transistor

117 Articles

FET: Fun Endeavors Together

May 2, 2023 by 42 Comments

Last time, we’ve looked over FET basics, details, nuances and caveats. Basics aren’t all there is to FETs, however – let’s go through real-world uses, in all their wonderful variety! I want to show you a bunch of cool circuits where a friendly FET, specifically a MOSFET, can help you – and, along the way, I’d also like to introduce you to a few FETs that I feel like you all could have a good long-term friendship with. If you don’t already know them, that is!

Driving Relays

Perhaps, that’s the single most popular use for an NPN transistor – driving coils, like relays or solenoids. We are quite used to driving relays with BJTs, typically an NPN – but it doesn’t have to be a BJT, FETs often will do the job just as fine! Here’s an N-FET, used in the exact same configuration as a typical BJT is, except instead of a base current limiting resistor, we have a gate-source resistor – you can’t quite solder the BJT out and solder the FET in after you have designed the board, but it’s a pretty seamless replacement otherwise. The freewheel (back EMF protection) diode is still needed for when you switch the relay and the coil produces wacky voltages in protest, but hey, can’t have every single aspect be superior.

The reason you can drive it the same way is quite simple: in the usual NPN circuit, the relay is driven by a 3.3 V or a 5 V logic level GPIO, and for small signal FETs, that is well within Vgs. However, if your MCU has 1.8 V GPIOs and your FET’s Vgs doesn’t quite cut it, an NPN transistor is a more advantageous solution, since that one will work as long as you can source the whatever little current and the measly 0.7 V needed.

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Transistors That Grow On Trees

April 28, 2023 by 16 Comments

Modern technology is riddled with innovations that were initially inspired by the natural world. Velcro, bullet trains, airplanes, solar panels, and many other technologies took inspiration from nature to become what they are today. While some of these examples might seem like obvious places to look, scientists are peering into more unconventional locations for this transistor design which is both inspired by and made out of wood.

The first obvious hurdle to overcome with any electronics made out of wood is that wood isn’t particularly conductive, but then again a block of silicon needs some work before it reliably conducts electricity too. First, the lignin is removed from the wood by dissolving it in acetate, leaving behind mostly the cellulose structure. Then a conductive polymer is added to create a lattice structure of sorts using the wood cellulose as the structure. Within this structure, transistors can be constructed that function mostly the same as a conventional transistor might.

It might seem counterintuitive to use wood to build electronics like transistors, but this method might offer a number of advantages including sustainability, lower cost, recyclability, and physical flexibility. Wood can be worked in a number of ways once the lignin is removed, most notably when making paper, but removing the lignin can also make the wood relatively transparent as well which has a number of other potential uses.

Thanks to [Adrian] for the tip!

FET: The Friendly Efficient Transistor

April 25, 2023 by 57 Comments

If you ever work with a circuit that controls a decent amount of current, you will often encounter a FET – a Field-Effect Transistor. Whether you want to control a couple of powerful LEDs, switch a USB device on and off, or drive a motor, somewhere in the picture, there’s usually a FET doing the heavy lifting. You might not be familiar with how a FET works, how to use one and what are the caveats – let’s go through the basics.

Here’s a simple FET circuit that lets you switch power to, say, a USB port, kind of like a valve that interrupts the current flow. This circuit uses a P-FET – to turn the power on, open the FET by bringing the GATE signal down to ground level, and to switch it off, close the FET by bringing the GATE back up, where the resistor holds it by default. If you want to control it from a 3.3 V MCU that can’t handle the high-side voltage on its pins, you can add a NPN transistor section as shown – this inverts the logic, making it into a more intuitive “high=on, low=off”, and, you no longer risk a GPIO!

This circuit is called a high-side switch – it enables you to toggle power to a device at will through a FET. It’s the most popular usecase for a FET, and if you’re wondering more about high-side switches, I highly recommend this brilliant article by our own [Bil Herd], where he shows you high-side switch basics in a simple and clear way. For this article, you can use this schematic as a reference of how FETs are typically used in a circuit.

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MOSFET Heater Is Its Own Thermostat

March 29, 2023 by 16 Comments

While we might all be quick to grab a microcontroller and an appropriate sensor to solve some problem, gather data about a system, or control another piece of technology, there are some downsides with this method. Software has a lot of failure modes, and relying on it without any backups or redundancy can lead to problems. Often, a much more reliable way to solve a simple problem is with hardware. This heating circuit, for example, uses a MOSFET as a heating element and as its own temperature control.

