SEMICONDUCTOR MATERIALS UNIVERSITI MALAYSIA PERLIS SCHOOL OF ELECTRICAL SYSTEMs ENGINEERING Chapter 1 SEMICONDUCTOR MATERIALS ELECTRONIC DEVICES EET109 PERANTI ELEKTRONIK

CONTENT 1.1 Semiconductor, conductors and insulators 1.2 Energy bands 1.3 Covalent bonding 1.4 Conduction in semiconductors 1.5 N-type and P-type semiconductors 1.6 The diode 1.7 Biasing the diode 1.8 Voltage-current characteristic of a diode 1.9 Diode models 1.10 Testing a diode

WHAT IS ELECTRONIC DEVICES?? Such as diodes, transistors, and integrated circuits (ICs). Made of a semiconductor material. To understand how these devices work, we should have a basic knowledge of the atoms structure and the interaction of atom particles !!

Insulators Conductors Semiconductors material does not conduct electrical current valence electron are tightly bound to the atom – very few free electron Conductors material that easily conducts electrical current. The best conductors are single-element material (e.g copper, silver, gold, aluminum) Only one valence electron very loosely bound to the atom- free electron Semiconductors material between conductors and insulators in its ability to conduct electric current in its pure (intrinsic) state is neither a good conductor nor a good insulator most common semiconductor- silicon(Si), germanium(Ge), and carbon(C) which contains four valence electrons. (1.1) INSULATORS, CONDUCTORS & SEMICONDUCTORS

History Of Semiconductor Devices Fig. 1-32a & b forward and reverse bias schematic diagram

Electron Shells and Orbits Electrons near the nucleus have less energy than those in more distant orbits. Each distance (orbits) from the nucleus corresponding to a certain energy level. In an atom, the orbits are group into energy bands – shells Figure 1-1 Bohr Model (1.2) ENERGY BANDS

Energy Bands (1.2) ENERGY BANDS

Energy gap-the difference between the energy levels of any two Energy Bands Energy gap-the difference between the energy levels of any two orbital shells Band-another name for an orbital shell (valence shell=valence band) Conduction band –the band outside the valence shell where it has free electrons. (1.2) ENERGY BANDS

1.3 COVALENT BONDING Covalent bonding – holding atoms together by sharing valence electrons Sharing of valence electron produce the covalent bond To form Si crystal

The atom are held together forming a solid substrate. Result of the bonding: The atom are held together forming a solid substrate. The atoms are all electrically stable, because their valence shells are complete. The complete valence shells cause the silicon to act as an insulator-intrinsic (pure) silicon. In other word, it is a very poor conductor. Fig. 1-8 Covalent bonding Covalent bonding in an intrinsic or pure silicon crystal. An intrinsic crystal has no impurities. Covalent bonds in a 3-D silicon crystal COVALENT BONDING (cont..)

1.4 CONDUCTION IN SEMICONDUCTOR Figure 1-10 Energy band diagram for a pure (intrinsic) silicon crystal with unexcited (no external energy such as heat) atoms. There are no electrons in the conduction band. This condition occurs only at a temperature of absolute 0 Kelvin.

Conduction Electrons and Holes Absorbs enough energy (thermal energy) to jumps Fig. 1-9 Intrinsic Silicon a free electron and its matching valence band hole – electron-hole pair Recombination-when a conduction electron loses energy and fall back into hole in valence band Figure 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free (also called conduction electrons) CONDUCTION IN SEMICONDUCTOR (cont..)

Conduction Electrons and Holes Fig. 1-9 Intrinsic Silicon Figure 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes. CONDUCTION IN SEMICONDUCTOR (cont..)

Electrons and Holes current Electron current free electrons Fig. 1-9 Intrinsic Silicon Apply voltage When a voltage is applied, free electrons are free to move randomly and attracted toward +ve end. The movement of electrons is one type of current in semiconductor and is called electron current. Figure 1-13 Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. CONDUCTION IN SEMICONDUCTOR (cont..)

Electrons and Holes Current movement of holes Fig. 1-9 Intrinsic Silicon Figure 1-14 Hole current in intrinsic silicon. CONDUCTION IN SEMICONDUCTOR (cont..)

