All about Transistors (what you should know): Switching States

Transistors are mainly used for switching and amplification applications. 

Transistor (Bipolar Junction Transistor - BJT): This three-layer semiconductor device is similar to 2 diodes back to back. It has 3 contacts: Base (B), Emitter (E), and Collector (C).

BJT

A through-hole TO-92 PNP Transistor 3906.

A through-hole TO-92 NPN Transistor 2222A.

• In a transistor, we push/pull current into the Base and then that turns on the collector circuit. Saying this, Base is the control of this current-controlled electronic switch. So, as we change the Base current, the Collector and Emitter current changes dramatically (gets amplified).

PNP Transistor in forward bias.

NPN Transistor in forward bias.

• We forward bias the base-emitter junction and reverse bias the base-collector junction. This means, we put a positive voltage on the base and we drive current into the base which makes this transistor gets turned-on (current flows from R1 to R2). 

• NPN Transistor showing two different depletion regions (shaded) when the Base-Emitter diode region is forward biased. When the +ve and -ve charges flow across the depletion region, there is a potential energy gap. As the B-E region is forward biased, e- and h+ move across the Base-Emitter depletion layer and the depletion width narrows down. As the e- charges move across D1 (B-E region), they punch in through D2 depletion region (B-C region) and the collector starts getting crowded with the negative charge. Since, we have a conventional current Ic flowing from right to the left through D2, the current adds-up to the Base current. Now, how do we know how much is the current flow?
• The NPN and PNP junction transistors can be used in mainly three different modes of operation: Saturation, Cut-off, Active. The NPN transistors follow this:
• Saturation mode (acts like an on/closed switch, and Vb > Ve, Vb > Vc). 
• Cut-off (part of saturation - when completely saturated, acts like an off/open switch with some leakage current). In this, Vb < Ve and Vb < Vc, Ib = 0.
• Active mode: The transistor acts like a current amplifier. So, the equation {Ic = (BETA) . Ib} is valid in active mode. Mandatory in the active region: Vc > Vb > Ve. The Collector is greater than the Emitter. Mark it!
• To push the circuit into the Saturated region, we increase the Vb.
• Also, Ie = Ib + Ic

Transistor as a Switch. When IB (base current) is injected, C closes to E and completes the circuit. The Collector current is a function of the Base current.

• Amplification application of transistors: We put a signal, say S0, to the amplifier (a circuit made of transistors in such a way that it amplifies/enlarges the S0 value by a certain factor). For this, the transistor must be set in the active mode (setting it up in this mode is not trivial) and once we put the signal in, we get a larger signal out. Amplifiers are easier to construct/build by using the MOSFETs as they have certain behavior and you won't have to set up the DC Load Line (to attain the Q-point) to active region (which is easier than in BJT and would take a lesser amount of time). To know more about amplification characteristics you may research and study more on the internet/literature and refer to the transistor I-V characteristics.

A Transistor Circuit with a momentary push-button switch added to control the transistor (Control the Emitter specifically) circuit implementation on the Breadboard

The other types of Transistors are FETs. For example, N-Channel MOSFET enhancement mode (Current Rating 2-3A) Circuit which turns Copper-Nickel294 wire coil to Red Hot (Current flow = 1A) on applying a Gate Voltage:

Learn about MOSFETs: Click Here!