the diode itself and by 

the effectiveness of the package heat sink (see Section 3.3). 

The lower limit of current is more difficult to define because the 

series resistance of the ohmic contact increases non-linearly as 

the diode area decreases. 

3.2.4 Output Power 

The output power is determined primarily by the diode area and 

the doping level. The highest power/efficiency is achieved 

using an integral heat sink (IHS) construction. This has the 

advantage of a thick, gold plated heat sink formed directly onto 

the epitaxial region and a minimal thickness of remaining 

substrate, thereby reducing the parasitic resistance. Further 

improvement in output power can be obtained at higher 

frequencies by the use of a graded-gap, current injected Gunn 

diode. 

The upper limit on power handling is mainly determined by the 

dissipation of DC power through the package heat sink and the 

fact that the diode impedance becomes low and is difficult to 

match. 

3.2.5 FM Noise and dF/dT 

FM noise measurements are given as noise to carrier ratios (N/ 

C) versus carrier offset modulation frequency, f m . The 

frequency deviation, Df, is calculated from: 

N/C = 20.log (Df.H2 /f m ) 

where values of (N/C) are given in dBc/Hz. 

In the graded-gap Gunn diode, the variation in position at which 

domain formation occurs is much less than in a conventional 

device. Associated with this is a reduction in the variance of the 

first intervalley scattering effect for electrons entering the 

cathode region and hence a reduction in noise. 

The factors controlling FM noise and the 

frequency-temperature coefficient are complex and are 

resolved by work involving both the diode and oscillator 

design. The equivalent circuit of both the diode and the cavity 

form a total circuit concept and there is a trade-off between the 

parameters in each component. Oscillators and diodes need to 

be designed together to achieve given second order aspects of 

the specification. 

Second order effects such as noise, chirp, dF/dT, etc., bear no 

direct relationship to the primary characteristics of frequency 

and power. The secondary parameters can show wide variation 

between diodes, which in all other primary respects are 

identical. 



3.2.6 Pulse Diodes 

These diodes are even more cavity dependent than CW diodes. 

Whereas a conventional CW diode requires as low a series 

resistance as possible, this is not true in pulse diodes. The 

constant re-nucleation of domains at the start of each pulse 

requires closer control of the electric field at the cathode. 

Various methods of achieving this are available, including 

adding series resistance with a disc or by partially alloying the 

contacts to provide some non-ohmic behaviour. Alternatively, a 

graded-gap, current injected Gunn device may be used. 

Frequency change during the pulse (chirp) is also affected by 

these measures as is the dF/dT with which it correlates. 

Pulse diode noise is normally defined as the degradation in the 

Fourier sin(x)/x display of the RF power pulse from the ideal 

rectangular pulse values. The factors that control pulse noise 

and chirp are complex and the solution in any particular case is 

arrived at by modifications both to the diode processing and 

oscillator design. 

3.2.7 Special Operating Conditions 

In all cases where the optimum unique frequency and power 

design parameters have to be modified to achieve some special 

condition of low or high temperature operation, restricted 

voltage or current consumption, or broadband tuning, then 

some loss of optimum performance results. There is always a 

trade-off to be made to meet a special operating condition. 

3.2.8 Low Harmonic Diodes 

Diodes can be designed to meet low harmonic generation limits 

(if required by local regulations) by modifying the material 

specification. The extent to which the harmonics can be 

reduced, however, is limited by the cavity design and depends 

on the measurement technique. 

3.2.9 Broadband Operation 

Diodes can be specially designed by modifying the material 

properties and controlling the encapsulation parasitics to give a 

broadband tuning performance in a carefully defined cavity. 

This is achieved at the expense of output power and efficiency. 

3.2.10 High Temperature Operation 

Marconi Applied Technologies’ standard range of Gunn diodes 

can be used up to heat sink temperatures of 85 8C. At higher 

temperatures it is necessary to reduce the transit region doping 

level in order to ensure long life. This means that for the same 

current flowing through the device a larger die is used and 

greater cooling is achieved by the increased area of contact 

with the heat sink and the larger surface area for radiation. This 

affects other properties of the diode and may require further 

modifications to be made to the specification. 

3.3 Mounting and Heat Sink Considerations 

The increase in temperature between the diode heat sink and 

the semiconductor transit region is defined by: 

DT =R y (P in 7P out ) 

The thermal drop between the ambient and the diode heat sink 

must also be taken into account to avoid exceeding the 

maximum transit region temperature of *250 8C. The transit 

region temperature may be computed as follows: 

T tr =T amb + DT case + DT 

=T amb + DT case +R y (P in 7P out ) 

where: T tr = transit region temperature (4250 8C). 

T amb = ambient temperature. 

DT case = temperature difference between the diode 

heat sink and ambient at the operating power. 

R y = diode thermal resistance. 

In well designed packages, the temperature difference, DT case , 

is usually less than 30 8C for an input power of about 15 W. 

4 PERFORMANCE CHARACTERISTICS 

4.1 Limiting Conditions of Use 

Temperature: 

operating (for standard types) . . . . 740 to +85 8C 

storage . . . . . . . . . . . . 755 to +150 8C 

Operating voltage . . . . each type is individually rated. 

Application of a bias voltage in excess of this value may lead to 

degradation in performance. 


