Editorial: Turn Up the Heat with an Electrical-Thermal MMIC Design Flow
High-power and heat often go hand-in-hand. If you agree with this statement, then how well do you think you know and understand thermal influences on monolithic microwave integrated circuit (MMIC) designs?
Let’s take this quiz and find out: (True or False)
- Substrate vias are often thermally ineffective.
- A thin epi-layer can have a big effect on junction temperature.
- A passivation layer can influence junction temperatures.
- Pro-rata steady-state thermal analysis is not a reliable predictor of duty cycle performance.
If you answered “True” to all of these questions – well-done! (If not, you might find these application notes quite helpful .)
Indeed, most high-power power amplifier (PA) designers do know these hidden, best practice techniques; however, the lessons usually come from trial and error as well as being burned (yes, pun intended). In other words, high-power RF components – like PAs – not only produce high-power but also generate significant heat as a byproduct. Thus, long before a MMIC goes into production, it is better to know how the thermal profile will affect the electrical performance of the device.
Up until now, a dedicated MMIC electrical-thermal co-design flow largely did not exist. So, when it came to a electrical-thermal design flow, the MMIC designer had two choices:
- Run electrical simulations and ignore thermal altogether (aka “Buyer beware”); or
- Run two disparate point tools – one thermal and one electrical – and evaluate results independently (i.e. “Not my problem”).
Neither of these options is attractive, as they leave the door open for failure given the cause and effect nature of electrical-thermal interactions when the PA is operational.
For example, with integrated transceiver design, balancing thermal effects of the PA on the receiver LNA electrical performance with a small die size can be a chicken or the egg exercise in the best case scenario and a shot-in-the-dark in the worst case. When an integrated electrical-thermal co-design flow is used, the real world behavior of the MMIC is fully in the control of the designer. The advantages are then two-fold:
- Faster design turn-around since the MMIC designer is not dependent upon someone else to run simulations.
- Quicker ‘what-if’ electrical-thermal simulations to determine the optimum design such as the most compact active designs possible for a particular application (bias/power level/efficiency).
Imagine what’s possible now when thermal sweeps can be analyzed as a function of PA bias and the loop can be closed between thermal and electrical effects, not as a manual exercise, but by directly exchanging data between simulations. Furthermore, take DC analysis and chip layout into thermal simulations and back again to the electrical domain for subsequent harmonic balance, transient or even system simulations—a whole new world of design possibilities is uncovered.
With AWR Connected™ for SYMMIC (see Figure 1), a viable electrical-thermal co-design solution targeted at MMIC designers exists. What was not thought possible before or simply ignored previously is now a solid reality. This product flow makes it possible to take AWR’s Microwave Office™ electrical designs into CapeSym’s SYMMIC software package for additional thermal analysis and vice-versa. Together, AWR and CapeSym provide high-power RF designers with the ability to obtain optimal electrical performance with proper consideration given to thermal operating properties as well – resulting in next-generation RF products and systems designed more robustly and reliably.
Figure 1: Electrical-thermal co-design analysis for MMICs.
Oh, and also imagine what else is no longer possible — magic smoke (blue smoke) escaping during test (see Figure 2)! With an electrical-thermal co-simulation design flow, MMIC designers no longer need to wait until the device is built to find out that transistors often operate at very different temperatures in a MMIC, and as a result can and often do cause electrical performance problems for the circuit.
- Figure 2: Blue smoke on a macro-scale.
Catch a demo of AWR Connected for SYMMIC electrical-thermal co-design flow on AWR.TV, or better yet, download an evaluation copy at awrcorp.com and try it for yourself.
Sherry Hess brings to AWR more than 15 years of EDA experience in domestic and international sales, marketing, support, and management. Sherry holds BSEE and MBA degrees from Carnegie Mellon University in Pittsburgh, Pennsylvania.
