Bob York. Transistor Basics - MOSFETs

Transcription

1 Bob York Transistor Basics - MOSFETs

2 Transistors, Conceptually So far we have considered two-terminal devices that are described by a current-voltage relationship I=f(V Resistors: Capacitors: Inductors: Diodes: I V / R I C dv dt I 1 Vdt L / I I s e V nv T 1 V I(V Transistors add a third terminal to control the current flow through the device. The two most common types of transistors are: Field-Effect Transistors (FETs: voltage-controlled current flow Bipolar Junction Transistors (BJTs: current-controlled current flow Control Terminal V c or I c V I(V, V c or I(V, I c FETs BJTs In ECE, we will not discuss the physics of device operation in depth. The transistor is simply a black box with certain well-defined terminal properties.

3 MOSFETs There are many types of FETs but all share some common features and nomenclature. Key points: Every FET has a gate, drain, and source Current flows between the drain and source. The gate is the control terminal. The DC gate leakage current is negligible, I g 0 Start with n-channel enhancement MOS (NMOS (MOS=Metal-Oxide-Semiconductor. If we take the source as the voltage reference (ground, the drain current will depend on the gate voltage and drain voltage as shown : NMOS Drain Gate Gate Source Drain s (, Source Current-Voltage Characteristic for NMOS Drain Current I g 0 G D S Common-source configuration Gate Voltage Drain Voltage

4 Common-Source NMOS Characteristic N-channel Enhancement MOS vs for specific values of = -V t Ohmic or Triode region Saturation region = V tn +.0 = V tn Increasing I g = 0 G D S Device is off no current flows vs in saturation: V V V ds gs tn I K ( V V d n gs tn = V tn = V tn V tn (cutoff I-V Curves are described analytically by: K V V V V V V V I K V V V V V 0 Vgs V tn n ( gs tn ds ds ds gs tn d n ( gs tn ds gs tn Important observations: V t No current flows for < V tn. V tn is called the Threshold voltage Once the drain voltage exceeds -V tn, a constant current flows that depends on For enhancement-mode NMOS the gate threshold voltage is positive V tn >0

5 MOSFET Saturation Region The saturation region is especially important. The NMOS device is in saturation when the following conditions are satisfied: Vds Vgs Vt Vgs Vt When the device is in saturation the drain current is given by: I K V V d n gs t Device #1 K n and V t are the important device parameters. K n depends on some material constants and the device size/geometry Device # It is difficult to control K n and V t precisely, so two different discrete devices may have significant differences in these parameters V t1 V t Later we will explore some circuit techniques to deal with this issue Note: state-of-the-art devices may follow a different behavior: where α is closer to 1 I K V V d n gs t

6 NMOS Saturation - Examples In the following, the devices have parameters: V t 1V K n 5mA/V Consider: V g =3V +10 V Here we have: I 5mA/V 3V1V 0mA d Vds 10V Vgs 3V so Vds Vgs Vt and Vgs Vt Thus device is in saturation and +5 V 5 ma V out Here we have: V V V so ds gs t V V V ds gs out Device is in saturation so I d V 1V gs 5mA= 5mA/V From this we find Vgs V

7 Common-Source PMOS Characteristic P-channel Enhancement MOS Similar characteristics to PMOS except currents and voltages are reversed V sd =V sg +V tp Ohmic or Triode region Saturation region V sg =V tp +.0 I g = 0 V sg G S D Vsd V sg =V tp V sg =V tp V sg =V tp Increasing V sg V sg V tp (cutoff By convention the threshold voltage for enhancement-mode PMOS is taken as negative vs V sg in saturation: V sd K V V V V V V V I K V V V V V 0 Vsg V tp p ( sg tp sd sd sd sg tp d p ( sg tp sd sg tp Device is off no current flows I K ( V V d p sg tp -V tp V sg

8 PMOS Saturation - Examples In the following, the devices have parameters: V tp 1V K p 5mA/V Consider: V g =6V V sg +10 V V sd Here we have: I 5mA/V 4V1V 45mA d Vsd 10V Vsg 4V so Vsd Vsg Vtp and Vsg Vtp Thus device is in saturation and +5 V Here we have: V V V sd sg out V sg Vsd V V V so sd sg tp Device is in saturation so I d 0mA= 5mA/V V 1V sg 0 ma V out From this we find Vsg 3V V 5V3V V out

9 Depletion-Mode FETs Enhancement-mode devices are normally off devices, since no current flows when =0. A certain applied gate voltage is required to turn on the device and get current flowing Depletion-mode devices are normally on. They conduct current at =0, and an applied gate voltage is required to stop the current flow and turn them off N-channel Depletion-mode MOS P-channel Depletion-mode MOS I g = 0 G D S I g = 0 V sg G S D Vsd symbol symbol vs in saturation: I K ( V V d n gs tn Device is off no current flows ss Threshold voltage has the opposite sign in comparison to enhancement devices. Otherwise the characteristics are similar. vs V sg in saturation: I K ( V V d p sg tp Device is off no current flows ss V tn -V tp V sg

10 MOSFET Construction Source Gate L g W g Gate oxide Drain Key parameters: L : gate length W : gate width c g ox : oxide capacitance density : carrier mobility in semiconductor g Semiconducting substrate Body connection Saturation current parameter: N-channel P-channel 1 W 1 W 1 W 1 W Kn ncox kn Kp pcox kp L L L L g g g g g g g g Engineers control whether a device is an enhancement or depletion device by adding carefully-controlled amounts of impurities ( dopants in the semiconductor Enhancement Devices V 0 Vgs Vt gs Depletion Devices V 0 Vgs Vt gs No charge carriers exist under the gate, so no current flow is possible An applied field allows charge to accumulate under the gate allowing current to flow Charge carriers naturally accumulate under the gate, allowing current to flow The applied field depletes the charge in the channel, cutting off the flow of current

11 JFETs N-ch JFET Drain Gate Source Gate Drain s (, JFETs are another type of depletion-mode FET. They are constructed differently but otherwise behave much like a depletion MOSFET, except that can never exceed zero volts. The maximum current at =0 is ss. JFETs can be made in both n-channel and p-channel versions. Some high-speed compound semiconductor devices (GaAs MESFETs and HEMTs behave like JFETs Source N-ch JFET Ohmic or Triode region Saturation region = 0 ss I g = 0 G D S = V t = V t Increasing = V t V t (cutoff

12 FET Family Tree Field-Effect Transistors JFET, MESFET MOSFET Depletion-mode (normally on Enhancement-mode (normally off Depletion-mode (normally on n-ch p-ch n-ch p-ch n-ch p-ch

13 Discrete Device Example: N7000 This is a popular discrete NMOS device that we will use in the ECE lab. From the data sheet: N7000 Gate A Drain Source Id, ma Vt.35V K 0mA/V n Measured N7000Data Data Model Vgs, Volts The data sheet specifies that V t is between 0.8V and 3V, with a typical value of.1v. Such a wide range of expected V t is typical of many discrete devices. Representative data for small currents is shown at left