Semiconductor Detectors Explained: Energy Bands & Physics

This section covers semiconductor doping in detail. N-type semiconductors are created by adding Group 15 elements (like phosphorus) to germanium. These dopants have 5 valence electrons while germanium has 4, creating an extra free electron that conducts electricity. P-type semiconductors are created by adding Group 13 elements (like boron) to germanium. These dopants have only 3 valence electrons, creating a 'hole' that can accept electrons from neighboring atoms. In n-type materials, free electrons are majority charge carriers (negative charge). In p-type materials, holes are majority charge carriers (positive charge). Current flows in the direction of hole movement, while electrons move in the opposite direction to fill holes.

Semiconductor doping involves intentionally introducing impurity atoms from different periodic table columns into a crystalline lattice to control electrical conductivity; n-type doping (using column V elements like phosphorus or arsenic) adds extra electrons that become weakly bound (binding energy ~0.05 eV) and easily donated to the conduction band at room temperature, while p-type doping (using column III elements like boron or gallium) creates holes that can move through the lattice, with carrier concentrations following the relationship n ≈ ND+ for n-type and p ≈ NA- for p-type, and the temperature-dependent behavior showing freeze-out at low temperatures, extrinsic conduction at moderate temperatures, and intrinsic conduction at high temperatures where thermal excitation overwhelms doping effects.

Semiconductor materials are classified into N-type and P-type based on their charge carriers. N-type semiconductors have electrons as majority carriers and holes as minority carriers, while P-type semiconductors have holes as majority carriers and electrons as minority carriers. The type of semiconductor is determined by the impurity atoms added: pentavalent impurities (such as phosphorus, arsenic, or antimony) create N-type semiconductors by adding extra electrons, while trivalent impurities (such as boron, aluminum, gallium, or indium) create P-type semiconductors by creating holes. This fundamental principle of doping is essential for understanding semiconductor behavior and electronic device operation.

Semiconductors are fundamental electronic materials. Doping transforms intrinsic semiconductors into extrinsic ones by adding impurities. N-type semiconductors are created by doping with Group 5 elements like phosphorus (5 valence electrons), which donate extra electrons making them majority carriers. P-type semiconductors are created by doping with Group 3 elements like boron (3 valence electrons), which create holes making them majority carriers. At thermal equilibrium, n × p = ni². In doped materials, majority carrier concentration equals dopant concentration, while minority carrier concentration is much lower.

N-type semiconductors are created by doping pure silicon with pentavalent impurities (Phosphorus, Arsenic, Antimony) that have 5 valence electrons. When a pentavalent atom replaces a silicon atom, it forms 4 covalent bonds but has one extra electron that becomes free. This extra electron is the majority charge carrier. The dopant atom becomes a positive ion (donor atom). P-type semiconductors are created by doping pure silicon with trivalent impurities (Boron, Aluminum, Gallium) that have 3 valence electrons. When a trivalent atom replaces a silicon atom, it can only form 3 covalent bonds, leaving one bond incomplete (a hole). A neighboring silicon atom provides an electron to fill this hole, creating a positive ion (acceptor atom) and a free hole. In N-type semiconductors, free electrons are the majority carriers and holes are the minority carriers. In P-type semiconductors, holes are the majority carriers and free electrons are the minority carriers. Both types remain electrically neutral overall.

A P-N junction forms when P-type and N-type semiconductors are joined. Electrons diffuse from N to P while holes diffuse from P to N, creating a depletion layer depleted of mobile charge carriers. Fixed ions remain: positive in N-type and negative in P-type regions, creating an electric field opposing further diffusion. The barrier potential is 0.7V for silicon and 0.3V for germanium. Forward bias (positive to P, negative to N) reduces barrier potential and depletion width, allowing current flow. Reverse bias (positive to N, negative to P) increases barrier potential and depletion width, blocking majority carrier flow. Donors donate electrons, acceptors accept electrons, creating the fixed charges.

A P-N junction is formed by joining p-type and n-type semiconductors in a single crystal. The junction is the boundary between the p-type and n-type regions. When a P-N junction is formed, majority carriers diffuse across the junction: electrons from n-side diffuse to p-side, and holes from p-side diffuse to n-side. This diffusion creates a region depleted of free charge carriers called the depletion region. The depletion region contains only immobile ions: positive ions on the n-side and negative ions on the p-side. This region has no free charge carriers to conduct current. The immobile ions create an electric field called the built-in electric field, pointing from the n-side (positive ions) to the p-side (negative ions). The potential difference across the P-N junction is called the barrier potential (Vb), approximately 0.7 V for silicon and 0.3 V for germanium at equilibrium. This barrier potential prevents majority carriers from diffusing across the junction and is responsible for the rectifying behavior of P-N junctions.

