An IMPATT diode (IMPact ionization Avalanche Transit-Time diode) represents a type of high-powered semiconductor diode that is typically implemented in high-frequency microwave devices. This technology allows the generating of signals at frequency ranges from a few GHz to a few hundred GHz. IMPATT diodes have negative resistance and are used as oscillators to create microwaves. One of their key advantages is their high power compared to other types of microwave diodes. The diode is often called Read diode in honor of W.Т. Read, who proposed its structure and principles in 1958. A forward-biased PN junction was used as a method of injection for the carrier.
IMPATT diode principles of operation
Standard PN junctions and IMPATT diodes have similar I-V characteristics (Fig. 1a). When the “turn on” voltage is reached, the diode will start to conduct in the forward direction and block reverse current flows. However, if the voltage is higher than the breakdown point, it leads to avalanche breakdown and reverse current flows.
Microwave generation is possible in IMPATT diodes with different types of semiconductor structures. However, the IMPATT diode principles of operation are convenient to illustrate on the example of p+-n-i-n+-diode structure (Fig. 1b). Here, the regions of avalanche multiplication and the carrier drift are spatially separated. Maximum electric field strength takes place in the region of the p+-n-diode junction (Fig 1e).
The electric field decreases sharply in the n-region and remains almost constant in the i-region, which is completely depleted at high reverse voltages. If the reverse voltage increases, the electric field in the junction will exceed the value of the breakdown strength, and the impact ionization coefficient α will reach high values. Since α strongly depends on the electric field strength, the extent of the impact ionization region is small.
The avalanche increase in the number of free charges occurs in this region which called the avalanche multiplication region. The formed holes are carried by the internal electric field of the junction to the p+-region, and the electrons, getting to the i-region, are moving to the n+-region in a constant electric field (Fig. 1c). If the electric field strength in the slightly doped region is high, the electron velocity remains almost constant.
The depleted region, in which electron motion with constant velocity occurs, is called the drift zone. Since the differential electron mobility is close to zero, the volume charge density does not decrease with the electron motion.
IMPATT generator circuit
The current through the diode should be limited by a certain value. To implement this, a supply drives the diode through a limiting resistor. Also, the DC isolation from the radio frequency signal is done by a RF choke. The IMPATT diode should be integrated into a tuned circuit, typically, into a waveguide cavity. Oscillations arise in this circuit once the supply voltage is applied.
Such semiconductor diodes allow reaching much higher power levels compared to other devices in this frequency range that are used as negative resistance.