Avalanche breakdown Impact ionization causes an avalanche of current Occurs at low doping Zener tunneling Electrons tunnel from valence band to conduction band Occurs at high doping Tunneling wave decays exponentially in the classically forbidden region e1>e2 Tunneling is a wave phenomena. Tunneling and total internal reflection are used in a beam splitter. http://lampx.tugraz.at/~hadley/physikm/apps/snell/snell.en.php Zener tunneling Breakdown voltage is typically much lower than the breakdown voltage of an avalanche diode and can be tuned by adjusting the width of the depletion layer. Used to provide a reference voltage. Avalanche breakdown Tunneling Institute of Solid State Physics Technische Universität Graz metal - semiconductor contacts Photoelectric effect Workfunction Electron affinity Interface states Schottky barriers Schottky diodes Ohmic contacts Thermionic emission Tunnel contacts Photoelectric effect current hf0 = ef at threshold workfunction f threshold frequency f0 There is a dipole field at the surface of a metal. This electric field must be overcome for an electron to escape. Singh work function - electron affinity If fs < fm, the semiconductor bands bend down. If fs > fm, the semiconductor bands bend up. work function - electron affinity Electrons flow from a low work function material to high work function material. The high work function material becomes negatively charged. You have to push the electrons uphill into the low work function material. This determines the band bending. If fs < fm, the semiconductor bands bend down. If fs > fm, the semiconductor bands bend up. Singh p-type Walter Schottky Schottky contact / ohmic contact EF,m E metal F,s Schottky contact EF,s EF,m metal Ohmic contact: linear resistance 1 J 2 Rc -cm specific contact resistance: V n-type Schottky contact / ohmic contact Schottky contact EF,s EF,m metal EF,m metal EF,s Ohmic contact: linear resistance 1 J 2 specific contact resistance: Rc -cm V Interface states http://www.springermaterials.com/navigation/#n_240905_Silicon+%2528Si%2529 Schottky barrier Vbif s f m eN 2 D 2e Vbi V eND x E xx n W x V xx 0 xx e e n n n r 0 eND e 2 Like a one sided junction, the metal side is heavily doped. CV measurements 2e Vbi V xp eN A e ee N C A F m-2 xp2 VV bi 2 C 1/ 1 2Vbi V 2 C eNe A V GaAs has larger Eg and Vbi Thermionic emission 1901 Richardson Owen Willans Richardson Current from a heated wire is: 2 ef J ATR exp kB T Some electrons have a thermal energy that exceeds the work function and escape from the wire. Vacuum diodes diode Thermionic emission eV bi V Fermi function EF EE E E E f( E ) expF exp F exp exp kTB kT BB kT kT B E The density of electrons with enough energy to go over the barriers exp kB T Thermionic emission eV nth exp kB T eV Ism n th exp kB T Ims IV sm ( 0) eV II I I ekB T 1 sm ms s Schottky barrier Ism ~ 0 Ism > Ims Ims constant Thermionic emission eV II I I ekB T 1 sm ms s Nonideality factor = 1 Thermionic emission * 2 efb Is AAT R exp kB T A = Area * AR = Richardson constant * -2 -2 n-Si AR = 110 A K cm * -2 -2 p-Si AR = 32 A K cm * -2 -2 n-GaAs AR = 8 A K cm * -2 -2 p-GaAs AR = 74 A K cm Thermionic emission dominates over diffusion current in a Schottky diode. Schottky diodes Majority carrier current dominates. nonideality factor = 1. Fast response, no recombination of electron-hole pairs required. Used as rf mixers. Low turn on voltage - high reverse bias current eV I I ekB T 1 s Tunnel contacts For high doping, the Schottky barrier is so thin that electrons can tunnel through it. metal p+ p Degenerate doping at a tunnel contact metal n+ n Tunnel contacts have a linear resistance. Contacts Transport mechanisms Drift Diffusion Thermionic emission Tunneling All mechanisms are always present. One or two transport mechanisms can dominate depending on the device and the bias conditions. In a forward biased pn-junction, diffusion dominates. In a tunnel contact, tunneling dominates. In a Schottky diode, thermionic emission dominates. Institute of Solid State Physics Technische Universität Graz JFETs - MESFETs - MODFETs Junction Field Effect Transistors (JFET) Metal-Semiconductor Field Effect Transistors (MESFET) Modulation Doped Field Effect Transistors (MODFET) n JFET n-channel JFET n For NA >> ND 2(e Vbi V ) xn eND 2eV bi conducting at Vg = 0 Depletion mode h xn eND 2eV bi nonconducting at V = 0 Enhancement mode h xn g eND Power SiC JFET p p n n-channel (power) JFET depletion zone n-channel JFET drain n+ p+ depletion region p n p gate n+ JFETs are often discrete devices source MESFET Metal-Semiconductor Field Effect Transistors n Depletion layer created by Schottky barrier 2(e Vbi V ) xn eND Fast transistors can be realized in n-channel GaAs, however GaAs has a low hole mobility making p-channel devices slower. JFET n D n-channel JFET D G 2(e V V ) G S x bi n-channel JFET n eN S D p-channel JFET eN h2 Pinch-off at h = xn V D V = pinch-off voltage p 2e p At Pinch-off, Vp = Vbi -V. JFET The drain is the side of the transistor that gets pinched off. .