What Are TRAPATT Pulse Generators?

TRAPATT pulse generators are microwave power sources built around the TRAPATT (Trapped Plasma Avalanche Triggered Transit) diode, a semiconductor device that generates high-peak-power pulses at frequencies ranging from hundreds of megahertz into the tens of gigahertz. The TRAPATT diode is a close relative of the IMPATT (Impact Avalanche Transit Time) diode, but it operates in a distinct regime: a high-field avalanche zone sweeps rapidly through a reverse-biased p-n junction, filling the depletion region with a dense plasma of electron-hole pairs that become temporarily trapped at low field behind the zone. This trapped plasma state, not seen in conventional IMPATT operation, is the defining mechanism that gives the device its name and its unusually high conversion efficiency.

TRAPATT devices emerged from microwave semiconductor research in the late 1960s as engineers sought pulsed power sources that could generate short, high-peak-power bursts without the bulk of magnetrons or the complexity of traveling-wave tubes. The JEDEC dictionary definition of the TRAPATT diode describes it as a semiconductor device that, when biased into avalanche, exhibits negative resistance at frequencies below its transit-time frequency range due to the generation and dissipation of trapped electron-hole plasma.

Operating Principle

The TRAPATT mode is initiated when the electric field across a reverse-biased p+-n-n+ or n+-p-p+ structure exceeds the avalanche breakdown threshold. A thin avalanche zone nucleates at the high-field region and propagates across the depletion layer at a velocity faster than the carrier saturation velocity, leaving behind a dense plasma of electrons and holes. Because the field collapses behind the zone as the plasma fills the space, the carriers are swept out slowly, producing a current pulse that persists well after the zone has traversed the device. This collapse-and-refill cycle, occurring at microwave frequencies determined by device geometry and bias conditions, is responsible for the negative resistance that sustains oscillation. Efficiency values of 15 to 40 percent are typical in pulse operation, with peak efficiencies approaching 75 percent at frequencies near 0.6 GHz, figures that substantially exceed those achievable with IMPATT diodes in the same frequency range.

Device Structure and Performance

A TRAPATT diode is fabricated as a silicon or gallium arsenide p-n structure with a lightly doped n-type depletion region between heavily doped contact regions. The depletion width, typically 2.5 to 12.5 micrometers depending on the target frequency, determines the transit time and hence the operating frequency. In pulsed operation, TRAPATT diodes can switch very high currents with rise and fall times below one nanosecond. Research documented through OSTI on high-voltage picosecond pulse generation using avalanche diodes describes the circuit conditions, including fast-recovery pulse-forming networks, that extract high-peak-power pulses from these devices. Typical operating points for radar applications reach 1.2 kilowatts of peak power at 1.2 GHz, with the diode embedded in a resonant cavity or coaxial circuit that serves as both the frequency-determining element and the output coupler.

Applications

TRAPATT pulse generators have applications in systems requiring high-peak-power microwave bursts at moderate average power levels, including:

• Pulsed Doppler radar systems and low-power local oscillators for radar receivers
• Radio altimeters in civil and military aviation
• S-band phased-array radar transmitters where compact, high-efficiency sources are required
• Landing systems and precision approach aids using microwave ranging
• Avalanche transit-time device demonstrations in microwave engineering education and laboratory instrumentation