There are two types of transistors:

# Bipolar Junctions Transistors (BJT)

![[bipolar-junction-transistor|200x]]

The current flowing in from Base controls the much larger current flowing between Collector and Emitter. 

## Rules for NPN Transistors

**1. Polarity**
The collector must be more positive than the emitter.

**2. Junctions**
The base$\rightarrow$emitter and base$\rightarrow$collector connections behave like diodes.

**3. Maximum Ratings**
Any transistor has maximum ratings for $I_C$ ,$I_B$ and $V_{CE}$ that should not be exceded.

**4. Current Amplifier**
When rules 1-3 are obeyed, $I_C$ is roughly equivalent to $IB$ and can be written like:

$\displaystyle I_C = \beta I_B$

In the following example circuit we can see that for a $\beta$ of 100 the Ic current is aroung 100 times higher than the base current.

```circuitjs
$ 1 0.000005 11.086722712598126 50 5 43 5e-11
R 256 16 256 -48 0 0 40 2 0 0 0.5
g 256 144 256 208 0 0
t 192 80 256 80 0 1 -1.552 0.44799999999999995 100 default
R 144 80 64 80 0 0 40 0.44799999999999995 0 0 0.5
w 256 16 256 64 3
w 256 96 256 144 3
w 144 80 192 80 1
x 272 -5 289 -2 4 24 C
x 273 179 289 182 4 24 E
x 96 68 112 71 4 24 B
x 266 82 275 85 4 12 Ic
o 1 16 0 159745 0.0001 0.0001 0 2 1 3
38 3 F1 0 0 1.4 -1 Voltage
```

## NPN vs PNP Transistors

The names come from the way they are constructed
![[bjt-transistors.png]]

![[transistors-pnp-npn]]

# Transistor Modes

## Saturation
In this mode the transistor acts like a short circuit and current flows freely from emitter to collector.

## Cut-Off
In this mode the transistor acts like an open circuit and no current flows from emitter to collector.

When $V_C > V _B < V_E$ then the transistor acts in Cutoff Region

## Active
The current from collector to emitter is proportional to the current flowing into the base.

# Field-Effect Transistors (FET)

![[field-effect-transistor|200x]]