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IMPATT Diode Applications for High-Frequency Circuits

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • IMPATT diodes use an alternate doping layout compared to baseline PN diodes that imparts negative resistance and a high-power output in avalanche breakdown mode.

  • IMPATT diodes can operate in avalanche breakdown mode without failure.

  • These characteristics make IMPATT diodes exceptionally suitable for high-power and high-frequency circuits.

An axial-leaded diode.

IMPATT diode applications result from its ability to operate in the avalanche breakdown region without failure.

Diodes are fundamental circuit elements used to control the current flow, but many variants are available that are more or less viable for particular circuits. These changes arise due to alterations of the base PN junction that is the building block of all diodes (and all semiconductor devices). The impact ionization avalanche transit-time diode, or IMPATT diode, uses two heavily doped regions at the anode and cathode and additional N-type and undoped regions to produce high power at high frequencies. IMPATT diode applications tend to excel in power generation, typically at microwave frequencies and above, but they also include more general circuit functions.

Pros and Cons of IMPATT Diode Applications

Pro

Con

  • Extremely high frequency operating range

  • Small size

  • Reliable at high temperatures

  • Excellent power output

  • Small tuning range

  • Avalanche breakdown can cause excessive noise

IMPATT Diode Design and Operating Theory

An IMPATT diode differs from ordinary diodes due to its negative resistance and the high-power output it can maintain at high-frequencies. Naturally, the output has considerable phase noise issues, which is acceptable considering its operating conditions.  The large output power arises from the diode operating in the avalanche region, a state of rapid conduction that occurs during reverse biasing, which often precedes the thermal failure of the device. However, devices can operate within this region by carefully doping the PN junction that defines the diode structure. Moreover, additional devices that support thermal management (e.g., a heat sink) can help draw away and safely dissipate the generated heat to prevent damage to the diode or any surrounding materials.

While an IMPATT diode still relies on charge carriers – electrons and holes – to carry current, the avalanche breakdown supplements the number of free charge carriers. When an electron with sufficient energy strikes a silicon atom, it can dislodge one of the valence electrons; this process can continually cascade with the freed electrons, impacting other silicon atoms and restarting the process anew. The avalanche breakdown occurs in the N-doped region that is more electron-rich/hole-poor and creates a drift zone of high resistivity in the wake of the electrons moving toward the anode. 

An IMPATT diode exhibits negative resistance, but not all negative resistance is the same; this diode has negative differential resistance or an instantaneous negative ratio of the differential voltage and current. It’s possible to define components' negative resistance as voltage-  or current-driven: in the former case, voltage and current correspondence peak at some voltage before the current falls significantly with increasing voltage (before entering the saturation region). The latter – which includes IMPATT diodes – is a multi-valued function that broadly resembles an “S.”

The current control of the IMPATT diode also arises in oscillator circuits where it can withstand the high energy of microwave-level and faster frequencies. The diode is parallel with a current source and a series LC-tuned circuit in this topology. The LC-tuned circuit uses a low-impedance model to prevent oscillation outside the targeted resonant frequency. Unlike more common feedback oscillators, IMPATT diode circuits do not rely on feedback, which can lead to heavy distortion and clipping in high-frequency environments. In general, oscillation will only occur when the negative resistance magnitude is less than the resonator, and the frequencies are conjugate (i.e., opposite signs).

IMPATT Diode Applications Cover High Frequency and High Power

Much like normal diodes, IMPATT diodes fill several circuit roles. However, the added benefit of negative resistance offers greater functionality than a standard diode. 

  • High-frequency amplifier - IMPATT diodes make excellent amplifiers due to their high power output (both continuous signals and pulses in the kilowatt range) at frequencies well into the gigahertz range. However, it is worth noting that the avalanche breakdown process introduces significant phase noise; circuit designers must prove ample filtering to prevent distortion and power loss.

  • Generator - IMPATT diodes can generate microwave energy; as mentioned, they are excellent at high power and frequency settings. Their operation is also extremely cost-effective, and circuits containing these diodes possess exceptional reliability and stable performance. Medium-to-low power microwave circuits, like those found in speed guns, also benefit from the performance characteristics.

  • Local oscillator - Alongside a mixer, local oscillators change the frequency of an incoming signal by adding or subtracting the frequency of the input signal and that of the local oscillator. IMPATT diodes are uniquely situated for local oscillators as they are less susceptible to temperature variation during operation that can lead to unstable conditions, and their high power output ensures that they can drive downstream components in the circuit. Local oscillators are also paramount for satellite signal down-conversion, enabling received signals transmission over long cable lengths without incurring excessive signal loss. One caveat is the phase noise produced by the IMPATT diode, which can disrupt circuit timing.

Finally, IMPATT diodes are environmentally robust, as their normal operation has to withstand the generated heat associated with avalanche breakdown. While thermal routing will be a factor in underlying performance, this trait makes them highly suitable for devices in high-temperature settings.

Make an Impact on Board Design With Cadence Solutions

IMPATT diode applications allow for a great many circuit implementations due to their negative resistance characteristics and general high-performance capabilities. Continuing development of transmission protocols utilizing high-frequency millimeter-wave technology relies on both aspects to drive data speeds forward. The biggest caution with IMPATT diodes is the necessity for a strong focus on mitigating EMI for continued stability even at elevated frequencies. Cadence’s PCB Design and Analysis Software suite gives designers the tools to evaluate designs from an EMC standpoint as well as traditional circuit simulation. With the DFM-led Constraint Manager of OrCAD PCB Designer, designing boards to meet the myriad of performance benchmarks and industry requirements is easier than ever.

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