Bipolar Transistor

   Bipolar transistors, having 2 junctions, are 3 terminal semiconductor devices. The three terminals are emitter, collector, and base. A transistor can be either NPN or PNP.  See the schematic representations below:

     Note that the direction of the emitter arrow defines the type transistor. Biasing and power supply polarity are positive for NPN and negative for PNP transistors. The transistor is primarily used as an current amplifier. When a small current signal is applied to the base terminal, it is amplified in the collector circuit. This current amplification is referred to as H_FE or beta and equals Ic/Ib.
     As with all semiconductors, breakdown voltage is a design limitation. There are breakdown voltages that must be taken into account for each combination of terminals. i.e. Vce, Vbe,and Vcb. However, Vce(collector-emitter voltage) with open base, designated as Vceo, is usually of most concern and defines the maximum circuit voltage.
     Also as with all semiconductors there are undesireable leakage currents, notably Icbo ,collector junction leakage; and Iebo, emitter junction leakage. A typical collector characteristic curve is shown below:

     Note that the negative collector-emitter voltage tells you that the transistor is PNP.  Also that the output current increases with input or base current and varies very little with collector-emitter voltage.
Primary considerations when selecting a transistor are:
(a) Voltage ratings of all three junctions
(b) Power rating and thermal resistance
(c) Current handling capability and the transistor case size
(d) Leakage currents, mainly Icbo and Iebo
(e) Frequency response and /or switching times.
(f) Current gain (H_FE and h_fe)
(g) Temperature parameter variation.
(h) Saturation resistance
(I) h-parameters for linear applications

Diode - Rectifier

In this discussion the term diode and rectifier will be used interchangeably; however, the term diode usually implies a small signal device with current typically in the milliamp range; and a rectifier, a power device, conducting from1 to 1000 amps or even higher. Many diodes or rectifiers are identified as 1NXXXX . A semiconductor diode consists of a PN junction and has two(2) terminals, an anode(+) and a cathode(-). Current flows from anode to cathode within the diode. A diode and schematic representation are shown below.      An ideal diode is like a light switch in your home. When the switch is closed, the circuit is completed; and the light turns on.When the switch is open, there is no current and the light is off. This can be shown as:      However, the diode has an additional property; it is unidirectional, i.e.current flows in only one direction(anode to cathode internally).When a forward voltage is applied, the diode conducts; and when a reverse voltage is applied, there is no conduction. A mechanical analogy is a ratchat, which allows motion in one direction only. An ideal diode characteristic would be:      However, a typical diode characteristic is more like the following: Forward Voltage Drop , Vf      Notice that the diode conducts a small current in the forward direction up to a threshold voltage, 0.3 for germanium and 0.7 for silicon ; after that it conducts as we might expect. The forward voltage drop, Vf, is specified at a forward current, If. Leakage current      In the reverse direction there is a small leakage current up until the reverse breakdown voltage is reached. This leakage is undesireable, obviously the lower the better, and is specified at a voltage less the than breakdown; diodes are intended to operate below their breakdown voltage. Current Rating      The current rating of a diode is determined primarily by the size of the diode chip, and both the material and configuration of the package, Average Current is used, not RMS current. A larger chip and package of high thermal conductivity are both conducive to a higher current rating. Switching      The switching speed of a diode depends upon its construction and fabrication. In general the smaller the chip the faster it switches, other things being equal. The chip geometry, doping levels, and the temperature at nativity determine switching speeds . The reverse recovery time, trr, is usually the limiting parameter; trr is the time it takes a diode to switch from on to off. Conclusion The very minimal diode specifications are: (a)Maximum reverse voltage (b)Rated forward current (c)Maximum forward voltage drop (d)Maximum leakage current (e)Package style (f)Maximum reverse recovery time

