Junction Field Effect Transistor (JFET) Dr. M A Islam Assistant Professor EEE,IIUC

1. FET comes in several forms: JFET( Junction FET): The control (Gate) voltage varies the depletion with of a reverse-biased p-n junction. MESFET( Metal semiconductor FET): Junction is replaced by schottky barrier MISFET (Metal-insulator-semiconductor FET): Metal gate electrode separated by insulator MOSFET (Metal oxide semiconductor FET ): Uses Oxide layer as insulator

2. Transistor Operation General Operation: - Amplification - Switching The transistor is three-terminal device with the feature that the current through two terminal and control by third terminal. This control feature allows to amplify small ac signal or to switch device on and off state.

compared to a water spigot: 2. Transistor Operation The source of water pressure – accumulated electrons at the negative pole of the applied voltage from Drain to Source The drain of water – electron deficiency (or holes) at the positive pole of the applied voltage from Drain to Source. The control of flow of water – Gate voltage that controls the width of the n-channel, which in turn controls the flow of electrons in the n-channel from source to drain. 2.1 JFET operation can be compared to a water spigot:

3. Device Structure

Electrons in the n-type channel drift from left to right, opposite to current flow. The end of the channel from which electrons flow is called source (S), and end toward which they flow is called the drain. The p+ regions are called the gates. If the channel is p-type, holes will flow from source to drain, in the same direction as the current flow, and the gate region will be n+.

4. Type of JFET The channel is made of either N-type or P-type semiconductor material; an FET is specified as either an N-channel or P-channel device In N-channel devices, electrons flow so the drain potential must be higher than that of the Source (VDS > O) In P-channel devices, the flow of holes requires that VDS < 0

5. Operating Characteristics There are three basic operating conditions for a JFET: A. VGS = 0, VDS increasing to some positive value B. VGS < 0, VDS at some positive value C. Voltage-Controlled Resistor

A. VGS = 0, VDS increasing to some positive value Three things happen when VGS = 0 and VDS is increased from 0 to a more positive voltage: The depletion region between p-gate and n-channel increases as electrons from n-channel combine with holes from p-gate. Increasing the depletion region, decreases the size of the n-channel which increases the resistance of the n-channel. But even though the n-channel resistance is increasing, the current (ID) from Source to Drain through the n-channel is increasing. This is because VDS is increasing.

Pinch-off If VGS = 0 and VDS is further increased to a more positive voltage, then the depletion zone gets so large that it pinches off the n-channel. This suggests that the current in the n-channel (ID) would drop to 0A, but it does just the opposite: as VDS increases, so does ID.

Saturation At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS. • The ohmic value of the channel is at maximum.

6. I-V characteristics

JFET: I-V characteristics

Metal-Semiconductor Interfaces Metal-Semiconductor contact Schottky Barrier/Diode Ohmic Contacts MESFET

Two kinds of metal-semiconductor contacts: Rectifying Schottky diodes: metal on lightly doped silicon Low-resistance ohmic contacts: metal on heavily doped silicon

Device Building Blocks Schottky (MS) p-n junction HBT MOS

fBn Increases with Increasing Metal Work Function q f Bn E c v y M Si = 4.05 eV Vacuum level, y M : Work Function of metal c Si : Electron Affinity of Si Theoretically, fBn= yM – cSi

Schottky barrier height, fB , is a function of the metal material. Schottky Barriers Energy Band Diagram of Schottky Contact Metal Depletion layer Neutral region qfBn Ec Ef Ev qfBp N-Si P-Si Schottky barrier height, fB , is a function of the metal material. fB is the most important parameter. The sum of qfBn and qfBp is equal to Eg .

Schottky barrier heights for electrons and holes fBn + fBp  Eg fBn increases with increasing metal work function

Energy band diagram of an isolated metal adjacent to an isolated n-type semiconductor q(fs-c) = EC – EF = kTln(NC/ND) for n-type = EG – kTln(Nv/NA) for p-type

Energy band diagram of a metal-n semiconductor contact in thermal equilibrium. qfBn = qfms + kTln(NC/ND)

Measured barrier height fms for metal-Si and metal-GaAs contacts Theory still evolving (see review article by Tung)

Energy band diagrams of metal n-type and p-type semiconductors under thermal equilibrium

Energy band diagrams of metal n-type and p-type semiconductors under forward bias

Energy band diagrams of metal n-type and p-type semiconductors under reverse bias

Vbi = fms (Doping does not matter!) fBn = fms + kTln(NC/ND) Charge distribution Vbi = fms (Doping does not matter!) fBn = fms + kTln(NC/ND) electric-field distribution Em = qNDW/Kse0 E(x) = qND(x-W)/Kse0 W (Vbi-V) = - ∫E(x)dx = qNDW2/Kse0

Depletion Depletion width Charge per unit area q

Capacitance Per unit area: Rearranging: Or:

1/C2 versus applied voltage for W-Si and W-GaAs diodes

1/C2 vs V If straight line – constant doping profile – slope = doping concentration If not straight line, can be used to find profile Intercept = Vbi can be used to find Bn

Current transport by the thermionic emission process Thermal equilibrium forward bias reverse bias J = Jsm(V) – Jms(V) Jms(V) = Jms(0) = Jsm(0)

Schottky Diodes Reverse bias Forward biased V = 0 I V Forward bias Reverse biased

Schottky Diodes

4.19 Applications of Schottly Diodes V PN junction Schottky f B Schottky diode diode I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on fB . A larger I0 means a smaller forward drop V. A Schottky diode is the preferred rectifier in low voltage, high current applications.