The function of the circuit relies on a parasitic diode formed within the transistor itself, inherent in its construction. This diode is found in most power MOSFETs and conducts from the source to the drain. The key is that it conducts at a rate proportional to its temperature, so if the circuit is fed with AC, during the negative half of the voltage cycle this diode can be probed and used as a thermostat. In this build, it is controlled by a set of resistors attached to a voltage regulator, which turn the heater on if it hasn’t reached its threshold temperature yet.

In theory, these resistors could be replaced with potentiometers to allow for adjustable heat for certain applications, with plastic cutting and welding, temperature control for small biological systems, or heating other circuits as target applications for this type of analog circuitry. For more analog circuit design inspiration, though, you’ll want to take a look at some classic pieces of electronics literature.

Frequency Tells Absolute Temperature

January 18, 2023 by 21 Comments

It is no secret that semiconductor junctions change their behavior with temperature, and you can use this fact to make a temperature sensor. The problem is that you have to calibrate each device for any particular transistor you want to use as a sensor, even if they have the same part number. Back in 2011  1991, the famous [Jim Williams] noted that while the voltage wasn’t known, the difference between two readings at different current levels would track with temperature in a known way. He exploited this in an application note and, recently, [Stephen Woodward] used the same principle in an oscillator that can read the temperature.

The circuit uses an integrator and a comparator. A FET switches between two values of collector current. A comparator drives the FET and also serves as the output.  Rather than try to puzzle out the circuit just from the schematic, you can easily simulate it with LT Spice or Falstad. The Falstad simulator doesn’t have a way to change the temperature, but you can see it operating. The model isn’t good enough to really read a temperature, but you can see how the oscillation works

You can think of this as a temperature-to-frequency converter. It would be easy to read with, say, a microcontroller and convert the period to temperature.  Every 10 microseconds is equal to a degree Kelvin. Not bad for something you don’t have to calibrate.

Thermistors are another way to measure temperature. Sometimes, you don’t need a sensor at all.

It’s Not Easy Counting Transistors In The 8086 Processor

January 16, 2023 by 15 Comments

For any given processor it’s generally easy to find a statistic on the number of transistors used to construct it, with the famous Intel 8086 CPU generally said to contain 29,000 transistors. This is where [Ken Shirriff] ran into an issue when he sat down one day and started counting individual transistors in die shots of this processor. To his dismay, he came to a total of 19,618, meaning that 9,382 transistors are somehow unaccounted for. What is going on here?

The first point here is that the given number includes so-called ‘potential transistors’. Within a section of read-only memory (ROM), a ‘0’ would be a missing transistor, but depending on the programming of the mask ROM (e.g. for microcode as with a CISC x86 CPU), there can  be a transistor there. When adding up the potential but vacant transistor locations in ROM and PLA (programmable logic array) sections, the final count came to 29,277 potential transistors. This is much closer to the no doubt nicely rounded number of 29,000 that is generally used.

[Ken] also notes that further complications here are features such as driver transistors that are commonly found near bond wire pads. In order to increase the current that can be provided or sunk by a pad, multiple transistors can be grouped together to form a singular driver as in the above image. Meanwhile yet other transistors are used as (input protection) diodes or even resistors. All of which makes the transistor count along with the process node used useful primarily as indication for the physical size and complexity of a processor.

A Transistor? Memory? Wait, It’s Both!

December 18, 2022 by 13 Comments

What do you get if you cross graphene, hexagonal boron nitride, and tungsten diselenide? Well, according to researchers at Hunan University, you get a field effect transistor that can act as both a switching element or a memory cell. The partial floating-gate field-effect transistor or PFGFET uses 2D van der Waals heterostructures to deal with isolated atomic layers. The paper in Nature is unfortunately behind a pay wall, but you can read a summary over on [TechExplore].

The graphene acts as the gate, and the transistor can be switched between n-type behavior and p-type behavior. It can also be configured as a switching element or as a memory element similar to an EEPROM cell.

One advantage of having configurable transistor types is that a single transistor structure can produce CMOS or complementary circuits. Traditionally, a CMOS IC has two different transistor structures and often producing one of them requires extra effort.

The configuration takes place by applying a control voltage pulse. A negative control voltage produces a p-type FET and a positive voltage configures the same transistor as an n-type. If you don’t have access to the paper, the figures available online offer a good bit of insight into the device’s design.

If you want to learn more about ordinary MOSFETs, we talk about them often. You can also get the skinny on CMOS from [Bil Herd].