NPN Transistor
TRANSISTOR
Transistors are electronic switches that can use a small input signal (either voltage or current) to make a connection, allowing a much larger signal to do some work. Small devices like microcontrollers give a hard time providing enough power to make even small motors turn. But that same signal from the microcontroller can turn on a MOSFET (metal oxide semiconductor field-effect transistor) as well as drive high current devices, like the motors used in electric go-karts. BJTs (bipolar junction transistors) can amplify small input signals by a large factor.
NPN TRANSISTOR FUNCTION
NPN is one of the two types of bipolar transistors, consisting of a layer of p-doped semiconductor (the base, middle layer) between two n-doped layers. The NPN transistor amplifies the entry of the weak signal into the base and generates strong amplified signals at the end of the collector. In NPN transistors, the movement path of an electron is from the emitter to the collector region through which the current in the transistor is constituted. This type of transistor is often used in circuits, as their majority charge carriers are high mobility electrons compared to holes.
THE SYMBOL OF NPN BJT
PNP TRANSISTOR
The other type of BJT is the PNP, consisting of a layer of n-doped semiconductor between two layers of p-doped material. A small current leaving the base is amplified in the collector output. That is, a PNP transistor is “on” when its base is pulled low relative to the emitter.
The arrows in the NPN and PNP transistor symbols are on the emitter legs and points in the direction of the conventional current flow when the device is in forward active mode.
THE SYMBOL OF PNP BJT
CONNECTING BJT
The bipolar junction transistor, unlike led, capacitor or resistor, has three legs called an emitter (E), base (B) and collector (C). This means that interchanging the collector and the emitter of the transistor may leave the circuit inoperative.
CONNECTING NPN BJT
CONNECTING PNP BJT
What is a BJT?
The bipolar junction transistor is a solid-state device. In the BJTs the current flows in two terminals, emitter and collector and the amount of current is controlled by the third terminal i.e. base terminal. It is different from the other type of transistor i.e. field-effect transistors in which the output current is controlled by the input voltage. The basic symbols of the BJTs n-type and p-type are shown below.
Working Principle of Bipolar Junction Transistor
The BE junction is forward bias and the CB is a reverse bias junction. The width of the depletion region of the CB junction is higher than the BE junction. The forward bias at the BE junction decreases the barrier potential and produces electrons to flow from the emitter to the base. The base is thin and lightly doped; it has very few holes and less amount of electrons from the emitter; about 2%. This initiates the base current flow due to the combination of electrons and holes. The leftover large number of electrons will pass the reverse bias collector junction to initiate the collector current. By using KCL we can observe the mathematical equation.
IE = IB + IC
The base current is very less as compared to emitter and collector current.
IE ~ IC
NPN
IE = IB + IC
VCE = -VBC + VBE
DC β and DC α
β = Common-emitter current gain
α = Common-base current gain
β = IC α = IC
IB
Active:
- Most important mode of operation.
- Central to amplifier operation.
- The region where current curves are practically flat.
- Barrier potential of the junctions cancel each other out causing a virtual short.
- Cutoff Current reduced to zero.
- Ideal transistor behaves like an open switch.
Here the operation of a PNP transistor is the same as the NPN transistor; the only difference is only holes instead of electrons. The below diagram shows the PNP transistor in the active mode region.
Three Types of BJT Biasing
Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.
Construction of Transistor
Transistor is constructed by placing an oppositely doped semiconductor material between two similarly doped semiconductors. Or placing n-type material between two p-type materials which forms the pnp-transistor or by placing a p-type material between two n-type semiconductors which forms an npn-transistor.
Following are the different types of applications in BJT.
- Switching
- Amplification
- They are the building blocks of most of the electronic circuits, especially where audio, current or voltage amplification is required.
- NPN transistors are preferred over PNP transistors for amplification purpose because conduction carried out through mobility of electrons is better than conduction through mobility of holes.
- The core use of transistors includes switching applications or both as amplification and switching.
- There are a kind of transistors which produce current flow depending on the amount of light shined upon them, those are known as phototransistors.
- Bipolar Junction Transistors(BJT) can cause a greater current flow from the emitter to collector when a small amount of current is passed through the base.
- Field Effect Transistors act as voltage-controlled devices. Field Effect Transistors (FETs) have very high input impedance and it allows very little current through them. This is helpful for not causing the power source to load down as they are not disturbing the original circuit power elements on which they are connected to. FETs are cheaper and easier to manufacture and cause less loading.
- Heterojunction Bipolar Transistors (HBT) can provide faster switching speeds and are used in analog and digital microwave applications. They are priceless to fabricate and can provide better lithographic yield. They are used in mobile and laser drivers as power amplifiers.
- Darlington transistors have much higher ability to gain current. Because of its sensitivity, it can pick currents from human skin and that is why it is used in creating a touch-sensitive button.
- Schottky transistors diverts high input currents and prevents the transistors from saturating.
- Multiple emitter transistors are used in Transistor-Transistor Logic (TTL) and NAND logic gates.
- Dual gate MOSFETs are used in RF mixers/multipliers, RF amplifiers where two controlled gates are required in a series.
- Avalanche transistors have the capability of switching high currents in less than nanosecond transition time.