Switching Power Supply PN Junction Schottky rectifier Transformer rectifier 100kHz 110V/220V Hi-voltage Hi-voltage Lo-voltage 50A DC AC AC 1V DC AC MOSFET inverter utility power feedback to modulate the pulse width to keep V = 1V out

Applications of Schottky diodes Question: What sets the lower limit in a Schottky diode’s forward drop? Synchronous Rectifier: For an even lower forward drop, replace the diode with a wide-W MOSFET which is not bound by the tradeoff between diode V and leakage current. There is no minority carrier injection at the Schottky junction. Therefore, Schottky diodes can operate at higher frequencies than PN junction diodes.

Quantum Mechanical Tunneling Tunneling probability:

Note the difference with p-n junctions!! In both cases, we’re modulating the population of backflowing electrons, hence the Shockley form, but… V > 0 V > 0 V < 0 V < 0 Barrier is not pinned Els with zero kinetic energy can slide down negative barrier to initiate current Current is limited by how fast minority carriers can be removed (diffusion rate) Both el and hole currents important (charges X-over and become min. carriers) Barrier from metal side is pinned Els from metal must jump over barrier Current is limited by speed of jumping electrons (that the ones jumping from the right cancel at equilibrium) Unipolar majority carrier device, since valence band is entirely inside metal band

Let’s roll up our sleeves and do the algebra !! Jsm = 2qf(Ek-EF)vx dkxdkydkzvxe-(Ek-EF)/kT   (2p)3/W = 2q vx > vmin,vy,vz Vbi - V V > 0 Ek-EF = (Ek-EC) + (EC -EF) EC - EF = q(fBn-Vbi) Ek - EC = m(vx2 + vy2 + vz2 )/2 m*vmin2/2 = q(Vbi – V) kx,y,z = m*vx,y,z/ħ

This means… Jsm = q(m*)3W/4p3ħ3 dvye-m*vy2/2kT   ∞ -∞ dvze-m*vz2/2kT dvxvxe-m*vx2/2kT vmin (2pkT/m*) (kT/m*)e-m*vmin2/2kT = (kT/m*)e-q(Vbi-V)kT x e-q(fBn-Vbi)/kT dxe-x2/2s2 = s2p ∞ -∞   dx xe-x2/2s2 = s2e-A2/2s2 A = qm*k2T2/2p2ħ3e-q(fBn-V)kT = A*T2e-q(fBn-V)kT A* = 4pm*qk2/h3 = 120 A/cm2/K2

J = A*T2e-qfBN/kT(eqV/kT-1)

In regular pn junctions, charge needs to move through drift-diffusion, and get whisked away by RG processes MS junctions are majority carrier devices, and RG is not as critical. Charges that go over a barrier already have high velocity, and these continue with those velocities to give the current

Forward current density vs applied voltage of W-Si and W-GaAs diodes

Thermionic Emission over the barrier – low doping

Tunneling through the barrier – high doping Schottky barrier becomes Ohmic !!

Fermi Level Pinning q f Bn E c v y M Si = 4.05 eV Vacuum level, + - A high density of energy states in the bandgap at the metal-semiconductor interface pins Ef to a narrow range and fBn is typically 0.4 to 0.9 V Question: What is the typical range of fBp?

Ohmic Contacts

Ohmic Contacts Tunneling probability: - - x Silicide N+ Si Ev Ec , Ef fBn - x V Efm Ev Ec , Ef fBn – V Tunneling probability:

Ohmic Contacts

MESFET MESFET stands for metal–semiconductor field effect transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky (metal-semiconductor) junction is used. MESFETs are usually constructed in compound semiconductor technologies lacking high quality surface passivation such as GaAs, InP, or SiC, and are faster but more expensive than silicon-based JFETs or MOSFETs. Production MESFETs are operated up to approximately 45 GHz, and are commonly used for microwave frequency communications and radar. The first MESFETs were developed in 1966, and a year later their extremely high frequency RF microwave performance was demonstrated. From a digital circuit design perspective, it is increasingly difficult to use MESFETs as the basis for digital integrated circuits as the scale of integration goes up, compared to CMOS silicon based fabrication.

MISFET A MISFET is a metal–insulation–semiconductor field-effect transistor. MISFET is a more general term than MOSFET and a synonym to insulated gate field-effect transistor (IGFET). All MOSFETs are MISFETs, but not all MISFETs are MOSFETs. The insulator in a MISFET is a dielectric which can be silicon oxide (in a MOSFET), but other materials can also be employed. The generic term for the dielectric is gate dielectric, since the dielectric lies directly below the gate electrode and above the channel of the MISFET. The term metal is used for the gate material, even though it is usually highly doped polysilicon or some other nonmetal.