JFET Drain Current (Shockley)

Q: How do you calculate JFET Drain Current (Shockley)?

Use the formula Id = Idss × (1 − Vgs/Vp)². Shockley’s equation gives the drain current of a JFET in saturation as a function of gate–source voltage. Enter the values you know in the calculator above and it solves for the remaining variable.

Q: What is the formula for JFET Drain Current (Shockley)?

The JFET Drain Current (Shockley) formula is Id = Idss × (1 − Vgs/Vp)², where Id—Drain current (A); Idss—Drain current at Vgs = 0 (A); Vgs—Gate-source voltage (V); Vp—Pinch-off / cutoff voltage (V).

Id = Idss · (1 − Vgs/Vp)²

Quick answer

This calculator solves Id = Idss × (1 − Vgs/Vp)² for Drain Current. With Idss 44 mA, Gate-Source Voltage 12 V, Pinch-off Voltage 26.4 V, the result is 13.09 mA. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Idss

Gate-Source Voltage

Pinch-off Voltage

Drain Current (mA)

—

Formula

Id = Idss × (1 − Vgs/Vp)²

Description

Shockley’s equation gives the drain current of a JFET in saturation as a function of gate–source voltage. The current is maximum (Idss) at zero gate voltage and falls to zero as Vgs approaches the pinch-off (threshold) voltage. The square-law relationship gives JFETs their characteristic soft, low-distortion transfer curve used in audio and RF front ends.

Variables

Id — Drain current (A)
Idss — Drain current at Vgs = 0 (A)
Vgs — Gate-source voltage (V)
Vp — Pinch-off / cutoff voltage (V)

Practical Notes

For an n-channel JFET both Vgs and Vp are negative. The transconductance is gm = (2·Idss/|Vp|)·(1 − Vgs/Vp), largest near Vgs = 0.

Symbols and units

Symbol	Meaning	Default unit	Accepted units
Id	Drain Current	mA	pA, nA, µA, mA, A, kA
Idss	Idss	mA	pA, nA, µA, mA, A, kA
Vgs	Gate-Source Voltage	V	pV, nV, µV, mV, V, kV
Vp	Pinch-off Voltage	V	pV, nV, µV, mV, V, kV

Step by step

1. What you know: Idss 44 mA, Gate-Source Voltage 12 V, Pinch-off Voltage 26.4 V
2. The relation: Id = Idss × (1 − Vgs/Vp)²
3. Substitute: Idss = 44 mA, Vgs = 12 V, Vp = 26.4 V
4. Result: Drain Current = 13.09 mA

Related quantities from the same inputs

P = V × I = 0.528 W

Worked examples

Minimum range

Given: Idss 0.22 mA, Gate-Source Voltage 0.33 V, Pinch-off Voltage 0.726 V

Result: Drain Current = 65.45 µA

Low range

Given: Idss 2.2 mA, Gate-Source Voltage 3.3 V, Pinch-off Voltage 7.26 V

Result: Drain Current = 0.6545 mA

Typical

Given: Idss 44 mA, Gate-Source Voltage 12 V, Pinch-off Voltage 26.4 V

Result: Drain Current = 13.09 mA

High range

Given: Idss 1.1 A, Gate-Source Voltage 0.23 kV, Pinch-off Voltage 0.506 kV

Result: Drain Current = 0.3273 A

Maximum range

Given: Idss 11 A, Gate-Source Voltage 2.3 kV, Pinch-off Voltage 5.06 kV

Result: Drain Current = 3.273 A

Solving for each variable

Solve for Drain Current

Idss 0.22 mA, Gate-Source Voltage 0.33 V, Pinch-off Voltage 0.726 V → 65.45 µA

Solve for Idss

Drain Current 100 µA, Gate-Source Voltage 0.33 V, Pinch-off Voltage 0.726 V → 0.3361 mA

Solve for Gate-Source Voltage

Drain Current 100 µA, Idss 0.22 mA, Pinch-off Voltage 0.726 V → 0.2365 V

Solve for Pinch-off Voltage

Drain Current 100 µA, Idss 0.22 mA, Gate-Source Voltage 0.33 V → 1.013 V

How Drain Current changes with Idss

Idss (input)	Drain Current (result)
4.4 mA	1.309 mA
11 mA	3.273 mA
22 mA	6.545 mA
33 mA	9.818 mA
44 mA	13.09 mA
66 mA	19.64 mA
88 mA	26.18 mA
0.176 A	52.36 mA
0.308 A	91.64 mA
0.44 A	130.9 mA

The same result in other units

Unit	Value
µA	13090
mA	13.09
A	0.01309

Embed this calculator

Paste this into any page. It keeps working when this page updates, and credits the source.

<iframe src="https://vbatpower.com/embed/jfet-drain-current/" width="100%" height="460" style="border:1px solid #1f2a37;border-radius:12px" loading="lazy" title="JFET Drain Current (Shockley)"></iframe>
<p><a href="https://vbatpower.com/formulas/jfet-drain-current/">JFET Drain Current (Shockley)</a> Calculator by vbatpower.com</p>

Frequently asked questions

How do you calculate JFET Drain Current (Shockley)?

Use Id = Idss × (1 − Vgs/Vp)². With Idss 44 mA, Gate-Source Voltage 12 V, Pinch-off Voltage 26.4 V, Drain Current works out to 13.09 mA.

How do you calculate calculate the drain current for idss = 8ma, vp = −4 v and vgs = −2 v?

Rearrange for Drain Current: with Idss 2.2 mA, Gate-Source Voltage 3.3 V, Pinch-off Voltage 7.26 V, Drain Current is 0.6545 mA.

What is the formula for JFET Drain Current (Shockley)?

The formula is Id = Idss × (1 − Vgs/Vp)². Shockley’s equation gives the drain current of a JFET in saturation as a function of gate–source voltage.

How do you find Idss when you know the other values?

Rearrange for Idss: with Drain Current 100 µA, Gate-Source Voltage 0.33 V, Pinch-off Voltage 0.726 V, Idss is 0.3361 mA.

How do you find Gate-Source Voltage when you know the other values?

Rearrange for Gate-Source Voltage: with Drain Current 100 µA, Idss 0.22 mA, Pinch-off Voltage 0.726 V, Gate-Source Voltage is 0.2365 V.

What units does JFET Drain Current (Shockley) use?

Drain Current is shown in µA, mA, A. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Discrete Amplifiers formulas →

Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain JFET Drain Current (Shockley)

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App