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+---
+title: MOSFETs
+date: 2025-06-22
+author: W. D. Sadeep Madurange
+layout: post
+---
+
+Recently, I needed a low-power circuit for one of my battery-operated projects.
+Much of the system's power savings depended on its ability to switch off power
+to components, such as servos, electronically when not needed. That's how I
+stumbled upon MOSFETs, transistors capable of controlling circuits operating at
+voltages far above their own.
+
+## Acknowledgments
+
+This article is a summary of what I learnt about using transistors as switches.
+I'm not an electronics engineer, and this is not an authoritative guide. The
+circuits in this post must be considered within the context in which I've used
+them. All credits for the schematics belong to <a
+href="https://electronics.stackexchange.com/users/292884/simon-fitch"
+class="external" target="_blank" rel="noopener noreferrer">Simon Fitch</a>. 
+
+## Preamble
+
+For a typical MOSFET-based switch, we can connect a GPIO pin of a
+microcontroller to the gate of a logic-level N-channel MOSFET placed on the low
+side of the load and tie the gate and the drain pins of the MOSFET with a
+pull-down resistor. This would work as long as the power supplies of the
+microcontroller and the load don't share a common ground. Things become more
+complicated when they do (e.g., controlling power to a component driven by the
+same microcontroller).
+
+In that scenario, the source potential visible to the load is the difference
+between the gate and the threshold potentials of the MOSFET. For example, when
+the gate and the threshold potentials are 3.3 V and 1.5 V, the potential the
+load sees is 1.8 V. So, to use a low-side N-channel MOSFET, we need the gate
+potential to be higher than the source potential, which may not always be
+practical. The alternative would be a hide-side switch.
+
+## P-channel high-side switch
+
+The following schematic shows how a high-side P-channel MOSFET (M1) could
+switch power to a 6 V servo driven by a 3.3 V MCU.
+
+![P-channel high-side switching circuit](p_high_side.png)
+
+When the microcontroller outputs low, the M2 N-channel MOSFET stops conducting.
+The R1 resistor pulls the gate of the M1 P-channel MOSFET up to +6 V, switching
+the servo off. When the microcontroller outputs high on the GPIO pin, M2's
+source-drain connection starts conducting, causing M1's gate potential to drop
+to 0 V, which switches on power to the servo. 
+
+## N-channel high-side switch
+
+The P-channel high-side switch would be the typical architecture for our use
+case. However, if we have access to a potential high enough to safely raise the
+gate potential above the threshold such that their difference outputs the source
+potential required to drive the load, we can switch on the high side using an
+N-channel MOSFET:
+
+![N-channel high-side switching circuit](n_high_side.png)
+
+In the schematic, both M1 and M2 are N-channel MOSFETs. When the
+microcontroller output is low, M2 stops conducting. This causes the M1's gate
+potential to rise above the threshold, turning the servo on. Conversely, a high
+output on the GPIO line switches M2 on, which lowers M1's gate potential. This
+switches the servo off. The R2 pull-up resistor prevents the high impedance of
+the output pins at power-up from switching the servo on.
+
+Both topologies require M2 to act as a level converter between circuits
+containing the microcontroller and the servo, converting between 0 V and +6 V
+or +9 V. M2 is a low-power signal converter carrying less than a milliamp of
+current. The gate-source threshold voltage of M2 must be lower than the MCU's
+supply voltage. 2N7000, 2N7002, and BSS138 are popular choices for M2.
+
+The D1 flyback diodes used in the two topologies safeguard the MOSFET from
+voltage spikes caused by inductive loads such as servos.
+
+## A BJT alternative
+
+A Bipolar Junction Transistor (BJT) is a simpler, cheaper, and more widely
+available type of transistor that can be used as a switch.
+
+![BJT architecture](bjt.png)
+
+In the schematic, when the MCU outputs high, Q2 starts conducting. Q2 amplifies
+Q1's base current. Unlike MOSFETs, which are voltage-driven, BJTs are driven by
+base current. Resistors R3 and R4 must be chosen carefully to output the
+desired base currents. <a
+href="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/"
+class="external" target="_blank" rel="noopener noreferrer">"How to choose a
+transistor as a switch"</a> is an excellent guide on using BJTs as electronic 
+switches.
+
+## Which topology to choose?
+
+The professional community appears to prefer MOSFETs over BJTs. MOSFETs are
+more efficient when the switch is on. However, they are more challenging to
+drive, especially with a 3.3 V MCU, due to the V<sub>GS</sub> potentials
+required to achieve specified R<sub>DS(on)</sub> values (i.e., to turn them on
+fully).
+
+N-channel MOSFETs have lower on-resistance values, making them more efficient
+than P-channel ones. They are also cheaper. However, they are harder to drive
+on the high side as their gate potential must be higher than the source
+potential. This often requires extra circuitry such as MOSFET drivers.
+
+## Further reading
+
+ - <a href="https://www.embeddedrelated.com/showarticle/98.php"
+   class="external" target="_blank" rel="noopener noreferrer">Different MOSFET
+   topologies</a>
+ - <a href="https://www.embeddedrelated.com/showarticle/809.php"
+   class="external" target="_blank" rel="noopener noreferrer">How to read 
+   MOSFET datasheets</a>
+ - <a src="https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/"
+   class="external" target="_blank" rel="noopener noreferrer">How to use a 
+   transistor as a switch</a>
+ - <a src="https://forum.digikey.com/t/guide-to-selecting-and-controlling-a-mosfet-for-3-3-vdc-logic-applications/42606"
+   class="external" target="_blank" rel="noopener noreferrer">Guide to 
+   selecting and controlling a MOSFET for 3.3 VDC logic applications</a>
+ - <a src="https://forum.digikey.com/t/driving-a-large-relay-from-a-3-3-vdc-microcontroller-using-an-npn-darlington-transistor/41751"
+   class="external" target="_blank" rel="noopener noreferrer">Driving a large 
+   relay from a 3.3 VDC microcontroller using an NPN Darlington transistor</a>