Semiconductor Electronics

Chapter 15 Semiconductor Electronics: Materials, Devices And Simple Circuits Semiconductor basics Properties And Examples Band Theory of Solids Intrinsic Semiconductors Extrinsic SC ➤ Questions PN junction ➤ Energy band for n-type and p-type SC Drift speed for electron and holes PN junction ➤ Forward bias PN junction and Zener diode Questions based on PN junction ➤ Zener diode I-V characteristics ➤ Voltage regulation Application of PN junction Half wave rectifier ➤Full wave rectifier Special PN diodes Photodiode LED ➤ Solar cell Transistors ➤ Transistor's introduction Working of transistor alpha, beta parameters Input and output. ➤ Characteristics Logic gates Transistor as a switch Transistor as an amplifier Oscillatory circuit Digital electronics Boolean algebra Logic gates-And, Or, Not, Nor, Nand SEMICONDUCTOR BASICS Electronic instruments serve diverse purposes in fields ranging from telecommunication and entertainment to computers and nuclear physics. While their origins trace back to the era of vacuum tubes, today's advancements in electronics owe much to semiconductor devices. Semiconductor devices offer numerous advantages over their predecessors. They are compact, consume less power, have longer lifespans, and operate more efficiently. Moreover, they are cost-effective. These traits have spurred the widespread adoption of semiconductor devices, replacing vacuum tubes in almost every application. Take computers, for instance: early vacuum tube-based models were massive, requiring entire rooms and limited to basic calculations. In contrast, contemporary personal computers (PCs) are much smaller yet immensely powerful, thanks to semiconductor technology's strides. In subsequent sections, we will explore the foundational principles of semiconductors. This understanding will lay the groundwork for comprehending the functioning of various semiconductor devices. We will delve into specific devices like diodes and transistors, elucidating their workings and practical applications. Materials in the solid state are classified based on their electrical conductivity, typically characterized by their resistivity (p) or conductivity (ơ), where resistivity is the inverse of conductivity (p=1/6). (i) Metals: Metals demonstrate exceptionally low resistivity, indicating high conductivity. Their resistivity (p) typically falls within the range of 100-2)-10-) .m, corresponding to conductivity (ơ) values of approximately 10-10 S.m), (ii) Semiconductors: Semiconductors exhibit resistivity or conductivity values intermediate between metals and insulators. Their resistivity (p) generally ranges from 10-5-10 0.m, with conductivity (6) around 105-10 S.m (iii) Insulators: Insulators possess high resistivity, indicating low conductivity. Their resistivity (p) typically lies within the range of 1011-1019 2.m, corresponding to conductivity (a) values of approximately 100-110-10-19) S.m1, 1 Properties And Examples While the provided resistivity and conductivity values offer approximations to gauge magnitude, they may vary beyond the specified ranges. However, it's important to note that relative resistivity values aren't the sole criteria for distinguishing between metals, insulators, and semiconductors. Other factors will become apparent as we delve further into this topic. This chapter primarily focuses on semiconductor examination, which can be categorized as follows: (i) Elemental Semiconductors: Notably Silicon (Si) and Germanium (Ge). (ii) Compound Semiconductors, including: Inorganic Compounds: Such as Cadmium Sulfide (CdS), Gallium Arsenide (GaAs), Cadmium Selenide (CdSe), Indium Phosphide (InP), among others. Organic Compounds: Examples include Anthracene, doped phthalocyanines, etc. Organic Polymers: Like Polypyrene, polyaniline, polythiophene, etc. While conventional semiconductor devices predominantly utilize elemental and inorganic compound semiconductors like Si or Ge, advancements since 1990 have introduced semiconductor devices incorporating organic semiconductors and semiconducting polymers. This heralds the advent of futuristic technologies such as polymer electronics and molecular electronics. Band Theory of Solids Within the confines of an individual atom, each electron possesses a specific energy level dictated by its quantum state. Furthermore, electrons sharing identical quantum states exhibit uniform energy levels across all isolated atoms. When arranged in a crystalline structure, atoms are brought into close proximity, typically within a range of 2 to 3 angstroms. In this configuration, electrons undergo novel interactions due to the altered spatial arrangement and the collective influence of neighboring atoms. 1. Interacting with the electrons of adjacent atoms. 2. Interacting with the nuclei of neighboring atoms. Na11s 2s22p6 3s Crystal Energy Band Energy 3s 2p Energy Band 2s 1s Energy Band The N states corresponding to the angular momentum quantum number 1 = 0, initially exhibiting identical energy levels within individual isolated atoms, undergo dispersion and amalgamation to give rise to a continuous energy band. When considering a collection of N isolated sodium (Na) atoms, each possessing three 3s1 electrons, all N electrons within the 3s orbital share identical energy levels. Consequently, their energy distribution can be represented as a singular line, as depicted earlier. However, upon bringing these isolated atoms together within a crystal lattice, the interactions between neighboring atoms disrupt this uniformity. By infusing a minute quantity, typically a few parts per million (ppm), of a suitable impurity into the pristine semiconductor, its conductivity experiences a substantial enhancement. These materials are known as extrinsic semiconductors or impurity semiconductors. The deliberate introduction of a desired impurity is termed doping, with the impurity atoms referred to as dopants. This process yields a material termed a doped semiconductor. It's imperative that the dopant does not distort the original pristine semiconductor lattice, and occupies only a fraction of the original semiconductor atom sites in the crystal. This necessitates the dopant and semiconductor atoms to have closely matching sizes. There exist two categories of dopants utilized for doping tetravalent Si or Ge: (1) Pentavalent dopants (valency 5), such as Arsenic (As), Antimony (Sb), Phosphorous (P), among others. (ii) Trivalent dopants (valency 3), including Indium (In), Boron (B), Aluminium (Al), and similar elements. The selection of a dopant relies on the intended effect and the particular semiconductor material under consideration. The dopant element is chosen from neighboring groups in the Periodic table, ensuring that the dopant atom's size closely matches that of Si or Ge. Intriguingly, the utilization of pentavalent and trivalent dopants in Si or Ge yields two distinct semiconductor types, each possessing unique characteristics. Extrinsic semiconductor is of two types: n-type and p-type. (a) n-type semiconductor: When a pentavalent impurity atom, such as antimony, phosphorus, or arsenic, is inserted into a germanium (Ge) or silicon (Si) crystal, it occupies the position of a Ge (or Si) atom within the crystal lattice. Four out of the five valence electrons of the impurity atom establish covalent bonds, with each valence electron pairing up with one of the valence electrons from four neighboring Ge (or Si) atoms. Consequently, the inclusion of a pentavalent impurity to pure Ge (or Si) leads to a rise in the number of free electrons, thereby augmenting the conductivity of the crystal. DONOR ATOM (+4 FREE ELECTRON The impure Ge (or Si) crystal is designated as an 'n-type' semiconductor due to its surplus of 'negative' charge carriers, which are electrons. In this context, the impurity atoms are labeled 'donor' atoms since they supply or donate conducting electrons to the crystal, consequently shaping its electrical characteristics. The fifth valence electrons of the impurity atoms occupy distinct energy levels situated slightly below the conduction band. These energy levels are known as 'donor levels' and are positioned approximately 0.01 eV below the conduction band in germanium (Ge) and 0.05 eV below in silicon (Si). As a result, at room temperature, the "fifth" electrons associated with nearly all donor atoms experience thermal excitation from the donor levels to the conduction band. Upon reaching the conduction band, these electrons serve as charge carriers when subjected to an external electric field. In typical circumstances, the predominant source of electrons in the conduction band is the donor levels, with only a minor portion arising from the valence band. Consequently, the primary charge carriers accountable for conduction are the electrons provided by the donor atoms. Because of the restrained excitation from the valence band, the quantity of holes within this band remains minimal. 11 Electron energy E Conduction band 0.01eV (Ge) 0.05 eV (Si) E el energy levels] Valance band As a result, the contribution to current from the holes is restricted. In an n-type semiconductor, electrons are regarded as the 'majority carriers,' whereas holes are classified as the 'minority carriers. The provided illustration depicts an n-type semiconductor at temperatures exceeding 0 Kelvin. Within this semiconductor, there exists one electron-hole pair thermally generated in the conduction band, along with nine conduction electrons derived from donor atoms. Here are some significant details about P-Type Semiconductors: These semiconductors are produced by introducing a small quantity of trivalent impurity into a pristine semiconductor sample, such as germanium (Ge). 1) Majority Charge Carriers: Holes Minority Charge Carriers: Electrons Concentration Relationship: 2) I_{e} >> \mathfrak{h}_{h} n_{e} >> m_{h} 3) Conductivity σε > sigma = n_{c}*mu_{c}*e 4) Donor Energy Level: Positioned just below the conduction band. 5) Electron and Hole Concentration: In a doped semiconductor, the electron concentration (ne) and the hole concentration (ns) are not equivalent (unlike in an intrinsic semiconductor). This relationship can be expressed as n_{e}*n_{b} = n_{i} ^ 2 where n is the intrinsic concentration. In an n-type semiconductor, the concentration of electrons in the conduction band is nearly equivalent to the concentration of donor atoms (N_{d}) and is significantly greater than the concentration of holes in the valence band, i.e., n_{e} \approx N_{d} >> m_{b} Impurity Atom: 6) Referred to as the donor atom, it falls under the V group of the periodic table. Net Charge: 7) > The overall charge on an n-type crystal is neutral. 8) Immobile Charge: Carries a positive charge. (b) p-type semiconductor: When a trivalent impurity atom, such as boron, Aluminium, gallium, or indium, is added to a germanium (Ge) or silicon (Si) crystal, it replaces one of the Ge (or Si) atoms within the crystal lattice. The three valence electrons of the impurity atom form covalent bonds with one valence electron each from the adjacent Ge (or Si) atoms. As a result, a void is created, leading to the formation of a 'p-type' semiconductor. This designation stems from its excess of positive 'acceptor' atoms, which generate holes capable of accepting electrons. The impurity atoms occupy distinct vacant energy levels situated just above the uppermost portion of the valence band, known as 'acceptor levels'. 12 ACCEPTOR FREE HOLE E_{u} E v f ATOM +4