Lecture 4 n Junctions (Diodes) Wednesday 27/9/2017 n junctions 1-1
Agenda Continue n junctions Equilibrium (zero bias) Deletion rejoins Built-in otential Reverse and forward bias I-V characteristics Bias voltage Forward and reverse currents Reverse breakdown Secial Purose Diodes n junctions 1-2
Test Yourself Choose the correct answer for the following two statements: i. The resistance of. increase with rise in temerature degrees. 1. Insulators 2. Conductors 3. Semiconductors 4. Dielectrics ii. Charge carrier motion which is caused by an electric field due to an externally alied voltage is known as: 1. Carrier Drift 2. Thermal Motion 3. Carrier Diffusion 4. Brownian motion n junctions 1-3
Last Lecture Thermal equilibrium: balance between drift and diffusion deletion region We can divide semiconductor into three regions: Two quasi-neutral n and regions (QNR s) One sace charge region (SCR) n junctions 1-4
Equilibrium (zero bias) n junctions 1-5
The Deletion Aroximation Assume the QNR s are erfectly charge neutral Assume the SCR is deleted of carriers (deletion region) n junctions 1-6
Sace charge density n junctions 1-7
Electric Field The electric field is roortional to the integral of the sace charge density and will be zero in the (charge) neutral regions outside of the deletion region n junctions 1-8
Carrier Profiles: in thermal equilibrium (zero bias) In equilibrium: dynamic balance between drift and diffusion for electrons and holes inside SCR n junctions 1-9
Built-in Potential (zero bias) q E qd x 2 x 1 dv D n d dx d,, dv d D dx dx D V ( x2) V ( x1 ) ln n I I drift, drift, n I I diff, diff, n V kt kt 0 ln, V0 ln q n q N N A 2 ni D Because of the electric field across the junction, there exists a built-in otential. Its derivation is shown above n junctions 1-10
n junction under bias Uon alication of voltage Electrostatics uset: deletion region widens or shrinks Current flows With rectifying behavior Carrier charge storage n junctions 1-11
n junction in reverse bias When the n-tye region of a diode is connected to a higher otential than the -tye region, the diode is under reverse bias, which results in wider deletion region and larger built-in electric field across the junction n junctions 1-12
Carrier Profiles: under reverse bias n At reverse bias voltage deletion region widens Net drift current in SCR minority carrier extraction from QNRs Carrier flow is small because there are few minority carriers extracted from QNRs from the minority side - ve terminal attraction Holes (majority) electrons holes Electrons (majority) attraction + ve terminal n junctions 1-13
n junction in forward bias When the n-tye region of a diode is at a lower otential than the - tye region, the diode is in forward bias The deletion width is shortened and the built-in electric field decreased n junctions 1-14
Carrier Profiles: under forward bias n At forward bias voltage deletion region shrinks Net diffusion current in SCR minority carrier injection into QNRs Carrier flow can be high because lots of minority carriers are injected into QNRs from the majority side + ve terminal ushing Holes (majority) electrons holes Electrons (majority) ushing - ve terminal n junctions 1-15
IV characteristics of n juntions I D n V D Non-linear device n junction (Diode) current equation: I D I S V V ( 1) e T D n junctions 1-16
Physics of forward bias: Potential dro across SCR reduced by V minority carrier injection in QNRs Minority carrier diffusion through QNRs Minority carrier recombination at contacts to the QNRs (and surfaces) Large suly of carriers injected into the QNRs I D V D ev T n junctions 1-17
Physics of reverse bias Potential dro across SCR increased by V minority carrier extraction from QNRs Minority carrier diffusion through QNRs Minority carrier generation at surfaces & contacts of QNRs Very small suly of carriers available for extraction I saturates to small value n junctions 1-18
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n junctions 1-20
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Reverse Breakdown When a large reverse bias voltage is alied, breakdown occurs and an enormous current flows through the diode n junctions 1-22
Secial Purose Diodes Zener Light-emitting Photo Varactor Laser Schottky PIN Tunnel Current-regulator n junctions 1-23
Secial Purose Diodes Light Emitting Diode (LED) In a forward-biased -n junction, recombination of the holes and electrons requires energy ossessed by the unbound free electrons In Si and Ge, most of the energy is dissiated in the form of heat and hotons But in other material such as GaAs, the energy generate light but it is invisible for the eye to see (infrared) Color Green Orange Red Construction GaP GaAsP GaAsP Forward Voltage Other materials that emit light during forward-bias oeration n junctions 1-24 2.2 2.0 1.8
Secial Purose Diodes Photodiode a secial light sensitive diode with a clear window to the n junction oerated with reverse bias Reverse current increases with greater incident light Reverse current, ( ) I l Dark current 0 Irradiance, H n junctions 1-25
Secial Purose Diodes Schottky diode a metal-to-semiconductor contact diode that is used rimarily in high frequency and fast-switching alications has a low forward voltage dro and high efficiency but rather low reverse voltage rating. Metal-silicon junction n region Metal region Cathode n Anode n junctions 1-26
Secial Purose Diodes PIN diode a three layer diode consisting of and n layers searated by a narrow intrinsic layer In microwave alications, the in diode acts as a voltage-controlled resistor Certain tyes are used as hotodetectors in fiber otic systems A n region intrinsic region K region Anode i n Cathode n junctions 1-27
Secial Purose Diodes Tunnel diode has a characteristic curve that shows a negative resistance reading between B and C with a small forward voltage The negative resistance region is unstable. Taking advantage of this characteristic, the tunnel diode can be used in an oscillator circuit at microwave I frequencies F B Negativeresistance region A 0 Tunneling current C V F n junctions 1-28
Lecture Summary Covered material Continue n junctions Deletion rejoins Built-in otential (zero bias) Forward and reverse bias I-V characteristics Bias voltage Forward and reverse currents Reverse breakdown Secial urose diodes (LED) Material to be covered next lecture Diode equivalent circuits DC and AC analysis n junctions 1-29