---
title: BJT Bipolar Junction Transistor
url: https://doc.liz6.com/en/hardware/03-semiconductor/04-BJT
locale: en
area: hardware
tags:
- hardware
- semiconductor
date: 2026-06-30
modified: 2026-07-16
description: BJT Bipolar Junction Transistor Basic Concepts BJT (Bipolar Junction Transistor) — a semiconductor device that uses a small current to control a large current. …
---

# BJT Bipolar Junction Transistor

## Basic Concepts

**BJT** (Bipolar Junction Transistor) — a semiconductor device that uses a **small current** to control a **large current**.

```
NPN:                    PNP:
      C (Collector)              C
      │                      │
    ┌─┴─┐                  ┌─┴─┐
  B │   │   Ic           B │   │   Ic
  ──┤   ├──              ──┤   ├──
    │   │                  │   │
    └─┬─┘                  └─┬─┘
      │                      │
      E (Emitter)             E

Arrow direction = Forward current direction of the emitter junction
NPN: Arrow points outward  (N→P→N)
PNP: Arrow points inward   (P→N→P)
```

### BJT Essence
```
Consists of two back-to-back PN junctions:

NPN:  N ─ P ─ N
      E   B   C
      ↑   ↑
    BE junction  BC junction

BE junction forward biased + BC junction reverse biased → Amplification mode
Collector current Ic is controlled by Base current Ib:
  Ic = β × Ib

β (hFE): Current gain, typically 100~400
```

---

## Three Operating Modes

| Mode | BE Junction | BC Junction | Ic | Application |
|------|-------------|-------------|-----|-------------|
| Cutoff | Reverse biased | Reverse biased | ≈0 | Switch OFF |
| Active | Forward biased | Reverse biased | β·Ib | **Amplifier** |
| Saturation | Forward biased | Forward biased | Vcc/Rc | Switch ON |

### Switch Mode
```
Cutoff: Ib = 0 → Ic = 0 → Equivalent to open circuit
Saturation: Ib > Ic/β → Vce ≈ 0.1~0.3V → Equivalent to closed circuit

When used as a switch:
  Rb = (Vdrive - Vbe) / Ib
  Ib ≥ Ic / β_min × 1.5  (Ensure deep saturation)
  Vbe ≈ 0.7V (Silicon transistor)
```

### Amplification Mode
```
Ic = β × Ib
Ie = Ic + Ib = (β+1) × Ib

Vce = Vcc - Ic × Rc
```

---

## Basic Amplifier Circuits

### Common Emitter — Most Common
```
          Vcc
           │
           Rc
           │
        ┌──┴── Vout
        │
      C │
  ──┤├─B   NPN
  Rb   │
       E │
         │
        GND

Gain: Av = -gm × Rc  (Inverting!)
      gm = Ic / VT ≈ Ic/26mV (Room temperature)

Input Impedance: ≈ Rb ∥ rπ (Medium, kΩ range)
Output Impedance: ≈ Rc
```

### Common Collector / Emitter Follower
```
Voltage Gain ≈ 1 (Non-inverting)
High input impedance, low output impedance
Used as a buffer
```

---

## Small-Signal Model (Hybrid-π)

```
      B ──┬── rπ ───┬── C
         │          │
         │   ┌──────┤
         │   │  ↑   │
         └───┘ gm·Vπ│
                 │   │
                 E   │
                     │
                    GND

rπ = β / gm
gm = Ic / VT
ro = VA / Ic  (Early Effect)
```

---

## Biasing Circuits

### Fixed Bias (Simplest, large thermal drift)
```
Vcc → Rb → Base
Unstable, not recommended
```

### Voltage Divider Bias (Standard practice)
```
         Vcc
          │
          R1
          │
    ┌─────┼── Base
    │     │
    R1    R2
    │     │
    └─────┼── GND
          │
         Re (Emitter resistor — provides negative feedback, stabilizes operating point)
          │
         GND

Adding Re greatly improves temperature stability
Ce parallel to Re restores AC gain
```

---

## Comparison with MOSFET

| Feature | BJT | MOSFET |
|------|-----|--------|
| Control Variable | Current (Ib) | Voltage (Vgs) |
| Input Impedance | Low~Medium (kΩ) | Extremely High (pA leakage) |
| Transconductance gm | Ic/VT (Linear) | 2Id/(Vgs-Vth) (Square) |
| Switching Speed | Slow (stored charge) | Fast |
| On-State Voltage Drop | Vce(sat)≈0.1V | Rds(on)×Id |
| Noise | Low 1/f noise | Higher 1/f noise |
| Cost | Low | Low |
| ESD Sensitivity | Robust | Extremely fragile! |

**Selection Advice:**
- Switching → MOSFET (High efficiency)
- Low-noise amplification → BJT
- High-current driving → MOSFET
- Simple LED/relay driving → Either works fine

---

## Darlington Pair

```
Two BJTs cascaded, total β = β₁ × β₂ (can reach 1000~10000+)

      C
      │
  B ──┤  Q1
      │  ├── E₁ → Q2 Base
      │
      E → Q2 Emitter

Disadvantages: Vbe doubles (≈1.4V), slow speed
Typical: TIP122 (NPN), ULN2003 (7-channel Darlington array)
```

---
*Keywords: BJT, NPN, PNP, β, Common Emitter, Switch, Saturation, Transconductance, Bias, Darlington*