FROM DOPED SEMICONDUCTORS TO SEMICONDUCTOR DEVICES Materials Doping Electronic bands Junctions Architectures Electronic and optical devices

Isomorphous substitution Isomorphous substitution

HYDROGENIC MODEL FOR DOPANT IONIZATION LEVELS IN EXTRINSIC SEMICONDUCTORS Hydrogenic doping model of n-donors and p-acceptors Insert a neutral P in place of a neutral Si in substitutional site of the diamond lattice Extra electron, which cannot be placed in VB, is loosely associated with this center Much the same way electron in Bohr H electron is attached to the central proton

HYDROGENIC MODEL FOR DOPANT IONIZATION LEVELS IN EXTRINSIC SEMICONDUCTORS Interaction energy is decreased by the effective dielectric constant of Si ( = 12) compared to vacuum ( = 1) Also the effective mass m* of e in Si is about 0.2me Bohr energies and radii thus need to be modified for n-doped Si to take these accounts into effect

EH = -e4m*/8o22h2n2 rH = n2h2o/e2m* HYDROGENIC MODEL FOR DOPANT IONIZATION LEVELS IN EXTRINSIC SEMICONDUCTORS EH = -e4m*/8o22h2n2 rH = n2h2o/e2m* Combined effect of higher  and lower m* for Si 1st IP of P donor being about 200 x less than H (0.044eV) 0.53 Å Bohr radius expands to ~ 12.5 Å spanning many Si atoms

Continuous Tuning of the Fermi Energy of a Semiconductor by Varying the Dopant Concentration

SILICON SOLAR CELL p-dope with extra B holes n-dope with extra P electrons

GaAs LED p-dope with extra Ga holes N-dope with extra As electrons

SCHOTTKY BARRIERS, SEMICONDUCTOR-METAL JUNCTIONS Electron energy level diagrams showing the formation of a Schottky barrier between a metal and an n-type semiconductor At equilibrium (open circuit), VS is the potential drop across the space charge region

SCHOTTKY BARRIERS, SEMICONDUCTOR-METAL JUNCTIONS VS = EF(M) -EF(SC) Schottky diode behavior under forward and reverse bias  

V Schottky Barrier SC-Au Junction Solar Cell Photovoltaic effect in n-Si/Au V CB VB Ef n Au n-Si Voc e hv h Au interface n-Si Voc determined by E(F)Si - E(F)Au and photon to electron conversion efficiency  depends on efficiency of e-h separation (current generation) versus h - e recombination

INCREASING THE NUMBER OF JUNCTIONS, THE n-p-n JUNCTION TRANSISTOR By extending the concept of the single p-n junction diode we can move to the double junction n-p-n junction transistor, and see how amplification and switching action naturally emerges. Basis of the monumental discovery by Brattain, Schockly and Bardeen at Bell Laboratories in 1948, followed shortly after by the integrated circuit of Groves, also at Bell Beginning of the global information revolution

INCREASING THE NUMBER OF JUNCTIONS, THE n-p-n JUNCTION BIPOLAR TRANSISTOR Electrons are effectively pumped from the left n-region under forward bias through the p-region to the right n-region under reverse bias The same number of electrons are involved but their energy has been amplified from IV1 to IV2 that drives them round the circuit

INCREASING THE NUMBER OF JUNCTIONS, THE n-p-n JUNCTION BIPOLAR TRANSISTOR Voltage amplification V2/I = R2 V1/I = R1 V2/V1 = R2/R1   Can also function as a current amplifier and an electrical switch (bias voltage on base)

BAND DIAGRAM FOR n-p-n JUNCTION BIPOLAR TRANSISTOR Tuning the bending of the CBs and VBs in a two junction SC device by voltage biasing to create an electron pump!!! Forward and reverse voltage biasing causes the electrons to be pumped from the left n-type emitter to right n-type collector passing through the p-type base

METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR MOS-FET MOS-FETs are basic electronic switches found in silicon chips Each MOS-FET can be switched current on or current off, representing a "1" or "0", known as binary code All computation is done using different combinations of these two outputs to do calculations Modern chips contain millions of transistors allowing them to execute millions of calculations per second p-Si n-Si oxide

METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR MOS-FET The tiny MOS-FET devices consist of a conducting source, drain and gate with a metal oxide insulator between the gate and semiconductor p-channel MOS-FET - a threshold positive voltage applied to the gate repels holes from p-channel and allows electrons to flow between source and drain electrodes and thereby turns the device on at a critical gate voltage Removing gate voltage switches it off p-Si n-Si oxide

Need long excited state lifetime for inversion e-h radiative recombination 100% reflecting <100% reflecting

Narrow Eg(GaAs) sandwiched by wide Eg(n,p-AlxGa1-xAs) – Eg composition tuning QSE tuning of Eg