EXPERIMENT – 14(Open Ended Lab)
OBJECTIVES:
Study I-V Characteristics of Gunn diode in X-band.
Outcomes: Students are able to
1. Analyze the characteristics curve of GUNN DIODE.
EQUIPMENTS:
Gunn power supply, Gunn oscillator, Isolator, PIN modulator, Frequency meter, Variable
attenuator, Detector with tunable mount, Waveguide stands, SWR meter, Matched load
terminations, Cables and Accessories.
THEORY:
GUNN Diodes (Transferred Electron
Device)
Gunn diodes are negative resistance devices which are normally used as low power oscillator
at microwave frequencies in transmitter and also as local oscillator in receiver front
ends. J B Gunn (1963) discovered microwave oscillation in Gallium arsenide (GaAs),
Indium phosphide (InP) and cadmium telluride (CdTe). These are semiconductors having a
closely spaced energy valley in the conduction band as shown in Fig. 1.1(a) for GaAs. When
a dc voltage is applied across the material, an electric field is established across it. At low E-
field in the material, most of the electrons will be located in the lower energy central valley.
At higher E-field, most of the electrons will be transferred in to the high-energy satellite
L and X valleys where the effective electron mass is larger and hence electron mobility is
lower than that in the low energy valley. Since the conductivity is directly proportional to
the mobility, the conductivity and hence the current decreases with an increase in E-
field or voltage in an intermediate range, beyond a threshold value Vth as shown in
Fig. 1.1(c). This is called the transferred electron effect and the device is also called ‘Transfer
Electron Device (TED) or Gunn diode’. Thus the material behaves as negative resistance
device over a range of applied voltages and can be used in microwave oscillators.
Fig 1.1(b) Gunn Diode
Fig 1.1(a) Multi-valley conduction configuration
band energies of GaAs
Fig 1.1(c) Current-voltage characteristics of GaAs
The basic structure of a Gunn diode is shown in Fig. 1.2 (a), which is of n-type GaAs
semiconductor with regions of high doping (n+). Although there is no junction this is called a
diode with reference to the positive end (anode) and negative end (cathode) of the dc voltage
applied across the device. If voltage or an electric field at low level is applied to the
GaAs, initially the current will increase with a rise in the voltage. When the diode voltage
exceeds a certain threshold value, Vth a high electric field (3.2 KV/m for GaAs) is
produced across the active region and electrons are excited from their initial lower valley to
the higher valley, where they become virtually immobile. If the rate at which electrons are
transferred is very high, the current will decrease with increase in voltage, resulting in
equivalent negative resistance effect. Since GaAs is a poor conductor, considerable heat is
generated in the diode. The diode will be bonded into a heat sink (Cu-stud).
The electrical equivalent circuit of a Gunn diode is shown in Fig. 1.2 (b), where Cj and –
Rj are the diode capacitance and resistance, respectively, Rs includes the total resistance of
lead, ohmic contacts, and bulk resistance of the diode, Cp and Lp are the package
capacitance and inductance, respectively. The negative resistance has a value that typically
lies in the range –5 to –20 ohm.
(b)
Fig.1.2 Constructional details and electrical equivalent circuit of Gunn Diode.
(MICROELECTRONICS AND VLSI, 4January 2013, http://mmicroelectronics.blogspot.com/2013/01/gunn-diode.html)
Gunn Oscillator:
In a Gunn Oscillator, the Gunn Diode is placed in a resonant cavity. In this case the oscillation
frequency is determined by cavity dimension than by the diode itself. Although Gun
Oscillator can be amplitude-modulated with the bias voltage, we have used separate PIN
modulator through PIN diode for square wave modulation
PROCEDURE:
1. Set the components and equipment as shown in the Fig. 1.3.
2. Initially set the variable attenuator for maximum attenuation.
3. Keep the control knob of Gunn Power Supply as below:
Meter Switch - ‘OFF’
Gunn bias knob - Fully anticlockwise
Pin bias knob - Fully anticlockwise
Pin Mod Frequency - Any Position
4. Keep the control knob of VSWR meter as below:
Meter Switch - Normal
Input Switch - Low Impedance
Range dB Switch - 40 dB
Gain Control Knob - Fully clockwise.
5. Set the micrometer of Gunn Oscillator for required frequency of operation.
6. ‘ON’ the Gunn Power Supply, VSWR meter and Cooling fan.
Voltage-current characteristics:
1. Turn the meter switch of ‘Gunn power supply to voltage position.
2. Measure the Gunn diode Current Corresponding to the various voltage
controlled by Gunn bias knob through the panel meter and meter switch.
3. Plot the voltage and current readings on the graph as shown in Fig.1.4.
4. Measure the threshold voltage from the graph which corresponds to
maximum current.
NOTE: DONOT KEEP GUNN BIAS KNOB POSITION AT THRESHOLD
POSITION FOR MORE THAN 10-15 SECONDS. READING SHOULD BE OBTAINED
AS FAST AS POSSIBLE. OTHERWISE, DUE TO EXCESSIVE HEATING, GUNN
DIODE MAY BURN.
Gunn Power Supply
Slotted line with
Gunn PIN Modulator Frequency Variable tunable probe.
Oscillator Meter Attenuator
Isolator
Fig 1.3.Block Diagram Set-up for study of Gunn-Oscillator Matched load
Termination
Fig 1.4. V-I Curve of Gunn Oscillator
VSWR/CRO
Gunn Power Supply
Gunn PIN Modulator Frequency Slotted line with
Variable
Oscillator Isolator Meter tunable probe.
Attenuator
Matched load
Fig 1.5. Block diagram for wave propagation characteristics in X-band.
Termination
PRECAUTIONS:
1. The Gunn power supply should be handled carefully.
2. Detector should not be exposed to power > 15mW.
3. VSWR readings and frequency adjustments should be done properly.
4. All elements should be properly tuned.
Viva Question:
1. What is Gunn effect?
2. GaAs is used in the fabrication of GUNN diodes because.......................
3. The gunn diode is.................................
4. When the applied electric field exceeds the threshold value, electrons absorb more energy from the field and
become..............................
5. The electrodes of a Gunn diode are made of...........................
6. When either a voltage or current is applied to the terminals of bulk solid state compound GaAs, a differential
______ is developed in that bulk device.
7. The frequency of oscillation in Gunn diode is given by.................................
8. The modes of operation of a Gunn diode are illustrated in a plot of voltage applied to the Gunn diode v/s
frequency of operation of Gunn diode. a) true b) false
9. The Gunn diode is used in the range of.......................
10. Silicon and germanium are called ___________ semiconductors.
11. What is the another name of Gunn diode?
12. Draw the equivalent Circuit for GUNN?
13. What are the different modes in GUNN diode oscillator?
14. How many junctions are there in GUNN?
15. Explain the transferred electron effect in GUNN?
16. What are applications of GUNN?
___________________________________________________________________________________