Junction FET

The J-FET (Junction Field Effect Transistor ) and the MOS-FET (Metal-Oxide-Semiconductor FET) are voltage controlled devices: that is a small change in input voltage causes a large change in output current. FET operation involves an electric field which controls the flow of a charge (current ) through the device. In contrast, a bipolar transistor employs a small input current to control a large output current. The source, drain, and gate terminal of the FET are analagous to the emitter, collector,  and base of a bipolar transistor . The terms n-channel and p- channel refer to the material which the drain and source are connected. Below is the schematic symbol for the p-channel and n-channel JFET. A simplified n-channel JFET construction is shown below. Note that the drain and source connections are made to the n-channel and the gate is connected to the p material. The n material provides a current path from the drain to the source. An n-channel JFET is biased so that the drain is positive in reference to the source. On the other hand , a p-channel JFET with n material gate would be biased in reverse.      As with any reversed biased PN junction, a depletion region is formed which increases as the reverse gate voltage is increased. This depletion region, being devoid of majority carriers, reduces the channel drain-source current. See figure below:      As a result, the the drain-source current is controlled by the gate voltage. Referring to the figure below, a typical JFET characteristic curve; notice the effect of the gate-source voltage on the drain-source current, Ids. Notice the near linear relationship of drain current to drain voltage from zero to about one volt after that the JFET saturates. However, as the gate voltage is increased, the drain-source current is increased.      The figure of merit, Gfs, that is the ratio of drain-source current to gate voltage is the JFET transconductance. As with the bipolar transistor, there can be many relevant specifications, depending upon the JFET circuit application, however the most common parameters are: (a) Gate source breakdown voltage, BVgss (b) Gate reverse leakage current , Igss (c) Gate source cutoff voltage , Vgs(off) (d) Drain current at zero gate voltage , Idss (e) Forward transconductance, Gfs (f) Input capacitance, Ciss (g) Switching considerations (h) Drain-source on resistance, Rds(on)

MOSFETS

A FET with an oxide coating between gate and channel is called a MOSFET (metal- oxide semiconductor field effect transistor) The figure below shows the oxide, insulating the gate from the channel. As a result, the MOSFET has very high input resistance, higher than the JFET ; and as with the JFET, the gate controls the main or channel current, Ids.      Notice that a postive gate voltage will induce a negative charge in the n-channel, enhancing the drain-source current, Ids; while a negative gate voltage will induce a depletion region in the n- channel, thereby reducing the drain-source current, Ids. A MOSFET so constructed is a depletion/enhancement MOSFET. A typical n-channel MOSFET curve is shown below.       Figure (A), shows the transconductance, Gm, or effect of the gate voltage upon the drain to source current. Notice the gate-source voltage can be positive or negative. This would not be possible with a JFET. In figure (B), again notice the effect of increasing Vgs on Ids; notice also that for any given gate current, the drain-source voltage has little effect upon the drain current above 3 volts, since the MOSFET is in saturation.      The MOSFET is also made in an enhancement-only mode, where a gate signal only induces or enhances channel current ,the gate signal never depletes the channel current. Naturally there are p-channel enhancement MOSFETS, where a negative gate voltage enhances channel conductivity;  and n-channel enhancement mode MOSFETS where a positive gate voltage enhances channel conductivity. One final note, breakdown voltage in MOS devices do not depend upon p-n junction stress but rather upon the thickness and quality of the insulating oxide.When breakdown does occur, the oxide is punctured and the device is destroyed. The specifications for MOSFETS or any transistor are mutltitudinous, and so only the basics are listed. They are: (a) Breakdown Voltages (b) Forward transconductance (c) Drain- source on resistance , Rds(on) (d) Switching characteristics (e) Zero gate voltage drain current, Idss (f) Input capacitance, Ciss

SCR

The Silicon Controlled Rectifier (SCR) is simply a conventional rectifier controlled by a gate signal. The main circuit is a rectifier, however the application of a forward voltage is not enough for conduction. A gate signal controls the rectifier conduction. The schematic representation is:      The rectifier circuit (anode-cathode) has a low forward resistance and a high reverse resistance. It is controlled from an off state (high resistance) to the on state (low resistance) by a signal applied to the third terminal, the gate. Once it is turned on it remains on even after removal of the gate signal, as long as a minimum current, the holding current, Ih, is maintained in the main or rectifier circuit. To turn off an SCR the anode-cathode current must be reduced to less than the holding current, Ih. The characteristic curve is as shown below.      Notice the reverse characteristics are the same as discussed previously for the rectifier or diode, having a breakover voltage with its attending avalanche current; and a leakage current for voltages less than the breakover voltage. However, in the forward direction with open gate, the SCR remains essentially in an off condition (notice though that there is a small forward leakage) up until the forward breakover voltage is reached. At that point the curve snaps back to a typical forward rectifier characteristic. The application of a small forward gate voltage switches the SCR onto its standard diode forward characteristic for voltages less than the forward breakover voltage.      Obviously, the SCR can also be switched by exceeding the forward breakover voltage, however this is usually considered a design limitation and switching is normally controlled with a gate voltage. One serious limitation of the SCR is the rate of rise of voltage with respect to time, dV/dt. A large rate of rise of circuit voltage can trigger an SCR into conduction. This is a circuit design concern. Most SCR applications are in power switching, phase control, chopper, and inverter circuits. Major considerations when ordering a SCR are: (a) Peak forward and reverse breakdown voltages (b) Maximum forward current (c) Gate trigger voltage and current (d) Minimum holding current,Ih (e) Power dissipation (f) Maximum dV/dt

