Monolithic and Hybrid Microwave Integrated Circuits: What's the Difference?

Key Takeaways

• Microwave integrated circuits are designed to support operation at microwave frequencies in a compact package.

• Monolithic and hybrid microwave integrated circuits are two types of components that provide diverse functions for high frequency microwave systems.

• Monolithic microwave integrated circuits (MMICs) provide greater advantages at high frequencies, as they are built on semiconductors other than silicon.

Integrated circuits have been with us for decades, and they’ve only become more advanced over time. Today’s advanced RF products make use of microwave monolithic integrated circuits (MMICs) to provide standard RF capabilities in standardized packages. The other common type of RF component is a hybrid microwave integrated circuit (HMIC). Both monolithic and hybrid microwave integrated circuits have their place in today’s RF systems, but they provide different capabilities and functionalities.

Today, many components for RF systems are available as HMICs or MMICs and the terms are sometimes used as marketing tools to sell products. To reduce component count and total system cost, it’s a good idea to try and find integrated circuits to consolidate components into smaller packages. If you’re unsure of which type of component to use, here’s how HMICs and MMICs are different and which applications are best for these RF components.

Monolithic vs. Hybrid Microwave Integrated Circuits

Hybrid microwave integrated circuits (HMICs) and monolithic microwave integrated circuits (MMICs) are the two types of dominant integrated circuits used in microwave systems. Newer components for microwave applications are often advertised as being MMICs. The primary differences between MMICs and HMICs lie in how they are constructed:

• MMICs are built up from a single semiconductor die. All circuits and components are fabricated directly on the semiconductor die from a single base material. This is done with standard planar processes, although future MMICs will be designed as 3D circuits using heterogeneous integration.

• HMICs are built from multiple discretes, semiconductors, and integrated circuit blocks on a high-quality substrate. Devices are then connected to each other with metalized wires and contacts, giving a completely modular integrated circuit.

Both types of integrated circuits are then placed into packages with an epoxy or other material, and these packages can have standard IPC form factor (QFN, SOT, etc.). As these components operate at microwave frequencies, they are generally not through-hole components, as through-hole leads create high frequency signal integrity problems normally seen on via stubs. Unless an MMIC or HMIC comes in a customized package, they may be indistinguishable from a standard Si integrated circuit until you read a datasheet.

Materials for HMICs or MMICs

MMICs are fabricated from any semiconductor material that can be put through a planar process. While Si can be used as the substrate, today’s higher-end MMICs use some of the following materials:

• SiC (up to ~5 GHz)

• SiGe (up to ~80 GHz at low power)

• GaAs (up to ~100 GHz but low power)

• GaN-Si (up to 8 GHz)

• GaN-SiC (up to ~100 GHz at high power)

As an example application, today’s newer RF power amplifiers for mmWave applications have gone almost entirely GaN-SiC, as these materials offer very high thermal conductivity, which is needed to keep temperature low. MMICs dominate at higher frequencies (K band and higher) reaching mmWave.

HMICs may be fabricated from any of the following semiconductor materials:

• Si (Quartz, SOI, doped, or undoped substrates)

• III-V materials (GaAs or AlGaAs)

• Ceramics (alumina and beryllia)

• II-VI materials (InP)

• Woven PTFE/glass

Other materials may be used for portions of an HMIC as well. The last material in the above list should look familiar, as this is a PCB substrate material. In fact, PCB substrate materials are used to build prepackaged modules with SMD pads or castellated pads, which can then be bonded directly on a PCB. Commercial HMICs tend to operate up to the X or K bands at a range of output powers.

This hybrid microwave integrated circuit places multiple SMD components and modules into a complete package with bond wires.

Why Use HMICs and MMICs?

Before MMICs and HMICs, RF circuits were built entirely from discrete components and passives in a PCB layout. This becomes problematic at very high frequencies, where parasitics create undesired signal behavior and crosstalk in a PCB layout. HMICs help reduce parasitics and overall component count/size by making circuit blocks more compact in a single package. MMICs go a step further; by integrating everything on a single semiconductor die, component count and assembly count can be reduced while also taking advantage of more exotic semiconductor materials.

Making MMICs Hybrid with Multichip Modules

An alternative method for creating MMICs with hybrid functionality is to use multichip modules, or MCMs. These components integrate multiple ICs (called chiplets in the parlance of MCMs) onto a single semiconductor die. The structure of the MCM is built up vertically and horizontally with interconnecting wires deposited using standard metalization processes. This allows hybrid functionality to be brought into a single package by bonding chiplets onto an interposer substrate. The basic structure of an MCM is shown in the image below.

These packages follow the same idea as HMICs in that multiple components and circuits are bonded onto a high-quality substrate to form the package. The key difference in an MCM is that some of the blocks on an MCM may be monolithic circuits, including MMICs. This ability to integrate and place MMICs with other circuit blocks on a single substrate is key to driving further integration and reducing overall system size.

AMD’s Epyc MCM processor.

This structure is currently used on consumer-grade, gaming-grade, and server-grade CPUs and GPUs manufactured by Intel and AMD. The advantage of this structure is that it creates a single package that accommodates hybrid or monolithic components, rather than placing each component onto a PCB. This type of package enables more unique designs to be created with a modular approach, and it’s likely to extend to other groups of advanced components.

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