1.5 N-TYPE & P-TYPE SEMICONDUCTORS Doping The process of creating N and P type materials By adding impurity atoms to intrinsic Si or Ge to improve the conductivity of the semiconductor Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-) p-type material – a semiconductor that has added trivalent impurities n-type material – a semiconductor that has added pentavalent impurities

Trivalent Impurities: Aluminum (Al) Gallium (Ga) Boron (B) Indium (In) P-type N-type Trivalent Impurities: Aluminum (Al) Gallium (Ga) Boron (B) Indium (In) Pentavalent Impurites: Phosphorus (P) Arsenic (As) Antimony (Sb) Bismuth (Bi) Fig. 1-15 & 16 Pentavalent and Trivalent N-TYPE & P-TYPE SEMICONDUCTORS (cont..)

Boron, indium and gallium have 3 valence e- form covalent bond with 4 P-type semiconductor Trivalent impurities are added to Si or Ge to increase number of holes. Boron, indium and gallium have 3 valence e- form covalent bond with 4 adjacent silicon atom. A hole created when each trivalent atom is added. The no. of holes can be controlled by the no. of trivalent impurity atoms The trivalent atom can take an electron- acceptor atom Current carries in p-type are holes – majority carries electrons – minority carries (created during electron-holes pairs generation). Fig. 1-15 & 16 Pentavalent and Trivalent B impurity atom Trivalent impurity atom in a Si crystal

Pentavalent impurity atom in a Si crystal N-type semiconductor Pentavalent impurities are added to Si or Ge, the result is an increase of free electrons 1 extra electrons becomes a conduction electrons because it is not attached to any atom Number of conduction electrons can be controlled by the number of impurity atoms Pentavalent atom gives up an electron - call a donor atom Current carries in n-type are electrons – majority carriers Holes – minority carriers (holes created in Si when generation of electron- holes pair. Fig. 1-15 & 16 Pentavalent and Trivalent Sb impurity atom Pentavalent impurity atom in a Si crystal

Checkup Questions 1). What is the difference between conductors and insulators? 2). Why does a semiconductor have fewer free electrons than a conductor? 3). What is a crystal? 4). How a covalent bonds formed? 5). Which electrons are responsible for electron current in silicon? 6). What is a hole? 7). How is an n-type and a p-type material formed? ). What is the difference between intrinsic and extrinsic semiconductors?

1.6 THE DIODE n-type material & p-type material become extremely useful when joined together to form a pn junction – then diode is created Diode is a device that conducts current only in one direction. before the pn junction is formed  no net charge (neutral) since number of proton and electron is equal in both n-type and p-type. p region: holes (majority carriers), e- (minority carriers) n region: e- (majority carriers), holes (minority carriers)

Basic diode structure at the instant of junction formation showing only majority and minority carriers.

The Depletion Region THE DIODE (cont..) Fig 1-18 a & b depletion region THE DIODE (cont..)

The Depletion Region Summary: When an n-type material is joined with a p-type material: A small amount of diffusion occurs across the junction. When e- diffuse into p-region, they give up their energy and fall into the holes near the junction. Since the n-region loses electrons, it creates a layer of +ve charges (pentavalent ions). p-region loses holes since holes combine with electron and will creates layer of –ve charges (trivalent ion). These two layers form depletion region. Depletion region establish equilibrium (no further diffusion) when total –ve charge in the p-region repels any further diffusion of electrons into p-region. Hence, no current is flowing Fig 1-18 a & b depletion region THE DIODE (cont..)

Barrier Potential In depletion region, many +ve and –ve charges on opposite sides of pn junction. The forces between the opposite charges form a “field of forces "called an electric field. This electric field is a barrier to the free electrons in the n-region, need more energy to move an e- through the electric field. The potential difference of electric field across the depletion region is the amount of voltage required to move e- through the electric field. This potential difference is called barrier potential. [ unit: V ] Depends on: type of semiconductive material, amount of doping and temperature. (e.g : 0.7V for Si ; 0.3 V for Ge at 25°C). THE DIODE (cont..)

Energy Diagram of the PN Junction and Depletion Region Fig 1-18 a & b depletion region THE DIODE (cont..)

