week3-1

1

• Lectures 6, 7 and 8
• Transistor Function
• Jan. 17, 20 and 22, 2003

2
Topics

• Previous
• Transistor as switch
• Basic fabrication steps.
• Basic Transistor function
• Basic transistor behavior.

3
Transistor structure

• n-type transistor

4
Gate voltage and the channel
Vs0 V
Vt 0.7 V
gate
drain
source
Vgs gt Vt
Id
gate
drain
source
Vgs lt Vt
5
Drain current characteristics
6
Example 1

• V gs keeps 1.5V, 2.5V, 5V
• V ds changes

7
Drain current

• Linear region (Vds lt Vgs - Vt) !!!
• Id k (W/L)(Vgs - Vt)Vds - 0.5 Vds2)
• Saturation region (Vds gt Vgs - Vt)
• Id 0.5k (W/L)(Vgs - Vt) 2
• Text book is wrong!!!

8
What is related to the characteristics

• W/L ratio!!!
• V between Drain and Source
• V between Gate and Source
• Threshold voltage
• Material

9
Transistor layout

• n-type (tubs may vary)

L
w
10
0.5 ?m transconductances

• From a MOSIS process
• n-type
• kn 73 ?A/V2
• Vtn 0.7 V
• p-type
• kp 21 ?A/V2
• Vtp -0.8 V

11
Current through a transistor

• Use 0.5 ?m parameters. Let W/L 3/2. Measure at
  boundary between linear and saturation regions.
• Vgs 2V
• Id 0.5k(W/L)(Vgs-Vt)2 93 ?A
• Vgs 5V
• Id 1 mA

12
Explanation

• When V gs 2V, can current go beyond 93 uA (when
  V ds changes)?

13
Example 2

• When Vgs 2 V and V ds 2 V and 1.2 V
  respectively, calculate the I ds?
• W/L 8
• Kn 73 uA/V2

14
Example 3

• Inverter when transition time, what is the
  current? (important example)
• How N and P work?
• Inverter after transition time, what is the
  current?

15
Questions

• What is the real carrier in N transistor?

16
Gate voltage and the channel
Vs5 V
Vt -0.8 V
gate
drain
source
Vgs gt Vt
Id
gate
drain
source
Vgs lt Vt
17
Questions

• What is the real carrier in P transistor?

18
Transistor and digital Logic

• Combinational logic
• Depend on transistor inverter
• Sequential logic
• Depend on transistor inverter

19
Drain current characteristics
20
Review

• N and P transistors as switches
• Fabrication process
• Current Characteristics of N and P transistors

21
Example 4
22

• Lecture 9
• Transistor Parasitics Latch Up
• Jan. 24, 2003

23
Topics

• Transistor and basic fabrication steps.
• Basic Transistor function
• Basic transistor behavior.
• Latch up.

24
Basic transistor parasitics

• Gate to substrate, also gate to source/drain.
• Source/drain capacitance, resistance.

25
Basic transistor capacitance

• Gate capacitance Cg. Determined by active area.
• Source/drain overlap capacitances Cgs, Cgd.
  Determined by source/gate and drain/gate
  overlaps. Independent of transistor L.
• Cgs Col W
• Gate/bulk overlap capacitance.

26
Transistor gate capacitance

• Gate-source/drain overlap capacitance

gate
source
drain
overlap
27
Transistor source/drain parasitics

• Source/drain have significant capacitance,
  resistance.
• Measured same way as for wires.
• Source/drain R, C may be included in Spice model
  rather than as separate parasitics.

28
Transistor Resistance

• RU/I
• Future topic

29
Latch-up

• CMOS ICs have parasitic silicon-controlled
  rectifiers (SCRs).
• When powered up, SCRs can turn on, creating
  low-resistance path from power to ground. Current
  can destroy chip.
• Early CMOS problem. Can be solved with proper
  circuit/layout structures.

30
Parasitic SCR
circuit
I-V behavior
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Parasitic SCR structure
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Solution to latch-up

• Use tub ties to connect tub to power rail. Use
  enough to create low-voltage connection.

33
Tub tie layout
p
metal (VDD)
p-tub
34
Question
Why these tub ties are needed?
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