ECA1212 Introduction to Electrical & Electronics Engineering Chapter 4: Basic Semiconductor and Diode by Muhazam Mustapha, October 2011

Learning Outcome By the end of this chapter students are expected to: Explain some basic theory about charge transport in semiconductor Explain diode circuit operation

Chapter Content Physics of Semiconductor PN Junction and Diode Diode Circuits

Physics of Semiconductor

Three Categories of Material Based on their electrical conductivity, material can be categorized into 3 groups: Conductor Non-conductor Semiconductor This conductivity property is determined by the electronic structure in the outer most shell Electronic structure in the outer most shell, in turn, will determine the amount of energy needed by the outer most electron to be freed from the atom CO1

Three Categories of Material In a system of a large number of atoms come close together – in a compound or crystal, for example – the energy level of the outer shells will merge together to form BANDS For a material to conduct electricity, its electron in the outer band (VALENCE) must be able to go up to the CONDUCTION band The energy distance (gap) between the valence band and the conduction band is what determines the conductivity of the previous 3 categories of material CO1

Three Categories of Material Small Gap Overlap Conduction Band Electron Energy Big Gap Valance Band Conductor Semiconductor Non-conductor CO1

Conductor In a conductor, the conduction band and the valance band are overlapping This allows the electrons in outer most shell (valance band) to freely move among the system of atoms This free movement of electron is permitted without any external energy (or excitation) Metals are the material that posses this kind of conductivity CO1

Non-Conductor In a non- conductor, the conduction band and the valance band are far apart This disallows the electrons in outer most shell to freely move among the system of atoms It is almost impossible to push the electrons up to the conduction band without damaging the material structure Most of non-metallic material are non-conductor CO1

Semiconductor In between conductor and non-conductor, there exist a special type of material that possesses an intermediate electronic structure property The conduction band and the valance band are not overlapping but not far apart either This allows the electrons in its valance band to jump into the conduction band if they acquire enough energy The source of such energy could be from heat, electromagnetic rays, direct hit by another electron, etc CO1

Semiconductor The elements in the Periodic Table Group IV are the most common semiconductors The examples are: Carbon, Silicon and Germanium CO1

Electron Transport in Semiconductor We may view the crystalline structure of Group IV elements as follows: valance electron bonding 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ CO1

Electron Transport in Semiconductor Some of the electrons in valance band may gain some energy and become free free electrons 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ holes 4+ 4+ 4+ 4+ 4+ 4+ CO1

Electron Transport in Semiconductor The free electrons contribute to electric conductivity in the semiconductor material The covalent bond from where the electrons come out will now be lacking of an electron and become another electron transport medium called HOLES Holes made electron transport possible by allowing an electron in a neighboring covalent bond to jump into it and effectively create electrical movement CO1

Electron Transport in Semiconductor Holes transport phenomenon only exists in semiconductor material Under the influence of the same electric field, holes make a net movement in an opposite direction of electrons movement Electric field Free electrons Valance band electrons Holes CO1

Dopant Pure semiconductor material is called intrinsic In intrinsic semiconductor, the no. free electrons and holes will be balanced Dopant can be added to intrinsic semiconductor to alter the no. one of the transport carrier – either free electrons or holes Doped semiconductor is called extrinsic CO1

N-type Semiconductor Elements of Periodic Table Group V can be added as dopant (phosphorus, arsenic) These elements have an extra electron that cannot contribute to the covalent bond, hence it is freed These electrons do not need extra energy to be freed, hence they behave like free electrons in conductors This type of semiconductor with more free electrons than holes is called N-type semiconductor CO1

N-type Semiconductor CO1

N-type Semiconductor CO1 extra free electrons 4+ 4+ 5+ 4+ 4+ 4+ 4+ 4+

N-type Semiconductor In N-type semiconductor: The majority carrier is free electrons The atom (element) that contribute to the extra free electron is called DONOR atom CO1

P-type Semiconductor Elements of Periodic Table Group III can be added as dopant (boron, gallium) These elements lack an electron in the outer shell hence cannot create a complete covalent bond These bonds are effectively created with holes This type of semiconductor with more holes than free electrons is called P-type semiconductor CO1

P-type Semiconductor CO1

P-type Semiconductor CO1 extra holes 4+ 4+ 3+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+

P-type Semiconductor In P-type semiconductor: The majority carrier is holes The atom (element) that contribute to the extra holes is called ACCEPTOR atom CO1

PN Junction CO1

PN Junction What will happen if a P-type semiconductor is fabricated next to an N-type semiconductor? Will the extra free electrons in the N-type area cross over into the P-type and neutralize the extra holes? As a matter of fact, they do However, the crossing over causes charge imbalance CO1

PN Junction P N Depletion Region − − + + − − + + − − + + − − + + − − + Electron migration CO1

