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Microwave Devices Faculty of Engineering EMG2026 OBJECTIVE: EXPERIMENT 1 CHARACTERISTICS OF GUNN DIODE 1. To identify the principle of operation of Gunn oscillator. 2. To estimate the voltage current characteristic of Gunn diode. 3. To analyse the relationship between voltage across Gunn diode and frequency of the microwave signal generated. 4. To analyse the relationship between voltage across Gunn diode and output power. APPARATUS: Gunn power supply/SWR meter (737021), Gunn oscillator, PIN modulator, Cavity wavemeter, Isolator, Slotted line probe, Variable attenuator. INTRODUCTION: Gunn diode is a type of Transferred Electron Device (TED). Material like GaAs exhibit negative differential mobility (i.e. decrease in carrier velocity with an increase in electric field) when bias voltage is above a threshold level. This gives rise to negative resistance characteristic of the Gunn device (i.e. decrease in current with an increase in voltage, see Figure 1). The shape of the V-I characteristic suggests that the device may be used as negative resistance amplifier or oscillator. Unlike other diodes, the Gunn device does not require the presence of a p-n junction. The Gunn diode is a GaAs material with ohmic contacts on the two ends. GaAs L I (mA) V th V 1 V Figure 1: V-I characteristic of Gunn diode. Revision Sept 2010
Microwave Devices Faculty of Engineering EMG2026 A detailed analysis shows that the transit time T = L/v (where L is length of sample and v is velocity of charge carriers) of electron through the GaAs sample should be larger than the domain growth time, T D , to have the negative resistance characteristic. The domain growth time is given by T g = ε neµ . (1) e where n is charge carrier density, e the charge of an electron, µ e the mobility of electron in the negative slope region of the V-I characteristic, and ε the permittivity of the GaAs material. For L/v > T g , the length of sample should be εv L > . neµ e A more accurate analysis shows that the required criterion is nL > 10 12 cm -2 . The threshold electric field for GaAs is 3.2x10 5 V/m. The threshold voltage is therefore V th = 0 . 32× L (2) where L is in micro-meter. For a sample length of 30 µ m , the threshold voltage will be 9.6 volts. The frequency of oscillation is related to transit time and hence the length of the device. Shorter sample length will have a higher operating frequency. Since the threshold electric field is fixed, the operating bias voltage across the device decreases linearly as the design frequency is increased. When Gunn diode is placed in a resonant cavity, oscillation occurs at the frequency of the resonant circuit rather than at the intrinsic frequency (or transit time frequency). The resonant frequency of the resonant circuit can be several times the intrinsic frequency. Revision Sept 2010
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