GUNN DIODE

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Gunn diode

(JB Gunn discovered Gunn Effect at IBM in ’62 in GaAs, InP, and CdTe).

Gunn diode (Transferred electron device) is used in high-freq electronics. It has three regions: two heavily N-doped layers sandwiching a thin layer of lightly doped N-type material. Application of voltage across the sandwich creates a very high electrical field in the thin middle layer, forcing a current to flow through it and altering its conducting properties. Its resistivity increases, preventing further conduction and causing the current to fall. It means a Gunn diode has a region of negative differential resistance. This negative differential resistance fetches major application of Gunn diode in oscillators at microwave  freq and above. Apply DC to bias the Gunn diode into its negative resistance region, which cancels the positive resistance of the load circuit, thus creating a circuit with zero resistance, which will produce spontaneous oscillations.

Its oscillation freq governed by middle layer can be tuned with an electronic resonator in the form of a waveguide, microwave cavity or YIG sphere. Gunn diode mounted inside a cavity cancels the loss resistance of the resonator, and it oscillates at its resonant frequency, which can be tuned mechanically, by adjusting the size of the cavity. Gunn diodes are often used as oscillators in W-band (75-110 GHz). Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 THz.

In addition to the valence and conduction bands, GaAs has a sub-band at energy higher than the conduction band. The sub-band is empty until energy is supplied to elevate electrons to it. Such energy can come from K.E. of ballistic electrons. Those electrons in the conduction band which are moving with sufficient kinetic energy can reach this band. In GaAs, the mobility or drift velocity in the third band is lower than the mobility in the usual conduction band, so with a small increase in forward voltage, more and more electrons can reach the third band and the current decreases. It creates a region of negative incremental resistance in the V–I characteristics.

(These electrons may acquire the requisite energy from a strong electric field, or they may be injected with the right energy by a cathode. For the latter, the cathode material has to be chosen carefully; chemical reactions at the interface need to be controlled during fabrication and additional monoatomic layers of other materials inserted. With forward voltage applied, the Fermi level in the cathode moves into the third band, and reflections of ballistic electrons starting around the Fermi level are minimized by matching the density of states and using the additional interface layers to let the reflected waves interfere destructively.)

When the potential applied to the diode is high enough, the charge carrier density along the cathode becomes unstable, and will develop small slices of low conductivity and high field strength which move from the cathode to the anode. It is not possible to balance the population in both bands, so there will always be thin slices of high field strength in a general background of low field strength. So in practice, with a small increase in forward voltage, a slice is created at the cathode, resistance increases, the slice takes off, and when it reaches the anode a new slice is created at the cathode to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again.

The laboratory methods that are used to select materials for the manufacture of Gunn diodes include angle-resolved photoemission spectroscopy.

At low E-field most of the electrons will be located in the lower energy central valley Γ. 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 Γ 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 V_th as shown in Fig. 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 –ve resistance device over a range of applied voltages and can be used in microwave oscillators.

            

 where Cj is diode capacitance;

                       Rj is diode resistance; 
                      Rs is sum of lead, ohmic contacts, and bulk resistance of the diode,
                     Cp is package capacitance and Lp is package inductance. 
                    The value of negative resistance lies between –5 and –20 Ω.

It is an inexpensive oscillator of microwave power. Its I-V characteristics show that for voltages between 0V and ~1V, it behaves as a linear resistor. An increase in voltage takes it to threshold where the current stops increasing. Further increase in voltage takes the diode to negative resistance region where the curve slopes downward, which means that it is getting ready to oscillate. The operating point is usually about 4 times the threshold voltage. It often oscillates in W-band (75-110 GHz). 

Applications: As amplifiers, oscillators at µwave freq (highest output power of any semicon device). Used for airborne collision avoidance radar, anti-lock brakes, sensors to monitor the flow of traffic, car radar detectors, pedestrian safety systems, distance traveled recorders, motion detectors, slow-speed (22m/sec) sensors, traffic signal controllers, automatic door openers, automatic traffic gates, process control equipment to monitor throughput, burglar alarms and equipment to detect trespassers, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors.