Gunn Oscillator
Gunn Oscillator
Gunn Oscillator
Gunn oscillator
And electronics of TEDs
by Pejman Taslimi
represented to Dr. Hassani
Microwave Lab
6th November 2005 – Shahed university of Tehran
Introduction
The application of two-terminal semiconductor devices at microwave frequencies has
been increased usage during the past decades. The CW, average, and peak power outputs of
these devices at higher microwave frequencies are much larger than those obtainable with the
best power transistors.
The common characteristic of all active two-terminal solid-state devices is their
negative resistance. The real part of their impedance is negative over a range of frequencies.
In a positive resistance the current through the resistance and the voltage across it are in
phase. The voltage drop across a positive resistance is positive and a power of (I2R) is
dissipated in the resistance. In a negative resistance, however, the current and voltage are out
of phase by 180°. The voltage drop across a negative resistance is negative and a power of (-
I2R) is generated by the power supply associated with the negative resistance. In other words,
positive resistances absorb power (passive devices), whereas negative resistances generate
power (active devices).
The differences between microwave transistors and transferred electron devices
(TEDs) are fundamental. Transistors operate with either junction or gates, but TEDs are bulk
devices having no junctions or gates. The majority of transistors are fabricated from elemental
semiconductors, such as silicon or germanium, whereas TEDs are fabricated from compound
semiconductors, such as gallium arsenide (GaAs), indium phosphate (InP), or cadmium
telluride (CdTe). Transistors operate with “warm” electrons whose energy is not much
greater than the thermal energy (0.026 eV at room temperature) of electrons in the
semiconductor, whereas TEDs operate with ‘hot” electrons whose energy is very much
greater than the thermal energy. Because of these fundamental differences, the theory and
technology of transistors cannot be applied to TEDs.
Gunn Effect
A schematic diagram of a uniform n-type GaAs diode with ohmic contacts at the end
surfaces is shown in the figure.
J. B. Gunn observed the Gunn Effect in the n-type GaAs bulk diode in 1963, an effect best
exp1ained by Gunn himself, who published several papers about his observations. He stated
in his first paper that:
Above some critical voltage, corresponding to an electric field of 2000-4000 volts/cm,
the current in every specimen became a fluctuating function of time. In the GaAs
specimens, this fluctuation took the form of a periodic oscillation superimposed upon
the pulse current. . . . The frequency of oscillation is determined mainly by the specimen,
and not by the external circuit ....The period of oscillation was usually inversely
proportional to the specimen length and closely equal to the transit time of electrons
between the electrodes, calculated from their estimated velocity of slightly over 107
cm/s. . . . The peak pulse microwave power delivered by the GaAs specimens to a
matched load was measured. Value as high as 0.5 W at 1 Gc/s, and 0.15 W at 3 Gc/s,
were found, corresponding to 1-2% of the pulse input power.
Modes of Operation:
Since Gunn first announced his observation of microwave oscillation in the n-type
GaAs and n-type InP diodes in 1963, various modes of operation have been developed,
depending On the material parameters and operation conditions. The formation of a strong
space-charge instability depends on the conditions that enough charge is available in the
crystal and that the specimen is long enough so that the necessary amount of space charge
can be built up within the transit time of the electrons. This requirement sets up a criterion for
the various modes of operation of bulk negative-differential-resistance devices. Copeland
proposed four basic modes of operation of uniformly doped bulk diodes with low-resistance
contacts as shown in the figure below.
1. Gunn oscillation mode: this mode is defined in the region where the product of
frequency multiplied by length is about 107cm/s and the product of doping multiplied
by length is greater than 1012 cm-2. In this region the device is unstable because of the
cyclic formation of either the accumulation layer or the high-field domain. In a circuit
with relatively low impedance the device operates in the high-field domain mode and
the frequency of oscillation is near the intrinsic frequency. When the device is operated
in a relatively high-Q cavity and coupled properly to the load, the domain is quenched
or delayed (or both) before nucleating. In this case, the oscillation frequency is almost
entirely determined by the resonance frequency of the cavity and has a value of several
times the intrinsic frequency.
2. Stable amplification mode: This mode is defined in the region where the product of
frequency times length is about 107 cm/s and the product of doping times length is
between 1011 and 1012 cm-2.
