15 - ECE331 Intrinsic Transport in Doped Semiconductors At...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Lu ECE331 Intrinsic Transport in Doped Semiconductors At room temperature MOST of the carriers in extrinsic semiconductors are provided by the DOPANTS whose ionization energy is much SMALLER than the gap energy * At HIGHER temperatures however more and more carriers are excited across the energy gap and these eventually SWAMP the carriers provided by the dopants The extrinsic semiconductor then LOOKS like an INTRINSIC one 200 400 600 1 2 n / N D TEMPERATURE (K) FREEZE OUT EXTRINSIC INTRINSIC n i / N D LOW TEMPERATURES ROOM TEMPERATURE HIGH TEMPERATURES Lu ECE331 Carrier Drift in an Electric Field Having developed techniques for describing the statistics of carriers in semiconductors we would now like to consider the nature of carrier DYNAMICS under a variety of conditions * We first consider the DRIFT of carriers that is induced by the application of an ELECTRIC FIELD across the semiconductor * The electric field superimposes a slow net DRIFT on top of the RAPID but RANDOM motion that the charge carriers execute due to their THERMAL energy AT ROOM TEMPERATURE ELECTRONS AND HOLES IN A SEMICONDUCTOR UNDERGO A RANDOM THERMAL MOTION THAT IS GENERATED AS THE CARRIERS SCATTER FROM VARIOUS IMPERFECTIONS IN THE CRYSTAL STRUCTURE IN SPITE OF THIS RAPID MOTION THE NET CURRENT FLOWING THROUGH THE CRYSTAL IS ZERO IN THE ABSENCE OF AN APPLIED ELECTRIC FIELD WITH AN ELECTRIC FIELD APPLIED ACROSS THE CRYSTAL HOWEVER A SLOW DRIFTING MOTION IS SUPERIMPOSED ON THE RANDOM CARRIER WALKS GIVING RISE TO A NET CURRENT FLOW t o t o + t v d ×∆ t
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Lu ECE331 Carrier Drift in an Electric Field A simple classical analysis suggests that the DRIFT VELOCITY v d accumulated by the electron should vary LINEARLY with the magnitude of applied electric field E * If we assume that carriers in the crystal scatter after some average RELAXATION TIME τ their drift velocity may be easily estimated ) 1 . 7 ( * E v p d m q p = 10 5 10 6 10 7 1000 10 4 10 5 10 6 v d (cm/sec) E (V/cm) ELECTRONS HOLES Si, 300 K EQUATIONS 7.1 & 7.2 PREDICT A LINEAR DEPENDENCE OF THE DRIFT VELOCITY ON ELECTRIC FIELD WHICH IS INDEED FOUND AT MODERATE ELECTRIC FIELDS AT HIGHER FIELDS HOWEVER THE ELECTRON AND HOLE DRIFT VELOCITIES SATURATE AT THE SAME VALUE THE SATURATION OCCURS ONCE THE DRIFT VELOCITY BECOMES COMPARABLE TO THE THERMAL VELOCITY OF THE CARRIERS IN THIS REGIME ADDED ENERGY IMPARTED BY THE FIELD IS TRANSFERRED TO THE LATTICE RATHER THAN INCREASING THE CARRIER VELOCITY ) 2 . 7 ( * E v n d m q n = Lu ECE331 Carrier Mobility In MOST situations we will be interested in however Equations 7.1 & 7.2 generally hold and we therefore introduce the carrier quantity known as
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 7

15 - ECE331 Intrinsic Transport in Doped Semiconductors At...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online