My *nix world

The semiconductors: Diode, BJT, JFET, MOSFET

One important category in electronics is the Semiconductors. In the following we are going to talk about semiconductors, more exactly about four of them which you would most likely find in any electrical device nowadays:

  1. Diode
  2. BJT (the Bipolar Junction Transistor)
  3. JFET (the Junction-Gate Field Effect Transistor)
  4. MOSFET (the Metal-Oxide Field Effect Transistors)

Please keep in mind that I am not an expert in either chemistry or electronics, however these devices are so simple that is almost sad they are not taught in the kindergarten (children => sand => Silicon => semiconductor).

What is a semiconductor

A semiconductor is a type of material (eg. Silicon, Si, that can be found on Earth's soil as sand which represents ca. 15% of Earth's mass) that is neither a conductor (eg. a metal) nor a insulator (eg. rubber, glass, etc), is something in between, is a semiconductor. One thing about them is that their resistance decreases as the temperature is increased which is exactly opposite in case of a metal. With other words - and this is important - they tend to lose their semi-conductivity property with the increasing of temperature.

Due to its semi-conductivity the Silicon by itself is not very useful, but mixed with some other materials (like Phosphorus or Boron) it starts immediately to attract electrons or to repeal electrons, which is a very useful thing if you think the current means the flow of electrons through a material. The process in which the Silicon is mixed with Phosphorus or Boron is called doping (ie. injecting of a foreign substance in order to improve performance).

By doping a chunk of Silicon with Phosphorus we get a new material called N-type material, where N stands for Negative. Conversely, by doping a chunk of Silicon with Boron we get a new material called P-type material, where P stands for Positive. The fact that they are called negative N-type material and positive P-type material is directly related to their electrical charge: the N-type is negatively charged while the P-type is positively charged. To help you understand why is that we have to go a little bit at the atomic level.

An atom is made of particles that may or may not have an electrical charge: the proton which is regarded as a positive charge, the electron regarded as a negative charge and the neutron which is neither, it's neutral from this point of view. The protons and neutrons makes the atom nucleus while the electrons are orbiting the atom's nucleus somehow in the same way the communication satellites are orbiting the Earth. These electrons are positioned around the nucleus in different "orbitals" (energy levels). Naturally every energy level "has room" for a precise number of electrons. For instance the first energy level "has space" for only 2 electrons, the second energy level can host only 8 electrons, the third one 18 electrons, etc (read more about atomic orbital). An atom would naturally love to have 8 electrons on the last energy level. If it has only 3 then it thinks that it's easier to give these electrons than to get 5 electrons such that 3+5=8. When it has 5 electrons it thinks that it is easier to find and attract 3 electrons than to give its 5 electrons away. You see, the atom is smart, it is very efficient.

Now, a Silicon atom has only 4 electrons on its last energy level. It means that for it it's equal if it has to attract or to give these electrons in order to reach its ideal energy level. That is the first reason it was chosen as the main material for our semiconductor mixed material.

The Phosphorus on the other hand has 5 electrons on its last energy level. It would love to attract 3 electrons such that it would reach its ideal energy level. Since the Silicon wants to give or to get 4 electrons and the Phosphorus wants to get 3 electrons they negotiate an truce: they are going to share these electrons in pairs of two. So the new molecule is going to (4+5)/2 = 4 pairs of shared electrons and one free electron. Since the electron represents a negative charged particle we can say that this Silicon-Phosphorus material is going to have a negative charge carrier. Such mixed material is called N-type semiconductor material.

Below is shown Antimony (Sb) which like Phosphorus has also 5 electrons in the outer valence shell, thus is also used for building a N-type material:

The semiconductors: Diode, BJT, JFET, MOSFET


The Boron has only 3 electrons on its last energy level. It would love to give these electrons such that it would reach its ideal energy level. Since the Silicon won't be happy with only 3 electrons due the fact that it would love to get or give only 4 electrons, they negotiate a truce: they are going to share these electrons. So the Silicon-Boron molecule is going to have (4+3)/2=3 pairs of shared electrons and one pair that has an electron missing. Since one electron is missing and since the electron is regarded as a negative charge, minus * minus = plus, so the molecule would have a positive charge carrier. Please note that the missing electron is sometime referred as "a hole". The "hole" is the positive charge carrier in this case. Also such mixed material is called P-type semiconductor material.

The semiconductors: Diode, BJT, JFET, MOSFET


Please note that these materials are neither negatively or positively charged, they only have either a negative or positive charge carriers, something that allows the electrons to flow through such a material.

In conclusion, by using the doping process we made both the new N-type semiconductor material and the P-type semiconductor material to be able to conduct electricity.

