Lecture 11 OUTLINE pn Junction Diodes contd Narrowbase

Lecture 11 OUTLINE • pn Junction Diodes (cont’d) – Narrow-base diode – Junction breakdown Reading: Pierret 6. 3. 2, 6. 2. 2; Hu 4. 5

Introduction • The ideal diode equation was derived assuming that the lengths of the quasi-neutral p-type & n-type regions (WP’ , WN’) are much greater than the minority-carrier diffusion lengths (Ln , Lp) in these regions. Excess carrier concentrations decay exponentially to 0. Minority carrier diffusion currents decay exponentially to 0. • In modern IC devices, however, it is common for one side of a pn junction to be shorter than the minority-carrier diffusion length, so that a significant fraction of the “injected” minority carriers reach the end of the quasi-neutral region, at the metal contact. Recall that Dp = Dn = 0 at an ohmic contact In this lecture we re-derive the diode I-V equation with the boundary condition that Dp = 0 at a distance xc’ (rather than ) from the edge of the depletion region. EE 130/230 A Fall 2013 Lecture 11, Slide 2

Excess Carrier Distribution (n side) • From the minority carrier diffusion equation: • For convenience, let’s use the coordinate system: x’’ 0 0 x’ xc ' • So the solution is of the form: • We have the following boundary conditions: EE 130/230 A Fall 2013 Lecture 11, Slide 3

• Applying the boundary conditions, we have: • Therefore • Since this can be rewritten as • We need to take the derivative of Dpn’ to obtain the hole diffusion current within the quasi-neutral n region: EE 130/230 A Fall 2013 Lecture 11, Slide 4

Evaluate Jp at x=xn (x’=0) to find the injected hole current: Thus, for a one-sided p+n junction (in which the current is dominated by injection of holes into the n-side) with a short n-side: EE 130/230 A Fall 2013 Lecture 11, Slide 5

and Therefore if xc’ << LP: For a one-sided p+n junction, then: EE 130/230 A Fall 2013 Lecture 11, Slide 6

Excess Hole Concentration Profile If xc’ << LP: Dpn(x) Dpn is a linear function: slope is constant 0 Jp is constant 0 x'c x' (No holes are lost due to recombination as they diffuse to the metal contact. ) EE 130/230 A Fall 2013 Lecture 11, Slide 7

General Narrow-Base Diode I-V • Define WP‘ and WN’ to be the widths of the quasi-neutral regions. • If both sides of a pn junction are narrow (i. e. much shorter than the minority carrier diffusion lengths in the respective regions): e. g. if hole injection into J the n side is greater than electron injection JN into the p side: JP -xp EE 130/230 A Fall 2013 Lecture 11, Slide 8 xn x

Summary: Narrow-Base Diode • If the length of the quasi-neutral region is much shorter than the minority-carrier diffusion length, then there will be negligible recombination within the quasi-neutral region and hence all of the injected minority carriers will “survive” to reach the metal contact. – The excess carrier concentration is a linear function of distance. For example, within a narrow n-type quasi-neutral region: Dpn(x) 0 xn location of metal contact (Dpn=0) x WN ’ The minority-carrier diffusion current is constant within the narrow quasi-neutral region. Shorter quasi-neutral region steeper concentration gradient higher diffusion current EE 130/230 A Fall 2013 Lecture 11, Slide 9

pn Junction Breakdown C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 4 -10 Breakdown voltage, VBR VA A Zener diode is designed to operate in the breakdown mode: EE 130/230 A Fall 2013 Lecture 11, Slide 10

Review: Peak E-Field in a pn Junction E(x) -xp xn E(0) For a one-sided junction, where N is the dopant concentration on the lightly doped side EE 130/230 A Fall 2013 Lecture 11, Slide 11 x

Breakdown Voltage, VBR • If the reverse bias voltage (-VA) is so large that the peak electric field exceeds a critical value ECR, then the junction will “break down” (i. e. large reverse current will flow) • Thus, the reverse bias at which breakdown occurs is EE 130/230 A Fall 2013 Lecture 11, Slide 12

Avalanche Breakdown Mechanism R. F. Pierret, Semiconductor Device Fundamentals, Figure 6. 12 High E-field: if VBR >> Vbi Low E-field: ECR increases slightly with N: For 1014 cm-3 < N < 1018 cm-3, 105 V/cm < ECR < 106 V/cm EE 130/230 A Fall 2013 Lecture 11, Slide 13

Tunneling (Zener) Breakdown Mechanism Dominant breakdown mechanism when both sides of a junction are very heavily doped. VA = 0 VA < 0 Ec Ev Typically, VBR < 5 V for Zener breakdown EE 130/230 A Fall 2013 Lecture 11, Slide 14 C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 4 -12

Empirical Observations of VBR R. F. Pierret, Semiconductor Device Fundamentals, Figure 6. 11 • VBR decreases with increasing N • VBR decreases with decreasing EG EE 130/230 A Fall 2013 Lecture 11, Slide 15

VBR Temperature Dependence • For the avalanche mechanism: – VBR increases with increasing T, because the mean free path decreases • For the tunneling mechanism: – VBR decreases with increasing T, because the flux of valence-band electrons available for tunneling increases EE 130/230 A Fall 2013 Lecture 11, Slide 16

Summary: Junction Breakdown • If the peak electric field in the depletion region exceeds a critical value ECR, then large reverse current will flow. This occurs at a negative bias voltage called the breakdown voltage, VBR: where N is the dopant concentration on the more lightly doped side • The dominant breakdown mechanism is avalanche, if N < ~1018/cm 3 tunneling, if N > ~1018/cm 3 EE 130/230 A Fall 2013 Lecture 11, Slide 17