Semiconducting junctions Lecture 12 Semiconducting junctions

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).

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

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

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 ND donor atoms per m3 p-type NA acceptor atoms per m3 Consider temperatures ~300K Almost all donor and acceptor atoms are ionised. impurity atoms m - 3 N A D x a = 0 p type n (x) = N (x>0) = 0 (x<0) (x<0) = 0 (x>0) p-n interface at x=0.

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

Band Bending 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

Junction At equilibrium the Fermi level gradient equals zero!

p-n junction I IV characteristics :

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

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

Ec No bias Ev Forward Ec bias Reverse Ec bias Drift (thermally exc.) Diffusion (E-field) e- e- Ec No bias Ev Forward bias Ec j V Reverse bias Ec V V

Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT)) I-V Characteristics Hole current: diffusion Ipd = C1Npexp (-eVbi/(kT)) drift Ipu = CNpn = Ipd = C1Npexp (-eVbi/(kT)) at forward bias IpF = C1 Np exp (-e(Vbi- V) /(kT)) Ip = IpF - Ipu = C1Np exp (-e(Vbi- V) /(kT)) – C1Np exp (-eVbi/(kT)) = C1Npexp [-eVbi/(kT)][exp(eV/(kT)-1] =Ipd [exp(eV/(kT))-1] Electron current: In = Ind [exp(eV/(kT))-1 with Ind = C2Nn exp (-eVbi/(kT)) I = Io [exp(eV/(kT)-1] Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT))

Rectifier Ac transfers into dc a) b) I t

photodiode Based on the photovoltaic effect -solar cell -photodetectors

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

Zener diode Wykład VI

photodiode Short-circuit (U = 0) Isc = q Nph(Eg) Light is absorbed if ; EHP are created; electric field separates carriers Short-circuit (U = 0) ID (A) VD (V) - E C V EF hf Isc Isc = q Nph(Eg)

photodiode Open circuit Voc Id = Io [exp(eVoc /kT)-1] ID (A) VD (V) EC EV qVbi qVOC Voc Id = Io [exp(eVoc /kT)-1] This current balances photogenerated current, Isc Isc – Id = 0

Solar cell Transfers solar energy into electric energy P = I x U=I2 x R= U2/R

LED Ge Si GaAs

Semiconductor laser