# Transistor switching speed

I want to be able to identify the speed of a transistor in terms of switching. Most datasheets don't indicate that but I found this transistor from ST-Electronics. Is it the maximum switching frequency of the transistor or is it indicating something else?

I won't go further than a very simplified (1st order) model of what's going on to help develop a meaning for $$\f_t\$$. There are higher order models. But to a first order the following approximation should suffice:

simulate this circuit – Schematic created using CircuitLab

Bipolars are voltage controlled devices. Not current controlled. Given the above, $$\i_b=C_\beta\, s\, V_f\$$ and $$\i_c=g_m\,V_f\$$. For the purposes of understanding $$\f_t\$$, it occurs when $$\\vert\frac{i_c}{i_b}\vert=1\$$.

Derived directly from the above, $$\C_\beta=\frac1{2\pi}\frac{g_m}{f_t}\$$ then defines a relatively flat relationship between $$\f\$$ and $$\g_m\$$. As $$\I_\text{C}\$$ (or $$\g_m\$$ which is proportional) increases so does $$\f_t\$$ in linear fashion and thus the constant $$\C_\beta\$$ captures this.

$$\C_\beta\$$ will remain relatively constant for frequencies lower than the given $$\f_t\$$. That's why it can be listed on the datasheet as a parameter, in fact.

Once the peak $$\f_t\$$ is reached, however, $$\C_\beta\$$ increases proportional to the square of $$\I_\text{C}\$$.

In a sense, $$\C_\beta\$$ is kind of like $$\h_\text{FE}\$$ which appears to be relatively constant for any given operating point. (And both aren't relatively constant when certain things like $$\I_\text{C}\$$ exceed a certain level.)

In your device's case, they specify that a typical value of $$\f_t\$$ is $$\100\:\text{MHz}\$$ when operating at $$\I_\text{C}=100\:\text{mA}\$$ and $$\V_\text{CE}=10\:\text{V}\$$. At room temperature, you can work out that $$\g_m=\frac{100\:\text{mA}}{V_T\approx 25.9\:\text{mV}}\approx 3.85\:\mho\$$. So $$\C_\beta\approx 6\:\text{nF}\$$ for this device.

But keep very much in mind that both $$\f_t\$$ and $$\g_m\$$ are functions of a lot of other device parameters and the operating point. So it's important to understand those additional device parameters when evaluating it. The datasheet likely selected the more optimistic operating point for the device. So it's really not possible to use this simplified result as some kind of bright line understanding of the device. It's just one data point to give you a rough idea of the best case expectation, I think.

Note also that the datasheet specified $$\I_\text{C}=100\:\text{mA}\$$. There's a reason. For RF work, the bipolar is often operated at higher collector currents. The reason is to achieve higher $$\g_m\$$ such that $$\g_m\gg \frac1{R_\text{E}}\$$ since, in that case, $$\C_\beta\$$ can be neglected in the design.

Is it the maximum switching frequency of the transistor or is it indicating something else?

To expand the above would require acquiring a lot more data on some device, then writing up examples -- first, of emitter followers; then second of other stages -- and then analyzing them using the parameter. I'm not prepared to write that.

So I stop here.

Switching speed of transistor cannot be defined with exact number.

For high efficiency SMPS design you don’t want the transistor to be in linear region more than example 2% of overall time. On other hand in RF power transmitter you are happy with 10% because transistors are at the edge how fast they can be made.

Regarding Ft frequency in your datasheet the transistor is completely in linear region (not a power efficient).

If you wanna use 772 or 882 for hard-switched SMPS you can consider max frequency of 100-200kHz. In special soft-switching topologies they can go up to about 500kHz.

• Commented Jul 20 at 22:12