EDIT School 2024

Silicon Track Basics

1

Silicon Basics

• Silicon detectors are typically high resistivity >1 KW-cm “float zone” silicon
• The small energy gap between impurity “donor” or “acceptor” levels means most mobile electrons and holes are due to dopants (at RT)

This is why we are not overwhelmed with current when a bias is applied

​

2

Intrinsic

n-type

p-type

band

diagram

density of

states

Fermi-dirac

distribution

carrier

concentrations

Silicon Diode

A silicon diode is a semiconductor device that allows electricity to flow in one direction, while blocking it in the opposite direction. Silicon diodes are commonly used in electronic circuits and devices because of their versatility and unique properties. 

​

Here are some key properties of silicon diodes:

• How they work: A silicon diode has a p-type (positive) and n-type (negative) semiconductor material junction, called the "depletion region". When a positive voltage is applied to the p-side, the barrier decreases, allowing current to flow. When a negative voltage is applied, the barrier increases, blocking current. 
• Forward voltage drop: A small voltage drop across the diode junction when forward biased. 
• Breakdown voltage: The voltage before the diode starts conducting in reverse. 

​

3

Charge Drift and Collection

Diode depletion

• Silicon detectors have lightly doped bulk (usually n) and heavily doped contacts. Unusually large depleted area.
• Diffusion of charge carriers will form a local depleted region with no applied voltage
• As we apply external bias the depletion region (and the internal field) grows until it hits the bottom.

​

4

Very asymmetric doping in �our devices

pn junction�no external

fields

Device Characteristics

5

R. Lipton

Resistivity:

Depletion :

Electric Field:

electron, hole mobility

Effective carrier concentration

x = distance from junction D = silicon thickness

Device Testing - CV

There are a few standard tests which measure the basic properties of silicon detectors

• Detectors are normally operated fully depleted – all impurities ionized. This leaves a “space charge” region depleted of charge carriers that acts like a dielectric in the junction/ohmic capacitor

Assume this acts as a parallel plate capacitor (diode test structures):

​

​

6

R. Lipton

Device Testing - IV

A high quality detector will also have low leakage current – current driven by carrier generation in the depleted bulk.

• Strongly temperature dependent
• Avalanche breakdown at 3x10⁵ V/cm

Measure V_bias vs I curves

• Look for anomalous currents
• Breakdown voltage

Current can be affected by surface defects, problems with the ohmic contact, cut edge currents, impurities (especially metals), and physical

damage

7

R. Lipton

Test structures

Rapid rise until

depletion

Carrier Drift

8

R. Lipton

Electron, hole drift:

Drift time to an electrode:

Signal Shapes

9

R. Lipton

Spieler

Weighting potential for a 300 μm thick strip detector 50μm pitch

Simulation

A silicon detector is rather complex. There are commercial “TCAD” packages that can simulate silicon-based devices utilizing finite element analysis using the detailed semiconductor equations:

• Internal fields
• Coupling to electrodes
• Charge diffusion and drift
• Traps
• Surface effects

​

10

R. Lipton

Strip Detector

• Bias resistor integrated on each strip usually implanted polysilicon resistor
• Coupling capacitor formed by very thin (2500 A) dielectric. The dielectric is usually a “grown thermal” oxide supplemented with deposited layers formed by CVD (chemical vapor deposition)
• In addition there are “guard rings” which keep the field from reaching the damaged silicon near cut edges.
• “Microdischarge” breakdown can occur when fields near the implant increase due to charges in the oxide and potential of the coupling aluminization

​

11

R. Lipton

Si

n-

p+

n+

oxide

junction

Ohmic

contact

n, p and junctions

Silicon can be doped with donor or �acceptor impurities. This �corresponds to n (usually �phosphorus) or p (boron) type �silicon.

• The contact between the heavily� doped and lightly doped regions �provide the junction (p-n) and ohmic (n-n⁺ or p-p⁺) contacts
• Junction contacts are high field (possible breakdown) regions
• Ohmic contacts need to be heavily doped for a proper metal- semiconductor contact

12

R. Lipton Higgs Factory Workshop[ 11/16/2012

Semiconductor Surface

The surface is a crucial part of the detector design

• Junction side must be polished and defect-free
• Field must be ~0 at damaged cut edge
• Silicon dioxide used as insulator and passivation can accumulate charge (usually positive)
• An electron accumulation layer builds up near the positively charged silicon/silicon dioxide interface – this acts as an insulator on a p+/n or a conductor on a n+/p junction – need an additional “p-stop” to electrically isolate n strips on p conductor
• Detectors can also be sensitive to charge attracted to the surface – affected by humidity

​

13

R. Lipton Higgs Factory Workshop[ 11/16/2012

Electron concentration

1e11 oxide charge

n-type

Detector�Layout

14

R. Lipton

bias

resistor

DC test

pad

coupling

capacitor

aluminization

Guard

ring

bond

pad

strip

implant

Bias line

100μm

Wirebonding

Aluminum ultrasonic welding or gold thermocompression

• Aluminum wedge bonding achieves the finest pitch and does not need elevated temperature
• This is a bit of an art – getting good bonds depends on machine, operator, and especially the surface properties of the materials

​

​

​

15

R. Lipton

Interconnect

How we connect the detectors to the electronics, cool them, and mount them is increasingly important

• Interconnect is a common point of failure
• Determines pitch
• Can be expensive
• Adds capacitance and inductance
• Can determine geometry

​

​

16

R. Lipton

Hybrid Pixel Interconnect using bump bonds