4.2 Typical Performance Curves 

7502 

1.5 

1.5 

I TH 

0 1 2 

1.0 

1.0 

I TH 

CURRENT (A) 

0.5 

0 

Fig. 4.1 

0 1 V TH 2 

3 4 V op 5 

BIAS VOLTAGE 

a) 

CURRENT (A) 

0.5 

0 

BIAS VOLTAGE 

b) 

Typical DC Characteristic a) Standard Gunn Diode, b) Graded-gap Gunn Diode 

V TH 3 4 V op 5 

7503 

OUTPUT POWER PO (mW) 

300 

Df 

200 

300 

200 

100 

0 

7100 

7200 

7300 

100 

740 720 0 20 40 60 80 

CASE TEMPERATURE T (8C) 

P O 

FREQUENCY CHANGE Df (MHz) 


a) b) 

110 

Df 

100 

P O 

280 

80 

0 

90 

7120 

740 720 0 20 40 60 80 



30 

55 

40 


20 

10 

0 

740 

720 


0 

P O 

20 

40 

Df 

60 

200 

100 

0 

7100 

80 



50 

Df 

45 

40 

35 

P O 

740 720 0 20 40 60 80 


0 

740 

780 

7120 


c) d) 

Fig. 4.2 

Variation of Power and Frequency with Temperature for: 

a) DC1276H operating in the fundamental mode; 

b) DC1277G-T operating in the fundamental mode; 

c) DC1279D operating in the second harmonic mode; 

d) DC1279F-T operating in the second harmonic mode 



1000 

100 

1000 

100 

P out 

P out 

Z eff 

P out 

Z eff 

100 

10 

100 

P out (1) 

10 

P out (2) 

Fig. 4.3 

POWER (mW) 

10 

Z EFFICIENCY 

1 

30 100 


Variation of power and frequency with temperature 

for DC1276 and DC1277 series. 

PERCENTAGE (%) 

Fig. 4.4 

POWER (mW) 

10 

Z EFFICIENCY (1) 

1 

30 100 


Z EFFICIENCY (2) 

Variation of power and efficiency for: 1) DC1278 

and DC1279 series; 2) DC1279F-T series 

PERCENTAGE (%) 


+30 

+1 

CHANGE IN FREQUENCY (MHz) 

+20 

0 

POWER 

+10 

71 

0 

72 

710 

FREQUENCY 

73 

720 

8 9 10 11 12 74 

OPERATING VOLTAGE (V) 

CHANGE IN POWER (dB) 

Fig. 4.5 

Variation of output power and frequency with operating voltage 

15 

+3 

+2 

Fig. 4.6 


10 

5 

POWER 

VOLTAGE 

0 

73 

8 9 10 11 12 

OPERATING VOLTAGE (V) 

Effect of voltage tracking on the variation of output power with frequency on a wideband tunable waveguide 

oscillator. 

+1 

0 

71 

72 

CHANGE IN OUTPUT POWER (dB) 

I sat 

I op 

GUNN VOLTAGE 

CHANGE IN FREQUENCY (MHz) 

+10 

0 

710 

720 

Fig. 4.8 

730 

73 

720 710 0 +10 +20 +30 +40 +50 +60 +70 

TEMPERATURE (8C) 

FREQUENCY 

POWER 

+1 

Variation of frequency and power with temperature. 

0 

71 

72 

CHANGE IN POWER (dB) 

GUNN CURRENT 

V sat 

V op 

Fig. 4.7 

DC Gunn characteristics 



5 BIBLIOGRAPHY 

Microwave Oscillation of Current in III-V Semiconductors. 

Gunn J.B. 

Solid State Commun., 1, 88 (1963). 

Theory of the Gunn Effect. 

Kroemer H. 

Proc. IEEE, 52, 1736 (1964). 

The Gunn Effect. 

Hobson G.S. 

Clarendon, Oxford, 1974. 

Theory of Stable Domain Propagation in the Gunn Effect. 

Butcher P.N. 

Phys. Lett., 19, 546 (1965). 

Transferred Electron Amplifiers and Oscillators for Microwave 

Application. 

Sterzer F. 

Proc. IEEE, 59, 1155 (1971). 

Hot Electron Microwave Generators. 

Carroll J.E. 

Arnold, 1970. 

Fundamental Mode graded-gap Gunn Diode Operation at 77 

and 84 GHz. 

Dale I., Stephens J.R.P., Bird J. 

Conf. Proc. Microwaves ‘94, . 

Advances in Hot Electron Injector Gunn diodes. 

Spooner H., Couch N.R. 

GEC J. of Res., 7, 34 (1989). 

Hot Electron Injection by Graded Al X Ga 1-X As. 

Long A.P., Beton P.H., Kelly M.J., Kerr T.M. 

Electronics Letters, 22, 130 (1986). 

Hot Electron Injection : Concept to Product. 

Couch N.R., Kelly M.J. 

Physics World, 2, 37 (1989). 

Whilst Marconi Applied Technologies has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any 

use thereof and also reserves the right to change the specification of goods without notice. Marconi Applied Technologies accepts no liability beyond that set out in its 

standard conditions of sale in respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein. 

#2000 Marconi Applied Technologies Printed in England 

Gunn Diodes AN1, page 19

Marconi Applied Technologies Gunn Diodes Application Notes

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