Bipolar Transistors

Question 1

I_DE ? I_RB ?

Answer 1

Electron current entering the Emitter

Hole current entering the Base

Question 2

Name all five current components

Answer 2

I_DE I_DB I_RE I_RB I_R

Question 3

B equation and meaning?

Answer 3

I_c/I_b = Common Emitter current gain

Question 4

Define Emitter Efficiency and its equation

Answer 4

Ratio of current injected into base to total emitter current

I_DB/(I_DB+I_R+I_DE)

Question 5

Equation for L_pb ? (length of base?)

Answer 5

L_pb = sqrt(D_pb * t_pb)

Question 6

Advantages of lateral and Vertical Scaling?

Answer 6

Lateral - Reduced Capacitance, Reduced Density
Photolithography
Vertical - Reduced base width, Increased operating speed
Increased risk of punchthrough

Question 7

Polysilicon Emitter - How does it offer increased current gain?

Answer 7

I_de - Increase effective emitter width - increases current gain.
Recombination rate of holes is reduced.
Hole concentration falls to zero over a greater distance.

Question 8

Heterojunction bipolar transistors - How does it increase current gain? (Hint - relate to B)

Answer 8

B = I_db/I_de
Therefore increasing I_db increases current gain.
Key is intrinsic carrier density
n_i = sqrt(N_c*N_v)*exp((E_g)/2kT)
Therefore decrease the bandgap - E_g

Make Base and Emitter out of different materials!

Question 9

How do you make a Heterojunction bipolar transistor?

Answer 9

Introduce Germanium into the base.

Question 10

How is the Si-Ge grown?

Answer 10

Must be grown Epitaxically

Question 11

What are the problems with growing Si-Ge?

Answer 11

Lattice constant of SiGe is greater than Si.
Desired outcome is strained Alloy growth - i.e. SiGe takes up lattice constant of Si.
If there is too much Ge - SiGe can relax and and dislocations result.

Question 12

What is D_pb?

Answer 12

hole diffusion acceleration (cm^2/s)

Question 13

What causes Base Spreading Resistance - r_bb?

Answer 13

Base current flows laterally across to emitter.
Cause voltages drop and is reduced away from base.

Current density decreases away from base contact.

Thus lateral resistance is created.

Question 14

What is the definition of Base Spreading Resistance - r_bb?

Answer 14

Base spreading resistance is the ratio of average voltage drop to the base current I_B.

Question 15

What is the equation of Base Spreading Resistance?

Answer 15

r_b = Average(V_b)/I_b

Question 16

I_E ?

Answer 16

DE + R + DB

Question 17

I_B ?

Answer 17

DE + R + RB

Question 18

How is a polysilicon emitter formed?

Answer 18

Polysilicon emitter is put onto substrate and is arsenic doped.
This arsenic is then driven in and the contact and n+ region is self-aligned,

Question 19

What are the consequences of mask misalignment?

Answer 19

Parasitic Capacitances and degrading transistor performance.

Question 20

Benefits of a polysilicon emitter

Answer 20

Self-aligned emitter:

1. no parasitic capacitances/resistances
2. Saves space
3. Effective emitter width is increased, I_DE is reduced and current gain increases.

Question 21

Width of depletion region - eqn?

Answer 21

W_EB - eqn given in eqn sheet

Question 22

What are the additional three terms for W_EB?

Answer 22

1) Epsilon_o and _Si
2) V_o - V_EB
3) ((NAE + NDB)/(NAE*NDB))

Question 23

`Base Transport factor equation

Answer 23

Ic/Idb

Question 24

Equation for base transit time

Answer 24

Tr = Wb^2/(2*Dpb)

Question 25

Equation for built in voltage (in eqn sheet)

Answer 25

Vo = kt/q(ln(N/ni^2)