N-channel JFET construction and working

A JFET (junction field-effect transistor) is a unipolar device in which the current flows through a channel of one doping type, controlled by the reverse bias on a gate p–n junction.

Construction (n-channel)

A bar of lightly doped n-type silicon forms the channel, with ohmic contacts at the ends: the drain (D) at the top and the source (S) at the bottom. Heavily doped p⁺ regions are diffused into both sides (and connected together) to form the gate (G). The p–n junctions create depletion regions that extend into the channel.

          D
          |
     +----+-----+
     | p+  n  p+|
 G --|  channel |-- G
     | p+  n  p+|
     +----+-----+
          |
          S

Working principle

1. With VGS = 0 and a small positive VDS, electrons flow from source to drain through the channel; ID rises linearly with VDS (ohmic region), like a resistor.
2. The gate junction is reverse biased (VGS ≤ 0). A more negative VGS widens the depletion regions, narrows the channel, increases its resistance and reduces ID. So VGS controls ID, like a voltage-controlled resistor/current source.
3. For VGS = 0, as VDS increases, the reverse bias at the drain end is higher, so the depletion region is wider there. At VDS = |VP| (VP = pinch-off voltage) the channel pinches off near the drain, and ID saturates at IDSS (saturation current). Beyond that, ID remains almost constant (saturation/pinch-off region).
4. At VGS = VP (negative, e.g. -4 V) the channel is completely closed: ID = 0 (cut-off).
5. The gate current is almost zero (reverse-biased junction), so the input resistance is very high (10⁸ to 10¹⁰ Ω).
6. Breakdown (avalanche) occurs at large VDS, beyond the saturation region.

Characteristics

Drain characteristics (ID vs VDS for different VGS):

 ID
 IDSS|      _______  VGS = 0
     |   _-'
     |  /  _______  VGS = -1 V
     | /  /
     |/  /  _______  VGS = -2 V
     +--+------------ VDS
     ohmic| saturation  (breakdown)

Transfer characteristic (ID vs VGS) is a parabola from IDSS at VGS = 0 to zero at VGS = VP.

Drain current equation (Shockley)

In the saturation region:

ID = IDSS (1 - VGS/VP)²        for VP ≤ VGS ≤ 0

where IDSS = drain current at VGS = 0 and VP = pinch-off voltage (negative for an n-channel JFET). Transconductance: gm = ∂ID/∂VGS = (2 IDSS/|VP|)(1 - VGS/VP) = gm0 (1 - VGS/VP), where gm0 = 2 IDSS/|VP|.

Condition for operating in the pinch-off (saturation) region: VDS ≥ VGS - VP (= VGS + |VP|).