Bipolar Junction Transistors

Question 1

Describe the Principle of Operation of a BJT

Answer 1

For a pnp device, illumination of reverse bias p-n junction produces EHPs. Holes on n-type side drift to p side due to built-in field of the junction. This forms a current independent of applied voltage, aka a photodiode. A second forward-biased p-n junction is added to do so for all minority carries.

Question 2

Describe the doping levels of the base, emitter and collector

Answer 2

Emitter is Heavily Doped

Base is less doped than the emitter, but more than the collector.

Collector is lightly doped.

Question 3

Describe the Operation of A PNP BJT in forward Active Mode

Answer 3

The emitter-base junction is forward biased in this mode, whilst the base-collector junction is reverse biased.

Holes are injected from the emitter to the base and electrons are injected from the base to the collector. This generates a large emitter current. In turn Due to the reverse-biased junction at the Collector-base boundary, a small reverse saturation current is produced. An additional electron flow is generated from the base to the emitter.

Under ideal conditions we assume that there is no recombination current in the base as the holes pass through.

Question 4

Describe the operation of an npn BJT in normal active mode

Answer 4

The same as pnp with reverse polarities

Question 5

Calculate Emitter Current

Answer 5

I_E = I_En +I_Ep

I_En is electrons injected from base to emitter

I_Ep is holes injected from base-emitter.

Question 6

Define Emitter Injection Efficiency

Answer 6

The current generated due to hole flow from the emitter divided by the total emitter current.

Question 7

What is the base transport factor aka B?

Answer 7

Fraction of injected holes that travel successfully across the base to the collector.

This can be affected by the gap between the transistor and the width of the base, too wide a base and recombination occurs before the electrons leave the base. The same rule occurs with too target a gap between the base and the collector

Question 8

What is the collector current.

Answer 8

In general,

I_C = BI_Ep + I_CB0

I_CBO being the reverse saturation current

However, as this current is small we normally ignore it.

Question 9

What is the current transfer ratio aka Common Base Current Gain 𝛼?

Answer 9

This is the ratio between the emitter current and the collector current. It is normally indicated as 𝛼.

Question 10

Draw the band diagram of a BJT at thermal equilibrium

Answer 10

Question 11

Describe common-base configuration

Answer 11

The base lead is common to the input and output of the circuit.

No current amplification in this configuration, with a good transistor.

However, if R_in is low, R_out is made high then because

I_E≈ I_C in a well-designed transistor, the voltage is amplified.

Question 12

The equation for voltage amplification in common-base configuration

Answer 12

Question 13

Describe the common emitter configuration

Answer 13

The emitter is the common terminal between the input and outputs of the circuit

Question 14

What is the base current?

Answer 14

Can be defined as the total of all currents which are injected by the base.

Question 15

What is common-emitter current gain?

Answer 15

Current amplification between collector current and base current.

Question 16

What equations are used to calculate the current and voltage amplifications?

Answer 16

Question 17

What are the key features of a basic analogue n+pn bipolar integrated circuit transistor?

Answer 17

1. All three contacts on the front
2. Buried n+ layer to provide low resistance path from active area to collector
3. Electrical Isolation-reverse biased on junctions
4. Highly doped regions just below Emitter and Collector contacts

Question 18

Steps For Fabrication of A Basic analogue n+pn bipolar integrated circuit transistor

Answer 18

1. Buried layer formation (arsenic or antimony implant)
2. Epitaxial deposition of n-type collector
3. Isolation formation (boron implant and drive-in)
4. Base region formation (boron implant and drive-in)
5. Emitter and Collector region formation (phosphorus implant and drive-in)
6. Contact Formation (Al deposition, etching, then anneal in H2/N2ambient)

Question 19

Describe the Key Features of a high speed n+pn bipolar circuit transistor

Answer 19

1. Oxide isolation over pn junctions

• Lower parasitic capacitance allowing for faster switching
• Takes less silicon area -> improving packing densities

2. Self-aligned process means minimal separation between base and emitter. Specified by the thickness of oxide spacer.

3. Use of polysilicon for a heavily doped ‘extrinsic’ p+ base forms a collar around the lightly doped intrinsic base. This lowers the base series resistance.

Question 20

Draw a pnp transistor in forward active mode

Answer 20

Question 21

How do you calculate the cut off frequency of a BJT?

Answer 21

The cutoff frequency is defined as the frequency at which the transistor current gain falls to 0.707 of its gain at low and medium frequencies.

Question 22

What are the terminal current evaluation equations?

Answer 22

Question 23

What are the terminal current evaluation assumptions?

Answer 23

1. Steady State Conditions 𝜕𝑛/𝜕𝑡=𝜕𝑝/𝜕t =0
2. No external generation of carriers G_n = G_p = 0
3. Uniform doping in all regions
4. Conductivities in neutral regions are sufficiently high to ensure all voltages are dropped across depletion regions.
5. No generation or recombination in depletion regions
6. Low level injection conditions

1.

Question 24

What effect does a narrow base assumption have on the BJT?