Integrated Circuit Devices Professor Ali Javey Summer 2009 PN Junctions Reading: Chapter 5

PN Junctions A PN junction is present in every semiconductor device. Donors N-type P-type – + I V I N P V Reverse bias Forward bias diode symbol A PN junction is present in every semiconductor device.

Energy Band Diagram and Depletion Layer of a PN Junction N-region P-region Ef (a) Ec Ec Ef (b) Ev Ev Ec Ef (c) A depletion layer exists at the PN junction. n  0 and p  0 in the depletion layer. Ev Neutral Depletion Neutral N-region layer P-region Ec Ef (d) Ev

Doping Profile of “Idealized Junctions”

Qualitative Electrostatics Band diagram Built in-potential From e=-dV/dx

Formation of pn junctions When the junction is formed, electrons from the n-side and holes from the p-side will diffuse leaving behind charged dopant atoms. Remember that the dopant atoms cannot move! Electrons will leave behind positively charged donor atoms and holes will leave behind negatively charged acceptor atoms. The net result is the build up of an electric field from the positively charged atoms to the negatively charged atoms, i.e., from the n-side to p-side. When steady state condition is reached after the formation of junction (how long this takes?) the net electric field (or the built in potential) will prevent further diffusion of electrons and holes. In other words, there will be drift and diffusion currents such that net electron and hole currents will be zero.

Equilibrium Conditions Under equilibrium conditions, the net electron current and hole current will be zero. E-field N-type P-type NA = 1017 cm3 ND = 1016 cm3 hole diffusion current net current = 0 hole drift current

Built-in Potential N-region P-region E c q V bi qB (b) E f E qA v

The Depletion Approximation We assume that the free carrier concentration inside the depletion region is zero. We assume that the charge density outside the depletion region is zero and q(Nd-Na) inside the depletion.

Field in the Depletion Layer On the P-side of the depletion layer,  = –qNa d E qN = - a dx e s qN ( x ) = E a ( x - x ) e p s E On the N-side,  = qNd qN = + E ( x ) d ( x x ) e n s

Field in the Depletion Layer The electric field is continuous at x = 0. Naxp = Ndxn A one-sided junction is called a N+P junction or P+N junction

Depletion Width

EXAMPLE: A P+N junction has Na=1020 cm-3 and Nd =1017cm-3 EXAMPLE: A P+N junction has Na=1020 cm-3 and Nd =1017cm-3. What is a) its built in potential, b)Wdep , c)xn ? Solution: a) b) c)

Reverse-Biased PN Junction

Forward Biased PN Junction

Junction Breakdown I Forward Current V , breakdown B voltage V Small leakage Current A Zener diode is designed to operate in the breakdown mode.

Quantum Mechanical Tunneling Potential energy barrier E x d

Tunneling Breakdown (a) Ec Dominant breakdown cause when both sides of a junction are very heavily doped. Ef Ev (b) Filled States - Empty States Ec Ev I (c) V Breakdown

Avalanche Breakdown impact ionization avalanche breakdown

The PN Junction as a Temperature Sensor What causes the IV curves to shift to lower V at higher T ?

Other PN Junction Devices–From Solar Cells to Laser Diodes Also known as photovoltaic cells, solar cells can convert sunlight to electricity with 15-30% energy efficiency

Solar Cells short circuit I Dark IV light N P I Eq.(4.9.4) - 0.7 V V E sc - 0.7 V V E Solar Cell c IV Eq.(4.12.1) Maximum E – I power-output v sc + (a) (b)

p-i-n Photodiodes Only electron-hole pairs generated in depletion region (or near depletion region) contribute to current Only light absorbed in depletion region contributes to generation Stretch depletion region Can also operate near avalanche to amplify signal

Light Emitting Diodes (LEDs) LEDs are typically made of compound semiconductors Why not Si