When a P-type semiconductor material is joined with a N-type semiconductor material we get a new device that is called a PN-material. The middle layer of it is called the PN-junction. This PN-junction is going to be in electrical equilibrium, it won't have any charge carriers and thus it wouldn't naturally conduct current. So that region, called also depletion region (depleted of charge carriers), is a natural potential electrical barrier, a natural insulator (of about 0.6-0.7V) which won't allow further electrons to flow from one region to the other  (ie. from N to P or vice-versa). However, if we would apply an external potential energy greater than the PN-material's electrical barrier potential of 0.7V then we could overcome that potential electrical barrier, the barrier would collapse, and by doing that we would allow the current to flow from one region to the other.

What is a diode

A diode is nothing more than the PN-junction device described above. It's electrical symbol looks like this:

The semiconductors: Diode, BJT, JFET, MOSFET

By connecting a battery which would have a potential energy greater than the depletion region's potential electrical barrier (eg. 0.6V) with its negative terminal to the N-type material and with its positive terminal to the P-type material then we are pushing the electrons within the N-type material to the point where the N-region would be somehow negatively charged. On the other hand the positive terminal of the battery would start attracting the electrons from the P-type material letting the P-type region somehow negatively charged. When these happen then the depletion region would collapse and the current would start flowing from the negative terminal towards the positive terminal through the PN-material. This type of connection is called forward bias.

The semiconductors: Diode, BJT, JFET, MOSFET


If we would connect the battery on reverse, ie. with its positive terminal to the N-type material and its negative terminal to the P-type material, then the above phenomenon would also reverse, thus would oppose the current flow. This type of connection is called reverse bias.

The semiconductors: Diode, BJT, JFET, MOSFET


So the function of a diode is basically to allow the current flow in one direction while opposing the current in the reverse direction. The barrier potential energy is also called forward voltage VF which in case of a Silicone diode would be ca. 0.6V. As the current flow increases the semiconductor internal temperature increases to the point that it starts be less and less resistive and finally get damaged. The maximum forward current that can handle is called forward current IF. The maximum temperature at which the PN-junction is still stable is called junction temperature TJ. Naturally a diode opposes the current flow when connected on reversed bias. The maximum voltage it can oppose the flow is called reverse voltage VR.

Normally when we choose a diode we would be interested of how much current it can safely handle, about its maximum forward and reverse current and eventually its operating temperature range.

Here is a little demo of a diode circuit in action:

Please watch a very good YouTube tutorial by Ben Eater. Also please read the Semiconductor tutorials on

What is a transistor

Another type of semiconductor device is the transistor. The transistor consists of a sandwich of P-N-P materials or N-P-N materials, each layer of the sandwich being doped in different proportions. They are like two diodes in parallel that are arranged back to back. They are used in almost any electrical device nowadays, are extremely cheap and nonetheless can be extremely small. As of 2017 the smallest transistor used in electrical devices is only 10nm small with NVidia Tesla GPU chip containing ca. 21 billions transistors.

The most common types of transistors are: BJT, JFET and MOSFET. Or at least these are the ones I'm going to talk about next.

What is a BJT

A very common type of transistors is the Bipolar Junction Transistor (BJT). It can be designed either as a P-N-P sandwich (revers bias) or as a N-P-N sandwich (forward bias), each region having the following names: E=emitter, B=base, C=collector. In order to turn ON the transistor we need to overcome the emitter-base potential barrier of 0.7V. So by applying a forward voltage of 0.7V or more between the emitter and base the current will start not only to flow between emitter and base but also between the emitter and collector.  When that's happening we say the transistor is switched ON.

The semiconductors: Diode, BJT, JFET, MOSFET


When the base is made of a N-type material it is a P-N-P transistor. When the base is made of a P-type material it is a N-P-N transistor. The BJT electrical symbol looks like above, depending if it's a N-P-N (right) transistor or a P-N-P (left) transistor. Remember that the arrow always points to the N-type material.

The base function of a transistor is the electrical switch. Another function of the transistor is of a current amplifier (small input voltage between base-emitter gets amplified as current flow between emitter and collector).

A little demo of a circuit with a transistor as a switch:

Please watch this good YouTube tutorial about BJT by 00retrobrad00.

The N-P-N type BJT

When the transistor is connected in forwarded bias to a power source between the emitter and the base and that power source has a potential energy under a certain threshold (eg. 0.7V) then the "emitter-collector switch" is open, there is not current flow between the emitter and collector. When that threshold is exceeded the transistor is switched on, ie. the current starts flowing from the emitter to the collector. Keep in mind that the current needed to overcome that potential barrier is smaller in comparison with the current that starts flowing between the emitter-collector.

