Advanced Electronics: Understanding Design Challenges For Space Applications Part 2

October 3, 2019 Francesco Poderico

In the previous article we saw that when a particle that hits a BJT with enough energy, part of this energy may get absorbed by the component and may create unwanted current between the emitter and the collector. If this issue is neglected, we may have some malfunctions. This phenomenon is called SEE (Single Event Effect). It is the responsibility of the designer to design the board in a way that will be SEE tolerant.  As I wrote in the previous article, I’m not assuming that the reader has a degree in microelectronics or physics, and in doing so we may lose some details, however, this is a small price to pay to explain a such difficult subject in “simple terms”. 

What are we going to learn?

•    How to estimate SEE (Single Event Effect) current

•    What is the LET (Linear Energy Transfer)

•    We understand why consumer electronic will get damaged when hit by radiations.

What is the LET?

The Linear Energy Transfer (LET) is a design parameter that the system engineer should provide to the designer engineer, this parameter is not easy to estimate, for example it depends on the orbits, the altitudes, etc.  However, when is known, is usually provided at the beginning of the project. 

To understand what LET is let’s have a look at figure the below. As an example, let’s consider a high energized particle hitting a component… and let’s see what may happen “inside” the component.

Figure 1. A particle is going to hit a component

When charged radiation passes through our BJT (MOS), it slows down and loses some kinetic energy. Part of this energy is lost due to Columbian forces and part of the energy loss is due to radiation losses. Therefore, we can write the total energy loss per unit of length as: 

Figure 2. A charged particle hit an atom, part of energy is lost due to Columbian forces and release an electron from the atom

The first term of the eq.1 (𝑑𝐸/𝑑𝑥)𝑐𝑜𝑙 is called Linear Energy Transfer (LET) and represent the energy lost in the material due only to Columbian forces. In the space industry the LET is defined in respect of the material density as well, and therefore the unit used is [MeV cm²/mg].

Question: “How much energy is needed to create an electron hole?”  

Answer: “for material like silicon is only 3.6 eV “ 

Please notice that on different material the above value is different, for example on air is 34 eV, on Si02 is 17 eV, etc. 

We need the above information to calculate the current created by a particle in a component (BJT or MOS).

Let’s see how… let’s assume we want to estimate the SEE current when LET is known, for example, LET = 100 MeV cm²/mg.

Since we know that we need 3.6eV to create an ion, we can start to calculate the number of ions created: n = LET/3.6.

Since the charge of an electron is equal to q =〖1.6〗^(-19)  C, from the definition of LET we can write that

Where dx is the length of the critical path we are evaluating the effect of SEE current, for example on a BJT dx would be the distance between the emitter and the collector, (in a MOS will be the length of the channel).

OK, we know the total charge created in our BJT, but what about the current created between the collector and the emitter? 

Now, a particle in the Si will travel at a relativistic speed (for example at 33% of the speed of the light) therefore by knowing the geometry of the transistor we know the pulse duration dt = dx/(0.33 c) where c is the speed of the light.

So we and we can finally calculate the SEE current

Please notice that even if this current is very high, its ON only a short period of time.

How do we interpret the [eq. 4] ? How do we prevent our board from getting damaged?

There are many ways to prevent our board from getting damaged.  Let’s consider the following circuit:

Figure 3. In normal conditions the current is low

Let’s assume Q1 is “ON”, while Q2 is not conducting at all.  What happens when Q2 is hit by a high energized particle?

As we saw earlier, the higher is the LET of the particle, the higher will be the current created, between the collector and the emitter of the component hit. 

If we compare figure 3 to figure 4 we should not find it hard to convince ourselves that in figure 4 we can even damage our circuit. How do we avoid this?

Figure 4. High current due to SEE

One way to avoid damaging our circuit is to insert some element that limits the current from Q1 to Q2. We can do this in many ways, the easiest way is just to add a resistor. The bigger the resistor, the lower is the maximum current that can travel, on the other side we don’t want to add a large resistor, since this will affect the bandwidth of our design.

What have we learned?

We have learned that in space when a particle hits a BJT (or a MOS) we may get a large current.

We have also learned how to estimate the current in terms of amplitude (eq.4) and duration. And with a tool like Spice (Altium has an excellent Spice simulator) is possible to verify the effect of the current pulse. We have also seen that it is possible to minimize the effect of this current by limiting the current using a passive element like a resistor.

Talk to an Altium expert today to learn more.

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