The Gunn-diode Fundamentals and Fabrication PDF

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The Gunn-diode: Fundamentals and fabrication Conference Paper · October 1998 DOI: 10.1109/COMSIG.1998.736992·Source: IEEE Xplore

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The Gunn-diode: Fundamentals and Fabrication Robert van Zyl, Willem Perold, Reinhardt Botha* Department of Electrical and Electronic Engineering, University of Stellenbosch, Stellenbosch, 7600 e-mail: [email protected] * Department of Physics, University of Port Elizabeth, Port Elizabeth, 6000 e-mail: [email protected] relatively few electronic engineers understand clearly the principles behind the Gunn-effect. The aim of this paper is to give the reader an overview of the underlying theory of the Gunn–effect and how it is utilised in Gunn-diodes to produce a.c. power [2], [3]. Concepts which will be discussed include the negative differential mobility phenomenon in GaAs, Gunn-domain formation and the Keywords — Gunn-diode, Gunn-effect, transferred electronbasic Gunn-diode structure. A typical simulation of a Gunneffect, GaAs, energy band, Monte Carlo particle simulation. diode in a cavity will also be presented.

Abstract — A short tutorial on the Gunn-diode is presented. The principles underlying Gunn-oscillations are discussed briefly and illustrated by relevant simulations. The simulation of a typical Gunn-diode in a cavity is also presented. In conclusion, the fabrication process of low power Gunn-diodes is discussed.

I. INTRODUCTION JB (Ian) Gunn discovered the Gunn-effect on 19 February 1962. He observed random noise-like oscillations when biasing n-type GaAs samples above a certain threshold. He also found that the resistance of the samples dropped at even higher biasing conditions, indicating a region of negative differential resistance. As will be explained later, this leads to small signal current oscillations. In Figure 1 part of the famous page from one of Gunn’s laboratory notebooks is shown with the entry “noisy” on the line for 704 volt. Describing it as the “most important single word” he ever wrote, it laid the foundation for what was to become a major mode of a.c. power generation. Due to their relative simplicity and low cost, Gunn diodes remain popular to this day. It is, however, also true that

The University of Stellenbosch, in conjunction with the University of Port Elizabeth, is currently fabricating GaAs Gunn-diodes for research purposes. The aim is to optimize Gunn-diodes for a.c. output at W-band frequencies. A review of this manufacturing process will be given. The simulations in this paper have been performed by a Monte Carlo particle simulator developed at the University of Stellenbosch. A short review of the Monte Carlo simulation of semiconductors is given in [4]. II. THE GUNN-EFFECT IN THE STRICT SENSE A. The Energy Band for GaAs To understand the Gunn-effect it is necessary to have some insight in the behaviour of electrons in a crystal lattice, and most importantly, the allowed energy states electrons can occupy. These are dictated by the energy band structure of a semiconductor which relates an electron’s energy as a function of its wave vector k. The band structure for GaAs is shown in Figure 2. Both the valence (negative electron energy) and conduction (positive electron energy) bands are shown. Only the conduction bands need to be considered for the study of electron dynamics, since electrons in the valence bands are stationary. Energy bands are very complex structures. It is, however, clear from Figure 2 that for realistic electron energies (E t0, indicated by the dashed curve. At this point in time, the domain has grown sufficiently to ensure that electrons at both points C and D move at the same velocity, 1, as is clear from the bottom graph in Figure 6.

V0

Biased GaAs sample of length L

anode

cathode

0

A 0

-

+ B

+

C

-

D

L

t = t0

EH2

t > t0

EH1

Fig. 5. The simulated steady-state average drift velocity of electrons in bulk GaAs as a function of the applied electric field at 300K. The region of NDR is indicated.

E0 EL1 EL2

L

0

Distance from cathode

C. The formation of Gunn-domains The question of exactly how the NDR phenomenon in GaAs results in Gunn-oscillations can now be answered with the aid of Figure 6. A sample of uniformly doped n-type GaAs of length L is biased with a constant voltage source V0. The electrical field is therefore constant and its magnitude given by E0 = V0/L. From the bottom graph in Figure 6 it is clear that the electrons flow from cathode to anode with constant velocity 3. It is now assumed that a small local perturbation in the net charge arises at t = t0, indicated by the solid curve in Figure6. This non-uniformity can, for example, be the result of local thermal drift of electrons. The resulting electrical field distribution is also shown (solid curve). The electrons at point A, experiencing an electric field EL1, will now travel to the anode with velocity 4. The electrons

4 3 2 1

EL2

EL1 E0 EH1 EH2

Electric field Fig. 6. A graphical illustration of the formation of Gunn-domains.