How a NPN transistor works:

Using a BJT transistor as an amplifier

The P-N-P transistor works the same as the N-P-N except that in case of P-N-P we supply a forward voltage between base and emitter which will open the transistor such that an even greater current will start flowing from collector to emitter. Please keep in mind that the N-P-N transistors are the easiest to use when an electrical switch is needed.

This is a little demo circuit of a transistor as an amplifier:

Please watch a very good YouTube tutorial by Ben Eater. Also please read the NPN tutorial on Transistors.

Example: amplify an electret microphone.

The semiconductors: Diode, BJT, JFET, MOSFET

An electret microphone is a type of electrostatic capacitor-based microphone, which eliminates the need for a polarizing power supply by using a permanently charged material - Wikipedia



Since this microphone is basically an capacitor-like device it needs current to power on. Usually, depending on which model you have, it would require anything between 2 to 5 VDC, would draw as much as 0.5mA and would normally have a sensitivity of -45dB (the higher the voltage, the better its sensitivity).

Note: It's minus 45dB because its sensitivity is below the human hearing threshold, which is a cool thing about microphones as they sense sounds that we cannot hear. If we amplify a sound we normally cannot hear it becomes "hearable". And that's what we are going to do next.

We are going to use a 5VDC power supply to power this microphone and that means we also have to limit the current that it draws (eg. 0.5mA) by using a resistor in series with the microphone. The microphone output would be an signal that would oscillate up and down, like a sine wave and thus it's an AC signal.

Ohm's law: R1 = U/I = 5V/0.5mA = 10KΩ

Since our power source is a DC power supply it outputs DC signals while our microphone outputs AC signals. We don't want to capture and later amplify the DC signal by mistake so we have to filter it. For that we are going to use in series a coupling capacitor with the value of C1 = 100nF (read more).

By using an oscilloscope we can probe our circuit and capture the AC signal:

The semiconductors: Diode, BJT, JFET, MOSFET

By using our voice while speaking into the electret microphone a weak AC signal (around 300mV peak to peak) is generated. If we were to capture/record this audio signal on a sound card then play it we would hear probably a small/weak sound. Of course you could amplify it at the computer level but why not building our own amplifier thus making the circuit independent?

By using a small signal transistor we could amplify the input AC signal at least 50 times, which would be cool! The ubiquitous 2N2222A transistor has a DC current gain characteristics of hFE=30-325 times which would be just fine for this project. However, better BJT transistors are available like 2N3403 with a gain of or hFE=180-540, the 2N5306 with a gain of at least hFE=7000 or MPSA14 with a gain of at least hFE=20000.

By applying a low current at the base we get an amplified  current flow between the emitter and the collector. The microphone signal can thus be amplified  by connecting a BJT transistor's Base terminal in series with the capacitor C1. Since the Emitter is the one that emits electrons we are going to connect the BJT's Emitter terminal to the negative rail of the power supply and the Collector terminal to the positive rail of the power supply.

Next we need to use another resistor R2 = 10KΩ between the Collector and the positive rail. By doing that we inevitably pull-up the voltage (V=IR) while the transistor pulls-down the voltage since it's connected to the negative power rail. Since our AC signal oscillates bellow zero the pull-up resistor R2 will cut the voltage below zero which leads to loosing some parts of the audio signal. In order to overcome this we have to pull up the AC signal below the Base and Collector terminals so we add a new resistor R3=100KΩ.

In order to make sure that the final signal is clean waveform we could add another C2=100nF capacitor between the Collector terminal and the power supply positive rail.

And the circuit is completed!

The semiconductors: Diode, BJT, JFET, MOSFET

If we want to test it we should add a speaker or to capture the output signal by a sound-card. All we have to do is to connect a speaker between the C2 capacitor and the power supply negative rail and we are done!

By connecting the oscilloscope probe between the capacitor C2 and the ground we can measure the amplified voltage:

The semiconductors: Diode, BJT, JFET, MOSFET

And here is a sound recorded with the above amplifier:


Note: please note that this circuit is far from being a good audio amplifier, it was created only to test/confirm the BJT amplification characteristics.

What is a JFET

Another useful type of transistor is the Junction Field Effect Transistor - JFET. Since its name implies the existence of a field effect I automatically think to a kind of material where an external electric field is applied, thus it leads me to the voltage concept.

One difference we might quickly notice about JFETs is the electrical terminals names. On BJT transistor we were used with their Base-Emitter-Collector electrical terminals names whereas on JFET the Gate-Source-Drain names are preffered. So basically it's the "same man, different hat". However, the JFET's inner design is a bit different, though.