PN Junction The N-type area that loses electron will have more positive charge, and vice versa This will then create a voltage difference at the interface between the P- and N-type material in the polarity that is the same direction to the electrons migration from N to P This voltage difference creates an electric field that will then prevent further migration of electrons to P-type when it reaches certain voltage value CO1

PN Junction The electron-hole neutralized area (but with effective positive or negative charge), is called depletion region The width of the depletion region is very small but it depends on the concentration of dopant The voltage different across the junction is around 0.75V – depletion voltage (VD) CO1

Diode CO1

Diode By creating a PN junction we basically create a diode Diode symbol in circuit: Diode is mostly used in rectifier circuits (circuits that allow current to flow only in 1 direction) The direction of current flow is the same as the direction of the triangle (explained in succeeding slides) CO1

Reverse Biasing Diode Reverse biasing a diode means we are applying voltage across its to make the depletion region larger This is done by applying a +ve potential to the N side of the diode and –ve potential to the P side of the diode The +ve potential will then attract the electrons in N-type side toward the terminal, and the –ve potential will attract the holes in P-type side toward the other terminal CO1

Depletion Region Increased Reverse Biasing Diode Depletion Region Increased − − − + + + e- − − − + + + e- − − − + + + e- − − − + + + P N − − − + + + e- − − − + + + e- − − − + + + − − − + + + e- − − − + + + CO1

Reverse Biasing Diode Since the holes and electrons are attracted toward the opposite terminals, the area of depletion region increased The whole process is like the process of charging a capacitor because the electrons and the holes attracted to the terminals are like the charges that accumulated on capacitor plates Hence, at final stage, there is no current flow through the diode – just like capacitor CO1

Forward Biasing Diode Forward biasing a diode means we are applying voltage across its to make the depletion region smaller This is done by applying a –ve potential to the N side of the diode and +ve potential to the P side of the diode The –ve potential will then repel the electrons in N-type side toward the depletion region, and the +ve potential will repel the holes in P-type side toward the depletion region from the side CO1

Depletion Region Decreased Forward Biasing Diode Depletion Region Decreased − + e- − + e- − + e- − + P N − + e- − + e- − + − + e- − + CO1

Forward Biasing Diode Even though the depletion region is smaller now, but there is will still no current flowing until the 0.75V voltage (barrier) due to the electron migration is offset by the external potential Once the voltage barrier is passed, the depletion region vanishes, the current then flows with very little resistance Hence, when current is flowing through the diode at forward biased, the diode is basically a short circuit CO1

Forward Biasing Diode P N anode cathode current flow e- e- e- e- e- e- > 0.75V CO1

I-V Characteristics On reverse bias voltage, there is zero current flowing On forward bias, there will be current flowing after the 0.75V voltage barrier is overcome At a very high reverse bias voltage, a junction breakdown will take place and current will flow in reverse direction – beyond the scope of this course Refer to the graph in the next slide CO1

I-V Characteristics I Breakdown Reverse Bias Forward Bias V CO1 VD = 0.75V V CO1

Ideal Model I Reverse Bias Forward Bias V ON (Forward Bias) OFF (Reverse Bias) CO1

Offset Model + − I Reverse Bias Forward Bias V ON (Forward Bias) VD − I Reverse Bias Forward Bias VD V ON (Forward Bias) OFF (Reverse Bias) + VD − CO1

Diode Circuits CO1

Half-Wave Rectifier Rectifier is a circuit that changes AC current to DC The process involves diodes as diodes only allows current to flow in one direction Half-wave rectifier only allows the positive (half) part of an AC current to flow through it CO1

Half-Wave Rectifier + + vS RL vL − − vS vL t t CO1

Full-Wave Rectifier Full-wave rectifier requires two diodes and a transformer with a center tap The diodes still allow only one way current through them – one diode allows one half wave The arrangement of the two diodes channels the current to flow into the load in same direction Center tap makes the transferred power is half, but since both half wave are allowed to flow, it doubles back the power – making it the same power as the half wave CO1

Full-Wave Rectifier + vS RL − vS vL − + + vS vL t t CO1

Bridge Rectifier Bridge rectifier is a full wave rectifier that doesn’t require a center tapped transformer Four diodes are arranged as shown in the next slide Only two diodes flowing current during each half wave The two half waves are channeled through the load in the same direction making the load to experience the same voltage polarity through it CO1

Bridge Rectifier + RL vS + vL − − CO1

Bridge Rectifier RL + vS vL − + − vS vL t t CO1

Bridge Rectifier + RL vS − + vL − vS vL t t CO1

Bridge Rectifier Capacitor can be used to reduce ripple in rectifiers + + vL vS RL − − vL Capacitor discharge Capacitor re-charge t CO1