Field-effect transistor (FET)s

• Transistors are essential semiconductor devices.

• Two major types:

  • Bipolar Junction Transistor (BJT)

  • Field-Effect Transistor (FET)

• BJTs use both electrons and holes (bipolar)

• FETs use only one type of charge carrier (unipolar).

• A voltage-controlled semiconductor device.

• Controls current flow using an electric field.

• Two main types:

  • Junction Field-Effect Transistor (JFET)

  • Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)

• Consists of three terminals: Gate, Source, and Drain.

• Operates with reverse-biased pn junction to control current in a channel.

• Two types based on channel structure:

  • N-channel JFET: Current flows through an n-type channel.

  • P-channel JFET: Current flows through a p-type channel.

• High input impedance, making it useful in high-impedance amplifiers.

• Reverse-biased gate-source junction controls current flow.

• \(V_{DD}\) provides drain-to-source voltage, allowing current flow from drain to source.

• \(V_{GG}\) sets reverse-bias voltage, producing a depletion region along the pn junction.

• The depletion region controls channel width and resistance.

• Wider depletion region towards the drain end due to greater reverse-bias voltage.

• Voltage-Controlled Device: JFET operates as a voltage-controlled, constant-current device.

• Drain Characteristic Curve:

  • \(I_D\) increases with \(V_{DS}\) initially (Ohmic region) but becomes constant in the active region.

  • Pinch-off voltage (\(V_p\)) is where \(I_D\) becomes constant.

• Breakdown Region:

  • Occurs when \(I_D\) increases rapidly beyond safe limits.

  • JFET must be operated below breakdown voltage.

• \(V_{GS}\) Controls \(I_D\):

  • Increasing negative \(V_{GS}\) reduces \(I_D\) due to narrowing of the channel.

  • The cutoff voltage (\(V_{GS}\text{(off)}\)) is where \(I_D\) is approximately zero.

  • For an n-channel JFET, the more negative \(V_{GS}\) is, the smaller \(I_D\) becomes.

  • \(V_{GS}\)(off) and \(V_p\) are equal in magnitude but opposite in sign.

• Transfer characteristic curve: \(V_{GS}\) and \(I_{D}\) relationship.

• Also known as the transconductance curve.

• Key characteristics:

  • \(I_{D} = 0\) when \(V_{GS} = V_{GS(off)}\)

  • \(I_{D} = I_{DSS}\) at \(V_{GS} = 0\)

  • \(I_{D}\) follows a square-law relationship:

\[I_{D} \approx I_{DSS} \left( 1 - \frac{V_{GS}}{V_{GS(off)}} \right)^2\]

• \(I_{D}\) can be determined for any \(V_{GS}\) if \(V_{GS(off)}\) and \(I_{DSS}\) are given.

• An example of how the transfer characteristic curve (blue) of an n-channel JFET is developed from the JFET drain characteristic curves (green).

• \[\begin{aligned} g_m & = \frac{\Delta I_D}{\Delta V_{GS}} \\ g_m & = g_{m0} \left(1 - \frac{V_{GS}}{V_{GS(off)}} \right)\\ g_{m0} &\Rightarrow~ \text{transconductance at}~ V_{GS} = 0 \end{aligned}\]
  \(\Delta V_{GS}\)\(\Delta I_D\)Forward Transconductance (g\(_m\)):
• \[g_{m0} = \frac{2 I_{DSS}}{|V_{GS(off)}|}\]
  is unknown, it can be estimated as: When

• The purpose of biasing is to set a proper Q-point.

• Three types of biasing in the active region:

  • Self-bias

  • Voltage-divider bias

  • Current-source bias

• Ensures gate-source junction is reverse-biased.

  \[\begin{aligned} V_{GS} &= -I_D R_S \Leftarrow \text{n-channel JFET}\\ V_{GS} &= +I_D R_S \Leftarrow \text{p-channel JFET} \end{aligned}\]

• The Q-point is determined by plotting the load line.

• \[V_{GS} = -I_D R_S\]
  The load line equation:

• Intersection of load line and transfer characteristic gives \(I_D\) and \(V_{GS}\).