Q1. Write the truth table output of a NAND gate for inputs A = 1, B = 1.
A NAND gate gives Y = NOT (A AND B).
A AND B = 1 AND 1 = 1
Y = NOT 1 = 0
Semiconductor Electronics — Class 12
The chapter behind every phone and computer. Learn semiconductors, diodes and logic gates, and compute a transistor's current gain.
Intrinsic & extrinsicPure semiconductors are intrinsic; doping makes them extrinsic — n-type (extra electrons) or p-type (extra holes).
p-n junction diodeAllows current in one direction only (forward bias) and blocks it in reverse — used to convert AC to DC (rectification).
RectifierA half-wave rectifier uses one diode; a full-wave rectifier uses two (or a bridge of four) to use both halves of the AC cycle.
Logic gatesCircuits that perform Boolean logic: OR, AND, NOT, NAND, NOR. NAND and NOR are "universal" gates.
Transistor current-gain calculator Interactive
Enter the collector and base currents to find the common-emitter current gain β.
Current gain, β = I_C/I_B—
A small base current controls a much larger collector current — that is how a transistor amplifies.
Numericals practice Interactive
Transistor current-gain problems with steps.
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Solved numericals
Q2. A transistor has I_B = 20 µA and I_C = 4 mA. Find β and I_E.
β = I_C/I_B = 4×10⁻³ / 20×10⁻⁶ = 200
I_E = I_B + I_C = 0.02 + 4 = 4.02 mA
Formula sheet
| Quantity | Formula | SI unit |
|---|---|---|
| Current gain (common-emitter) | β = I_C / I_B | — |
| Emitter current | I_E = I_B + I_C | ampere (A) |
| OR gate | Y = A + B | logic |
| AND gate | Y = A · B | logic |
| NOT gate | Y = Ā | logic |
Common mistakes & exam wins
- A diode conducts only in forward bias — the basis of rectification (AC → DC).
- NAND and NOR are universal gates: any logic circuit can be built from them alone.
- In a transistor, a tiny base current controls a large collector current (β = I_C/I_B).
- n-type has electrons as majority carriers; p-type has holes.
Frequently asked questions
What is the difference between n-type and p-type semiconductors?
n-type is doped with pentavalent atoms giving extra electrons (majority carriers); p-type is doped with trivalent atoms giving extra holes (majority carriers).
What is a rectifier?
A circuit using diodes to convert alternating current (AC) into direct current (DC); it can be half-wave or full-wave.
Why are NAND and NOR called universal gates?
Because any other logic gate (AND, OR, NOT, etc.) can be built using only NAND gates or only NOR gates.
What is the current gain of a transistor?
In the common-emitter configuration, β = I_C/I_B — the factor by which a small base current is amplified into the collector current.