Tunnel Diode and Back Diode

TUNNEL DIODE      A tunnel diode is a semiconductor with a negative resistance region that results in very fast switching speeds , up to 5 GHz. The operation depends upon a quantum mechanic principle known as "tunneling" wherein the intrinsic voltage barrier (0.3 Volt for Germanium junctions) is reduced due to doping levels which enhance tunneling. Refering to the curves below, superimposing the tunneling characteristic upon a conventional P-N junction, we have:      Resulting in a composite characteristic which is the tunnel diode characteristic curve.      The negative resistance region (between points A and B) is the important characteristic for the tunnel diode. In this region, as the voltage is increased, the current decreases; just the opposite of a conventional diode. The most important specifications for the tunnel diode are the Peak Voltage (Vp), Peak Current (Ip) , Valley Voltage (Vv), and Valley Current (Iv). BACK DIODE      A Back diode is a tunnel diode with a suppressed Ip and so approximates a conventional diode characteristic See the comparison below:      The reverse breakdown for tunnel diodes is very low, typically 200mV, and the TD conducts very heavily at the reverse breakdown voltage. Referring to the BD curve the back diode conducts to a lesser degree in a forward direction . It is the operation between these two points that makes the back diode important. Forward conduction begins at 300 mV (for germanium) and a voltage swing of only 500mV is required for full range operation.

Typical tunnel diodes supplied by American Microsemiconductor

Part  Number	Ip Peak     Point   Current (mA)	IV Valley Point Current Max. (mA)	C Capacitance       Max.     (pF)	VP Peak Point Voltage Typ. (mV)	VV Valley Voltage Typ. (mV)	Vfp Forward Peak Voltage Typ. (mV)	RS Series Resist. Max. (ohms)	-G Negative Conductance (mhosx10-3)	fRO Resistive Cutoff Frequency Typ. (GHz)
1N3712	1.0 + 10%	0.18	10	65	350	500	4.0	8 Typ.	2.3
1N3713	1.0 + 2.5%	0.14	5	65	350	510	4.0	8.5 + 1	3.2
1N3714	2.2 + 10%	0.48	25	65	350	500	3.0	18 Typ	2.2
1N3715	2.2 + 2.5%	0.31	10	65	350	510	3.0	19 + 3	3.0
1N3716	4.7 + 10%	1.04	50	65	350	500	2.0	40 Typ.	1.8
1N3717	4.7 + 2.5%	0.60	25	65	350	510	2.0	41 + 5	3.4
1N3718	10.0 + 10%	2.20	90	65	350	500	1.5	80 Typ.	1.6
1N3719	10.0 + 2.5%	1.40	50	65	350	510	1.5	85 + 10	2.8
1N3720	22.0 + 10%	4.80	150	65	350	500	1.0	180 Typ.	1.6
1N3721	22.0 + 2.5%	3.10	100	65	350	510	1.0	190 + 30	2.6

TYPICAL ULTRA HIGH-SPEED SWITCHING TUNNEL DIODES

SUPPLIED BY AMERICAN MICROSEMICONDUCTOR

Part Number	IP        Peak point current (mA)	IV Valley Point Current Max. (mA)	C Capacitance       Max.        (pF)	VP Peak Point Voltage Typical (mV)	VV Valley Voltage Typical (mV)	Vfp Forward Voltage Typical (mV)	RS Series Resist. Typical (ohms)	T    Rise   Time Typical (psec.)
TD-261	2.2 + 10%	0.31	3.0	70	390	500-700	5.0	430
TD-261A	2.2 + 10%	0.31	1.0	80	390	500-700	7.0	160
TD-262	4.7 + 10%	0.60	6.0	80	390	500-700	3.5	320
TD-262A	4.7 + 10%	0.60	1.0	90	400	500-700	1.7	350
TD-263	10.0 + 10%	1.40	9.0	75	400	500-700	1.7	350
TD-263A	10.0 + 10%	1.40	5.0	80	410	520-700	2.0	190
TD-263B	10.0 + 10%	1.40	2.0	90	420	550-700	2.5	68
TD-264	22.0 + 10%	3.80	18.0	90	425	600 Typ.	1.8	185
TD-264A	22.0 + 10%	3.80	4.0	100	425	550-700	2.0	64
TD-265	50.0 + 10%	8.50	25.0	110	425	625 Typ.	1.4	100
TD-265A	50.0 + 10%	8.50	5.0	130	425	640 Typ.	1.5	35
TD-266	100 + 10%	17.50	35.0	150	450	650 Typ.	1.1	57
TD-266A	100 + 10%	17.50	6.0	180	450	650 Typ.	1.2	22