It is important to note that the sample had to be biased in the NDR region (see Figure 5) to produce a Gunn-domain. Once a domain has formed, the electric field in the rest of the sample falls below the NDR region and will therefore inhibit the formation of a second Gunn-domain. As soon as the domain is absorbed by the anode contact region, the average electric field in the sample rises and domain formation can again take place. The successive

formation and drift of Gunn-domains through the sample leads to a.c. current oscillations observed at the contacts. In this mode of operation, called the Gunn-mode, the frequency of the oscillations is dictated primarily by the distance the domains have to travel before being annihilated at the anode. This is roughly the length of the active region of the sample, L. The value of the d.c. bias will of course also affect the drift velocity of the domain, and consequently the frequency. The process of domain growth, drift and absorption at the anode is verified by the simulation results for a 5 µm GaAs sample shown in Figures 7 and 8. The sample is uniformly doped with concentration 1015cm-3 and biased at 5V. The frequency of oscillation is roughly 25GHz. 0

The doping profile of the Gunn-diode is shown in Figure 9. An active region is sandwiched between highly doped anode and cathode regions. These highly doped regions ensure good ohmic contacts with the external circuit. A 50% notch in the doping is included to provide an initial high electric field near the cathode. The reason for the notch will be explained later. The oscillator circuit is as the parallel resonant circuit shown in Figure 10. The simulated voltage and current waveforms are given in Figure 11. From these graphs it is evident that the oscillator generates in the order of 140mW at 70GHz with an efficiency of 2.4%. These values are typical of Gunn-diode 15

-1 Millions

oscillators.

10

-2 5 -3 0

-4 -5

-5 0

1

2

3

4

5

Millions

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

10

-2 5 -3 0

-4

-5

-5 0

1

2

3

4

5

15

0 -1 Millions

1

15

0 -1

10

-2 5 -3 0

-4

-5

-5 0

1

2

3

4

5

0

15

-1 Millions

0

10

-2 5 -3 0

-4 -5

-5 0

1

2

3

4

5

Fig. 8 The simulated field distribution for the dipole distibutions in Figure 7. Note the growth in the peak value and the subsequent drop in the field throughout the rest of the sample to below the NDR region shown in Figure 5.

III. SIMULATION OF A MILLIMETER-W AVE GUNN-EFFECT OSCILLATOR A typical application of a Gunn-diode in a cavity will now be discussed. A high frequency oscillator (70 GHz) has been chosen since it reveals an important aspect in the understanding of the high frequency limit inherent to Gunn-

Fig. 7 The simulated net charge concentration in a 5µm GaAs sample biased at 5V. The distributions are shown at four successive time instances to illustrate the formation, drift and absorption at the anode of a dipole domain in a Gunn-diode.

oscillators operating at these frequencies. The formation and drift of the dipole domains are illustrated with the sequence of field distributions in Figure 12. A “dead zone” is clearly evident near the cathode where no dipole domains form. Electrons injected at the cathode are initially confined to the central valley of the conduction

Electric Field [MV/m]

2 0 -2 -4 -6

t=0

-8

-10 0.15

0.25

1.9

Distance from cathode

2.0

0

0.5

1

1.5

2

1

1.5

2

1

1.5

2

1

1.5

2

[microns]

5×1015 cm-3 1×1016 cm-3 1.25×1017 cm-3

Fig. 9. The doping profile of the simulated Gunn-diode.The active region is sandwiched between the highly doped anode and cathode regions. A notch in the doping appear at the cathode.

Electric Field [MV/m]

0

0 -2 -4 -6

t = 3ps

-8

-10 0

0.5

Electric Field [MV/m]

2 0 -2 -4 -6

t = 5.6ps -8

-10

Fig. 10. The circuit schematic for the simulated Gunn-diode in a cavity. The diode is biased with a 3V d.c. power supply. The oscillator feeds into a 23 load.

Terminal voltage [V] / current [A]

6

v(t)

5

Electric Field [MV/m]

0

0.5

0 -2 -4 -6

t = 10ps -8

-10

4

0

0.5

Distance from cathode [microns]

i(t)

3

Fig. 12. The simulated sequence of fields for the Gunn-oscillator described in the text clearly shows a dead zone at the cathode.

2 1

0

0

5

10

15

20

25

30

Time [ps]

Fig. 11. The simulated voltage v(t) and current i(t) waveforms for the Gunn-oscillator desribed in the text. v(t) and i(t) are defined in Figure 10.

band. They do not immediately gain enough energy to be transferred to the upper L-valley. This results in a delayed domain formation and a consequent dead zone in the region of the cathode. The presence of a dead zone in the diode impacts negatively on the efficiency of the oscillator, because the length of the active region in which the domain can grow, decreases. Smaller domains translate into smaller output power. The existence of a dead zone affects high frequency (< 30 GHz) Gunn-oscillators the most, since the physical lengths of these diodes are of the order a few micron, roughly the same as the dead zone. Optimising Gunn-diodes invariably involves decreasing the dead zone by encouraging domain nucleation as near to the cathode as possible. The doping-notch is one way of reducing the dead zone,

since it forces a high electric field at the notch. This stronger field will accelerate the electrons faster than would otherwise be the case. The electrons will therefore gain enough energy for transfer to the L-valley in a shorter time and distance.

