There are three main types of the Transistors.
Bipolar Junction Transistor (BJT)
Junction Field Effect Transistor (JFET)
Metal Oxide Semiconductor FET (MOSFET)
In Bipolar Junction Transistors there are further two types which are NPN and PNP Transistors.
In Junction Field Effect Transistor (JFET) there are further two types which are N-channel and P-channel transistors.
Similarly in Metal Oxide Semiconductor FET (MOSFET) there are also further types which are depletion type (n-and p-channel) and enhancement type (n-and p-channel)
Now we will see all the above stated types of transistors in some details. So that your concept must be clear about these basic elements of the electronics technology.
As stated earlier that there are further two types as NPN and PNP.
A small input current and a positive voltage applied @ its base (with VB>VE) allows a large current to flow from collector to emitter.
A small output current and a negative voltage @ its base (with VB<VE) allows a much larger current to flow from emitter to collector as you can see in below figure.
NPN BJT Working environment :
Now let know about the working principle of NPN Bipolar Junction transistor with the example of below given figure. When no voltage is applied to the transistor’s base, the electrons in the emitter are prevented from passing to the collector side due to the PN Junction. Similarly if the negative voltage is applied on the base of transistor, things get even more dangerous as the PN junction between the base and emitter becomes reversed biased. This results in the formation of a depletion region that opposes to current flow. All above experiment you can see in this Figure.
Now I tell you another example of working principle of NPN Bipolar Junction Transistor.
If a positive voltage (>0.6V) is applied to the base of an NPN transistor, the PN junction between the base and emitter becomes forward-biased. During forward bias, escaping electrons are drawn to the positive base.
• Some electrons exit through the base, but because the p-type base is so thin, the onslaught of electrons that leave the emitter get close enough to the collector side that they begin jumping into the collector. Increasing the base voltage increases the emitter-to collector electron flow.
• Recall, positive current flow is in the direction opposite to the electron flow →current flows from collector to emitter.
NPN Transistor in a Complete Circuit
In NPN Transistor no current passes from collector to emitter when the base is not activated. This whole process we can see in the below diagram in more details.
When VB > VE we have an operating circuit.
Current passes from collector to emitter when base is activated.
Experiment of Transistor ( LED On/Off)
In this experiment we can see an LED, an NPN Transistor and one switch. When we change the positions of the switch as on/off, it turns the LED on/off. As you can see this process in the below diagram.
As discussed earlier about the JFETs but here we can discuss in some details about this type of transistors. Junction field effect transistors (JFET) like BJTs are three lead semiconductor devices. Normally JFETs are used in those switches which are controlled electrically, in current amplifiers and in voltage controlled resistors. The interesting and important point to be considered about BJTs and JFETs is they do not require any bias current, they are controlled by using only a voltage.
JFETs are normally on when VG – VS = 0.
When VG – VS ≠ 0, then JFETs become resistive to current flow through the drain-source pair → “JFETs are depletion devices.”
After discussing about the main theme of JFET, now let us see the different types of JFETs. There are only two types of JFETs.
In N-Channel JFET, a –ve voltage applied @ its gate (with VG < VS) reduces current
Flow from drain to source. It operates with VD > VS.
In P-Channel JFET, a +ve voltage applied @ its gate (with VG > VS) reduces current flow from source to drain. It operates with VS > VD.
JFETs have very high input impedance and draw little or no input current. Ff there is any circuit/component connected to the gate of a JFET; no current is drawn away from or sunk into this circuit.
MOSFET Stands for Metal Oxide Semiconductor FET. It is similar to JFET. A metal oxide insulator is placed @ the gate to obtain high input impedance @ the gate. The gate input impedance is approximately 1014Ω. Use of insulator as described above yields a low gate-to-channel capacitance. If too much static electricity builds up on the gate, then the MOSFET may be damaged.
There are two types of MOSFET.
- Enhancement Type
- Depletion Type
Enhancement type is normally off, thus no current flows through drain-source channel when VG = VS.
When a voltage applied @ the gate causes VG ≠ VS the drain-source channel reduces resistance to current flow.
Depletion type is normally on, thus maximum current flows through drain-source channel when VG = VS.
When a voltage applied @ the gate causes VG ≠ VS the drain-source channel increases resistance to current flow.