Energy Diagram of the PN Junction and Depletion Region Energy level for n-type (Valence and Cond. Band) lower than p- type material (difference in atomic characteristic : pentavalent & trivalent) and significant amount of overlapping. Free e- in upper part conduction band in n-region can easily diffuse across junction and temporarily become free e- in lower part conduction band in p-region. After crossing the junction, the e- loose energy quickly & fall into the holes in p-region valence band. THE DIODE (cont..)

Energy Diagram of the PN Junction and Depletion Region As the diffusion continues, the depletion region begins to form and the energy level of n-region conduction band decreases due to loss of higher-energy e- that diffused across junction to p-region. Soon, no more electrons left in n-region conduction band with enough energy to cross the junction to p-region conduction band. Figure (b), the junction is at equilibrium state, the depletion region is complete diffusion has ceased (stop). Create an energy gradient which act as energy ‘hill’ where electron at n-region must climb to get to the p-region. The energy gap between valence & cond. band – remains the same THE DIODE (cont..)

1.6 BIASING THE DIODE No electron move through the pn-junction at equilibrium state. Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer. Two bias conditions : forward bias & reverse bias The relationship between the width of depletion layer & the junction current Depletion Layer Width Junction Resistance Junction Current Min Max

Forward bias 1. Voltage source or bias connections (VBias) are + to the p region and – to the n region. 2. Bias voltage (VBias) must be greater than barrier potential (0.3 V for Germanium or 0.7 V for Silicon). The depletion region narrows. R – limits the current which can prevent damage to the diode Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse Diode connection

Flow of majority carries and the voltage across the depletion region Forward bias The negative side of the bias voltage (VBias) push the free electrons in the n-region -> pn junction. Flow of free electron is called electron current. Also provide a continuous flow of electron through the external connection into n-region. Bias voltage (VBias) imparts energy to the free e- to move to p-region. Once in p-region, those electrons loss energy-combine with holes in valence band. Flow of majority carries and the voltage across the depletion region

Flow of majority carries and the voltage across the depletion region Forward bias Positive side of bias voltage (VBias) source attracts the e- left end of p-region. Holes in p-region act as medium or pathway for these e- to move through the p-region. e- move from one hole to the next toward the left. The holes move to right toward the junction. This effective flow is called hole current. Flow of majority carries and the voltage across the depletion region

The Effect of Forward bias on the Depletion Region Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse As more electrons flow into the depletion region, the number of +ve ion is reduced. As more holes flow into the depletion region on the other side of pn junction, the number of –ve ions is reduced. Reduction in +ve & -ve ions causes the depletion region to narrow.

The Effect of the Barrier Potential During Forward Bias Electric field between +ve & -ve ions in depletion region creates “energy hill” that prevent free e- from diffusing at equilibrium state -> barrier potential (0.7 V for silicon; 0.3 V for germanium) When apply forward bias, free e- provided enough energy to climb the hill and cross the depletion region. Electron got the same energy = barrier potential to cross the depletion region. An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse

Reverse bias Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse Diode connection Reverse bias - Condition that prevents current through the diode Voltage source or bias connections (VBias) are –ve to the p material and +ve to the n material Current flow is negligible in most cases. The depletion region widens than in forward bias.

Shot transition time immediately after reverse bias voltage is applied +ve side of bias pulls the free electrons in the n-region away from pn junction cause add. +ve ions are created, widening the depletion region. In the p-region, e- from –ve side of the voltage source (VBias) enter as valence electrons e- and move from hole to hole toward the depletion region, then created add. –ve ions. As the depletion region widens, the availability of majority carriers decrease.

Reverse Current Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse Extremely small current exist – after the transition current dies out caused by the minority carries in n & p regions that are produced by thermally generated electron hole pairs. Small number of free minority e- in p region are “pushed toward the pn junction by the –ve bias voltage.

Reverse Current e- reach wide depletion region, they “fall down the energy hill” combine with minority holes in n -region as valence e- and flow towards the +ve bias Voltage – create small hole current. The conduction band in p region is at higher energy level compare to conduction band in n-region, e- easily pass through the depletion region because they require no additional energy.

