Class 12 Physics · Chapter 14 NotesSemiconductor Electronics: Materials, Devices and Simple Circuits

Solids are broadly grouped by their electrical resistivity. Metals have very low resistivity (about 10⁻² to 10⁻⁸ Ω m), insulators have very high resistivity (about 10¹¹ to 10¹⁹ Ω m), and semiconductors lie in between (about 10⁻⁵ to 10⁶ Ω m). A deeper explanation comes from energy bands. When atoms come together in a solid, their closely spaced energy levels form bands. The valence band holds the valence electrons, while the conduction band lies above it. In metals the two bands overlap or the conduction band is partly filled, so electrons move easily. In insulators the gap Eg is large (more than about 3 eV) and almost no electron can cross it. In semiconductors Eg is small (roughly 0.2 eV to 3 eV), so thermal energy at room temperature can lift some electrons into the conduction band.

• Metalconduction and valence bands overlap or conduction band partly filled.
• InsulatorEg > 3 eV, conduction band empty.
• Semiconductorsmall Eg, some electrons reach the conduction band at room temperature.
• Energy gap valuesC (diamond) 5.4 eV, Si 1.1 eV, Ge 0.7 eV, Sn 0 eV (metal).

A pure semiconductor such as silicon or germanium is called an intrinsic semiconductor. Each atom has four valence electrons and forms covalent bonds with four neighbours in a diamond-like lattice. At absolute zero all bonds are intact and the material behaves like an insulator. At higher temperatures, thermal energy breaks some bonds, freeing an electron and leaving behind a vacancy called a hole. The hole behaves like a particle with effective positive charge. In a pure semiconductor the number of free electrons equals the number of holes, that is ne = nh = ni, where ni is the intrinsic carrier concentration. Both electrons and holes move under an applied electric field, so the total current is I = Ie + Ih. Generation of carriers is balanced by recombination, keeping the concentrations steady at equilibrium.

• Intrinsic semiconductorne = nh = ni.
• Holevacancy in a covalent bond with effective positive charge.
• Total current I = Ie + Ih.
• At T = 0 K an intrinsic semiconductor behaves as an insulator.

The conductivity of a pure semiconductor at room temperature is too low for practical devices, so a controlled amount of impurity is added. This process is called doping and the impurity atoms are dopants. Pentavalent atoms such as As, Sb and P act as donors: four of their electrons bond with neighbouring Si or Ge atoms and the fifth is loosely bound, needing only about 0.01 eV in Ge and 0.05 eV in Si to become free. Such a material is an n-type semiconductor, where electrons are majority carriers and holes are minority carriers (ne >> nh). Trivalent atoms such as B, Al and In act as acceptors: they leave a hole in the bond, giving a p-type semiconductor where holes are majority carriers (nh >> ne). In both cases the crystal remains electrically neutral, and the product of carrier concentrations satisfies ne nh = ni².

• Dopingdeliberate addition of a small amount of suitable impurity.
• n-typepentavalent donor impurity, electrons are majority carriers.
• p-typetrivalent acceptor impurity, holes are majority carriers.
• Carrier relationne nh = ni².

A p-n junction is formed when one part of a semiconductor wafer is doped p-type and an adjacent part n-type. Because of the concentration difference, holes diffuse from the p-side to the n-side and electrons diffuse from the n-side to the p-side. This leaves immobile ionised donors on the n-side and ionised acceptors on the p-side, creating a space-charge region called the depletion region, about one-tenth of a micrometre thick. The resulting electric field causes a drift current opposite to the diffusion current. Equilibrium is reached when the two currents become equal and there is no net current. The potential difference across the junction that opposes further diffusion is called the barrier potential.

• Diffusionmovement due to concentration gradient.
• Driftmovement due to the junction electric field.
• Depletion regionspace-charge region without free carriers.
• At equilibrium, diffusion current equals drift current.

A p-n junction with metallic contacts at both ends is a semiconductor diode, a two-terminal device. Under forward bias the p-side is connected to the positive terminal and the n-side to the negative terminal. The applied voltage opposes the barrier potential, the depletion layer narrows, and the effective barrier becomes (V0 − V). Current rises sharply once the voltage crosses the threshold or cut-in value, about 0.2 V for germanium and 0.7 V for silicon. Under reverse bias the barrier becomes (V0 + V), the depletion region widens, and only a small reverse saturation current flows, almost independent of voltage. If the reverse voltage exceeds the breakdown voltage, the current increases sharply and the diode may be damaged. Dynamic resistance is rd = ΔV/ΔI.

• Forward biasbarrier reduced to V0 − V, large current (mA).
• Reverse biasbarrier increased to V0 + V, small current (μA).
• Threshold voltageabout 0.2 V for Ge and 0.7 V for Si.
• Dynamic resistance rd = ΔV/ΔI.

Since a diode conducts mainly in forward bias, it can convert alternating voltage into unidirectional voltage. In a half-wave rectifier a single diode in series with the load conducts only during the half cycle in which it is forward biased, so the output appears for one half of the input cycle. A full-wave rectifier uses two diodes with a centre-tap transformer; each diode conducts during alternate half cycles, so output is obtained during both halves. The rectified output is pulsating, so a capacitor connected across the load is used as a filter. The capacitor charges to the peak value and discharges slowly through the load, giving a steadier dc voltage. The time constant depends on the product of capacitance and load resistance.

• Half-wave rectifierone diode, output for one half cycle.
• Full-wave rectifiertwo diodes with centre-tap transformer, output for both half cycles.
• Capacitor filter smooths the pulsating output.
• Larger capacitance gives a larger time constant and steadier dc.

A bipolar junction transistor is a three-terminal device with two p-n junctions, formed as either n-p-n or p-n-p. The three regions are the emitter, base and collector. The base is thin and lightly doped, which allows the emitter current to be transferred largely to the collector, so the collector current is close to the emitter current. A small change in base current produces a much larger change in collector current, which is the basis of amplification. Logic gates such as AND, OR and NOT are digital circuits that give a definite output for given inputs and are the building blocks of digital electronics. Many transistors, diodes and other components fabricated together on a single small chip form an integrated circuit, which is compact, reliable and widely used in modern electronic systems.

• Transistor terminalsemitter, base, collector.
• Typesn-p-n and p-n-p.
• Base is thin and lightly doped, enabling current transfer.
• Integrated circuitmany components fabricated on one chip.