Technische Universität Graz Institute of Solid State Physics Schottky diodes JFETs - MESFETs - MODFETs
Quasi Fermi level When the charge carriers are not in equilibrium the Fermi energy can be different for electrons and holes. EFn E c n Nc exp kt B Ev EFp p Nv exp kt B
Technische Universität Graz Institute of Solid State Physics metal - semiconductor contacts Photoelectric effect Workfunction Electron affinity Interface states Schottky barriers Schottky diodes Ohmic contacts Thermionic emission Tunnel contacts
Photoelectric effect hf 0 = e at threshold current workfunction f threshold frequency f 0
Singh There is a dipole field at the surface of a metal. This electric field must be overcome for an electron to escape.
work function - electron affinity If s < m, the semiconductor bands bend down. If s > m, the semiconductor bands bend up.
Singh
p-type Walter Schottky Schottky contact / ohmic contact E F,m metal E F,s Schottky contact E F,m metal E F,s Ohmic contact: linear resistance specific contact resistance: R c J V 1 -cm 2
n-type Schottky contact / ohmic contact E F,s Schottky contact E F,m metal E F,m metal E F,s Ohmic contact: linear resistance specific contact resistance: R c J V 1 -cm 2
Interface states
http://www.springermaterials.com/navigation/#n_240905_silicon+%25
Schottky barrier en D E xxn r 0 W x n 2 Vbi en V D V en D x 2 2 xx n
CV measurements x p 2 Vbi en V A e NA C x 2 V V p 2 2 1 bi C bi V V e N A -2 F m GaAs has larger E g and V bi 1/C 2 ev V Nv ln ( T k T ) N A bi b B
Thermionic emission 1901 Richardson Owen Willans Richardson Current from a heated wire is: J A 2 RT exp e kt B Some electrons have a thermal energy that exceeds the work function and escape from the wire.
diode Vacuum diodes
Thermionic emission Fermi function ev bi V E F EF E E F E E f( E) exp exp exp exp kt B kt B kt B kt B The density of electrons with enough energy to go over the barriers E exp kt B
Thermionic emission n th ev exp kt B I sm ev nth exp kt B I ( 0) ms Ism V ev kt B I Ism Ims Ims e 1
Schottky barrier I I sm > I sm ~ 0 ms I ms constant
Thermionic emission ev B I I e kt sm Ims I s 1 Nonideality factor = 1
Thermionic emission I s * e 2 b AART exp kt B A = Area A R* = Richardson constant n-si A * R = 110 A K -2 cm -2 p-si A * R = 32 A K -2 cm -2 n-gaas A * R = 8 A K -2 cm -2 p-gaas A * R = 74 A K -2 cm -2 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 B I I e kt s 1
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.
Technische Universität Graz Institute of Solid State Physics 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 N A >> N D 2 ( ) x n Vbi V en D Depletion mode h x n 2Vbi en D conducting at V g = 0 Enhancement mode h x n 2V bi en D nonconducting at V g = 0
depletion zone n-channel (power) JFET
Power SiC JFET p n p
n-channel JFET drain p+ n+ depletion region p n p gate n+ source JFETs are often discrete devices
MESFET Metal-Semiconductor Field Effect Transistors n Depletion layer created by Schottky barrier x n 2 ( Vbi V) en D 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 S n-channel JFET x n 2 ( Vbi V) en D G S p-channel JFET 2 Pinch-off at h = x en n Dh Vp 2 V p = pinch-off voltage At Pinch-off, V p = V bi -V.
The drain is the side of the transistor that gets pinched off. JFET