FET vs BJT: Key Differences, Characteristics, Equations & Applications (with MCQs)

“Write the differences between FET and BJT” is one of the most repeated questions in electronics viva and written exams — BCS (EEE), PGCB, BPDB, BREB, DPDC, BTCL and university finals. To answer it confidently you need to understand why they differ: a BJT is controlled by current, a FET by voltage. This post builds that intuition with an animation, then gives a complete comparison table, equations, solved examples and MCQs.

⚡ Key takeaways

• BJT (Bipolar Junction Transistor): current-controlled, I_C = β I_B; conduction by both electrons and holes (bipolar).
• FET (Field-Effect Transistor): voltage-controlled, the gate voltage changes channel width; conduction by one type of carrier (unipolar).
• FET has very high input impedance (10⁸–10¹⁴ Ω), lower noise, better thermal stability and smaller size → dominates ICs (CMOS).
• BJT has higher transconductance/voltage gain for the same current and is common in analog and RF amplifiers.

📑 Contents

1. What is a BJT?
2. What is a FET?
3. Current control vs voltage control (animation)
4. Symbols and terminals
5. FET vs BJT – full comparison table
6. Key equations
7. Solved examples
8. Where each is used
9. Practice MCQs
10. বাংলায় দ্রুত রিভিশন

1. What Is a BJT?

A Bipolar Junction Transistor is a three-layer semiconductor device (NPN or PNP) with two p-n junctions and three terminals: Emitter (E), Base (B) and Collector (C). In the active region, the base-emitter junction is forward-biased and the collector-base junction reverse-biased. A small base current I_B controls a much larger collector current I_C = β I_B (β typically 50–300). Both electrons and holes take part in conduction — hence “bipolar”.

2. What Is a FET?

A Field-Effect Transistor has a conducting channel between Source (S) and Drain (D); the Gate (G) voltage creates an electric field that widens or narrows the channel. Since the gate draws practically no current (reverse-biased junction in a JFET, insulating oxide in a MOSFET), the FET is voltage-controlled. Only majority carriers (electrons in n-channel, holes in p-channel) carry the current — hence “unipolar”.

• JFET – junction gate; depletion-mode only.
• MOSFET (IGFET) – insulated gate; enhancement type (normally OFF) or depletion type (normally ON).

3. Current Control vs Voltage Control (Animation)

Left: in an n-channel JFET, making V_GS more negative widens the depletion regions (hatched), squeezing the channel and reducing I_D along the square-law curve — no gate current flows. Right: in an NPN BJT, increasing the base current (red arrow) lets proportionally more electrons cross from emitter to collector, so I_C rises linearly with I_B.

Animation 1 – JFET: voltage controls a channel. BJT: current controls current.

4. Symbols and Terminals

Fig. 1 – Circuit symbols. Arrow on the BJT emitter shows conventional current direction; arrow on the FET gate/body points toward an n-channel.

5. FET vs BJT – Full Comparison Table

Basis	BJT	FET
Control	Current-controlled (IB controls IC)	Voltage-controlled (VGS controls ID)
Carriers	Bipolar — electrons and holes	Unipolar — majority carriers only
Terminals	Emitter, Base, Collector	Source, Gate, Drain (+ Body in MOSFET)
Types	NPN, PNP	JFET (n/p), MOSFET (enhancement/depletion, n/p)
Input impedance	Low to moderate (≈ 1 kΩ – few kΩ in CE)	Very high (JFET ≈ 10⁸–10¹⁰ Ω, MOSFET ≈ 10¹⁰–10¹⁴ Ω)
Gain parameter	Current gain β (hfe)	Transconductance gm
Voltage gain	Higher (larger gm for same current)	Lower
Transfer characteristic	Linear: IC = β IB	Square law: ID = IDSS(1 − VGS/VP)²
Noise	More (shot noise at junctions)	Less — preferred for low-level RF front-ends
Thermal stability	Poor — IC rises with temperature → risk of thermal runaway	Better — ID falls with temperature at high current (negative tempco), no thermal runaway
Size & fabrication	Larger	Smaller, simpler — ideal for VLSI
Offset at zero current (switch)	VCE(sat) ≈ 0.2 V	No offset; behaves as a resistor RDS(on)
Power consumption (as switch)	Needs continuous base current	Gate needs charge only when switching
Radiation resistance	Less	More (JFET)
Static-charge sensitivity	Robust	MOSFET gate oxide easily damaged by ESD
Cost	Low (discrete)	Low in ICs; power MOSFETs moderate

6. Key Equations

BJT

I_E = I_B + I_C, I_C = β I_B, α = I_C/I_E = β/(1 + β), β = α/(1 − α)

JFET (Shockley’s equation)

I_D = I_DSS (1 − V_GS/V_P)², g_m = g_m0(1 − V_GS/V_P), g_m0 = 2I_DSS/|V_P|

Enhancement MOSFET (saturation)

I_D = k (V_GS − V_T)² for V_GS > V_T

7. Solved Examples

Example 1 (BJT). A transistor has β = 100 and I_B = 20 µA. Find I_C, I_E and α.

1. I_C = 100 × 20 µA = 2 mA
2. I_E = 2 + 0.02 = 2.02 mA
3. α = 100/101 = 0.990

Example 2 (JFET). I_DSS = 10 mA, V_P = −4 V. Find I_D and g_m at V_GS = −2 V.

1. I_D = 10 (1 − (−2)/(−4))² = 10 × (0.5)² = 2.5 mA
2. g_m0 = 2 × 10/4 = 5 mS; g_m = 5 × (1 − 0.5) = 2.5 mS

Example 3. At what V_GS does the above JFET conduct 5 mA? 5 = 10(1 − V_GS/(−4))² ⇒ 1 + V_GS/4 = 0.707 ⇒ V_GS ≈ −1.17 V.

8. Where Each Is Used

BJT is preferred for	FET/MOSFET is preferred for
Discrete audio and analog amplifiers needing high gain	Digital ICs — microprocessors, memories (CMOS)
RF amplifiers and oscillators (SiGe HBTs)	High-impedance input stages, electrometers, buffer amplifiers
Current mirrors, precision analog (bandgap references)	Power switching: SMPS, inverters, motor drives, chargers
Low-cost switching of small loads	Analog switches, choppers, voltage-variable resistors

The IGBT combines both: a MOSFET input (voltage-controlled, high input impedance) with a BJT-like output (low conduction loss at high current) — used in VFDs, traction and HVDC converters.

9. Practice MCQs (click to see answer)

1. A FET is a ______ controlled device. (a) Current (b) Voltage (c) Power (d) Resistance

✔ (b) Voltage.

2. BJT is called bipolar because: (a) It has two junctions (b) Both electrons and holes carry current (c) It has two supplies (d) It works in two regions

✔ (b) Both electrons and holes take part in conduction.

3. Input impedance of a MOSFET is of the order of: (a) 1 kΩ (b) 100 kΩ (c) 1 MΩ (d) 10¹⁰ Ω or more

✔ (d) 10¹⁰ Ω or more.

4. Thermal runaway is associated with: (a) BJT (b) JFET (c) MOSFET (d) Diode only

✔ (a) BJT.

5. Shockley’s equation relates: (a) I_C and I_B (b) I_D and V_GS (c) V_CE and I_C (d) I_E and V_BE

✔ (b) I_D = I_DSS(1 − V_GS/V_P)².

6. If β = 99, α is: (a) 0.99 (b) 0.9 (c) 1.01 (d) 100

✔ (a) 0.99 = 99/100.

7. Which device is used in CMOS digital ICs? (a) BJT (b) JFET (c) MOSFET (d) UJT

✔ (c) MOSFET.

8. A FET has ______ noise compared with a BJT. (a) More (b) Less (c) Same (d) Infinite

✔ (b) Less.

9. Enhancement MOSFET conducts only when: (a) V_GS = 0 (b) V_GS > V_T (c) V_GS < V_P (d) Gate is open

✔ (b) V_GS > V_T (normally-off device).

10. The gain parameter of a FET is: (a) β (b) α (c) g_m (d) h_ie

✔ (c) Transconductance g_m.

10. বাংলায় দ্রুত রিভিশন

📝 পরীক্ষার আগে এক নজরে

• BJT: কারেন্ট-নিয়ন্ত্রিত (I_C = βI_B); ইলেকট্রন ও হোল দুটোই বহন করে (বাইপোলার); টার্মিনাল E, B, C।
• FET: ভোল্টেজ-নিয়ন্ত্রিত (গেট ভোল্টেজ চ্যানেলের প্রস্থ নিয়ন্ত্রণ করে); শুধু সংখ্যাগুরু বাহক (ইউনিপোলার); টার্মিনাল S, G, D।
• FET-এর ইনপুট ইম্পিড্যান্স খুব বেশি, নয়েজ কম, তাপীয় স্থিতিশীলতা ভালো, আকারে ছোট — তাই IC-তে MOSFET ব্যবহৃত হয়।
• BJT-এর ভোল্টেজ গেইন বেশি, কিন্তু থার্মাল রানঅ্যাওয়ের ঝুঁকি আছে।
• সূত্র: α = β/(1+β); JFET: I_D = I_DSS(1 − V_GS/V_P)²।

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References: Boylestad & Nashelsky, Electronic Devices and Circuit Theory; Sedra & Smith, Microelectronic Circuits. Images and animations: original work by Totalecer.