Another, more successful, method is the injection of “hot” or energetic electrons directly into the cathode region. This is accomplished by inserting a heterojunction between the cathode contact and the active region of the diode [6]. A detailed discussion on hot electron injection is not within the scope of this tutorial. In essence, when an electron traverses a heterojunction of the correct type, it gains almost immediately a certain amount of energy dictated by the heterojunction. If this energy exceeds the gap, , transfer to the L-valley, and consequently Gunn-domain formation, is possible. Heterojunctions are typically 50nm in length, implying a drastic reduction in the dead zone and a subsequent improvement in efficiency. IV. FABRICATION OF GaAs GUNN-DIODES

The authors are currently in the process of manufacturing 10 GHz Gunn-diodes for research purposes. The aim is to apply the experience gained in this process to the development of efficient Gunn-diodes operating at frequencies in excess of 100 GHz. A chronological outline of the fabrication process is discussed below with a graphical representation of the process given in Figure 13.

Growth of diode structure Anode

AuGe layer contact layer active region layer doping-notch buffer layer substrate

AuGe layer

A. Growth of diode structure

Cathode

Define individual contacts by etching

The diode layers have been grown at the Department of Physics, University of Port Elizabeth, by a process known as Metalorganic Vapour Phase Epitaxy (MOVPE). Growth was performed in a horizontal, laboratory scale quartz reactor, capable of accepting a 2x2cm2 piece of substrate. The diode structures were grown on a 250µm GaAs:Si substrate with doping density n=1.3x1018 cm-3. This was followed by a 0.6µm buffer layer (n=1.4x1018 cm-3), a 0.3µm undoped injection layer (n=1.1x1015 cm-3 ) which serves as doping-notch, a 10µm undoped active region layer (n=2.5x1015 cm-3) and a 0.6µm Si-doped contact layer (n=1.4x1018 cm-3). The GaAs substrate was placed on a molybdenum susceptor, which was heated to 670oC before growth. Trimethylgallium and arsine (10% in H2), diluted in a H2 carrier gas, were used as source materials. n-Type doping of the contact layers was achieved by introducing SiH4 gas into the reactor. Growth rate is approximately 10 per second.

Define individual diodes by etching 100µm

400µm

Fig. 12. Step-by-step fabrication of low power Gunndiodes

The doping levels were determined from electrochemical capacitance-voltage profiling of the grown structures and Hall measurements on calibration layers. CV-profiling also provided an independent measurement of layer thicknesses. Metal contacts were thermally evaporated onto both sides of the structure to provide good electrical contact with the external circuitry. These metal contacts consist of three layers, namely a 80nm layer of AuGe sandwiched between two layers of 10nm Ni. It was found that these contacts disintegrate at currents exceeding 20mA, because they are so extremely thin. Additional AuGe had to be evaporated onto the existing contacts to a depth of 0.7µm. B. Etching and scribing of individual diodes Individual diodes are defined on the grown structures by a standard photolithographic procedure. A mask defines the desired metal contacts at the anode (top) side of the structures. Contacts with a 100µm diameter have been etched. The unwanted AuGe metal was etched away using a mixture of iodine crystals, potassium-iodide and water. The unwanted GaAs was etched away using a mixture of methanol, phosphoric acid, and H2O2. The GaAs had to be etched to a depth of at least 10µm to ensure that the active region is of the same dimensions as the metal contacts. The individual diodes can now be cut out using a diamond edge scriber. Each diode is of the order 400µm in diameter. C. Packaging

Fig. 13. Packaged low power Gunn-diode.

The diodes are now mounted in containers of suitable size. The packaging of an individual diode is shown in Figure 14. The diode is bonded to the gold plated copper base of the bottom external metal contact using a highly conducting epoxy. The two external contacts are separated by a ceramic spacer. 25µm gold bonding wires connect the top diode contact with the top lid. The wire is bonded onto the diode contact with the same conducting epoxy. D. Experimental results Experimental results will be presented at the conference. V. CONCLUSIONS The Gunn-effect in bulk GaAs and how this phenomenon is harnessed in the generation of a.c. power has been discussed. The fabrication of low power Gunn-diodes has

been dealt with briefly. It is the desire of the authors that this tutorial will have brought home an appreciation for these devices which have served us so well over the past three decades. REFERENCES [1] John Voelcker, “The Gunneffect”, IEEE Spectrum, p.24, July 1989. [2] BG Bosch, RWH Engelmann, Gunn-effect Electronics, Pitman Publishing, London, 1975. [3] JE Carroll, Hot Electron Microwave Generators, Edward Arnold Publishers, London, 1970. [4] RR van Zyl, WJ Perold, “The Application of the Monte Carlo Method to Semiconductor Simulation”, Trans. SAIEE, pp. 58-64, June 1996. [5] K Tomizawa, Numerical Simulation of Submicron Semiconductor Devices, Artech House, London, 1993. [6] Z Greenwald et al, “The Effect of a High Energy Injection on the Performance of mm Wave Gunn Oscillators”, Solid-State Electronics, Vol. 31, No. 7, pp. 1211-1214, 1988.

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