Transistor #

The fundamental active component for amplification and switching. Breadpad supports BJTs (Bipolar Junction Transistors), MOSFETs, JFETs, and more with full SPICE model customization.

Basic Properties #

Terminals: 3 (varies by type)
- BJT: Collector, Base, Emitter
- MOSFET: Drain, Gate, Source
- JFET: Drain, Gate, Source
SPICE Designation: Q (BJT), M (MOSFET), J (JFET)
Layout: Fixed 3 consecutive tie points

Supported Transistor Types #

Bipolar Junction Transistors (BJT) #

NPN: Current flows from collector to emitter
PNP: Current flows from emitter to collector
Lateral PNP: Specialized PNP for ICs

Field Effect Transistors (FET) #

N-Channel MOSFET: Enhancement and depletion modes
P-Channel MOSFET: Enhancement and depletion modes
N-Channel JFET: Depletion mode only
P-Channel JFET: Depletion mode only

Specialized Types #

MESFET: Metal-semiconductor FET
IGBT: Insulated Gate Bipolar Transistor

SPICE Model Parameters #

Breadpad allows full customization of SPICE parameters:

Common BJT Parameters #

IS: Saturation current (1e-14 A)
BF: Forward beta/gain (100)
BR: Reverse beta (1)
VA: Early voltage (100V)
CJE: B-E junction capacitance
CJC: B-C junction capacitance
TF: Forward transit time
TR: Reverse transit time

Common MOSFET Parameters #

VTO: Threshold voltage (1V)
KP: Transconductance parameter
LAMBDA: Channel-length modulation
RD: Drain resistance
RS: Source resistance
CGS: Gate-source capacitance
CGD: Gate-drain capacitance

SPICE Netlist Format #

BJT example:

.MODEL Q1_MODEL NPN (IS=1e-14 BF=100 VA=100)
Q1 collector base emitter Q1_MODEL

MOSFET example:

.MODEL M1_MODEL NMOS (VTO=1 KP=0.1 LAMBDA=0.01)
M1 drain gate source bulk M1_MODEL

Common Transistor Models #

Small Signal BJTs #

2N3904 (NPN)

IS=6.734e-15
BF=416.4
VA=74.03
General purpose amplifier

2N3906 (PNP)

IS=1.41e-15
BF=180.7
VA=18.7
Complementary to 2N3904

Power BJTs #

2N3055 (NPN)

IS=2e-10
BF=50
Power amplifier, 15A

Small Signal MOSFETs #

2N7000 (N-Channel)

VTO=2.0
KP=0.15
200mA switching

BS250 (P-Channel)

VTO=-3.0
KP=0.1
Complementary to 2N7000

Power MOSFETs #

IRF540 (N-Channel)

VTO=4.0
High current capability
Low RDS(on)

Circuit Applications #

Amplifier Configurations #

Common Emitter/Source

High voltage gain
Phase inversion
Medium input/output impedance

Common Collector/Drain

Unity voltage gain
High input impedance
Low output impedance
Buffer/follower

Common Base/Gate

Unity current gain
Low input impedance
High output impedance
High frequency

Switching Applications #

Digital logic gates
Power switching
Motor control
LED drivers
Relay drivers

Analog Applications #

Differential pairs
Current mirrors
Voltage references
Oscillators
Active filters

Biasing Considerations #

BJT Biasing #

Fixed bias: Simple but temperature dependent
Voltage divider: More stable
Emitter feedback: Good stability
Collector feedback: Moderate stability

MOSFET Biasing #

Gate voltage: Set VGS > VTO
Self-bias: Source resistor
Voltage divider: Fixed gate voltage
Current source: Stable operation

Temperature Effects #

BJT Temperature Dependencies #

VBE: -2mV/°C
Beta: Increases with temperature
Leakage: Doubles every 10°C

MOSFET Temperature Dependencies #

VTO: -2 to -4mV/°C
Mobility: Decreases with temperature
RDS(on): Increases with temperature

Design Tips #

Operating Point: Ensure proper DC bias before AC analysis
Thermal Runaway: Use emitter/source degeneration
Frequency Response: Include junction capacitances
Switching Speed: Consider charge storage effects
Power Dissipation: P = VCE × IC (BJT) or VDS × ID (FET)
Safe Operating Area: Check SOA curves

Parameter Extraction #

To match real transistors:

Measure DC characteristics
Extract IS, BF/VTO from curves
Add capacitances for AC response
Verify with transient analysis

Simulation Tips #

Convergence: Start with default models, add complexity
Initial Conditions: Set IC for faster convergence
Temperature: Use TEMP parameter for thermal analysis
Substrate Connection: Connect MOSFET bulk properly
Model Levels: Higher levels = more accuracy, slower simulation