We could picture a JFET transistor as a large cylindrical block of a N-type semiconductor material surrounded by a small band of P-type material. The role of the P-type material band is to choke that large cylindrical N-type block thus limiting the current flow through the cylinder. The stronger the choke the less current flow and vice-versa. The ends of the N-type material cylindrical block are called Source and Drain. The P-type material band is called Gate.

The semiconductors: Diode, BJT, JFET, MOSFET


In normal conditions the current would flow naturally from the Source to the Drain. If we want to decrease the current flow between these two terminals we have to choke that P-type cylindrical block, we have to close somehow that Gate such that the current between the Source and the Drain gets limited. By connecting the Gate in reversed bias we create a depletion region around the Gate so the channel between the Source and Drain gets smaller. The larger the voltage applied at the Gate the larger the depletion region, thus the smaller the Source-Drain channel (and so the current flowing through that channel). Normally the minimum voltage required to overcome the P-N barrier is about 0.7V. When that value is reached it starts creating a depletion region between the P-type band and the N-type channel.

The JFET transistor can be built either as an NPN device (having a N-type channel, se image above) or as a PNP device (having a P-type channel).

A JFET can work in three different modes:

  • Common-Source (CS) mode, when an input voltage is applied at the Gate and the output current flows towards the Drain
  • Common-Gate (CG) mode, when an input voltage is applied at the Source, the Gate being connected at 0V, the output current flows towards the Drain
  • Common-Drain (CD) mode, when an input voltage is applied at the Gate and the output current flows towards the Source

Please watch a very good YouTube tutorial by 00retrobrad00. Also please read the JFET tutorial on Transistors.

What is a MOSFET

Another type of transistor is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). This type of transistor differs from the other types of FETs by having the Gate terminal isolated electrically from the N-type channel by a Metal Oxide insulator, the resistance being in the region of Mega-ohms. Being isolated the current will never flow through the Gate as in the case of the other transistors.

There are two types of MOSFETS:

  • depletion mode MOSFET (choke the N-type channel in order to reduce the current flow)
  • enhancement mode MOSFET ("widen" the N-type channel in order to enhance the current flow)

These are similar in construction, to switch from one to another all we have to do is to change the polarity at the Gate terminal. The construction model of a MOSFET looks like in the image below. Please note that the Gate is not connected electrically to the N-type channel, it is isolated electrically by a Metal-Oxide insulator. Being isolated the current will NEVER flow through the gate as in the case of the other transistors.

The semiconductors: Diode, BJT, JFET, MOSFET


The most common MOSFET mode is the Enhancement-mode MOSFET.

Here is how a MOSFET works:

Depletion-mode MOSFET

In this mode the transistor is switched ON without the need of a voltage at the Gate terminal. By applying a negative voltage at the Gate terminal (with respect to the Source terminal) that is over a certain threshold level (eg. VTH=3V) then the electrical field created in that region would repeal the electrons in the nearby of the N-type channel, thus creating a depletion region (remember, the Gate is separated of an N-type material channel). The larger the depletion region, the wider the channel and thus the smaller the current flow through the channel. It comes a point at which no matter how many volts we apply at the Gate terminal, the channel will be completely depleted so no current will flow through that channel from the Source to the Drain terminal.

Enhancement-mode MOSFET

In contrast to the above Depletion-mode which by default operates like an closed circuit, the Enhancement-mode MOSFET operates by default like an opened circuit (so no current flow between Source-Drain). To operate a MOSFET in Enhancement-mode all we have to do is to reverse the polarity at the Gate terminal. So by applying a positive voltage (with respect to the Source terminal) the electrical field created in that region would attract the electrons within the N-type channel, thus the channel becomes larger. The larger the channel the greater the current flow from the Source to the Drain. It comes a point at which no matter how many volts we apply at the Gate terminal, the channel will be completely enhanced so the rate of the current could not be further increased.

This is a demo of a MOSFET used as a switch in a circuit:

..and this is the same MOSFET used as an amplifier:

Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.

The following two tabs change content below.
The semiconductors: Diode, BJT, JFET, MOSFET

Eugen Mihailescu

Founder/programmer/one-man-show at Cubique Software
Always looking to learn more about *nix world, about the fundamental concepts of math, physics, electronics. I am also passionate about programming, database and systems administration. 16+ yrs experience in software development, designing enterprise systems, IT support and troubleshooting.

Leave a Reply

Your email address will not be published. Required fields are marked *


Get every new post on this blog delivered to your Inbox.

Join other followers: