ECE 142: Electronic Circuits Lecture 3: Semiconductors

Transcription

1 Faculty of Engineering ECE 142: Electronic Circuits Lecture 3: Semiconductors

2 Agenda Intrinsic Semiconductors Extrinsic Semiconductors N-type P-type Carrier Transport Drift Diffusion

3 Semiconductors A semiconductor is a material with conducting properties between those of a good insulator (e.g. glass) and a good conductor (e.g. copper). The most commonly used semiconductor is silicon.

4 Semiconductor Elements in the Periodic Table Group III Group IV Group V Boron (B) Carbon (C) Nitrogen (N) Aluminium (Al) Silicon (Si) Phosphorus (P) Gallium (Ga) Germanium (Ge) Arsenic (As) Indium (In) Tin (Sn) Antimony (Sb)

5 Semiconductors Each silicon atom has an outer shell with four valence electrons and four vacancies (It is a tetravalent element). In intrinsic (pure) silicon, atoms join together by forming covalent bonds. Each atom shares its valence electrons with each of four adjacent neighbours effectively filling its outer shell.

6 Intrinsic Semiconductors

7 Intrinsic Semiconductors The structure has zero overall charge The complete nature of the structure means that at absolute zero temperature (0 K) none of the electrons is available for conduction thus far the material is an insulator.

8 Intrinsic Semiconductors At room temperature some of the electrons are able to acquire sufficient thermal energy to break free from their bond. Whenever an electron leaves its position in the lattice it leaves a vacancy known as a hole. The process is known as electron-hole pair generation

9 Intrinsic Semiconductors

10 Intrinsic Semiconductors A freed electron can move through the body of the material until it encounters another broken bond where it is drawn in to complete the bond or recombines.

11 Intrinsic Semiconductors At a given temperature there is a dynamic equilibrium between thermal electron-hole generation and the recombination of electrons and holes As a result the concentration of electrons and holes in an intrinsic semiconductor is constant at any given temperature. The higher the temperature the more electronhole pairs that are present.

12 Intrinsic Semiconductors n = conduction electron density (cm -3 ) p = hole density (cm -3 ) n i = intrinsic carrier concentration (cm -3 ) depends on temp and material n p = n i 2, n = p n = p = n i

13 Intrinsic Semiconductors Two mechanisms for conduction become possible when a bond breaks: 1. Due to the movement of the freed electron. 2. Due to neighbouring electrons moving into the hole leaving a space behind it. (This can be most simply thought of as movement of the hole, a single moving positive charge carrier even though it is actually a series of electrons that move.

14 Intrinsic Semiconductors

15 Intrinsic Semiconductors

16 Intrinsic Semiconductors

17 Intrinsic Semiconductors

18 Intrinsic Semiconductors

19 Intrinsic Semiconductors holes electrons current

20 Intrinsic Semiconductors When an electric field (voltage) is applied, the holes move in one direction and the electrons in the other. However both current components are in the direction of the field. The conduction is ohmic, i.e. current is proportional to the applied voltage (field)

21 Intrinsic Semiconductors For an intrinsic semiconductor the number of electron and hole carriers, and thus the conductivity, increases rapidly with temperature. This is not very useful. Hence we dope the material to produce an extrinsic semiconductor.

22 Extrinsic Semiconductors Instrinsic conduction is very small (see example). Conductivity levels can be raised and controlled by doping with minute levels of impurity atoms to give extrinsic or doped semiconductors. Extrinsic semiconductors may be further divided into either n-type or p-type

23 N-type Semiconductors An n-type impurity atom has five outer (valence) electrons, rather than the four of silicon. Only four of the outer electrons are required for covalent bonding. The fifth is much more easily detached from the parent atom. As the energy needed to free the fifth electron is smaller than the thermal energy at room temperature virtually all are freed.

24 N-type Semiconductors EXTRA ELECTRON FREE AT ROOM TEMP. +4 N D = donor implant density n = p + N D n p = n i If N D >> n i > p n N D p n i2 / N D

25 P-type Semiconductors Here the doping atom has only three electrons in its outer shell. It is relatively easy for an electron from a neighbouring atom to move in, so releasing a hole at its parent atom. The freed hole is available for conduction. The energy needed to free the electron from its parent is usually small compared to the thermal energy so each impurity atom contributes one hole for conduction (fully ionised).

26 P-type Semiconductors N A = acceptor implant density +3 A neighbouring electron can move here. This creates a hole where the electron came from. p = n + N A n p = n i 2 If N A >> n i > n p N A n n i2 / N A

27 Carrier Transport 1. Carrier Drift 2. Carrier Diffusion

28 Carrier Drift The process in which charged particles move because of an electric field is called drift. Charged particles within a semiconductor move with an average velocity proportional to the electric field. The proportionality constant is the carrier mobility. Hole velocity Electron velocity v v h e = µ E p = µ E n Notation: µ p hole mobility (cm 2 /V s) µ n electron mobility (cm 2 /V s)

29 Drift Current Drift current is proportional to the carrier velocity and carrier concentration: v h t A = volume from which all holes cross plane in time t p v h t A = # of holes crossing plane in time t q p v h t A = charge crossing plane in time t q p v h A = charge crossing plane per unit time = hole current Hole current per unit area (i.e. current density) J p,drift = q p v h

30 Conductivity and Resistivity In a semiconductor, both electrons and holes conduct current: J J J p, drift tot, drift tot, drift = = = qpµ E q( pµ The conductivity of a semiconductor is Unit: mho/cm The resistivity of a semiconductor is Unit: ohm-cm J p p, drift p + J J n, drift n n, drift = + nµ ) E = qn( µ E) qpµ E p σe n + qnµ E σ qp µ + qnµ 1 ρ σ p n n

31 Resistivity Example Estimate the resistivity of a Si sample doped with phosphorus to a concentration of cm -3 and boron to a concentration of cm -3. The electron mobility and hole mobility are 700 cm 2 /Vs and 300 cm 2 /Vs, respectively.

32 Electrical Resistance I + V _ W homogeneously doped sample t L Resistance R V I = L ρ Wt (Unit: ohms) where ρ is the resistivity

33 Carrier Diffusion Due to thermally induced random motion, mobile particles tend to move from a region of high concentration to a region of low concentration. Analogy: ink droplet in water Current flow due to mobile charge diffusion is proportional to the carrier concentration gradient. The proportionality constant is the diffusion constant. J p = qd p dp dx Notation: D p hole diffusion constant (cm 2 /s) D n electron diffusion constant (cm 2 /s)

34 Diffusion Examples Linear concentration profile constant diffusion current x p = N 1 L Non-linear concentration profile varying diffusion current x p = N exp L d J p, diff dp = qdp dx N = qdp L J p, diff dp = qdp dx qdpn = exp L d x L d

35 Diffusion Current Diffusion current within a semiconductor consists of hole and electron components: J J p, diff tot, diff = qd = q( D p n dp dx dn dx n, diff dp dx The total current flowing in a semiconductor is the sum of drift current and diffusion current: D p J ) = qd J tot = J p, drift + J n, drift + J p, diff + J n, diff n dn dx

36 The Einstein Relation The characteristic constants for drift and diffusion are related: D = µ kt q kt Note that 26mV at room temperature (300K) q This is often referred to as the thermal voltage.

37 Faculty of Engineering ECE 142: Electronic Circuits Lecture 4: Semiconductor Technology

38 Silicon Run I Video

39 Basic Fabrication Processes Oxidation Photolithography Deposition Ion Implantation