3. LSA oscillation mode: This mode is defined in the region where the product of
frequency times length is above 107 cm/s and the quotient of doping divided by
frequency is between 2 x 104 and 2 x 105.
4. Bias-circuit oscillation mode: This mode occurs only when there is either Gunn or LSA
oscillation, and it is usually at the region where the product of frequency times length is
too small to appear in the figure. When a bulk diode is biased to threshold, the average
current suddenly drops as Gunn oscillations begin. The drop in current at the threshold
can lead to oscillations in the bias circuit that are typically 1 kHz to 100 MHz.
A Gunn device is also called a transferred-electron device since the negative resistance
arises from the transfer of electrons from the low to the high-energy band.
The oscillations that occur in materials with the energy band structure noted above was
discovered by J. B. Gunn. The possibility of obtaining negative differential resistance had
been predicted earlier by Ridley and Watkins.
There are two principal modes of operation that result in oscillations for a Gunn
device. When the applied voltage exceeds the
threshold value, a dipole domain (a region of
electron concentration and depletion) forms near
the cathode end with most of the voltage drop
appearing across the high-resistance part of the
device. A short section of the input region is in the
low-energy high-mobility state and electrons leave
the cathode with a large velocity. At the point in
the material that separates the high-mobility and
low-mobility states, electrons accumulate on the left side and are depleted on the right side by
virtue of the different motilities. This dipole arrangement of charge is shown pictorially in
figure.
This dipole domain sweeps across the device and when it arrives at the anode, the
device is in a high-mobility state and a new dipole domain forms at the cathode end and
moves toward the anode. This mechanism is self-repeating and represents an oscillation with
a period equal to the transmit time. This mode of oscillation has a low efficiency (a few
percent) of power generation and a frequency that cannot be controlled by the external
circuit. This mode of oscillation is called the transit time mode or Gunn mode.
The second mode of oscillation is the limited-space-charge (LSA) mode. Operation of a
Gunn oscillator in the LSA mode can produce several watts of power with efficiencies of
around 20 percent or more. The power outputs that have been obtained decrease with
frequency and are below 1 W at frequencies greater
than 10 GHz. Output power of several milliwatts
can be obtained at 100 GHz.
In the LSA mode the Gunn device is incorporated as
part of a resonant circuit. The frequency of the
resonant circuit is adjusted so that it is several times
greater than that of the transit-time mode. As a
consequence, dipole domains do not have sufficient
time to form and the device operates essentially as a
negative-resistance device. The dc bias is adjusted to
a value somewhat greater than the threshold
voltage. The RF voltage of the oscillations will build
up to a peak-peak value approximately equal to the
voltage increment over which the device resistance
is negative as shown.
The resonator loading, represented by the
resistor R, is adjusted to a value about 20 percent
greater than the maximum negative resistance of the
device. This will ensure that oscillations will start.
The amplitude of the oscillations will build up until
the average negative resistance of the Gunn device becomes equal to the resonator resistance
R. If the resonator frequency is adjusted to a value slightly above that of the transit-time
mode, the Gunn device will operate very much like the basic Gunn mode, but the dipole
domain will be quenched before it arrives at the anode by the negative-going oscillation
voltage. This type of operating mode is called a quenched-domain mode. Oscillations can also
occur by adjusting the resonator frequency, so that it is lower than the frequency of the Gunn
mode. In this case the dipole domains have sufficient time to sweep across the device and
arrive at the anode. However, the initiation of a new dipole domain is delayed until the
oscillation voltage rises above the threshold value. This mode of operation is called the
inhibited or delayed mode.
There is sufficient capacity between the post and the surrounding waveguide to
provide an adequate low-impedance RF bypass capacitance and thus RF currents do not flow
through the bias voltage supply. Fine tuning of the cavity can be obtained by means of a
tuning screw.
Summery
As described earlier, if the applied field is less than
threshold the specimen is stable. While, however, the
field is greater than threshold, the specimen is unstable
and divides up into two domains of different conductivity
and different electric field but the same drift velocity. The
figure shows the stable and unstable regions.
References:
• Microwave devices and circuits – Samuel Y. Liao – ISBN:0135812070
• Foundations for microwave engineering – Robert E. Collin – ISBN:0-07-011811-6