^SCs Versus VT devices 

Semiconducting devices are the basic building blocks of all the modern electronic circuits & have following advantages over conventional vacuum tubes:

1. don’ require a heating battery & thus set in to operation as soon as the circuit is switched on.
2. require comparatively low voltage for their operation.
3. don’t produce any humming noise during their operation.
4. shock proof, small & compact in size, cheaper than vacuum tubes, have a very long life & are free from vacuum deterioration trouble.

Limitations

1. higher noise level.
2. can’t handle as much as power as ordinary tubes can.
3. poor response in high frequency range.
4. temperature sensitive (maxi. tolerance 50 ⁰C).

^CEA

Output is 180⁰ out of phase with input.

Transconductance is also called mutual conductance.

As β > > α, thus the ac power gain of a CEA is much larger than that of a CBA. Remember the transistor is not generating any power. The energy for the higher ac power at the output is supplied by the battery.

Saturation state:

Both the junction are forward biased & here I_C is maximum & does not depend on the input current I_B.

Cut off state:  Both the junctions are reverse biased as a result I_C = 0. Between cut off & saturation state a transistor works as switch as here it turns over rapidly from OFF state (i.e. I_C = 0 or cut off) to the ON state (i.e. I_C is maximum or saturation state).

Active state: Emitter base junction is forward biased and the collector base junction is reverse biased. A transistor works as an audio amplifier in this regions.

Relation between α and β:

As the value of I_B is about 1 – 5 % of I_E or I_C is 95 – 99 % of I_E, α is about 0.95 and 0.99 and β is about 20 to 100.  It is found that α and β are independent of current if the emitter base junction is forward biased and the collector base junction is reverse biased. Also the above definitions of α and β do not hold when both the junctions of a transistor are forward biased or reverse biased.

The CE configuration is frequently used as it gives high current gain as well as voltage gain.

^Transistor

1. Emitter is the section on one side of transistor that supplies charge carriers. It is of moderate size & heavily doped. Collector is the section on the other side of transistor that collects the charge carriers. It is moderately doped but larger in size. Base is the middle section of transistor that forms two pn junctions with emitter and collector. It is very thin and lightly doped so as to pass most of the emitter injected charge carriers to the collector.
2. A transistor is also called Bipolar Junction Transistor (BJT), as in it current is due to both majority & minority charge carriers.
3. Input of a transistor is always forward biased, output is reverse biased & the common terminal is grounded.
4. Current in both npn & pnp transistor is I_E = I_B + I_C. As the base current I_B is very small thus I_E » I_C._. A transistor transfer almost the same electric current from low resistance path to high resistance path and hence is named as transistor. This transistor action makes it useful in transistor amplifier.
5. A transistor can be used as a switch, amplifier, oscillator & NOT gate.

^Photodiodes

Light incident near the junction of reverse biased (because the fractional change due to the photo-effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in the forward bias current) diode produce electron hole pairs which are moved by the junction electric field (electrons towards n end & holes near the p end).

Thus p-side becomes positive and n-side becomes negative giving rise to emf. When an external load is connected, a current flows which can be controlled by changing the intensity of light falling on it. It is used in, light operated switches, optical counters, reproduction of sound from film in move projectors & in photo detectors to detect radiations.