Problem solving tips

## Semiconductor Electronics: Materials, Devices And Simple Circuits

- Want to score better marks in exams? Have a look at some problem solving tips.
1
Tip 1: Problems based on calculating number of charge carriers:
Some important points to be kept in mind while solving such problems:
• At thermal equilibrium, in all cases: be it intrinsic or extrinsic. This is law of mass action.

• The material always possesses an overall charge neutrality.

Usually, the density of dopant is given as (parts per million) for given number of intrinsic semiconductor atoms per , i.e.
atoms of dopant are there for every one million () atoms of intrinsic semiconductor in one of volume.
We calculate the number of dopants ( or )as

When the semiconductor is doped, we assume the thermally generated electrons and holes to be negligibly small as compared to those produced by doping, i.e.,
• if doped with donor atoms (pentavalent impurities)

• if doped with acceptor atoms (trivalent impurities)

2
Tip 2: Problems based on Zener diode as a voltage regulator
For solving the parameters asked for voltage regulator circuits, we have to remember only these basic laws:
• Ohm's Law

• Kirchhoff's voltage law

• Kirchhoff's current law

NOTE: The value of should be such that the current through the Zener diode is much larger than the load current. The reverse current should be atleast greater than the minimum required to operate the zener diode in breakdown region.
Let us write down these equations for the circuit:
• By applying KCL at the junction of Zener diode branch and load branch:

• By KVL in input loop:

• By KVL in output loop:
3
Tip 3: Problems on CE transistor amplifier:
Step 1: Firstly, we draw the circuit diagram and connect battery at :
• input such that base-emitter is forward biased and

• output such that collector-base is reverse biased.
Step 2: We consider all the ac quantities for input side once and then for output side.
1. For input side, the ac voltage , current and resistance are related through ohm's law:

2. Similarly for output side, the ac voltage , current and resistance are related through ohm's law:

Step3: Then we relate the respective ac quantities on output side with the ac quantities on input side. We repeat the same for dc quantities.
AC: output input
DC: output input

1. Current gain

• The ac current gain relates ac output current and ac input current

• The dc current gain relates dc output current and dc input current

2. Voltage gain

• The ac voltage gain relates ac output voltage and ac input voltage

Current gain and voltage gain again be related similarly to Ohm's law.
Voltage gain=Current gain x Resistance gain

3. Power gain
We know P=VI. Similarly
Power Gain=Voltage Gain x Current Gain
Step 4: If parameters like and are asked, we apply KVL in input and output loop and solve the equations.
• KVL in input loop,
• KVL in output loop,