When creating electronics projects, you might have wondered what the difference is between the two most used transistor types: bipolar junction transistors (BJT) and metal-oxide-semiconductor field-effect transistors (MOSFET) and what to choose for your project. You must consider many different factors to arrive at a decision. These include power level, drive voltage, efficiency, cost, and switching speed, amongst other things. This guide will discuss some of the differences between a PNP bipolar transistor and a P-channel MOSFET, based on their characteristic curves so you can make the right decision.

 

 

For this project, Digilent Analog Discovery 2Analog Discovery 2 is used along with a transistor tester and a model of BJT and MOSFET transistors. Both transistors can be found in the myParts KitmyParts Kit from Texas Instruments.

Analog Discovery 2Analog Discovery 2 is a USB oscilloscope, logic analyzer, and multi-function instrument that allows users to measure, visualize, generate, record, and control mixed-signal circuits of all kinds. This test and measurement device is small enough to fit in your pocket, but powerful enough to replace a stack of lab equipment, providing engineering professionals, students, hobbyists. and electronic enthusiasts the freedom to work with analog and digital circuits in virtually any environment, in or out of the lab.

 

The Transistor Tester AdapterTransistor Tester Adapter is a module that allows you to add the functionality of a curve tracer to your test equipment suite. The Transistor Tester Adapter allows you to analyze the characteristics of discrete semiconductor devices like diodes, NPN and PNP transistors, and both P-Type and N-Type FETs. The Transistor Tester Adapter is equipped with the 2×15 MTE Connector, which makes it compatible with Analog Discovery 2.

 

Connect the Transistors

Plug one transistor into the adapter at a time to do the respective measurements. When working with the MOSFET, connect the device's Drain into the adapter's Collector (labeled “C”), the Gate into the Base (“B”), and the Source into the Emitter (“E”).

 

AdapterBJTMOSFET
Collector (“C”)CollectorDrain
Base (“B”)BaseGate
Emitter (“E”)EmitterSource

 

Plot the characteristic curve in WaveForms Curve Tracer

The Curve Tracer instrument allows you to work with the Transistor Tester Adapter and uses the relays of the platform to perform the specific measurements. With the Transistor Tester, you can plot the characteristics curve of a diode, NPN transistor, PNP transistor, N-channel FET, and P-channel FET directly. Check the Digilent Reference page for more information about the Curve Tracer.

 

 

Compare BJT with MOSFET

 

We compare the behavior of BJT and MOSFET in two different conditions.

 

collector-emitter (drain-source) voltage is close to 0

The drain current of the MOSFET starts to rise shortly after the drain-source voltage rises above zero. The length of the linear region depends on the gate-source voltage: the higher the voltage, the later the current saturates. The slope of the curves is a moderate one, increasing with the gate-source voltage. The collector current of the BJT starts to increase at a higher collector-emitter voltage, but the slope is more abrupt. The saturation region ends at a specific collector-emitter voltage for any base current, so the length of the saturation region depends only on the model of the transistor. The curves for different base currents in this region are very close.

 

Higher collector-emitter (drain-source) voltages

Once the drain current of the MOSFET saturates, it remains constant for any drain-source voltage, its value will depend only on the gate-source voltage. It can be observed, that the gate-source voltage is higher (up to 5V) than the voltage applied to the resistance connected to the base of the BJT (up to 2.5V). This is because the gate-source voltage of the MOSFET must be higher than a threshold voltage to turn the device on (which is around 3.5V in this case).

 

The BJT, like the MOSFET, has a base-emitter voltage limit, under which it is considered turned off (around 0.7V in this case), but this limit is much lower than the MOSFET's threshold voltage.

 

 

Find all the project details on the Digilent Reference Page.