V-I Characteristic for Forward Bias 1-8 Voltage-Current Characteristic Of A Diode V-I Characteristic for Forward Bias When a forward bias voltage is applied, there is current called forward current, IF . In this case with the voltage applied is less than the barrier potential (<0.7 V) so the diode for all practical purposes is still in a non-conducting state. Current is very small. Increase forward bias voltage – current also increase. FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

V-I Characteristic for Forward Bias With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly. But the voltage across the diode increase only gradually above 0.7 V. this is due to voltage drop across internal dynamic resistance of semiconductive material. Fig. 1-26b measurements with meters FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

dynamic resistance r’d decreases as you move up the curve Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode - Increase to the right - Increase upward After 0.7V, voltage remains at 0.7V but IF increase rapidly. Normal operation for a forward-biased diode is above the knee of the curve. zero bias Fig. 1-26b measurements with meters Below knee, resistance is greatest since current Increase very little for given voltage, Resistance become smallest above knee where\ a large change in current for given change in voltage.

V-I Characteristic for Reverse Bias VR increase to the left along x-axis while IR increase downward along y-axis. When VR reaches VBR , IR begin to increase rapidly. Breakdown voltage, VBR. -not a normal operation of pn junction devices. -the value can be vary for typical Si. -Cause overheating and possible damage to diode. Fig. 1-26b measurements with meters Reverse Current

Combine-Forward bias & Reverse bias  CompleteV-I characteristic curve Fig. 1-26b measurements with meters

Temperature Effects on the Diode V-I Characteristic Forward biased diode : for a given value of Barrier potential decrease as T increase. For reverse-biased, T increase, IR increase. Reverse current breakdown – small & can be neglected Fig. 1-26b measurements with meters

Diode Structure & Symbol 1-9 DIODE MODELS Direction of current cathode anode Diode Structure & Symbol

The Practical Diode Model The Complete Diode Model The Ideal Diode Model DIODE MODEL Fig. 1-33 ideal diode curve The Complete Diode Model

The Ideal Diode Model Ideal model of diode- simple switch: Closed (on) switch -> FB Open (off) switch -> RB Barrier potential, dynamic resistance and reverse current all neglected. Assume to have zero voltage across diode when FB. Forward current determined by Ohm’s law Fig. 1-33 ideal diode curve

The Practical Diode Model Adds the barrier potential to the ideal switch model is neglected From figure (c): The forward current [by applying Kirchhoff’s voltage law to figure (a)] By Ohm’s Law: Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V Represent by produced across the pn junction Open circuit, same as ideal diode model. Barrier potential doesn’t affect RB Fig. 1-33 ideal diode curve

The Complete Diode Model Complete model of diode consists: Barrier potential Forward dynamic resistance, Internal reverse resistance, The forward voltage consists of barrier potential & voltage drop across r’d : The forward current: acts as open switch in parallel with the large acts as closed switch in series with barrier potential and small Fig. 1-33 ideal diode curve

Example 1 (1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current. Fig. 1-33 ideal diode curve 1.0kΩ 1.0kΩ 10V 5V

Example 1 Ideal Model: Practical Model: (c) Complete model: Fig. 1-33 ideal diode curve

Typical Diodes Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in. Fig. 1-33 ideal diode curve

1-10 Testing A Diode By Digital MULTIMETER Testing a diode is quite simple, particularly if the multimeter used has a diode check function. When the diode is working If diode is good, will show approximately 0.7 V or 0.3 V when forward biased. When checking in reverse bias, typically get a reading “OL” or internal voltage source, 2.6V.

When diode is failed open, open reading voltage is 2.6V or “OL” When the diode is Defective When diode is failed open, open reading voltage is 2.6V or “OL” for both forward and reverse bias. If diode is shorted, meter reads 0V in both forward and reverse bias. Fig 1-38 DMM check w/electrode labels

OHMs Function Select OHMs range Good diode: Forward-bias: get low resistance reading (10 to 100 ohm) Reverse-bias: get high reading (0 or infinity) Fig 1-38 DMM check w/electrode labels

Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. P-materials are doped with trivalent impurities N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately 0.3 V for a Germanium and 0.7 V for Silicon for conduction to take place.

Summary A diode conducts when forward biased and does not conduct when reverse biased The voltage at which avalanche current occurs is called reverse breakdown voltage. Reverse breakdown voltage for diode is typically greater than 50V. There are three ways of analyzing a diode. These are ideal, practical, and complete. Typically we use a practical diode model.