The depletion region forms when electrons diffuse from N-type to P-type and holes diffuse from P-type to N-type. As electrons leave the N-type region, they leave behind positively charged donor ions. As holes leave the P-type region, they leave behind negatively charged acceptor ions. This creates a region with no free charge carriers, only fixed ions. Barrier potential (built-in potential) is the voltage that develops across the depletion region due to the electric field. It acts as a barrier that prevents charge carriers from crossing the junction under zero bias conditions. The P-type side has higher potential (positive ions) and the N-type side has lower potential (negative ions). A PN junction is formed by doping a single crystal of semiconductor material on one side with pentavalent impurities (P-type) and on the other side with trivalent impurities (N-type). Biasing refers to the external voltage applied to a PN junction. Forward bias connects the positive terminal to the P-type side and negative terminal to the N-type side. Reverse bias reverses these connections. The depletion region width changes with applied bias - it widens under reverse bias and narrows under forward bias.

This section explains PN junction formation and operation. When p-type and n-type materials join, electrons diffuse to fill p-side holes and vice versa, creating an induced electric field with positive charges on the n-side and negative on the p-side. The depletion region forms where carriers are depleted, with a built-in potential of ~0.5 eV across ~10^-6 m width. Forward bias connects positive to p-side and negative to n-side, reducing the electric field, narrowing the depletion region, and allowing majority carrier diffusion current to dominate. Reverse bias connects positive to n-side and negative to p-side, increasing the electric field, widening the depletion region, and causing drift current to dominate, ideally resulting in zero net current.

PN junction is the fundamental building block of semiconductor devices, applicable to both diodes and transistors. A PN junction forms when P-type semiconductor (doped with trivalent impurities creating holes as majority carriers) is joined to N-type semiconductor (doped with pentavalent impurities creating electrons as majority carriers). The boundary creates a PN junction diode with anode (P-side) and cathode (N-side). The depletion region forms due to diffusion of majority carriers - holes from P-region diffuse into N-region and recombine with electrons, while electrons from N-region diffuse into P-region and recombine with holes. This creates a region depleted of mobile charge carriers containing only fixed ions: negative acceptor ions in P-region and positive donor ions in N-region. These uncovered charges create an electric field that opposes further diffusion, establishing a barrier potential. The barrier potential prevents majority carriers from crossing the junction, acting as a one-way valve. The depletion region width depends on doping levels - higher doping results in thinner depletion region. Forward biasing connects battery positive to P-type and negative to N-type, reducing the barrier potential and narrowing the depletion region. This allows majority carriers to diffuse across the junction, creating large current flow. For silicon diodes, threshold voltage is 0.7V; for germanium, it's 0.3V. Below threshold, minimal current flows; above threshold, current increases exponentially.

This lecture introduces the interaction of radiation with matter, focusing on alpha, beta, and gamma radiation. Using an analogy of three people with different body sizes (thick, medium, thin), the instructor explains that alpha particles are the heaviest and most ionizing but least penetrating, beta particles are lighter with moderate properties, and gamma rays are the lightest with the highest penetration power. Alpha particles are helium nuclei with +2 charge, beta particles are high-speed electrons with -1 charge, and gamma rays are photons with no charge or mass. The instructor emphasizes that understanding these fundamental properties is essential for studying radiation behavior.

Alpha particles are helium nuclei (2 protons, 2 neutrons) with +2 charge, highly ionizing but with low penetrating power, stopped by paper or skin. Beta particles are high-energy electrons with -1 charge, moderately penetrating, and can eject electrons from atoms through electrostatic repulsion. Gamma rays are electromagnetic radiation with no mass or charge, having high penetrating power. The interaction of electromagnetic radiation with matter depends on energy: low-energy causes photoelectric effect, medium-energy causes Compton scattering, and high-energy causes pair production. These interactions determine how radiation deposits energy in matter.

Ionizing radiation interacts with matter through two main mechanisms: charged particles (electrons, alpha particles, protons) cause direct ionization via collisions with electrons and nuclei, losing energy progressively and creating the Bragg peak for heavy particles, while photons (X-rays and gamma rays) interact through three processes—photoelectric effect (complete absorption by inner-shell electrons), Compton scattering (partial energy transfer to outer-shell electrons), and pair production (photon conversion to electron-positron pair when energy exceeds 1.022 MeV)—with attenuation following an exponential law I = I₀e^(-μx).

Ionizing radiation is energy transfer from a source that can remove electrons from atoms, creating ions; it includes three main types—alpha particles (helium nuclei, +2 charge, high ionizing power, low penetration), beta particles (electrons/positrons, -1/+1 charge, moderate ionizing power, moderate penetration), and gamma rays (electromagnetic waves, no charge, low ionizing power, high penetration)—with an inverse relationship between ionizing power and penetrating power, and three interaction outcomes with matter: partial energy absorption, complete energy absorption, or no interaction (pass-through).

Ionizing radiation includes alpha, beta, and gamma rays, all capable of giving electrons enough energy to escape atoms or molecules. An alpha particle is a helium nucleus (2 protons, 2 neutrons) - highly ionizing but weakly penetrating, stopped by paper or a few centimeters of air. A beta particle is a fast-moving electron with medium ionizing and penetrating ability, stopped by a few millimeters of aluminum. Gamma radiation consists of high-energy photons emitted from energetic nuclei - weakly ionizing but highly penetrative, requiring concrete or lead to significantly reduce intensity.