A full lesson: a cartoon movie to start, then what a vacuum valve really does, why solid-state devices took over, resistivity and conductivity on one giant ruler, the heat test that separates metals from semiconductors, the semiconductor family, NEET shortcuts, problems to solve yourself, and 12 MCQs.
A valve needs a red-hot cathode, a vacuum and about 100 V, and it lets electrons go only one way. A silicon chip does the same control inside a solid, with no heater and no vacuum. Silicon sits between copper and glass on the resistivity ruler: a semiconductor.
Dadi's old wooden radio has a dial that glows orange. When she switches it on, nothing happens for half a minute. Then a hum, then the news. Look through the back and you see glass bottles with red-hot wires inside: vacuum valves (vacuum tubes). Arjun's phone plays the same news the moment he taps it, and it never gets that hot. Inside the phone are billions of tiny switches made of silicon, a semiconductor.
This lesson answers two questions. What did the valve do, and why did solid-state devices replace it? And what exactly is a semiconductor: how do we tell it apart from a metal like copper and an insulator like glass?
Try at home. Switch on an old filament bulb and an LED bulb of the same brightness. After a minute, hold your palm near each (do not touch). The filament bulb is hot because it heats a wire to glow, just like a valve heats its cathode. The LED works inside a solid and stays cool. That waste heat is one big reason valves lost.
"A valve is a one-way door for electrons, but it needs a heater, a vacuum and a big voltage. A semiconductor does the controlling inside a solid crystal. That is the whole story of this chapter's first page."
"Why do you think Dadi's radio needs to warm up, but a transistor radio starts at once?"
In a valve, electrons must first be boiled out of a cathode by heating it red hot (thermionic emission). Heating takes time. In a semiconductor the charge carriers are already present inside the solid, so nothing has to warm up.
A vacuum diode has two electrodes in an evacuated glass bulb: the cathode (heated by a filament) and the anode or plate. Electrons boiled off the hot cathode cross the vacuum to the plate only when the plate is positive. They can never go from plate to cathode, so the device conducts one way, like a valve in a water pipe. A triode adds a third electrode, the grid, which controls the flow. Heat the cathode, change the plate voltage, and try the grid.
Electrons would collide with air molecules, lose energy and the hot filament would burn out. So the bulb is evacuated.
The plate is cold, so it emits no electrons. Electrons can travel only cathode → plate, and only when the plate is positive.
Valves are bulky, waste power heating the cathode, need about 100 V or more, have a limited life and are less reliable.
"Remember the valve by its three demands: heat, vacuum, high voltage. Every one of them disappears in a semiconductor device."
From the 1930s, scientists found that in some solids the number of charge carriers and the direction of their flow can be controlled. Heat, light or a small applied voltage changes how many mobile carriers a semiconductor has. The carriers are produced and moved inside the solid itself. Warm the silicon slab, shine light on it, and reverse the small battery.
| Feature | Vacuum valve | Semiconductor device |
|---|---|---|
| Carriers come from | a heated cathode | inside the solid |
| Needs a vacuum? | yes | no |
| Working voltage | about 100 V or more | a few volts |
| Size, power, life | bulky, high power, wears out | tiny, low power, long life, reliable |
"NCERT's key line: in a semiconductor device the supply and flow of charge carriers are within the solid itself. Write it word for word."
The first way to sort solids is by how well they conduct. Resistivity ρ (in Ω m) is the resistance of a 1 m cube; conductivity σ = 1/ρ (in S m⁻¹ or Ω⁻¹ m⁻¹). The values span about 27 powers of ten, so we use a log scale. Drag the marker along the ruler (or use the slider) and pick real materials.
Low resistivity means high conductivity. Copper: ρ = 1.7 × 10⁻⁸ Ω m, so σ = 5.9 × 10⁷ S m⁻¹.
The same ρ gives different R for different shapes. A bar 1 cm long with 1 mm² cross-section has R = ρ × 0.01 / 10⁻⁶ = ρ × 10⁴ Ω.
Conductivity ranges are just the inverses: metals 10² to 10⁸, semiconductors 10⁵ to 10⁻⁶, insulators 10⁻¹¹ to 10⁻¹⁹ S m⁻¹.
These are only orders of magnitude, and the metal and semiconductor ranges overlap. Resistivity alone cannot always decide. The band picture (Lesson 2) and the heat test (Stage 5) can.
"Learn the ranges as powers of ten: metals minus two to minus eight, semiconductors minus five to six, insulators eleven to nineteen. For conductivity just flip the signs."
Put a sample on a hot plate and watch its resistivity. A metal conducts slightly worse when hot: ρ rises slowly. A semiconductor conducts much better when hot: ρ falls very fast, because heat creates many more free carriers. Choose a material and raise the temperature.
Carrier number is fixed and huge; hotter ions vibrate more and scatter electrons more. Copper: α ≈ 0.0039 K⁻¹.