Transient Voltage Supressor

Transient Voltage Suppressors (TVS) are semiconductor devices designed to provide protection against voltage and current transients.The silicon TVS is designed to operate in the avalanche mode and uses a large junction area to absorb large transient currents. Operation in the avalanche mode insures a low impedance ; also the TVS is characterized by a fast response time. The TVS is available as unipolar or bipolar(that is it can suppress transients in one direction or in both directions. The typical characteristic curve for a bipolar TVS is shown below: Referring to the curve:      The breakdown voltage (Vbr) is the point the TVS device enters avalanche, a high conductance region. This Vbr is measured at test current It. A circuit with a TVS protection would obviously operate below the voltage, Vrwm, also referred to as the working voltage. Ir is the maximum current measured at the working voltage. The maximum peak pulse current for a TVS is Ipp. The maximum clamping voltage,Vc,is the maximum voltage across the TVS when it is subjected to Ipp.

Triac

The triac is a three terminal semiconductor for controlling current in either direction. Below is the schematic symbol for the triac. Notice the symbol looks like two SCRs in parallel( opposite direction) with one trigger or gate terminal.The main or power terminals are designated as MT1 and MT2 . (See the schematic representation below) When the voltage on the MT2 is positive with regard to MT1 and a positive gate voltage is applied, the left SCR conducts. When the voltage is reversed and a negative voltage is applied to the gate, the right SCR conducts. Minimum holding current, Ih, must be maintained in order to keep a triac conducting.      A triac operates in the same way as the SCR however it operates in both a forward and reverse direction. To get a quick understanding of its operation refer to its characteristic curve below and compare this to the SCR characteristic curve. It can be triggered into conduction by either a PLUS (+) or MINUS (-) gate signal.      Obviously a triac can also be triggered by exceeding the breakover voltage. This is not normally employed in triac operation. The breakover voltage is usually considered a design limitation. One other major limitation, as with the SCR, is dV/dt, which is the rate of rise of voltage with respect to time. A triac can be switched into conduction by a large dV/dt. Typical applications are in phase control, inverter design, AC switching, relay replacement, etc. Major considerations when specifying a triac are: (a) Forward and reverse breakover voltage. (b)Maximum current (c) Minimum holding current (d) Gate voltage and gate current trigger requirements. (e) Switching speed (f) Maximum dV/dt Varactor Diode      The varactor diode symbol is shown below with a diagram representation.           When a reverse voltage is applid to a PN junction , the holes in the p-region are attracted to the anode terminal and electrons in the n-region are attracted to the cathode terminal creating a region where there is little current.This region ,the depletion region, is essentially devoid of carriers and behaves as the dielectric of a capacitor.      The depletion region increases as reverse voltage across it increases; and since capacitance varies inversely as dielectic thickness, the junction capacitance will decrease as the voltage across the PN junction increases. So by varying the reverse voltage across a PN junction the junction capacitance can be varied .This is shown in the typical varactor voltage-capacitance curve below.      Notice the nonlinear increase in capacitance as the reverse voltage is decreased. This nonlinearity allows the varactor to be used also as a harmonic generator. Major varactor considerations are: (a) Capacitance value (b) Voltage (c) Variation in capacitance with voltage. (d) Maximum working voltage (e) Leakage current

Zener Diode

Refer to the characteristic curve of a typical rectifier(diode) in the figure below. The forward characteristic of the curve we have previously described above in the DIODE section. It is the reverse characteristics we will discuss here.      Notice that as the reverse voltage is increased the leakage current remains essentially constant until the breakdown voltage is reached where the current increases dramatically. This breakdown voltage is the zener voltage for zener diodes. While for the conventional rectifier or diode it is imperative to operate below this voltage; the zener diode is intended to operate at that voltage, and so finds its greatest application as a voltage regulator. The basic parameters of a zener diode are: (a) Obviously,the zener voltage must be specified. The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available.   (b) A tolerance of the specified voltage must be stated. While the most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available . A test current (Iz) must be specified with the voltage and tolerance. (c) The power handling capability must be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1 , 5, 10, and 50 Watts.