2 The PN-JunctionOne of the simplest bipolar devices, important for the understanding of more complex devices (bipolar = both electrons and holes contribute to device characteristics).
3 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
5 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 m3p-type NA acceptor atoms per m3Consider temperatures ~300K Almost all donor and acceptor atoms are ionised.impurity atoms m-3NADxa= 0ptypen(x) = N(x>0) = 0 (x<0)(x<0) = 0 (x>0)p-n interface at x=0.
6 Electron and hole transfer n-type semiconductorp-type semiconductorECVmElectronsHolesElectron and hole transferConsider bringing into contact p-type and n-type semiconductors.n-type semiconductor: Chemical potential, m (Fermi level) below bottom of conduction bandp-type semiconductor: Chemical potential, m above top of valence band.Electrons diffuse from n-type into p-type filling empty valence states.
7 Band BendingElectrons 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
8 JunctionAt equilibrium the Fermi level gradient equals zero!
10 The principle working of a pn-junction P-dopedN-dopedPositively charged holes + negatively charged immobile acceptorsNegatively charged electrons + positively charged immobile donorsP-dopedN-dopedholes-+electronsNo electrons or holes, only charged donors/acceptors (DEPLETION LAYER)
11 The principle working of a pn-junction +-electronsholesNo VoltageP-dopedN-dopedIIV characteristics :Current+-electronsholesForward biascurrentVoltage“No” current(Leakage current)Large current-+electronsholesReverse bias“no” currentCircuit symbol:
12 Ec No bias Ev Forward Ec bias Reverse Ec bias Drift (thermally exc.) Diffusion (E-field)e-e-EcNo biasEvForwardbiasEcjVReversebiasEcVV