MOSFET Switching Characteristics¶

Book-grounded switch interpretation

• OFF: \(V_{GS}<V_{th}\), \(I_D\approx0\), so the drain rises to \(V_{DD}\).
• ON: sufficient gate overdrive moves the device to the low-\(V_{DS}\) ohmic endpoint, where \(V_{DS(on)}\approx I_DR_{DS(on)}\).
• During a transition the operating point can pass through saturation, but the desired steady ON state is deep triode, not saturation.

Source figure: Sedra/Smith, Microelectronic Circuits (7th ed.), PDF p. 410.

Exam-ready answer

An enhancement n-channel MOSFET is a voltage-controlled majority-carrier device. Its drain characteristics are a family of \(I_D\) versus \(V_{DS}\) curves drawn for increasing values of \(V_{GS}\).

1. Cutoff: for \(V_{GS}<V_{th}\), no inversion channel exists and \(I_D\approx0\). The device behaves as an open switch.
2. Triode or ohmic region: for \(V_{GS}>V_{th}\) and \(0\le V_{DS}<V_{GS}-V_{th}\), a channel exists and

For small \(V_{DS}\) the MOSFET behaves approximately as a voltage-controlled resistance; this is the ON region used in switching.

1. Pinch-off boundary: at \(V_{DS}=V_{GS}-V_{th}\), the channel pinches off near the drain.
2. Saturation: for \(V_{DS}\ge V_{GS}-V_{th}\), the ideal drain current is nearly independent of \(V_{DS}\):

Increasing \(V_{GS}\) increases channel charge and shifts the characteristic upward. In a practical MOSFET, channel-length modulation gives a slight positive slope in saturation. Thus a switching MOSFET uses cutoff for OFF and deep triode for low-loss ON operation.

Practice target: 8–9 minutes; about 1–1.5 handwritten pages including the labeled graph.

Exam-ready answer

A BJT switch is a current-controlled bipolar device driven between cutoff and saturation, whereas a MOSFET switch is a voltage-controlled majority-carrier device driven between cutoff and the low-resistance triode region. Their different carrier and drive mechanisms determine switching speed and loss.

Switching operation. When a BJT is overdriven, both junctions become forward biased and excess base charge accumulates. Removing the input does not immediately stop collector current; reverse base drive or a Schottky clamp is used to shorten \(t_s\). A MOSFET stores no minority charge, but its gate is not lossless dynamically. The driver first raises \(V_{GS}\) to \(V_{th}\), then supplies the Miller charge while \(V_{DS}\) changes, and finally raises the gate to the full drive voltage. A low-impedance gate driver therefore reduces transition time and overlap loss.

For either device, an approximate hard-switching overlap loss is

where \(V\) is the blocked voltage (V), \(I\) the switched current (A), and \(f_s\) the switching frequency (Hz). Thus faster edges lower switching loss but increase \(dv/dt\), \(di/dt\) and electromagnetic interference.

Concise example: at \(I=10\,\text{A}\), a BJT with \(V_{CE(sat)}=0.2\,\text{V}\) dissipates \(2\,\text{W}\), while a MOSFET with \(R_{DS(on)}=20\,\text{m}\Omega\) also dissipates \(10^2(0.02)=2\,\text{W}\). Hence the MOSFET is not automatically lower-loss; current, voltage rating and temperature-dependent resistance matter. If that MOSFET has \(Q_g=40\,\text{nC}\), \(V_{GS}=10\,\text{V}\) and \(f_s=100\,\text{kHz}\), its gate-drive loss is only \(P_g=40\,\text{mW}\), but the driver must still supply the peak charge current.

Conclusion: MOSFETs are generally preferred for high-frequency, low-to-medium-voltage switching because they eliminate minority-carrier storage and need no continuous input current. BJTs remain useful where cost, gain or a nearly fixed saturation drop is favourable; device choice must compare total conduction, switching and drive losses at the actual operating point.

Practice target: 16–18 minutes; draw both switching mechanisms, reproduce the comparison table, and include the loss relations.