Heat breaks bonds and releases carriers. Their number grows roughly exponentially, so ρ falls fast. Silicon's ρ drops about 25 times from 27 °C to 77 °C.
Its band gap is so large that ordinary heating frees essentially no electrons. It stays an insulator.
"When resistivity numbers overlap, ask: what happens on heating? Metal, resistance up. Semiconductor, resistance down. That is a favourite NEET statement question."
"What would a pure semiconductor behave like at absolute zero, 0 K?"
Like an insulator. At 0 K no bonds are broken, so there are no free carriers at all.
Elemental semiconductors are silicon and germanium (group 14). Compound semiconductors pair elements from either side of group 14: group 13 with 15 (GaAs, InP), or group 12 with 16 (CdS, CdSe). There are also organic (anthracene, doped phthalocyanines) and polymer semiconductors (polypyrrole, polyaniline, polythiophene). Tap one or two tiles to build a semiconductor.
"For NEET, remember the two words: elemental means Si and Ge, compound means a pair like GaAs or CdS. Organic ones like anthracene and polymers like polyaniline are semiconductors too."
Diode: cathode + plate. Triode adds the grid. The Greek prefix is the count.
Only the hot cathode emits. Plate negative: zero current, however large the voltage.
For σ flip the signs of the exponents: σ = 1/ρ.
Either metal or semiconductor. Look for a second clue: temperature behaviour or band gap.
Heat a semiconductor and R falls; heat a metal and R rises. At 0 K a pure semiconductor is an insulator.
Elemental | inorganic compound | organic | polymer. Only Si and Ge are elemental in NCERT.
"Three things to carry into the exam hall: the three resistivity ranges, the overlap, and the heating rule. Most questions on this page are answered by one of them."
Cover the solution, write the formula, and solve on paper. Open the hint only if you are stuck for more than a minute.
Pure silicon at room temperature has ρ = 2.3 × 10³ Ω m and copper has ρ = 1.7 × 10⁻⁸ Ω m. Find the conductivity of each and how many times better copper conducts.
σ = 1/ρ, then divide.
A bar 1 cm long with a cross-section of 1 mm² is made of (a) pure silicon, (b) copper. Find its resistance in each case.
R = ρ L / A with L = 10⁻² m and A = 10⁻⁶ m².
Three samples have conductivities A: 10⁵ S m⁻¹, B: 10⁻¹⁴ S m⁻¹, C: 10 S m⁻¹. Classify each using the NCERT ranges.
Convert to ρ = 1/σ first, then compare with the three ranges.
Dadi's radio has 5 valves. Each heater runs at 6.3 V and 0.3 A. How much power goes only into heating the cathodes? How much energy is that in one hour?
P = V I for one heater, then multiply by 5. E = P t.
The 1945 computer ENIAC used about 17 500 valves and drew about 150 kW. A modern phone chip has about 1.5 × 10¹⁰ transistors and uses about 5 W. Find the power per switch in each case and their ratio.
Divide total power by the number of switches.
Use the NCERT ranges: metals 10⁻² to 10⁻⁸, semiconductors 10⁻⁵ to 10⁶, insulators 10¹¹ to 10¹⁹ Ω m. Write the range first, then click.
Score: 0 / 12
| If the question says… | Write this first | The trap |
|---|---|---|
| "in a vacuum diode electrons flow" | cathode → anode only | Saying "both ways if the voltage is reversed" |
| "number of electrodes in a triode / pentode" | 3 / 5 | Counting the heater filament as an extra electrode |
| "advantage of semiconductor devices" | compact, low power, low voltage, long life, reliable | Picking "needs a heated filament" or "needs vacuum" |
| "resistivity of an insulator / metal" | 10¹¹–10¹⁹ / 10⁻²–10⁻⁸ Ω m | Mixing up ρ and σ exponents |
| "conductivity is given" | ρ = 1/σ, then compare | Comparing σ with the ρ ranges directly |
| "ρ = 10⁻⁴ Ω m, classify" | overlap: metal or semiconductor | Answering firmly "metal" or "semiconductor" |
| "on heating, resistance of Si / Cu" | Si decreases · Cu increases | Thinking all resistances rise with temperature |
| "elemental semiconductor" | Si, Ge | Choosing GaAs, CdS (compound) or carbon (diamond is an insulator) |
Lesson 2 explains why the three classes exist: energy bands and the band gap E_g. Lesson 3 shows where the carriers in a semiconductor come from: broken bonds, electrons and holes.
Values used: ρ at 27 °C: copper 1.7 × 10⁻⁸, nichrome 1.1 × 10⁻⁶, germanium 0.46, pure silicon 2.3 × 10³, glass ~10¹², fused quartz ~7.5 × 10¹⁷ Ω m. Stage 3 and 5 curves use a simple intrinsic-carrier model (n ∝ T^1.5 e^(−E_g/2kT)) with E_g(Si) = 1.1 eV, E_g(Ge) = 0.7 eV.
Follow the four steps: understand it, remember it, try it, then prove it with past papers.