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Lecture 12 Semiconducting junctions

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The PN-Junction One of the simplest bipolar devices, important for the understanding of more complex devices (bipolar = both electrons and holes contribute to device characteristics).

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Semiconductor devices: Inhomogeneous semiconductors All solid-state electronic and opto-electronic devices are based on doped semiconductors. In many devices the doping and hence the carrier concentrations are non-homogeneous. In the following section we will consider the p-n junction which is an important part of many semiconductor devices and which illustrated a number of key effects

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+ + + + + - - - - - A Diode Nonlinear I-V characteristics V I Forward bias Reverse bias n p + + + + - - - - - + + + - - - AA + + + + - - - - - + - +

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The p-n semiconductor junction: p-type / n-type semiconductor interface We will consider the p-n interface to be abrupt. This is a good approximation. n-type N D donor atoms per m 3 p-type N A acceptor atoms per m 3 Consider temperatures ~300K Almost all donor and acceptor atoms are ionised. impurity atoms m -3 N A N D x x = 0 p-typen- N D (x) = N D (x>0) = 0 (x<0) N A (x) = N A (x>0) = 0 (x<0) impurity atoms m -3 N A N D x x = 0 p-typen- impurity atoms m -3 N A N D x x = 0 p-typen- N D (x) = N D (x>0) = 0 (x<0) N A (x) = N A (x 0) p-n interface at x=0.

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Electron and hole transfer Consider bringing into contact p-type and n-type semiconductors. n-type semiconductor: Chemical potential, (Fermi level) below bottom of conduction band p-type semiconductor: Chemical potential, above top of valence band. Electrons diffuse from n-type into p-type filling empty valence states. n-type semiconductorp-type semiconductor E C E V E C E V E C E V E C E V Electrons Holes

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Electrons diffuse from n-type into p-type filling empty valence band states. The p-type becomes negatively charged with respect to the n-type material. Electron energy levels in the p-type rise with respect to the n-type material. A large electric field is produced close to the interface. Dynamic equilibrium results with the chemical potential (Fermi level) constant throughout the device. Note: Absence of electrons and hole close to interface -- depletion region Band Bending

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Junction At equilibrium the Fermi level gradient equals zero!

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p-n junction I IV characteristics :

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The principle working of a pn-junction P-doped N-doped Negatively charged electrons + positively charged immobile donors Positively charged holes + negatively charged immobile acceptors + - No electrons or holes, only charged donors/acceptors (DEPLETION LAYER) electrons holes P-doped N-doped

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The principle working of a pn-junction + - electrons holes No Voltage P-doped N-doped + - + - electrons holes Forward bias current - + + - electrons holes Reverse bias no current No current (Leakage current) Large current Current Voltage Circuit symbol: I IV characteristics :

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EcEc EvEv EcEc EcEc e-e- e-e- Drift (thermally exc.) Diffusion (E-field) No bias Forward bias Reverse bias V V V0 0 j

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I-V Characteristics Hole current: diffusion I pd = C 1 N p exp (-eV bi /(kT)) drift I pu = CN pn = I pd = C 1 N p exp (-eV bi /(kT)) at forward bias I pF = C 1 N p exp (-e(V bi - V) /(kT)) I p = I pF - I pu = C 1 N p exp (-e(V bi - V) /(kT)) – C 1 N p exp (-eV bi /(kT)) = C 1 N p exp [-eV bi /(kT)][exp(eV/(kT)-1] =I pd [exp(eV/(kT))-1] Electron current: I n = I nd [exp(eV/(kT))-1 with I nd = C 2 N n exp (-eV bi /(kT)) I = Io [exp(eV/(kT)-1] Io = I nd + I pd = (C 1 N p + C 2 N n ) exp (-eV bi /(kT))

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Rectifier Ac transfers into dc a) b) I t

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Based on the photovoltaic effect -solar cell -photodetectors photodiode

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Avalanche diode Powielanie lawinowe (V przebicia >6Eg/e) p n - elektrony uzyskują energię - - + aby kreować pary elektron-dziura przez zderzenie nieelastyczne

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Wykład VI Zener diode

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Light is absorbed if ; EHP are created; electric field separates carriers Short-circuit (U = 0) - E C E V E C E V F 0 hf I sc = q N ph (Eg) I D (A) V D (V) I sc photodiode

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Open circuit ECEC EVEV ECEC EVEV qV bi qV OC I D (A) V D (V) V oc I d = Io [exp(eV oc /kT)-1] I sc – I d = 0 This current balances photogenerated current, I sc

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Solar cell Transfers solar energy into electric energy P = I x U=I 2 x R= U 2 /R

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LED GeSiGaAs

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Semiconductor laser

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