EXPERIMENT:
Aim: Apparatus:
To study V-I characteristics of Gunn Diode (For MT 9001) Microwave Bench: -Gunn diode -Isolator -Pin modulator -Variable attenuator (20 db) -Frequency meter -Slotted section -Detection mount
Gunn Power Supply SWR Meter CRO Connectors and Cables as Required
Theory: 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 figure for GaAs. When dc voltage is applied across the materials, most of the electrons will be located in the lower energy central valley T. At higher E-field, most of the electrons will be transferred into the high energy satellite L & X valleys where the effective electron mass is larger & hence electron mobility is lower than that in the lower energy T-valley. Since the conduction conductivity is directly proportional to the mobility, the conductivity & hence the current decreases with an increase in E-field or voltage in an intermediate range, beyond a threshold value Vetch. This is called transferred electron effect & the device is also called Transfer Electron Device or Gunn Diode. Thus, the material behaves as negative resistance device over a range of applied voltages & can be used in Microwave oscillators. The basic structure of a Gunn diode is shown in fig (a), which consists 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 +ve 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 diode voltage exceeds a certain threshold value Vth, a high electric field is produced across the device where they become virtual immobile.
If the rate at which electrons are transferred is very high the current will decrease with increase in voltage; resulting in a equivalent negative resistance effect. Since GaAs is a poor conductor, Considerable heat is generated in the diode. The diode should be well bounded into heat sink. The electrical equivalent circuit of a gun diode is shown in figure where Cj and -Rj are the diode capacitance and resistance respectively. Rs includes the total resistance of a 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 ohms. The Gunn oscillator is based on negative differential conductivity effect in bulk semiconductor, which has two conduction bands minima separated by an energy gap (greater than thermal agitation energies). A disturbance at the cathode gives rise to high field region, which travels towards anode & so on. The time required for domain to travel from Cathode to anode (Transit time) gives oscillation frequency. In a Gunn oscillator, Gunn diode is placed in resonant cavity. In this case oscillators frequency is determined by cavity diminution than by diode itself. Although Gunn oscillator can be amplitude modulated with the bias voltage. We have used separate pin modulator through pin diode for square wave modulation capability. A message of square wave modulation capacity is modulation depth i.e. Output Ratio between ON & OFF state.
Procedure:
Connect the components as shown in Block Diagram shown below
Keep the Gunn Power Supply knob in counter clockwise position. Ensure that the cooling fan is switched on and directed towards the Gunn Oscillator and PIN Modulator. Switch on the Gunn Power Supply. Observe the current and voltage of the Gunn Oscillator on the LED display Now keep the V/I switch of Gunn Power Supply in I position and slowly vary the biasing voltage of the Gunn Diode by means of the pot and observe the values of current and voltage by changing the switch from voltage to current mode.
Note down the readings in observation table given below. Turn the Gunn bias control knob to counter clockwise position. Switch off the Gunn Power Supply. Plot the graph of V-->I for Gunn Diode and observe the negative resistance characteristics.
Observations: Sr. No. Voltage V volts Current I mA 1 2 3 4 5 6 7 8 9 10 Conclusion: _______________________________________________________________ _______________________________________________________________ _______________________________________________________________ _______________________________________________________________ _______________________________________________________________ _______________________________________________________________
