An IMPATT Diode or «IMPact ionization Avalanche Transit-Time diode» represents a type of a high power semiconductor diode which is typically implemented in high-frequency microwave devices. This technology allows generate signal at frequency range from a few GHz to few hundred GHz. In the IMPATT diodes a negative resistance caused by carriers transit time used as an oscillators in order to create microwave oscillations. One of the key advantages is the high-power capability comparing to other types of diode generators. The IMPATT diode is often called the «Read diode» in honor of W. Т. Read, who proposed the structure and principles of the device in 1958. A forward biased PN junction was used as a method of injection for the carrier.
IMPATT diode principle of operation
Standard PN junction and IMPATT diodes have a similar I-V characteristic (Fig.1a). When activation voltage is reached, the diode starts to conduct in forward direction and block reverse current flow. However, when voltage is higher than the breakdown level, the avalanche breakdown occurs and current flows in the reverse direction.
Microwave generation is possible in IMPATT diodes with different types of semiconductor structure. However, the IMPATT diode principle of operation is convenient to illustrate on the example of p+ — n — i — n+ — diode structure (Fig. 1b). Here, the regions of the avalanche multiplication and the carrier drift are spatially separated. Maximum electric field strength takes place in the region of p+ — n — diode junction (Fig 1e).
Electric field decreases sharply in the n-region and remains almost constant in the i-region. This region is completely depleted at high reverse voltages. When reverse voltage increases, electric field in the junction at some point will exceed the value of the breakdown. The breakdown causes the impact ionization coefficient α to reach high values. Since α strongly depends on electric field strength, the extent of the impact ionization region is small.
Avalanche increase in the number of free charges occurs in it. This region is called avalanche multiplication region. Formed holes are carried by the internal electric filed of the junction to the p+-region, and the electrons, getting to the i — region, are moving to n+ — region in a constant electric field (Fig. 1c). If the electric field strength in the slightly doped region is high, electron velocity remains almost constant.
Depleted region, in which electron motion with constant velocity occurs, is called drift zone. Since differential electron mobility is close to zero, the volume charge density does not decrease, when electrons move.
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 to rich much higher power levels comparing to other devices at this frequency range.